The Boatowner's Guide to Corrosion: A Complete Reference for Boatowners and Marine Professionals 3999906967

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“The best explanation that III BOSTON PUBLIC LIBRARY f-r-” 1 ij .

r of Boatowner’s

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06967 515 3

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have

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Mechanical and Electrical Manual

The Boatowner's Guide to Corrosion

A COMPLETE REFERENCE FOR BOATOWNERS AND MARINE PROFESSIONALS

Everett Collier

International Marine

/

McGraw-Hill

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— wife and sons,

in that

order

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is

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:

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Includes bibliographical references and index. ISBN 0-07- 1550 19-4 1.

Boats and boating

C.'athodic protection.

— Ciorrosion.

2.

Ships

Title.

1.

VM951.C585 2000 623.8'48— dc2

00-033488

1

/

1

1

should

lill

1-800-262-4729 -800-722-4726

WARNlNCi:

Repairing and maintaining your boat can expose you, your boat, and your equipment to potentially dangerous situations and substances. Reference to brand names does not indicate endorsement of or guarantee the safety of using these products. In using this book, the reader releases the author, pub-

and

from

any loss or injury, including death, allegedly caused, in whole or in part, by relying on information contained in this lisher,

book.

Paperback ISBN 0-07- 47544-3

book should

Marine

International

responsi-

this

distributt)r

liability for

1

Contents

List of Tables

Introduction

/

Appendix 1. Abbreviations and Symbols / 220

iv

v

/

Acknowledgments

1.

3.

10

Appendix

5.

Weights of

Corrosion Processes

/

Self-

24

Galvanic Corrosion

34

/

Corrosion

42

/

Aluminum and Aluminum /

58

/

/

Copper and Copper Alloys

88

/

12.

Other Marine Metals

13.

Cathodic Protection Systems

14.

Corrosion Avoidance

15.

Coating to Prevent Corrosion Corrosion

9.

/

/

/

97

106

Propulsion Systems

/

1

1

and Electronic Systems

Plumbing Systems

/

1

80

2

Masts, Spars, and Rigging

/

1

13

5

Deck Gear

.

/

129

20. 1

102

/

89 /

1

99

/

167

/

230

Common

278

/

Appendix

7.

Properties of Metals

280

Appendix

8.

Glossary

Index

83

/

Equivalent Grades

Appendix 6. Wire Table: Standard Annealed Copper Wire (AWG)

Bibliography

75

and Nickel Alloys

18. Electrical

Materials

17

/

/

227

/

10. Nickel

1

/

Theory

9.

7.

Anodes

UNS

Alloys

1

Quantity, Size, and Placement

4.

Electrochemical or

16. Hull

3.

Appendix

Iron and Iron Alloys

.

Appendix

of Zinc

1

8.

1

Conversion Eactors

4

7. Electrolytic

1

2.

/

Corrosion 6.

/

222

Appendix

Chemical Theory

Basic Electrical

4. Basic 5.

Theory

Basic Molecular

2. Basic

vi

/

/

309

/

303

/

289

/

279

Tables

22-1. 3-

Atomic Symbols and Weights of Elements

/

42-2.

Common

8-

11

Common

4

Steels (percentage

Chemical Formulas and Molecular Weights / 5 2-3. Elements and Their Properties / 6 4. Elements and Their Valence Numbers / 7 1. Resistivity of Selected Common Metals /

9-

1.

Galvanic Series of Metals

(ABYC

in

Seawater

Standard E-2)/ 20

Chemical Contents / 62 8-2. Chemical Properties of Cor-Ten Steel 8-3. Factors That Affect Stainless Steels in Water / 67

5.

/

63

Resistance of Stainless Steels to Pitting /

Alloy Designations

Foss

in Tensile

/

/

1

88

/

Bronze Family and 92

1-2.

UNS

Designations/

A7-1. Chemical Properties of Fow-Carbon

28 A7-2. Mechanical Properties of Plain Carbon (irades

/

Steels/

282

1

A7-3. Mechanical Properties of Standard Alloy Steels (Annealed)

/

282

A7-4. iVlechanical Properties of Standard Alloy Steels

(Normalized)

/

282

A7-5. C'omposition of Martensitic Stainless Steels (percentage by weight)

/

282

A7-6. C'omposition of Ferritic Stainless Steels (percentage by weight)

/

283

A7-1 1. Mechanical Properties of HighPerformance Austenitic Stainless Steels

/

Performance Duplex Stainless Steels A7-14. Nominal Composition of 6%

/

284

Superaustenitic Alloys /

284

A7-15. Mechanical Properties for 1000, 2000, and 3000 Series Wrought Alloys / 285

Major Copper Alloy Families and Their Designations

284

(percentage by weight)

81

UNS

/

Molybdenum

Strength after 7 Years of

Exposure / 79 9-3. Average Weight Foss and Maximum Depth of Pitting for Aluminum Plate Specimens after Immersion in Seawater 11-1.

Common

A7-13. Mechanical Properties of High-

71

76 9-2. Average Atmospheric Corrosion Rates and 1.

283

284 A7-12. Composition of High-Performance Duplex Stainless Steels (percentage by weight) / 284

67

Corrosion: Pitting Index

/

Marine Grades of Austenitic Stainless Steels and Their Chemical Compositions (percentage by weight) / 283 A7-9. Common Marine Grades of Austenitic Stainless Steels and Their Mechanical Properties / 283 A7-10. Composition of High-Performance A7-8.

weight)

8-4. Chloride Concentrations in Various /

by weight)

Austenitic Stainless Steels (percentage by

8-1.

Waters

A7-7. Composition of 18-8 Austenitic Stainless

A7-16. Mechanical Properties for Popular

Marine Alloys / 285 A7-17. Nominal Chemical and Mechanical Properties of Marine Nickel-Base Alloys / 285 A7-18. Nominal Chemical and Mechanical Properties of Representative Copper Alloy Pypes / 285 A7-19. Nominal Cdiemical and Mechanical Properties of Representative High-Copper Alloy Fypes / 286 A7-20. C^temical and Mechanical Properties of the Marine Brasses / 286 A7-21. Cdiemical and Mechanical Properties of the Marine Bronzes / 287 A7-22. C'hemical and Mechanical Properties of the Copper-Nickel Alloys / 288

Introduction

Countless excellent articles on various aspects of

marine corrosion, written by knowledgeable and experienced professionals, have appeared in marine periodicals and trade magazines. Much valuable information on the subject is contained in the many fine books that have been published on the broader subjects of boatbuilding, maintenance, and repair. A wealth of valuable information, current with the present state of technology in metals and alloys, coatings and

problems that typically occur in these systems, and some solutions. The second group is composed of boatyard and marina owners and operators, repairers, and surveyors to whom the folks in the first group turn when they have a problem they can’t solve. This group must therefore take a more thorough and analytical approach to problem solving based on a more in-depth understanding of the corrosion process. These readers may

barriers, exists in the literature of other related

find the basic foundational information in chap-

communities such as the chemical process and the petrochemical industries. However, nowhere in the current marine literature is there one place where all this useful information is brought together for ready reference by the weekend boaters who can’t use

ters

industrial

their boats, or the

boatyard operators to

they turn for help, or the boatbuilders

—

whom

— back-

yard or professional who would like very much to head off corrosion problems before they occur.

This book

is

written and organized princi-

pally for three groups within the marine

munity.

First

weekend

sailors, sportfishers,

fishers

who

are

the

recreational

com-

boaters,

and commercial

either don’t have or can’t afford to

spend the time and must solve their corrosion problems quickly and as simply as possible. They are not corrosion or chemical engineers and are not interested in the underlying chemistry and physics of corrosion science. They simply have a problem and want to solve it as quickly as possible. These readers can safely skip over the first five chapters, which are devoted to the underlying physics and chemistry and the various types of corrosion. Depending on the nature of the problem, readers can go directly to chapters 16 through 2 each of which deals with a specific 1 ,

vessel system, the corrosion

1

through 7 helpful before going on to the

later chapters.

The third group consists of designers, builders, and restorers who may not only have to solve existing problems but also have the opportunity to take steps to prevent or mitigate corrosion problems and extend the trouble-free life

of the boat. This group will also find

much

useful information in chapters 8 through 15,

which cover in some depth the various marine metals and coatings, including the more recent alloys and formulations that are currently available. This group will also find much useful information

in

There are

the appendices. (at least)

two schools of thought on

the broad, generic definition of corrosion.

One

would limit the definition to metals; the other would include any and all materials of construction. The latter group would define corof these

rosion as the deterioration of a material of con-

struction or of

its

properties as a result of

exposure to an environment. This group seems to have history on its side, and it is this definition that has been followed in this book. Finally, would leave you with the following bit of humorous doggerel written by a former I

president of the National Association of Ck)rrosion Faigineers

(NACF^.

1

recommend

this little

V

vi

Introduction \erse nor only for that

lies

buried

in

its

it,

humor

just

but for the truth

below the humor.

mostly Fe 203 And when the ocean meets the shore.

We We We We We We We

You’ll find there’s Fe 204

But heaven knows we’ll never stop

Mighty ships upon the ocean Suffer from severe corrosion. Even those that stay at dockside Are rapidly becoming oxide. Alas, that piling in the sea Is

.

Cause when the wind is salt and Things are getting awfully rusty.

gusty.

can measure can halt

it

We

can

or arrest

can gather can coat

it.

it

examine and

it.

cathodically protect

it.

it

it.

can spray

dissect

can pick

it.

it.

and weigh

We

it.

test

up and drop

it.

it.

No

doubt about it. Most of us would starve without it.

So here’s to

T. R. B.

rust!

Watson

Acknowledgments

One

of the

more

difficult tasks in writing a

book such

as this

is

the need

and the desire to acknowledge those who have contributed to the project. There are those who contributed from their technical and scientific knowledge and experience, others from their vast store of experience in the marine industry, and still others from their unique and proprietary knowledge of their products and ser\ ice. k'inally, there are those whose contributions were made over the years through informal teaching and training in boatyards, boatshops, magazine articles, and books too numerous to count but too valuable to ignore. In order to avoid the risk of offending by omission. I’ll not attempt to identify them here the list would be too long in any case but they know who they are and they have here, again, my thanks.

—

it.

—

—

1

a

Chapter

Basic Molecular

Theory

Corrosion is a chemical reaction, or more precisely, an electrochemical reaction. The chemical properties of matter are largely determined by the electrons around the nucleus of its atoms, particularly the outer or valence electrons.

Un-

derstanding corrosion requires an elementary grasp of the structure of materials.

ATOMS AND MOLECULES The atom

is

electrically neutral;

has no elec-

charge. That’s because the electrical charges

trical

of the particles that comprise the

each other. The core of the atom It

it

consists of

two types

atom balance is

the nucleus.

of particles

— protons

carrying a positive charge and neutrons carrying

no charge. Thus, the net charge on the nucleus positive.

MATTER Matter

This positive charge

the stuff that everything

is

is

made

of

even people. Matter can exist as a solid, a liquid,

around the nucleus

out from the core or nucleus. Each shell can

these forms, but primarily

hold only a specific number of electrons maximum of 2 electrons in the first or lowest en-

which the corrosion is taking place). The gases, mostly hydrogen and oxygen, help us explain what happens to all the liquids (the seawater in

in the

corrosion process. Matter

can also be classified according to tion:

whether

it’s

its

composi-

an element^ a compound, or a

mixture.

going to sound a little like “the footbone’s connected to the shin-bone, the shinbone’s connected to the knee-bone,” but a basic This

is

understanding of the skeletal structure of matter is

necessary

if

we’re going to be able to deal

fectively with corrosion

on

like

will be dealing

We

with solids (the metals that are corroding) and

matter involved

something

the layers of an onion, each increasingly farther

in the

but that’s not our problem.) in all

in “shells”

plasma form,

can also exist

with matter

balanced by negatively

helps to think of electrons as orbiting

cles. It

(It

is

charged electrons, the atom’s remaining parti-

metals, woods, plastics, petunias, soda pop, and

or a gas.

is

ef-

problems encountered

—

ergy shell, 8 for the second shell, least

cupied but not simplest

is

is

close, the

then the second

filled first,

in-

filled. Let’s

atom of them

all,

Hydrogen has only one the

take a look at the

atom

the hydrogen

l-l).

(fig.

first shell

a

as

most

electron,

shown. Since the

maximum

of

two

logically expect to find an

shown

bottom of it all. So let’s focus on atoms since that is where electrons hang out.

requires the

The number of electrons actually present in the shells of an atom depends upon the specific element, and a shell may be empty or may be oc-

trons, and, alas, electrons. electrons that are at the

shell

8 for the

nermost, and so on.

trons

those rascally

it

energy to orbit a nucleus that

Matter consists of molecules. Molecules consist of atoms. Atoms consist of protons, neuIt is

for the fourth. Because

innermost

hold

a boat.

32

third,

I

in its first shell. in figure 1-2,

and

it is

first shell

electrons,

in

can

we might

atom with two

elec-

Indeed, the helium atom,

has two electrons

in its inner-

shell.

Were we to look for an atom with three electrons, we would expect to find two of these in

1

2

Basic Molecular

Chapter!

Theory valence

electrochemical) reactions. This

ical (or

is

called

The valence of an atom is determined by the number of electrons in its outermost, or valence, shell. When the outermost shell is completely filled, the atom is in its most stable state

valence.

and

least inclined to enter into

tions. In

chemical reac-

combining with other atoms,

it

will tend

to gain or lose valence electrons in order to attain a stable configuration.

Molecules are made up of two or more atoms.

The atoms may be of the same element or of two or more different elements. There are 110

known

elements, fundamental substances that

consist of

or

The hydrogen atom, with

Fig. 1-1.

just

and one in the second. L.ithium is such an atom. Additional electrons would this

second

shell until

of 8 electrons, giving us an lO electrons.

it

held a

maximum

atom with

An atom with

1

1

a total of

electrons

would

have 2 electrons

in the first shell, 8 in the sec-

ond

in the third shell

shell,

and

1

— and so on.

OK so far? Now let’s consider the tendency of an atom to gain or lose electrons and, amounting to the

same

all

matter.

atoms, carry no electrical

like

charge.

first shell

go into

combination, constitute

Molecules,

a single valence

electron.

the

in

atoms of only one kind. These, singly

thing,

its

tendency to participate

in

chem-

IONIZATION more fundamental concepts we should talk about. The first, ionization^ is the process by which an atom or molecule acquires a charge. Atoms or molecules that have either gained or lost one or more electrons are known

There are

a couple

as ions. Ions consequently bear positive or negative charges,

and

significantly for us, the flow

of ions constitutes an electrical current. Ioniza-

fundamental to the corrosion process. We’ll talk more about it in chapter 2. tion

is

CHEMICAL BONDING The second concept bonds are

is

chemical bonding. Chem-

atoms of matter together. The forces that cause the bonding between atoms result from the interactions between charged particles the positive protons and the negative electrons. The atoms of some elements become more stable when they acquire one or more electrons; others become more stable when they give up electrons. ical

links that join the

—

Due

to their

give

up electrons.

compelling tendency to maintain electrical neutrality, atoms can’t simply take on t)r

they try Fig. 1-2.

trons.

The helium atom, with

its

two valence elec-

forming chemical bonds, to attain increased stability without givIn

ing up their electrical neutrality. There are a

number

of different types of chemical bonds, but

— Chapten for our purposes vve can consider

two

basic

covalent bonds and ionic bonds. The specific nature of the bonds formed depends

is

•

called an ionic bond.,

Basic Molecular

Theory

and the overall

electri-

An exam-

types

cal

upon

form ionic bonds are sodium (Na) and chlorine (Cl), which combine to form sodium chloride (Na"^Cl~), or salt.

the nature of the elements involved. This

an important consideration

in

is

corrosion.

charge of the molecule

is

neutral.

ple of relevant elements that

Covalent Bonding Atoms

of elements that

become more

stable by

gaining electrons can maintain electrical neutrality

by sharing electrons with other atoms of

same or smiilar elements. When atoms form chemical bonds by sharing electrons, this is called covalent bonding. The molecules formed the

by covalent bonding are designated with a sub-

showing the number of atoms. For examwhen two hydrogen {H) atoms combine by

script ple,

sharing electrons, the resulting molecule

ignated into

O2

,

H

is

des-

oxygen atoms combine nitrogen atoms into N 2 2.

Similarly,

.

Ionic

Bonding

Atoms

of elements that

become more

losing electrons can give

stable by

up electrons to atoms

become more stable by gaining them. However, the atom giving up the electron becomes a positive ion while the atom gaining an electron becomes a negative ion. Since oppoof elements that

sitely

the

charged bodies are attracted to each other,

two atoms

are

now bound

together by what

SUMMARY To sum up what we’ve covered so far, atoms are tiny energy systems bound together by the force of attraction between the positive nucleus and the negative electrons. There are

1

10

known

el-

fundamental substances that consist of atoms of only one kind and, singly or in various combinations, constitute all matter. Atoms link together to form various compounds through chemical bonding. An atom is in its most stable (lowest energy) state and least inclined to gain or lose electrons when its outermost shell is filled. An atom becomes a positive ion when it loses a valence electron and a negative ion when it gains an electron. Ions and electrons are the charge ements.,

carriers that constitute electrical current flow,

and ionization

is

a critical part of the corrosion

process. In

chapter 2

cesses by to

we examine

the chemical pro-

which atoms and molecules combine

form various materials.

3

Chapter 2

Theory

Basic Chemical

Now

that

parts

and

we have

defined

all

the pieces and

their peculiarities, let’s see

go together. This requires

a light

how

they

review of a few

basics in chemical theory.

atomic weight of an element

is

a quantity equal

atomic weight of the element expressed in grams. For example, the atomic weight of sodium is 23. This means that 23 grams repreor to put it sents 1 atomic weight of sodium differently, 1 mole of sodium weighs 23 grams. Table 2-1 gives the symbols and the approximate atomic weights for some of the common elements that we may encounter in the marine to the

—

SYMBOLS In chemistry, is

an abbreviation

—called

a

symbol

—

used to designate a particular chemical element.

The symbol is usually the first letter or the first letter and another letter of the English name of the element. For example, O is the symbol for oxygen, H is the symbol for hydrogen, and Cl is the symbol for chlorine. In some cases, however, the symbol is derived from the Latin name of the element. We see this in the symbol for sodium, Na, which comes from natrium^ sodium’s Latin name.

environment.

FORMULAS As we discussed in chapter 1, atoms combine to form molecules. Molecules are made up of two or more atoms, either of the same element or of different elements. The formula for a given mol-

Each symbol also represents a definite quantity of the element the gram-atomic weighty or mole. The mole is an important concept in

—

chemistry. In dealing with chemical reactions

must he able involved that

we

in

to

account for

the reaction.

start

all

That

of

Common

of the materials

is, all

of the atoms

with before the reaction takes place

work with than 6.02

2-1

Atomic Symbols and Weights Elements

we

must he present or accounted for in the materials resulting from the reaction. We do this by keeping track of the atoms using their atomic weights and the mole. d he term mole can he confusing. Its meanings range from a small pointy-nosed animal to a stone breakwater that encloses a harbor, and in between we have international spies, tunneling machines, and a spicy Mexican sauce. Let’s he clear as to what we mean when we use the term here. A mole is a quantity, like a dozen. A dozen is 12 anythings. A mole is 6.02 x 1()-’ anythings. The term mole is used because it’s a whole lot easier to

Table

x lO-L

The symbol of an element represents atomic weight or mole of that element. So, the gram-

Chemical

Element

Atomic Symbol

Atomic Weight (grams)

hydrogen

H

carbon

C

12

nitrogen

N

14

oxygen

0

16

sodium

Na

23

aluminum

Al

27

silicon

Si

28

chlorine

Cl

35-5

titanium

Ti

47-9

manganese

Mn

54-9

iron

Fe

55.8

nickel

Ni

58.7

copper

Cu

63.5

zinc

Zn

65.4

molybdenum

Mo

95-9

tin

Sn

1

1

4

118.7

6

Chapter! ecLile consists

of the symbols for

elements, and

if

all

Table 2-2

the included

the molecule contains

more

Common

Chemical Formulas and Molecular Weights

than one atom of any element, the formula incorporates a subscript following that element to indicate the

number

Molecular

of atoms.

Compound

Formula

sodium chloride

NaCl

58.5

water

H^O

18

hydrogen peroxide

H.O,

34

sodium hydroxide

NaOH

40

copper oxide

CuO

79-5

aluminum oxide (alumina)

AI,Oj

102

Fe,03

159-6

A

chemical formula also represents 1 grammolecular weight, or mole. For example, the for-

mula for a hydrogen molecule is H 2 and its gram-molecular weight is 2 grams. We know this because hydrogen atoms (H) have a 1 gramatomic weight each, and there are two such atoms in Hi. The formula for water is H 2 O, and its grammolecular weight is 18 grams. Two hydrogen atoms at gram each plus one oxygen atom at grams give us a total of 8 grams. Similarly, a sodium chloride (salt) molecule, represented by the formula NaCl, consists of one atom of sodium (Na) and one of chlorine (Cl). The atomic weight of the sodium atom is 23 and the atomic weight of the chlorine atom is 35.5, so the total molecular weight of sodium chloride is 58.5. Use of the mole is not limited to atoms. We

Theory

Basic Chemical

•

,

oxide (rust)

ferric

Weight

1

1

1

neutral, having

no

electrical charge.

Protons have

charge (+1.602 x

a unit positive electrical

10"'*^).

Electrons, 1/1837 the mass of a proton, have a unit negative electrical charge (-1.602 x

The

lO"'*^).

electrons are located outside the nucleus,

arranged

in

number

shells. Shell

Atoms

increasingly distant orbits, called is

1

the innermost shell.

are electrically neutral since they con-

an equal number of protons (positive charge)

also use this term to express the quantity of

tain

Where a mole of atoms represents the number of atoms in a gram-atom, a mole of molecules represents the number of mole-

and electrons (negative charge). The number of

molecules.

cules in a gram-molecule.

We

protons or the number of electrons

atomic number. Notice that as the atomic numbers of the atoms increase, electrons fill the

shells in a prescribed fashion.

or to the gram-molecular weight of a poly-

electrons.

atomic (more than one atom) element, such as Fi 2 or to the gram-molecular weight of a compound such as NaCl. Notice that in H 2 we would have 2 moles of hydrogen atoms or mole of hydrogen molecules. Table 2-2 gives some of the common chemical formulas and molecular weights that will be of interest to us in the marine environment.

of 8

,

1

referred to

as the

can use the term mole to refer to the gram-atomic weight of a single atom [monatomic) element, such as Na,

,

is

most

shell

is

filled first

and

The

is filled

first

or inner-

when

it

has 2

The second shell can hold a maximum electrons. The third shell can hold a maxi-

mum

1

8.

There are exceptions to

ample, as

in

argon, potassium, calcium, and iron)

of

this (for ex-

having to do with subshells and energy levels. However, that is more detail than is necessary for our purpose. What is significant here for us is

that

it is

the electrons in the outer or valence

shell that are

involved

in

the corrosion process

and whether the particular atom involved considers that shell to be

ATOMIC STRUCTURE We said

composed of three electrons, protons, and neu-

earlier that

basic particles

—

There

an atom

is

and neutrons are of roughly equal size and make up the nucleus. The total of the number of protons and neutrons is referred to trons. Protons

as the 7nass

number. Neutrons are

electrically

is

no

full.

truly satisfactory

shells pictorially; figure 2-1 fied representation of the

of an

atom

to

show

the

a greatly simpli-

concept.

The tendency

to participate readily in chemical re-

actions or changes electrons in

is

way

its

is

related to the

outermost

number

of

shell.

Table 2-3 gives the properties of the elements

5

6

Chapter!

•

Basic Chemical

Theory

valence number = +3

in

atomic number =

mass number

13

order of their atomic and mass numbers. The

neutrons

in

is

the total

number

of protons and

the nucleus.

Note that the outermost electron shells for hewith 2, 8, lium, neon, and argon are all “filled” and 8 electrons, respectively. These elements belong to a group called the noble or inert gases. Noble gases are extremely stable and show little

—

tendency to enter into chemical reactions. They are very

seldom found

thing in nature seeks to

we can conclude from

compounds. Since everyattain a more stable state,

in

the noble gases that

elements combine, they seek to

fill

their

when

outermost

electron shells, thus attaining greater stability.

COMBINING CAPACITY Fig. 2-1.

The combining capacity

Electron shell structure of an aluminum

atom.

ber of electrons

it

of an

can gain,

atom

lose,

is

the

num-

or share in the

Table 2-3

Elements and Their Properties

Number

Nucleus

Atomic

Mass

Number

Number

p = protons n = neutrons

1

ip

1

of Electrons

in Shells

Element

Symbol

hydrogen

1

1

helium

H He

2

4

2p + 2n

2

lithium

Li

3

7

3P +

4n

2

1

beryllium

Be

4

9

4P

+ sn

2

2

boron

B

5

11

5P +

6n

2

3

carbon

C

6

12

6p + 6n

2

4

nitrogen

N

7

14

7P + 7n

2

5

oxygen

0

8

16

8p + 8n

2

6

fluorine

F

9

19

2

7

neon

Ne

10

20

9P + ion lop + ion

2

8

sodium

Na

11

23

lip + i2n

2

8

1

magnesium

Mg

12

24

i2p + i2n

2

8

2

aluminum

Al

13

27

13 P +

140

2

8

3

silicon

Si

14

28

14 P + 140

2

8

4

phosphorus

P

15

31

15P + i6n

2

8

5

sulfur

S

16

32

i6p + i6n

2

8

6

chlorine

Cl

17

35

17P + i8n

2

8

7

argon

Ar

18

40

i8p + 22n

2

8

8

potassium

K

19

39

19P + 2on

2

8

8

1

calcium

Ca

20

40

2op

+

2on

2

8

8

2

iron

Fe

26

56

26p

+

3on

2

8

14

2

copper

Cu

29

64

29 P + 35 H

2

8

18

1

zinc

Zn

30

65

3 op + 3 Sn

2

8

18

2

2

4

3

Basic Chemical

Chapter 2 Table 2-4

Theory

o

o

Elements and Their Valence Numbers Element

Symbol

Valence

Number

aluminum

Al

+3

calcium

Ca

+2

chlorine

Cl

-1

copper

Cu

+1,

fluorine

F

-1

iron

Fe

+ 2, +3

lithium

Li

+1

magnesium

Mg

+2

oxygen

0

-2

potassium

K

+1

sodium

Na

+1

sulfur

S

-2

zinc

Zn

+2

+2

cLb Lewis dot diagram of a chlorine molecule depicting covalent bonding. Fig. 2-2.

Atoms may

more

attain a

sharing electrons. This

is

stable structure by

called covalefit

bond-

Each atom exerts the same amount of attractive force on the bonding electrons, so there is no strong tendency for one atom to lose an ing.

electron or for another to gain an electron. Fig-

ure 2-2 depicts covalent bonding in the chlorine

forming of chemical bonds. This is the valence number. The atom that loses electrons has a pos-

molecule. The trons

— those

o’s

represent the valence elec-

in the

outermost

shell

— of one

combining capacity, or positive valence number. The atom that gains electrons has a negative combining capacity, or negative valence

chlorine atom, and the x’s represent the valence

number. Electron transfer refers to both the donating and the receiving of electrons since both processes occur simultaneously. An electron transfer forms oppositely charged ions the socalled ionic bond. Atoms may also combine by

positively charged particle called a positive ion.

itive

—

electron sharing. the result

is

When atoms

Chemists are able to predict valence numbers from the electronic structure of the atoms, as given in table 2-4.

COVALENT, IONIC, AND POLAR

BONDS Chemical bonds are the means by which atoms and molecules are held together to form the compounds and mixtures we’ll be dealing with.

More to the point, corrosion is the breaking of some bonds and the formation of new ones. Since the nature of the bonds, along with the

atoms and molecules involved, determines the nature and the rate of the reaction, we should spend a little more time on chemical bonds and the bonding process.

An atom An atom

that gives

up an electron becomes

a

that gains an electron has a negative

charge and

is

called a negative ion.

The

attrac-

tion between oppositely charged ions constitutes

an ionic bond or polar bond.

share electrons,

referred to as a covalent bond.

specific

electrons of the other chlorine atom.

Elements whose atoms give up electrons readily

are said to be metallic.

Those whose atoms

gain electrons readily are said to be nonmetallic.

An

bond results from the transfer of electrons from an atom of a metallic element to an atom of a nonmetallic element. In general, or 2 electrons in their outermost atoms with shells are metals and have a tendency to give up electrons. Those with 6 or 7 electrons in their outermost shells are nonmetals and have a tenionic

1

dency to gain electrons.

One more

clarifying concept

is

necessary.

and covalent bonds represent the two extremes in bond formation. Pure ionic or pure covalent bonds rarely exist in nature. All bonds are Ionic

covalent

in

the sense that electrons are being

The degree to which the bond is also ionic depends on the relative attractions the atoms have for the electrons. II one atom exerts a shared.

7

8

Chapters

•

Basic Chemical

Theory

greater attractive force, shared electrons will

spend more time nearer the nucleus of that atom, giving

it

a relatively negative charge. Its partner

atom assumes a relatively positive charge. Thus, the bond will have a definite ionic character. Consider the bonding between the oxygen atom and the hydrogen atoms in the water molor more precisely, H-O-H, where ecule, H 2 O

—

Oxygen has

the dashes represent the bonds.

a

greater attraction for electrons than hydrogen so

toward the oxygen and away from the hydrogen, and the

the shared electrons are displaced

bonds are

ionic.

Since the electrons in an ionic to the

atom with

bond are

closer

the greater attractive force, the

EQUATIONS A

chemical equation

means of

a graphic

is

playing what happens

dis-

chemical reaction, just as a mathematical equation shows what happens in a numerical reaction. In both equain a

shown on

tions, the starting quantities are

the

left-hand side and the resulting quantities are

shown on

the right.

The two

sides of a numeri-

equation are separated by an equal (=) sign. In the chemical equation, they are separated by a reaction arrow {^) that means “yields” or

cal

“forms.” In both types, all of the quantities on the left side of the equation must be accounted for

on the

right side of the equation.

molecule has a nonsymmetrical shape that allows one end to be relatively positive and the other end to be relatively negative. The molecule is said to be a polar or dipole molecule. This is

equation are referred to as the reactants. Those on the right side are referred to as the products

illustrated in figure 2-3.

present

When bonds

are formed, the products result-

The

on the

quantities

left

side of a chemical

of the reaction. Since atoms and other particles reactants are neither created nor

in the

destroyed

in the reaction,

they must be present

from the change are more stable than the starting material. The increased stability is accompanied by decreased potential energy, so en-

and accounted

ergy has been released. Conversely, energy

cate parenthetically the phase or state of the

ing

consumed when bonds

O

is

are broken.

the reaction.

form or

for in the products, or results, of

They may however, change

state, so

terials in the

it is

common

equation.

their

practice to indi-

The following

ma-

letters are

used to indicate each phase:

O

(s)

one

o

(I)

negative (g)

o

oxygen

(aq)

solid liquid

gas

aqueous (dissolved

in

water)

pole (-2) Let’s

two

CTP

A

CTP

+

B2

^

AB

positive

hydrogen poles (+i)

Lewis dot diagram of a water molecule depicting its nonsymmetrical shape. Because the oxygen atom has a greater attraction for electrons, the shared electrons are displaced toward the oxygen atom and away from the hydrogen atoms. This gives the water molecule its angular, nonsymmetrical shape, allowing the hydrogen ends to be positive and the oxygen atom at the apex to be negative —a polar or dipole molecule. This shape gives water Fig. 2-3.

its

consider a generalized reaction:

exceptional qualities as a solvent.

This equation says that element

A and

ele-

form the compound AB. Notice, however, that we have two B atoms on the left side of the equation and only one atom on the right. For the equation to be most useful, it must be balanced; that is, it must account for all of the atoms and other particles on both sides of the equation. This is done by using numerical coefficients before the symbols and formulas. Thus, the equation becomes

ment B combine

to

2A Let’s see

how

+ B2

this

^

2AB

would look

rusting of iron (Fe). Both water

in the

case of

and oxygen are

Chapter 2 required for the formation of iron rust. Rust

what

is

the chemists refer to as a hydrated (chem-

combined with water) which the amount of water is ically

oxide

ferric

variable, so

in

we

an .v as the coefficient for the water component. The formula would look like this: will use

2 Fe(s) + IV2 02(g) +

The equation reads solid Fe react with

O to form

^/i

1

molecular weight of solid

1

Fe

and the bonding

is

is

rust,

in a

Fe 703

+3 valence

predominantly

carries a negative charge while the

this polar

carry positive charges.

It is

the water molecule that

makes

an ionic substance

immersed

hydrogen ends

good solvent. sodium chloride

it

like

nature of

a

undergoes dissociation, dissolving into positive and negative ions, (salt) is

water,

in

it

which are then surrounded by water molecules. This solution

is

called an electrolyte. Electrolytes

are electrical conductors in which current

ionic.

For chemical analysis of a reaction to be most

we must account

oxygen molecule and two small hydrogen atoms. There are two H-O bonds that form an angle of about 105°. The result is a V-shaped molecule. The oxygen end, at the apex,

When

atomic weights of gaseous

ried

by ions rather than by

liquid to gas. Conversely, energy

sents a definite quantity of the element.

is

liberated in the

from gas to liquid to solid, which is what happened to the O 2 in our chemical reaction. We show this in the above equation as follows: shift

2 Fe

(s)

+

1

V2

O2

(g)

+ X

H2O

^

(s)

more

+ Fleat

rapidly at

You need not write or balance chemical equations to combat corrosion, higher temperatures.

we have covered

here will

in

the corrosion process

and

also occur in the electrolyte. Seawater, the

medium is

in

which most marine corrosion occurs,

an electrolyte.

SUMMARY Chemical symbols are used to designate a particular chemical element. The symbol also repre-

more atoms of

the

same or

Two

or

different elements

form molecules. Formulas show the elements and amounts of each element present

combine in a

Fg 203

Typically, the process proceeds

but the fundamentals

important factors

car-

oxygen, are

a metal. Dissolved gases, especially

for energy

is

free electrons, as in

on both sides of the equation as well. Energy levels can change in a chemical reaction. Energy is released when bonds are formed because the elements attain more stable configurations at lower energy levels. Energy is consumed when bonds are broken and elements attain higher-energy, less stable arrangements. Because the liquid and gas states are progressively less stable than the solid state, energy is absorbed in the change from solid to meaningful,

Theory

relatively large

Fe203(s)

that 2 atomic weights of

(ferric oxide). In this reaction,

state,

^

XH2O

Basic Chemical

•

to

molecule.

The combining capacity of an atom is the number of electrons it can gain, lose, or share in

forming chemical bonds. This

is

called the va-

lence number. Chemical bonds are the

means

by which atoms and molecules are held together

form compounds and mixtures. Chemical

enable you to understand the equations that will

to

describe the corrosion process

actions, including corrosion, involve the break-

in later

chapters.

re-

bonds and the formation of new bonds to form new compounds. The nature of these bonds as well as the specific atoms and molecules involved determine the nature and the ing of these

WATER Water

AS

AN ELECTROLYTE

sometimes referred to as the universal solvent. This is an overstatement, but water is the most widely used of all solvents. Under the right conditions, there are few substances that will not dissolve in water to some degree. Water plays an important part in many is

a solvent.

It is

chemical reactions, particularly those we’re terested

in-

tions are graphic

pens

in a

When

a

Chemical equa-

ways of describing what hap-

chemical reaction. an ionic substance

is

dissolved

in

water,

ions are released from the substance, and the so-

lution

becomes an

electrolyte,

in.

Water molecules are polar. Each consists of

rate of these chemical reactions.

and

this

Seawater is an the medium in which most

electrolyte. is

marine corrosion occurs.

9

Chapter

3

Basic Electrical

Knowledge of

Theory

the fundamentals of electricity

is

marine corrosion and how to deal with it. Adequate comprehension of this subject does not involve advanced mathe-

essential to understanding

matics or physics, and

we have

the necessary chemistry. little

It

already covered

really boils

further study of our friend

down

to a

and nemesis, the

This simply means that

charge (see (electrons)

fig.

3-1).

The individual charges

on each body

its

will be evenly distrib-

surface because of the mutual force

among

those charges. Because

ELECTROSTATICS

smaller, the distance

We

sion on

A

is

between the charges must

be smaller, making the aggregate force of repul-

have already discussed the structure of the atom and the nature of protons (positive charge) and electrons (negative charge). We said that an atom contained the same number of electrons and protons; therefore the charges are balanced, and the atom is neutral or without an overall electrical charge. Protons and electrons having opposite charges attract each other. This attraction holds the electrons around the nucleus. that there exists a force of attrac-

A

greater than on B.

If

we connect

a

wire between the two spheres, the greater repulsive force of

A

from

A would

is

would continue until the are the same. The flow of elec-

to B. This flow

repulsive forces

trons

cause a flow of electrons

called an electric current.

DIFFERENCE OF POTENTIAL:

VOLTAGE

between particles of opposite charge and a force of repulsion between particles of like charge. Like charges repel and unlike charges attract. This is the fundamental principle of elec-

both are negatively charged. One, however, has a greater negative charge than the

tricity.

to as the difference in potential, or voltage. Po-

tion

If

an object

(for

example, an atom) has more

electrons than protons,

it is

said to be negatively

Both bodies that

in figure 3-1

have excess

electrons'.,

is,

other. This difference in charge level

tential

earth,

is

referred

measured relative to the potential of which is considered to be neutral or zero. is

An

object with fewer electrons than

Positively charged bodies have a higher potential

protons has

a positive charge. In proximity,

than that of earth; the potential of negatively charged bodies is lower than earth. Difference of

charged. these

two

objects will be attracted to each other.

both objects have an excess of electrons (or protons), they have like charges, and the force between them is repulsion. Conversely,

if

The magnitude

in

is

of great importance

— there must be

a difference in potential for current to continue.

Notice that between two bodies

it is

the dif-

ference in potential relative to each other that

the magnitudes of

determines current flow. Lhe force that exists between two bodies having different levels of

is

and proportional to the distance between the two objects. To be more precise on the latter, the magnitude of the force vanes inversely as the square of the distance. the electrical charges

potential

directly pro-

of this force

portional to the difference

10

dis-

Suppose we have two objects of different sizes, each with the same magnitude of negative

of repulsion

Thus we saw

you increase the

tance between the bodies by a factor of 2, the force decreases by a factor of 4.

uted over

electron.

if

charge

is

of measurement letter

and the unit represented by the

referred to as the voltage.,

“V.”

is

the

volt.,

Chapter

Basic Electrical

•

3

Table

3-1

Resistivity of Selected

Common

Ohms

Material

Charged bodies.

17

brass

45

copper

10.4

iron

59

phosphor bronze

70

silver

CURRENT The flow

9.8

69

tin

of electrons between

two bodies

Metals

per Circular Mil-foot

aluminum

Fig. 3-1.

Theory

tungsten

33

zinc

36

is

called the current. For current to flow there

must be a difference in potential between the two bodies and a metallic conductor connected between them to carry the electrons. If the conductor connects a negative body to a positive body relative to earth electrons will flow from the negative body to the positive body. If

—

lev-

(magnitudes) of potential, electrons will flow

from the lower positive potential to the higher positive potential. Likewise,

if

The

resistance of

electrons will flow from the higher negative po-

Electrical resistance

is

OHM'S LAW

electrons through wires,

flow through pipes

easily than others. little

Because

resistance to the flow

good conductors of electric current. In contrast, elements that release few free electrons greatly oppose the flow of electrons. These elements are poor conductors, or of electrons, they are

is

in

many ways

is

easier to visualize than let’s

take advantage of

plumbing system we are

marily interested

in

pri-

three factors: pressure,

water flow, and resistance to water flow. the pressure on the system

crease.

chapter 2 that some elements release

house

this analogy. In the

amount

RESISTANCE

in a

similar to an electrical circuit. Because water

If

such elements present

let-

“R”, and the unit of measurement is the ohm. Typical resistivity values of some common metals at 68°F (20°C) are listed in table 3-1 (see also appendix 6, Wire Table).

outermost shell of the atoms. The unit of measurement for current is the ampere., signifying a flow of 6.28 X 10^^ electrons (one coulomb) per second. Current is represented by the letter “I.”

more

designated with the

ter

The plumbing system

free electrons

in-

crease in temperature.

lower negative potential. The electrons that make up the flow of current are the free electrons in the valence or

tential to the

in

with an

both bodies have

negative potential but of different magnitudes,

We saw

most of the ma-

terials of interest to us also increases

—

both bodies are positive, but with different els

cross section.

is

increased, the

of water flowing per unit time will inIf

the pressure

is

decreased, water flow

will decrease. If the pressure

moved, the flow of water rectly conclude that the

on the system

will stop.

We

is

re-

can cor-

water flow is directly proportional to the magnitude of the rate of

pressure. If

resistance to the flow of water

is

increased

(smaller pipes, clogs, closing the tap), the flow of

insulators.

water

Fwen the best conductor exhibits some resistance to current flow. The magnitude of that re-

creased, water flow will increase. T hus, the rate

sistance depends

on the element or elements

in

the conductor. Resistance varies directly with

length of the conductor and inversely with

its

will

decrease.

of water flow

is

If

the resistance

is

de-

inversely proportional to the

resistance in the system.

We

can expresses the relationship of these three factors with the mathematical equation

11

12

Chapter

Theory

Basic Electrical

•

3

rate of

our understanding of

water pressure

water flow =

resistance

This equation makes

electricity.

A

convenient

mnemonic

device for remembering these rela-

tionships

the

is

Ohm’s law

triangle:

easy to see that dou-

it

bling the pressure doubles the rate of flow, while

doubling the resistance cuts the flow

in half.

an electrical circuit the rate of flow is called current^ the pressure is called voltage^ and resistance to the flow is called resistance. So the In

equation would look

like

Putting your finger over the variable you

want or

=

I

compute

—V

voltage current =

to

R

resistance

I

reveals the arithmetic required,

—

V

V

X R or

or

R

I

Stated verbally, this relationship says that the

current

in

amperes

is

equal to the voltage

in

ohms. The amperes (amps), volts, and ohms

volts divided by the resistance in

symbols for are A, V, and Let’s

is

respectively.

how

12 volts

it

works.

we know

If

and the resistance

the volt-

6 ohms,

is

we

can calculate the current flow as 2 amperes:

through a resistance, a

a current flows

couple of things take place. The

up

in forcing the is

lost to the rest of the circuit.

and we use Ohm’s law to calculate it. As an example, pushing a 2-ampere current through a 6-ohm resistance will use up 12 volts of potential difference:

flow and the resistance and the voltage.

We

we want

to

The second thing

compute

done

is

in

that

happens

is

that

work

pushing the electrons through the

re-

When work is done, power is dissiWhen electrical power is dissipated, heat

sistance.

pated. voltage = current x resistance

V =

or

I

x R is

Verbally, voltage in volts

is

equal to current

amperes multiplied by resistance

same values but solving culation becomes the

we want

Ax6U

the current

arranging the equation to

If

x R

= 12 V

know

simplify that calculation by re-

V =

I

=2

already

This

called voltage drop,

is

6U

we

used

is

current through the resistance.

voltage drop (V) =

But perhaps

that a

first is

portion of the total potential difference

This voltage

plug some sample values into the equa-

tion to see

age

12,

When

2Ax6i)

in

in

ohms. Using

for voltage, the cal-

generated; that

These are referred to as IR pressed

losses

have a direct

voltage

R =

re-

expressed with the mathematical equation

ues, the

or

Power

R

Using our sample current and resistance val-

we can

again restate the equation as

current

watts.

watts = current squared x resistance or P = P x

12V

to solve for resistance,

resistance =

in

losses, typically ex-

lationship to resistance and to current squared. It is

=

the resistance heats up.

is,

computation P

—V

=

(2

is

amperes)^ x 6 ohms

P = 4 amperes x 6 ohms

I

= 24 watts

Our example becomes R =

— 12 V 2

= 6

BASIC DIRECT CIRCUITS

LI

A

This mathematical expression forms)

is

called

Ohm’s law and

is

(in all

three

the basis for

Up

(DC)

we have described current flow movement of electrons from a negative

to this point

as the

CURRENT

Chapters ohms

= 100

R,

nAAA/-

V

=

I

Basic Electrical

Theory

where R, = R, + R 2 + Rn

R,

total

^

•

12

(+)

V

150Q battery 12 volts

= 0.08 A

Now

let’s

resistance

is

Adding Fig.3-2A. Basic series circuit with one resistor.

the voltage (12 volts)

dropped across

is

All

resistor

R^.

see

what happens with

in parallel

sistance.

The

the additional

(see fig. 3-3).

a resistor in parallel

additional check-out lane

if

—

like

opening an

— actually lowers

total resistance

re-

(RJ presented to

the battery will be less than that of either of the parallel resistors alone. cuits

body toward

a positive body.

electron theory and

This

based on

what actually

in fact,

is,

is

is

The

the total resistance

sum of the

is

rule for parallel cir-

the reciprocal of the

reciprocals of each of the resistances

in the circuit.

The

reciprocal just

means the

takes place. However, before the development of electron theory, scientists

made an

arbitrary de-

cision to postulate current flow from positive

By the time the true nature of electron flow was understood, this erroneous expedient had been published in countless texts, handbooks, and manuals and was shown on all to negative.

R, =

100 ohms

50 ohms

kinds of machinery. So despite the later science,

we continue

to

assume that

electric current

flows from positive to negative.

Our

discussion

of electric circuits will follow this long-standing

Let’s see

how

work. Figure 3-2A de-

circuits

picts a basic electrical circuit

ohm

resistance

in series.

Basic series circuit with two resistors. Part of the voltage is dropped across and part is dropped across resistor R^. Fig. 3-2B.

convention.

and

a

1

composed of

we can determine

magnitude of the current flowing =

1

00-

2-volt battery connected

Using Ohm’s law,

I

a

R, =

the

50

ohms

^AAA^ —vAAA^

in this circuit:

— V

R,

R

=

100

Ohms

^ 12

V

'total

100^2

'total

(U

= 0.12 A

battery 12 volts

Since this

is

a series circuit, the current has

only one possible path, so the current flow

same

in all parts

of the circuit.

If

we add

is

the

addi-

(-)

tional resistance to this circuit, in series with re-

sistor

R|,

decrease

we would expect

(fig.

3-2B).

current by using the in

the

We

sum

the current to

can calculate the new of

Ohm’s law equation:

all

the resistors as Rj

Basic parallel circuit. Current split between the two branches in proportion to the resistance in the branches and all the voltage (12 volts) is dropped across the two resistors. Fig. 3-3.

13

1

14

Chapter

3

number

I

•

Theory

Basic Electrical

= 25

divided by the resistance, so the equa-

rWWn

tion for parallel resistance looks like this:

—

— —

1

1

1

+

R, =

R„

Ro

total

(+) If

R]

is

100 ohms and

R2

50 ohms, we get

is

1

R,

100

R3 =

battery

ohms

50

'

total

ohms

Rj = 75

12 volts 1

WW ww^ U

100 ohms

-WW

1

+

Rt

ohms

1

+

1

50

(-)

1

=

1

^

+

R,

100

Fig. 3-4. Series-parallel circuit. Total

50

through resistor 1

=

R,

f

3

R,

in

R,

R,

the parallel branch.

= 33.33 ohms parallel circuit

Now we total

dropped and the remainder dropped across

part of the total voltage (12 volts) being

across resistor

=

proportion to

splits in

the resistance of the individual branches, resulting

100 100

and then

R,

current passes

can use Ohm’s law to solve for the current flowing from the battery:

mine the

is

shown

in figure 3-4.

To

total resistance of this circuit,

solve the parallel branch

first

deter-

we

re-

and then add that

to the other resistance in series.

J_

j_ _ jL 12

V

33.33

is

_L

75 ^ 50 ^ 25

U

= 0.36 A

Since this

”

Rp

+

1

a parallel circuit, the current

f 1

from the battery splits, part traveling through R[ and part through R 2 We know that more current will flow through the smaller resistor. We can calculate these currents by first determining the voltage drop across the resistors as

—

-1

50 ^ 25,

11

R.

^

J50

.

voltage drop = current

(I,)

x resistance

= 0.36 amperes x 33.33

Since this

is

ohms =

resis-

total resistance of the circuit

R,= R, + Rp = 100 + 13.64 = If

total circuit resistance

and the applied voltage cuit current

1

12

R,

113.64

13.64

(RJ

)

= 0.12 A

50

= 0.24

An example

13.64 ohms,

1

A

and the voltage drop across R[

is

12

Vr^ =

A

Notice that the sum of the two currents equals the total current of 0.36 amperes. Real-world circuits may have resistances in in parallel.

is

ohms

12 volts, the total cir-

is

0.106 12 V

is

is

_V

1001

ohms

13.64

12 volts

the voltage across each resistor,

12 V

and

Rp 11

The

(R,)

we can compute the current through each tor using Ohm’s law as

series

150

of a series-

If

I

we then

X R = 0.106 X 100 = 10.6 V sid')tract

the voltage

across R, from the applied voltage,

dropped

we know

voltage across the parallel branch circuit: Vr^ = Vappned -Vr, = 12.0

V - 10.6 V =

1.4 Vr^

the

Chapters

Now we

can calculate the current

in

each of

=

— 1.4

=

= 0.056 A

1.4

appropriate range that gives the most

accurate reading.

25 lo

Theory

you are buying a meter, buy digital, preferably one designed and constructed for use in the marine environment. Your new multimeter will come with two test cables (one red, one black) and an instruction book. There are many variations among these

the legs of the parallel branch as

L

lects the

Basic Electrical

•

= 0.028 A

50

instruments, so

If

essential to read the instruc-

it is

With analog instruments, placing of the test probes and selection of appropriate ranges are critical. You must always select a

tion book.

range that

MEASUREMENTS Inasmuch

as

we

will

you expect

normally be troubleshooting

we can make direct amperes, and ohms if we

down

ing

measurements of volts, have the proper equipment. A multimeter^ also called a umltitester, measures voltage, current, and resistance or continuity. If you don’t have one, you should. You can spend just about anything you want for one of these useful little tools, from $10 or $15 at your local marine or electronics supplier on up to several hundred dollars. The differ-

the scale.

in sensitivity,

accuracy, range, and, of

Buy the

you can afford, and if you’re going to keep it on the boat a good idea select one intended for marine use. These are a little more rugged and sealed to keep mois-

course, quality.

best

—

—

We

you get

till

measure voltage

some other

good

idea to get in the

a reading in the center of

tial.

When we

we mean

relative to the potential of

point, usually earthy or ground.

potential of earth

that

is

the established zero poten-

say that voltage

tive

must have

a sensitivity of at

50,000 ohms/volt to read

reliably the very

levels of currents involved.

measure voltage accurately

in

It

should also

the millivolt (thou-

volts,”

10 volts positive with respect

it is

flow from ground to the more negative point.

We

physically create an earth connection, or ground,

To measure voltage with

in

the ground.

a meter,

we touch

— the red one we expect the voltage to be positive — to the point whose pomeasure. We touch the other we want to the reference point — most commonly ground. test

probe

if

to

Measuring Current To measure

the flow of current,

the meter into the circuit. This

we must

means

insert

the circuit

must be broken and the meter connected across the opening, so that the current flowing through

sandths of a volt) range.

more but have a number of attractive features. They are more accurate, particularly in the low DC voltage range of interest in corrosion testing, and they are more robust. Better models include internal protection against incorrect polarity and autoranging, meaning the meter automatically seDigital multimeters cost a

“+10

with respect to earth's potential. Current will

tential

it

is

from the +10 volt point to the zero potential of ground. Similarly, “-10 volts,” is 10 volts nega-

pensive, but they lack the sensitivity of a digital

multimeter and are inherently fragile. If you’re going to use an analog multimeter for corrosion

The

to earth's potential. In this case, current will flow

one

low

a

Measuring Voltage

You have a choice between digital and analog. Analog multitesters are the ones with the needle and numbered scale. These are relatively inex-

least

It’s

by burying a copper conductor

ture out.

troubleshooting,

to find.

habit of starting on the highest scale and switch-

circuits already in place,

ences are

higher than the voltage or current

is

little

the circuit has to pass through the meter. circuit lacks a

convenient disconnect,

it

the

11

may

be

necessary to cut a conductor or unsolder a connection to test the current. Ca)iinect the red

probe to the positive side of the

circuit, black to

remake the contaking the measurement.

the negative side.

nection after

Remember

test

to

15

16

Chapters

•

Theory

Basic Electrical

Measuring Resistance When measuring

resistance (ohms) there can be

you connect an analog merer in the ohms mode to a live circuit, you will in all probability burn out the meter. Most digital ohmmeters have some degree of internal protection against this, but you should still get in the habit of making certain that there is no power on the circuit. Power for the ohmmeter is from an internal battery. Because this battery discharges over time, analog ohmmeters must he calibrated each no power on the

circuit. If

time the meter

used (and each time the scale

is

is

changed) by touching the test probes together and turning the ohms adjust knob until the meter indicates zero ohms. When using a digital meter, check the meter and lead resistance by crossing the probes. When measuring very small resistance, you may need to subtract this from the meter reading.

BASIC AC CIRCUITS In

DC circuits the current

flows

in

one direction.

Alternating current (AC) reverses direction at regular intervals. Standard States switches

maximum

maximum flow in hack to maximum

rection to tion, then

from

AC

in

the United

flow

in

one

di-

the opposite direc-

flow

in

the original

direction 60 times per second. This

is

shown

graphically in figure 3-5.

Fig. 3-5.

Corrosion due to stray currents can involve the shoreside wiring as well as onboard AC circuitry. We’ll talk more about this in chapters 9 and 18. 1'ypically, our troid'jleshooting activities

time.

will he limited to the DC.' characteristics of AC' circuitry.

Because

AC

systems behave differently

and involve considerably higher voltages, they can be dangerous. There is one universally accepted ground rule where AC' circuits are involved; if you have any question about what you are doing, don't do it! Mistakes in AC' circuits can he fatal. Before doing anything on the boat that MAY involve AC' circuits, disconnect the shore-power cord and turn off any onboard sources of AC' power, including inverters,

if

present.

an

in

Alternating voltage magnitude versus

The graph shows the negative-positive cycle

AC

circuit.

SUMMARY Voltage, or potential, a

body has

or

is

a relative term; that

a certain potential relative to

some other body. Electrons

is,

ground

actually

move

body to the positive (less negative) body, hut by convention we assume the flow of an electric current to he from positive from

a negative

to negative.

The amount

of current that will

flow depends upon the magnitude of the differ-

and the resistance of the conducting path. Ohm’s law describes this

ence

in

potential (voltage)

relationship mathematically.

— ^

^

Chapter 4

Basic Corrosion Processes

Corrosion ranks right up there with athlete’s foot, fungi, and green hair mold. It is ugly, insidious, complex, and confusing. It thrives in dark, hard-to-get-at places, and just when you think you’ve got it licked, it flares up again and spreads like crazy.

Its effects

are often seriously

damaging and even dangerous.

On

a boat, signs of corrosion are everywhere

staining and deterioration of both

wood

(sur-

and metal, pitting of stainless steel, powdering on aluminum, flaking steel, and disintegrating brass screws (where someone has been foolish enough to use them below the waterline or where exposed to the weather). Corrosion shows up on propellers and shafts, rudders and stocks, struts, through-hulls and fastenings, and just about any other metallic fitprise, surprise)

ting you’re likely to find aboard.

Everyone who’s ever owned a boat knows what corrosion is, but there is considerable confusion about the proper terms to describe it, and even greater confusion about preventing it. For example, some authoritative sources say you should bond everything. Others say don’t bond at all. Still others say

bond some components

hut not others. Ciiven such divergent advice,

it

should he no surprise that some recommended

more problems Horror stories abound, and

cures have the effect of causing

than they solve.

you’ve probably heard or had your share of them. Corrosion of metals exposed to seawater or even a salt-air environment is all but inevitable, but corrosion can be prevented or controlled to the extent that it need not compromise the vessel’s performance or seaworthiness. It must be controlled. in

The

alternative

terms of repairs,

is

just

too expensive

lost fishing, sailing, or boat-

ing time,

and even physical danger.

WHAT

IS

CORROSION?

Corrosion is a broad and complex subject. For our purposes, we limit our examination to three basic categories of corrosion and the particular forms that are important for us as owners, operators, builders, repairers, or restorers of boats.

These three categories are simple electrochemical or single-metal corrosion; galvanic corro-

sometimes referred to as two-metal or bimetallic corrosion; and electrolytic or stray-

sion,

current corrosion.

A TROUBLESOME RESTORATION For

heart-rending

41-foot English cutter featured

your hands on issue 41 (July-August 1981) of WoodenBoat magazine and read about George Cadwalader's restoration

sawn oak and bent elm frames. The frames were tied together

a particularly

story, get

30-year-old mahoganyplanked double-ender. The beauti-

of

a

fully

designed and heavily built

across the keel timber by massive

wrought-iron floors, and the entire structure was tied together with iron and bronze straps. The boat was fastened with copper riv-

on the bent frames and naval brass screws on the sawn frames. Four years later (Wooder?/3 oat no. 70) Cadwalader completes his tale of the restoration, and he asks the question "Would do it again?" The answer is an emphatic "No!" ets

I

17

18

Chapter4

•

Basic Corrosion Processes

forms of corrosion, four components

In all

must be present

—an anode,

a cathode, a metal-

The

positive electrode

negative electrode

is

the anode.

is

and the these two

the cathode,

When

path for the electrons to flow through, and an

electrodes are connected by a wire, free electrons

electrolyte (or electrolytic path) for the ions to

flow through the wire from the anode to the

flow through. Both the anode and the cathode

cathode as an electrical current. (Don’t be confused by the convention that direct currents flow from positive to negative. We are more con-

lic

must he in contact with the electrolyte. Oxygen and hydrogen also must be available, either directly or as a result of chemical action and the resultant dissociation of water into its two con-

cerned with the effects of the flow than the

Current

rection.)

is

di-

also flowing through the

by both positive and negative

stituents.

electrolyte, carried

Notice that I did not (and will not) use the term electrolysis. Electrolysis is what happens to

ions traveling in opposite directions tive ions

the electrolyte, not to the metals involved.

negative ions toward the positive cathode. Thus,

It is

toward the negative anode, and the

frequently used incorrectly to describe the cor-

the circuit

rosion process.

later.

To help

get the terms straight,

let’s

take a look

— the posi-

is

We

complete.

For now,

let’s

talk

more about

this

define our three principal

categories.

one cell in a common storage battery. Figure 4-1 shows such a cell, known as a voltaic or gal-

Simple Electrochemical Corrosion

vanic

Simple electrochemical corrosion involves a

at

cell.

— either pure metal aluminum) or an alloy stainless bronze, brass) — contact with an

gle piece of metal

a

(e.g.,

iron, tin, steel,

(e.g.,

elec-

in

corrosion can occur in

trolyte. Electrochemical

wire

sin-

I

:

the atmosphere or under immersion.

(electron flow)

It

nor-

mally proceeds continuously and at an extremely slow rate.

lead

lead

(anode)

dioxide

(-)

(cathode)

r

(+)

Galvanic Corrosion two or more dismean metals widely

Cialvanic corrosion involves similar metals, by

separated

in

which

I

the galvanic table

— mild

steel

and

bronze, for example. The metals must be con-

sulfuric acid

—either by physconductor — and must be

nected by a metallic path ical

contact or by a

direct

in

contact with the same body of electrolyte. Notice that

corrosion, which

V_ Simple galvanic

J cell

as

in

a

common

battery.

wire. In the electrolyte, the current ions,

is

carried by

both positive and negative, traveling

site directions; positive ions

move toward

in

oppo-

the neg-

ative anode, and negative ions toward the positive

cathode.

t)f

is still

it is

in

simple electrochemical

taking place.

Electrolytic Corrosion Electrolytic corrosion also involves

stor-

The current is carried by electrons traveling from the anode to the cathode through the age

galvanic corrosion occurs,

addition to the process

electrolyte (ion flow)

Fig. 4-1.

when

two separate

metals exposed to the same body of electrolyte,

but

in this

case the corrosion

electrical current

is

caused by an

from an outside source flowing

between the two metals.

F’lectrolytic

corrosion

is

also called stray-current corrosion.

subsequent chapters we examine electrochemical, galvanic, and electrolytic corrosion in In

Chapter 4 greater depth. For the

moment we

concentrate

on the basic corrosion process.

Basic Corrosion Processes

•

influencing conditions. cis

LaQue shows

A

classic study

that the surface metal loss in

good quality marine bronze due corrosion

THE CORROSION PROCESS

We

mine iron ore, process it into steel, and make goods out of it. When those goods rust, the steel turns hack into iron oxide, and the cycle is complete. How about that? Nature’s been recycling since time began. We’re just now getting with the program. But rust is only one form of corrosion, a special case peculiar to iron and steel. Although the corrosion process

is

similar in

all

to saltwater

per year.

less

That’s definitely slow!

Someone described corrosion as nature’s way of reclaiming the metals humans have taken from the earth.

about 0.0002 inches or

is

by Fran-

other metals,

ELECTROCHEMICAL PROCESS Electrochemistry deals with chemical reactions that produce electric currents

and the chemical

reactions that are caused by those currents in

an

electrolyte.

The

currents because ized, that

is,

its

electrolyte conducts electric

molecules have become ion-

they have been changed into posi-

and negatively charged particles called ions, which have the property of conducting tively

we don’t call it rust. In fact, we have no one commonly accepted word for it ex-

electric currents.

cept possibly oxidation since oxygen, either

had open-heart surgery. The

from the

asked the doctor when he awoke was,

in

is

those metals

from the dissociation of water,

air or

a necessary agent.

Interestingly, the metal oxides that are a

product of corrosion can also become protective coatings. Take

aluminum,

for example.

When

aluminum is exposed to the atmosphere, a film of aluminum oxide (alumina) quickly forms on the surface. This film

bonded

is

so dense and so closely

to the metal surface that moisture

additional oxygen cannot penetrate

and

and cause further corrosion. Rust, on the other hand, is porous, flaky, and loosely bonded to the surface; it provides little or no protection to the base it

metal.

Corrosion can be defined as the destructive alteration of metal by reaction with

its

environ-

ment. For corrosion to take place, oxygen and hydrogen must be available, and the metal must be

in

contact with an electrolyte, that

is,

conductive medium. The rate

an

elec-

which corrosion takes place depends not only on the type of metal involved but also on the nature of trically

at

the electrolyte. In our case, the electrolyte ally seawater.

How

is

usu-

well seawater functions as

An

old boatbuilding friend of mine recently

some

atom is roughly 2 x 10“^ inches in diameter and about 3.5 X 10"^'' pounds. That’s 3.5 with 27 zeros in front. We’re talking about some pretty small to put

What

is

rent flow

extremely slow process in the absence of other

is

of significance

is

that

when

the cur-

by metallic conduction, the electrons

move from one atom to the next and no mass is lost. However, when the current flow is

just

and an ion is lost from a piece of metal, essentially the mass of an atom is lost from the body of the metal. That’s corrosion. ionic

ELECTROMOTIVE POTENTIAL AND THE GALVANIC TABLE Seawater, the electrolyte tially a

in

our case,

is

solution of water and salt (and

other components as well). The salt

about

an

of this into perspective, an

stuff here.

the water. Simple electrochemical corrosion

is

“When

I

chloride,

is

question he

you sawed through my rib cage. Doc, what happened to the sawdust?” know what you’re thinking: What happens to the ions and the electrons? Where do they go? We’ll get to that, but

dependent on its temperature, its salinity, whether it is stagnant or fast moving, and the amount and nature of any pollution in an electrolyte

first

NaCl, an

When NaCl

is

is

some

sodium

electrically neutral molecule.

dissolved

in

water,

its

molecules

separate into ions (Na^ and Cl") that can freely.

essen-

Placing a piece of metal

move

in this (or

any) electrolyte sets up spontaneous voltage (or

19

)

20

Chapter 4

Basic Corrosion Processes

•

between the The magnitude of this

electromotive potential force, emf

silver-silver chloride reference electrode.

metal and the electrolyte.

electrolyte

voltage

seawater flowing at between 8 and

13 feet per second (4.7 to 7.7 knots) at a tem-

primarily a function of the type of

is

is

The

in

perature between 50°F and 80°F (10°C and

simplified form, by the accumulation of negative

26.7°C). In either series, the absolute numeri-

chloride ions (Cl“) on the surface of the metal

cal value of a single electrode potential

in figure 4-2.

tle real

metal and the temperature. This

Since voltage

some

respect to

shown,

the

metal must be measured with

any two metals. The least noble metals those at the top of the table are the most active ones and corrode most easily. They are referred to as anodic and

reference electrode. In classical

—

measured under laboratory conditions with respect to a hydrogen is

are electrically

The marine industry takes comparative measurements under more realistic conditions. The

metals are those toward the bottom of the table.

These are more passive, that is, ically, and they are electrically

resulting series, called the Galvanic Series of

Seawater

(see table 4-1)

is

based on

—

more negative (or less positive) when immersed in seawater. The most noble

electrode (yielding the electromotive series).

in

lies in

lit-

numerical difference between the potentials of

chemistry the voltage

Metals

importance

significance. Their

of

is

a relative term, the voltage of

is

a single piece of

is

a

less active

less

chem-

negative (or

Table 4-1 Galvanic Series of Metals

in

Seawater (ABYC Standard E-2) Corrosion Potential Range

Metals and Alloys Least Noble (Anodic) or Most Active

magnesium and magnesium

-1.60 to -1.63

alloys

-0.98 to -1.03

zinc

aluminum

-0.76 to -1.00

alloys"''

cadmium

-0.70 to -0.73

mild steel

-0.60 to -0.71

wrought

-0.60 to -0.71

iron

-0.60 to -0.71

cast iron

13% chromium

stainless steel,

18-8 stainless steel, Type

304

Type 410 (active

(active

in still

in still

water)

-0.46 to -0.58

water)

-0.46 to -0.58

Ni-resist

18-8,

3% Mo

stainless steel.

Type 316 (active

water)

in still

6% Fe) (active in still water) aluminum bronze (92% Cu, 8% Al) Nibral (81.2% Cu, 4% Fe, 4.5% Ni, 9% Al, 1.3% Mg) Inconel

(78%

naval brass

Ni,

(60%

yellow brass red brass

-0.46 to -0.58

14.5%

Cr,

39% Zn) (65% Cu, 35% Zn)

(85%

Muntz metal (60%

40%

-0.31 to -0.42 -0.31 to -0.42

-0.30 to -0.40

15% Zn) Cu,

-0.35 to -0.46

-0.30 to -0.40

Cu,

Cu,

-0.43 to -0.54

-0.30 to -0.40

Zn)

-0.30 to -0.40

tin

-0.31 to -0.33

copper

-0.30 to -0.57

50-50

-0.28 to -0.37

lead-tin solder

28% Zn, 1% Sn) aluminum brass (76% Cu, 22% Zn, 2% Al) manganese bronze (58.5% Cu, 39% Zn, 1% admiralty brass (71% Cu,

silicon

bronze

(96% Cu max, 0.8%

Fe,

-0.28 to -0.36 -0.28 to -0.36 Sn,

1%

1.5% Zn,

Fe,

2%

Si,

0.3% Mn) 0.75% Mn, 1.6% Sn)

-0.27 to -0.34

-0.26 to -0.29

in

Volts

Chapter 4

more

These metals are said

positive) in seawater.

and they are more

to be cathodic^

on.

An

When two

or

more metals

are

immersed

same body of seawater and connected

gether electrically

wired together least

the metals need not be immersed.

wet or damp with seawater electrolyte and conduct ions.

flow from the in

doing so the anodic metal corrodes, sacrificing itself to protect the more noble metal. The extra electrons collecting on the cathodic metal increase the negative charge on the protected metal, making it more negative than its characleast

When

first.

Wood

that

will also act as

is

an

A LITTLE TECHNICAL DETAIL Two 1.

teristic potential as listed in the table.

The

cviode^ a cathode^ a metallic path for the

and an electrolytic path for the ions are the components of a galvanic cell. Note that

to-

noble metal to the most noble metal, and

farther apart the metals are in the table,

electrons,

in

— either by direct contact or

— electrons will

Basic Corrosion Processes

the faster the rate of corrosion.

resistant to

corrosion.

the

The

•

things

happen during the corrosion process:

Metal atoms are

lost

by the anode. This

is

the corrosion.

noble metal, the anode^ corrodes the least noble metal is consumed,

the next least noble metal will corrode,

2.

Ions are formed in the seawater. These can

form a protective coating on the cathode or produce alkalis that attack wood.

either

and so

Table 4-1 (continued) Galvanic Series of Metals

In

Seawater (ABYC Standard E-2) Corrosion Potential Range

Metals and Alloys bronze-composition G

bronze

ASTM B62

copper nickel

2%

Cu,

10% Sn) (85% Cu, 5% Pb, 5%

(through-hull)

bronze-composition

13% chromium

(88%

Sn,

5%

Zn)

-0.24 to -0.31

M (88% Cu, 3% Zn, 6.5% Sn, 1.5% Pb)

-0.24 to -0.31

Type 401 (passive)

-0.26 to -0.35

stainless steel.

(90%

-0.24 to -0.31

Zn,

10%

Ni)

20%

Ni,

Cu,

copper nickel (75% Cu,

-0.21 to -0.28

5%

Zn)

-0.19 to -0.25 -0.19 to -0.25

lead

copper nickel Inconel nickel

(78%

(70% Ni,

Cu,

30%

13.5%

Cr,

-0.18 to -0.23

Ni)

6%

-0.14 to -0.17

Fe) (passive)

-0.10 to -0.20

200

18-8 stainless steel, type

Monel 400, K-500 (70%

304 (passive)

-0.05 to -0.10

30%

-0.04 to -0.14

Ni,

stainless steel propeller shaft

Cu)

(UNS Si7400-#i7

18-8 stainless steel, type 316 (passive)

,

8c

UNS S304S2-#i9)

-0.03 to -0.13

3% Mo

-0.00 to -0.10

titanium

-0.05 to +0.06

Hastelloy C

-0.03 to +0.08

stainless steel shafting (bar)

(UNS S209io-#22)

-0.25 to +0.06

platinum

+ 0.19 to +0.25

graphite

+ 0.20 to +0.30

Most Noble (Cathodic) or Least Active "The range shown does not include

sacrificial

available that have a

maximum

Note: Values

were measured

a

in

table

aluminum anodes. Aluminum

alloy sacrificial

anodes are

corrosion potential of -1.100 volts. in

seawater flowing at 8 to

13 ft./sec. (4.7 to 7.7 kn.)

and within

temperature range of 50°P to 8 o°F.

Source: American Boat and Yacht Council,

"Recommended Standard

E-2,

Cathodic Protection of Boats."

in

Volts

21

— 22

Chapter 4

•

Basic Corrosion Processes

metal

two double-positive charged metal ions tion.

This

is

the physical wastage of the metal

the corrosion.

some

in solu-

The remaining metal

e.xcess electrons (e")

is left

with

and therefore with

a

negative charge.

At the cathode, a dissolved oxygen molecule (O 2 in the seawater combines with the excess electrons and with hydrogen ions (H^) to form hydroxyl ions (OH") in the seawater. This reac)

tion restores the electrical neutrality of the cath-

ode metal and of the electrolyte. For the scholars, the equation for the cathode reaction

looks like

O2

+ 2 H 2 O + 4e-

^

4(OH)-

For every anode reaction, a cathode reaction takes place, and every anode reaction involves

O

sodium

O

chloride ion (Cl")

ion

the loss of

(Na^)

two atoms of

The chemical

reaction that takes place in the

electrolyte looks like

o electron 2M^" + 4(OH)Fig. 4-2.

Metal (iron) immersed

in

seawater elec-

Negatively charged chloride ions in the electrolyte at the surface of the metal push electrons from surface metal atoms deeper into the body of the metal. The surface metal atom dissolves into the electrolyte as a positively charged ion, leaving the remaining metal more negatively charged. The more chloride ions gather at the surface of the metal, the more negatively charged the (anodic) metal becomes. trolyte.

The corrosion process involves

three basic

chemical reactions of significance for us. One takes place at the anode, one at the cathode, and one in the electrolyte. A look at some of the negative chemical details will he helpful in understanding some of the ill effects that can ac-

company corrosion. The following chemical equation describes what happens

anode:

at the

2M

^

2M** + 4e“

Basically this says that at the

anode an

ization reaction takes place in which, for

metals,

two metal atoms

(iVl)

the solid metal.

dissolve to

ion-

most form

—

^

2M(0H)2

—

The metal ions 2\F^ from the anode reactions combine with the hydroxyl ions from the cathode reactions l(OH)" to form a metal

—

hydroxide, which

may

—

precipitate out as a solid

and deposit on the surface of the metal. This is the white, powder-like coating on aluminum and the pinkish coating on bronze. These coatings, except in the case of iron or mild steel, form a protective coating and inhibit further corrosion, hlowever, where the hydroxyl ions are not captured or combined with the metal ions but are allowed to build up in the electrolyte, we have a different and not so happy result.

ALKALI

DAMAGE

Water can be either acidic or basic. When water contains more hydroxyl ions (OFl") than hydrogen ions (IE), it is said to be basic or alkaline. Seawater is normally slightly alkaline. If hydroxyl ions produced at the cathode are not balanced by the reactions at the anode, and if they are able to concentrate, say in stagnant water, the water can

become strongly

alkaline. Strongly

alkaline solutions can dissolve the lignin in

Chapter 4

wood. Lignin is what binds the cellulose and hardens and strengthens the wood This condition, called

cilkcili

•

electrical contact

cells.

•

stray electric currents

is

•

installation of sacrificial

wood around

•

number and

rapidly corroding through-hulls and fasteners.

We

talk

more about

this in

chapter

•

in the

start

is

an

case of

high-quality copper alloys (such as bronze).

Some

The

introducing other

of these “other influences” are

•

strength of the electrolyte

•

temperature of the electrolyte

•

whether the electrolyte is stagnant or moving pollutants and impurities in the electrolyte

•

Corrosion aboard boats

•

extremely slow process, especially

influences.

location of sacrificial anodes is

additionally influ-

enced by

absence of other influences, corrosion

problems begin when we

anodes

6.

OTHER FACTORS In the

between dissimilar metals

fibers

delignification,

the cause of the deterioration of

Basic Corrosion Processes

•

whether the system is bonded whether a lightning protection system

is

in-

stalled

By

performing

some

fairly

simple

and

you can determine the extent of your boat’s corrosion potential. There are some equally simple and straightforward methods for preventing or controlling the situation so that serious problems do not occur. Before discussing these, we first need a better understanding of what we are up against.

straightforward

tests,

23

— Chapter

5

Electrochemical or Self-Corrosion

We

talked in chapter 4 about the basic chemistry

stresses, heat treatment, impurities,

and physics of the corrosion processes. In this chapter we narrow our focus to simple electro-

ety of other factors can

chemical corrosion, also called single-metal cor-

local cathodes

rosion or self-corrosion.

tact

Basic electrochemical corrosion involves a single piece of metal

—either

alloy,

is

when

the metal surface

with an electrolyte (see

If

we assume

fig.

is

in

con-

5-1).

the presence of seawater,

we

such as stain-

rosion to take place, namely an anode, a cath-

almost imperceptible process unless affected by such other factors as dissimilar metals and stray currents. For now we’re going to concentrate on components that

corrosion

sites

produce dissimilar surfunction as local anodes and

have three of the four things necessary for cor-

bronze, or cast iron. Electrochemical

less steel,

Such

a vari-

such as

a pure metal,

aluminum, or an

iron, tin, or

face sites.

and

a very slow,

ode, and an electrolytic path for ion flow seawater.

The

electron flow,

final necessity, a metallic is

path for

provided by the metal piece

itself.

are subject to simple electrochemical corrosion in metal (M)

the absence of other influencing conditions. In this

chapter

we

also touch

on other

straight-

forward degradation processes affecting singlemetal systems in and around boats. These include metallurgically influenced decay, such as de-

some

alloying and intergranular corrosion, and

that are mechanically assisted, such as erosion, cavitation,

and

liquid metal embrittlement.

THE NECESSARY CONDITIONS We

always need an anode and

rosion to occur, but there

two

they be

is

a

cathode for cor-

no requirement that

different pieces of metal. In the case

of an alloy, the different metals

in

the alloy can

function as anodes and cathodes. In brass, for

example

— an alloy of copper and zinc — the zinc

can function as the anode and the copper as the cathode. But

how

an alloy?

does

this

occur when the metal

In the case of a

isn’t

pure metal, the anodes

and cathodes can be metallurgically dissimilar locations on the surface of the single piece. No metal is perfectly homogeneous throughout its structure. Differences in surface properties re-

24

sulting

from

the

manufacturing

process.

Simple electrochemical corrosion. The local anode, the local cathode, the body of the metal for the conduction of electrons, and the saltwater electrolyte for the conduction of ions provide the four requirements for corrosion. Electrons move from the local anode toward the local cathode, causing metal atoms to ionize and dissolve into the electrolyte. Sodium ions and hydroxyl ions may combine near the cathode to form sodium hydroxFig. 5-1.

ide (caustic soda).

Chapter

Electrochemical or Self-Corrosion

5

Described below are the various forms of single-metal corrosion, grouped by major distinctive features of the corrosion process.

ATMOSPHERIC CORROSION Atmospheric corrosion

is

the

more or

less uni-

form, steady loss of surface metal. Rust is a good example. Rust is a scaly, reddish-brown

hydrated (combined with water) ferric oxide that forms on iron and iron-containing materials by oxidation in the presence of moisture. Unlike oxide coatings formed by other metals, rust is brittle and easily flakes off the metal, thus offering little or no protection to the metal. Any and all carbon and alloy steels are subject to this form of corrosion. Under appropriate conditions, all metals exhibit atmospheric corrosion to some extent. It may also occur during galvanic or two-metal corrosion.

Zones of corrosion for steel piling in seawater and relative loss of metal thickness. The most aggressive area for corrosive attack is the waterline area, the so-called splash zone. The corrosion Fig. 5-2.

rate

in this

area can be as

much

as

two

to three

times the atmospheric corrosion rate because of the

areas can be as low as 0.18 mil/yr. (0.0046

water washing over the metal in this area. Tests have shown that cycling through immersion and exposure to air produces more pitting than does continuous immersion.

mm/yr.), with

ASM

Prediction of a specific corrosion rate

tremely

those

difficult.

Those

some In

ful

in

in arid

mil equaling 0.001 inch, while

in industrial

as high at

at

1

Average corrosion rates

ex-

is

about

areas

may

continuously fresh supply of oxygen-rich

Handbook,

salt

vol. 13, Corrosion, 1987, p. 893.

be nearly ten times

1.7 mil/yr. (0.043 mm/yr.).

marine environments can be higher

still

was

north, the ground-level corrosion rate

than twice as high at 42 mil/yr. (0.04

tnore

in./yr.).

12 mil/yr. (0.30 mm/yr.).

any given geographical location

it is

help-

to consider five separate corrosion zones: at-

mospheric, splash zone, tidal, immersion, and bottom or mud zone. These zones together with their relative corrosion rates are

ure 5-2. However,

it is

shown

in fig-

somewhat misleading

to

about average corrosion rates since they are critically dependent on many factors. As mentalk

tioned above,

in dry, salt-free air,

corrosion

is

es-

IMMERSION CORROSION Under immersion conditions, the corrosion rates of carbon and low-alloy steels are pretty much the same, ranging from less than 2 mil/yr. (0.05 mm/yr.) to more than 50 mil/yr. (1.25 mm/yr.). This is typically in the form of non-uniform attack and increased pitting. The specific nature of the attack is dependent on the following factors.

sentially negligible. In wet, salt-laden air in tropical climates, corrosion rates for

carbon and

Salinity

low-alloy steels can be nearly as great as those

While both freshwater and

immersed in seawater, reaching rates of as much as 50 mil/yr. 1 .27 mm/yr.). For example, in tests conducted at Cape Canaveral, Florida, at a location 60 yards from the ocean, the corrosion rate at ground level was 17.4 mil/yr. (0.43 mm/yr.), but it was only 6.5 mil/yr. (0.16 mm/yr.) at an elevation of 30 feet. Interestingly, at Daytona Beach, 65 miles to the

sive to metals,

for steel

(

water

is

the

it is

more

(fig.

5-3).

water are corro-

generally accepted that salt

What is not so comhow much more cor-

corrosive.

monly understood rosive

salt

is

just

The presence of

water does two important things

—

salt in the it

increases

the electrical conductivity of the water,

chloride ions tend to break films

formed by the metal.

down

and the

protective

25

26

Chapter

Electrochemical or Seif-Corrosion

•

5

Mud Zone Corrosion rates

mud

(fig.

5-2).

fall

off to a

The reason

is

minimum

in

clean

oxygen

the lack of

and the fact that the rust layer is not disturbed and does offer some protection to the metal. This should offer some small comfort to the operators of marinas and boatyards with steel pilings.

One 10

20

15

Concentration of Salt (NaCI), wt.

25

%

NaCI concentration on corrosion

Effect of

rate of iron. Notice that the corrosion rate reaches a

maximum

tration,

at

which

seawater. ASM Handbook,

of caution: corrosion rates can

be misleading. Typically, they are determined

035 Fig. 5-3.

word

last

about is

just

by measuring the weight loss of metal over some period of time, then averaging this number over the area of the test sample riod.

and over the time pe-

However, the more serious damage

is

percent

salt

(NaCI) concen-

caused by pitting corrosion since areas of intense

about the

salt

concentration

pitting constitute the potential failure sites of the

3.5

metal. Corrosion rates

in

vol. 13, Corrosion, 1987, p .893, fig.

1.

tell

us

about the

little

density or depth of the pitting.

BIMETALLIC (GALVANIC)

CORROSION

Moving Water The

velocity of the seawater also has an effect

on the corrosion say

1

rate. In

slowly moving water,

to 2 ft./sec. (0.304 to

0.609 m/sec.), a cor-

rosion rate of about 40 mil/yr.

(

l.O

mm/yr.)

may

be expected. Doubling the velocity of the water

would approximately double the corrosion rate. However, fouling by marine organisms may offset this effect. If these organisms, which attach themselves to the metal surface during slack periods in the tides, are not flushed off during the flow, then a

growth can develop

that, in effect,

Carbon and low-alloy

steels are susceptible to

when placed

bimetallic corrosion

in

contact

with most other marine metals, especially brass, bronze, and stainless steels, unless the area ratios are favorable.

A

large surface area of steel

relative to the area of the

would be

more noble metal

a favorable ratio,

bronze fastening

for

example,

in a large steel plate. If

a

the area

not favorable, the parts must be insulated from each other with some sort of barrier ratio

is

coating or material. In

reduces the velocity at the surface of the metal.

proper welds, the weld metal

is

not

suffi-

ciently dissimilar to cause galvanic corrosion.

Dissolved Oxygen and

Te}}i()eratiire

1

The more dissolved oxygen in the water, the higher we woidd expect the rate of corrosion. An increase in the temperature of the water also increases

its

corrosivity.

If

the dissolved

oxygen

content were to he held constant, the corrosion rate

would

about double

just

(55°F) increase

for every 30°C>

temperature. 4’he corrosion in seawater increases directly

if

the weld

is

not ground smooth, pro-

tective paint coatings will tend to be thinner

and

uniform over the weld bead, and rust will start to form here. Also, experience indicates that carbon steels should not be joined to weathering steels. These metals are sufficiently disless

similar to permit galvanic corrosion to occur.

in

rate of mild steel

with oxygen concentration, doubling for a dou-

oxygen concentration. In the marine environment, the doubling effect of dissolved oxygen concentration tends to dominate. bling of

lowever,

STRAY-CURRENT CORROSION Carbon and low-alloy

steels are likewise sus-

ceptible to corrosion resulting

from stray or

leakage currents unintentionally caused by

Chapters

On

Electrochemical or Self-Corrosion

•

faulty electrical systems. Typically, the current

les.

from a bare wire or exposed connection immersed in the bilge water tries to find the lowest resistance path back to ground at the source. Like bimetallic corrosion, stray-current corro-

and tuna towers are particularly susceptible in the corners, at mounting flanges or where the structure is secured at the deck or house, and where they contact other metal fittings, fasten-

sion involves both anodic and cathodic sites.

ings, or

The point where and enters the

the current leaves the metal

electrolyte

is

suffers the deterioration.

the anodic site and

The point where

the

current leaves the electrolyte and reenters the

metal

is

cathode, which

the

is

protected.

boats, metal structures like pulpits

all

deck machinery.

Crevice corrosion

is

best dealt with at the de-

and construction stage. The goal should be to avoid crevices wherever possible. If a crevice can’t be avoided, it should be open and exposed to the surrounding seawater and no deeper than sign

The anode and the cathode can be yards apart. The magnitude of the currents involved are

areas should be kept clean and free of debris. In

considerably greater than those involved

some

in bi-

metallic corrosion. Stray-current corrosion

is

es-

independent of such factors as oxygen concentration, acidity, or alkalinity. Prevention

it

has to be. In existing construction, crevice cases,

it

may

be necessary to lay

in a

weld

overlay to eliminate a crevice.

sentially

consists of designing, installing, a

proper

and maintaining

electrical system.

PITTING CORROSION Pitting corrosion

corrosion.

It

is

an intense form of localized

occurs where a surface irregularity

become more anodic than the surrounding surfaces. The in-

CREVICE CORROSION

or scratch causes a minute area to

This type of corrosion occurs at or immediately

tensity of pitting corrosion

adjacent to a crevice. For our purpose, a crevice

ure to the relatively small area of the anode (the

narrow opening, space, or gap between two metal surfaces, or even between a metal and a

and the large area of the cathode (the surrounding area). Pits can develop and propagate very quickly. It is an insidious form of corrosion

is

a

nonmetal. Cracks, seams, flaws, or faults

in the

surface of the metal can qualify as crevice cor-

rosion initiation

sites.

Fastenings and their

is

due

in large

meas-

pit)

since

it is

not always readily apparent.

Pits

can

occur under deposits, at defects or holidays

in

washers, welded lap joints, and even sealants,

paint coatings, and around bits of weld spatter.

coatings, and barnacles can cause crevice corro-

Even when the metal cross section

sion. cient)

A

small volume of stagnant (oxygen-defi-

seawater becomes trapped

in the crevice,

and the crevice becomes anodic. The

relatively

small crevice area being anodic to the large sur-

rounding cathodic area

results in

an unfavorable

cathode-to-anode ratio (see chapter 6). Fortunately, this form of corrosion is usually readily apparent from the rust stains, and the source of the corrosion

is

Likely locations for crevice corrosion on steel

where stagnant moisture can joints, narrow confined spaces

vessels are places collect, as in

around bolts, threaded fittings, vent trunks, hatch coamings, and other metal structure joints at the deck. Some of the critical points to check on sailboats are lower wire terminals, tings, chainplates,

swaged

fit-

and closed-body turnbuck-

not perfo-

rated, pits constitute stress concentration sites

subject to catastrophic failure under stress. Stainless steel

is

an alloy of iron and chromium

(and some other metals).

Where oxygen

is

pres-

chromium oxidizes and forms a coating of chromium oxide, which is cathodic. The chromium oxide coating adheres tightly to the ent, the

surface and protects the metal from corrosion.

The

easily traced.

is

stainless steel

Flowever,

—

if

is

then said to be “p^^ssive.”

the steel

is

deprived of sufficient

oxygen as occurs anywhere stagnant water accumulates the protective coating will begin to break down. Over time, pinholes develop in the coating. These points of exposed metal begin to function as anodes to the cathodic surfaces around them. The result is pitting. This kind of corrosion can be very intense since the corro-

—

27

28

Chapters

•

Electrochemical or Self-Corrosion

sive action

is

concentrated

in

such a tiny area.

clamps often exhibit pitting good reason for using double hose

Stainless steel hose

corrosion

—

a

clamps on through-hulls.

which corrosion proceeds. Anaerobic

bacteria,

such as the sulfur-reducing bacteria (SRB), con-

sume

the hydrogen. This depolarizes the cath-

ode, accelerating corrosion.

The aerobic counterpart of

Likely places for pitting corrosion to occur

the

SRB

is

the

and bearings, in stern tubes, under barnacles, and on keel bolts. Other areas to watch are between propeller shafts and propellers, in the threads

sulfur-oxidizing bacteria, which can create a sul-

under propeller nuts, inside the lower terminals is

There have been reports of microbiologically induced corrosion (MIC) in steel hulls that bottom out on mud flats for extended times, but ex-

cracked or peeled back, moisture will inevitably

perts are reluctant to attribute such degradation

be wicked up under the sheathing and cause pit-

to

ting corrosion of the wire rope.

stances could be simply localized corrosion (pit-

are inside stanchion bases, between shafts

of wire rope fittings, and in turnbuckle threads.

Also,

If

if

the plastic sheathing on lifelines

pits are shallow, cleaning

them thoroughly

by sandblasting and then recoating (repainting) will be

adequate, but once pitting has gotten

must be ground out and the face restored with weld metal. It is probably to assume that a pit is at least as deep as well started,

it

furic acid

environment. The acid promotes cor-

rosion, typically of the pitting type, beneath these colonies.

MIC. The corrosion damage

and crevice corrosion) that results from oxygen depletion similar to the corrosion that occurs under barnacles and other marine ting

—

A

sur-

growth.

best

the best preventive.

it

in these in-

good, well-maintained coat of paint

is

is

wide.

INTERGRANULAR ATTACK (WELD DECAY)

POULTICE CORROSION Poultice corrosion occurs where a soft, salt-

water-saturated, typically organic mass

is

in

contact with metal for long periods of time. Ex-

amples are damp insulation on pipes or exhaust system components, an accumulation of dirt and debris

in

the bilges or scuppers of a steel vessel,

and even wood decking

on metal deck plates. Poultice corrosion is a risk wherever seawater can be absorbed, become stagnant, and lie set

against a metal surface for a long time.

MICROBIOLOGICALLY INDUCED CORROSION When microorganisms form on

a metal surface they

can

localized colonies

facilitate corrosion.

two basic groups of bacteria: anaerobic, which thrive in the absence of oxygen and which are the most common type, and aerobic, which require oxygen to live. Both cause corro-

1 here are

sion, but of different kinds. 1

gen

he formation and buildup of atomic hydroat the

cathode

will

always

term ireld decay is misleading. It is the metal immediately adjacent to the weld and not the weld itself that deteriorates. W'eld decay is a form of intergranular attack (ICiA), a First of all, the

limit the rate at

condition that gets

name from

its

crystalline particles that

the individual

form the metal

—called

grains.

Austenitic stainless steels, such as those

in

the

300 series (dype 304, Type 3 16, etc.), contain moderate amounts of carbon. When they are welded, the dissolved carbon in the weld area migrates to the grain boundaries and forms

chromium to the

has a

carbides. Lhis leaves an area adjacent

weld that is depleted of chrome and thus much lower resistance to corrosion in the

grain boundaries.

It is

this area that

is

now

sus-

ceptible to corrosive attack or decay.

Weld decay

is

a particularly treacherous

form

of corrosion since steel suffering intergranular

corrosion

may

look sound but

seriously weakened.

Many

may

actually be

think that only the

austenitic stainless steels are susceptible to

weld

decay, but certain high-nickel alloys and

some

aluminum

form

alloys can also experience this

—

— Chapters

of corrosion. However, stainless steel Types

304L and

3 16L

— the L stands

for

low carbon

Electrochemical or Self-Corrosion

•

popularly referred to as dezincification. Dealloying also occurs in alloys containing

aluminum

[dealnnunificatton) and nickel {denickelifica-

are not susceptible to weld decay.

tion).

Brass

DEALLOYING (DEZI NCI FICATION) Dealloying involves the preferential disintegration of one particular

Common

component

of an alloy.

yellow brass, an alloy of copper and

an excellent example of this type of corrosion. Depending on the type of brass, the percentage of copper varies from 55 or 60 percent to 90 percent whereas the zinc content varies from 10 percent to 45 percent. The zinc, being higher in the galvanic series and therefore more anodic, functions as the anode. The copper, lower in the series and therefore more cathodic, functions as the cathode. The body of the brass object provides the metallic path for electrons. The seawater will carry the ion flow. We have everything we need for corrosion to zinc, offers

is

not

good choice

a

for fastenings in

exposed locations, especially below the waterline. Bronze, Monel, stainless steel, or even hotdipped galvanized fastenings are vastly better. Incidentally, keep in mind that the popular manganese bronze

is

Composed of zinc, some tin,

not really bronze.

58.5 percent copper, 39 percent some iron, and about 0.38 percent manganese, really a brass.

it’s

Consequently,

it is

subject to

dezincification, although not nearly as as

common,

much

so

or cartridge, brass.

— deriving name from graphite content — subject to a similar corroGray

cast iron

its

its

is

When

sion process.

dealloying takes place

gray cast iron, graphite dissolved.

is

The process

in

the constituent that is

is

referred to as de-

graph itization.

occur.

The anode

— that

away, leaving a

soft,

is,

the zinc

— will corrode

porous copper

shell.

This

EROSION CORROSION

is

Erosion corrosion

is

a

mechanically assisted

at-

tack in which velocity or abrasion by a flowing brass

medium

major factor in the deterioration. The corrosion is caused or accelerated by the is

movement

a

of the liquid

of the metal.

It

may

medium

over the surface

involve the removal of the

protective coatings that form naturally on the

surface of the metals. Turbulence or impinge-

ment may compound the problem. Typical situations involve fast-moving water for

example, the flow of seawater through pip-

pumps, or heat exchangers, bends or on surfaces that obstruct

ing systems, valves,

especially at

the flow. In severe cases, say a high-velocity

stream of seawater with sand or other solid particles

entrained,

it

can cause rapid thinning and

eventual penetration of the metal. This can be a serious failure visible, Fig. 5-4.

Dezincification.

The

zinc

is

selectively dis-

solved from zinc-containing alloys. This occurs most

commonly contain

in

less

copper-zinc alloys such as brasses that

than 85 percent copper.

and

mode

since the degradation

failure can be

is

not

sudden and cata-

strophic.

The flow

and fuel-system enough for erosion

rates in freshwater

piping typically arc not fast

corrosion to be a problem. Lhe practical signif-

29

— 30

Chapters

•

Electrochemical or Self-Corrosion

icance of erosion corrosion for us as owners and

operators of boats

engine cooling and

in the

is

exhaust systems. Here water velocities can be quite high, perhaps as much as 75 to 80 ft. /sec. (23 to 24 m/sec.), and the raw water stream may well contain solid particles. Piping here

com-

is

monly steel or iron. The exhaust injection elbow, the most susceptible point in the system, eventually will corrode through and should be checked annually. Other places to keep an eye on are at elbows, tees, valves, and heat exchanger

fittings.

Preventive measures consist of

the use of alloys with greater strength or corrosion resistance

—such

Monel

as stainless steel or

or the use of larger diameter pipe and fittings. In

one heat exchanger took out of a 30-year-old boat, the bronze outside shell showed little evidence of corrosion, but a pressure test revealed

STRESS-CORROSION CRACKING Stress-corrosion cracking (SCC)

CAVITATION CORROSION Cavitation

is

a particularly aggressive

form of

caused by the formation and collapse of vapor bubbles in a liquid against a metal surface. These rapidly collapsing vapor bubbles produce explosive shock waves, and the high pressures this generates tend to erosion corrosion that

is

enter microscopic cracks and pores on the metal’s surface

damage

is

and cause damage. Cavitation

similar to pitting except that the sur-

face in the pits tends to be

We

much more

corrosion and tensile

nection with propellers, where

pockmarking

it

in

con-

appears as

se-

edge of the blades. This results from the sudden formation and collapse of low-pressure bubbles due to the vere

at the trailing

prop’s high-speed rotation,

also occurs on

pump

types of surfaces

in

liquids subject to

1

many

contact with high-velocity in

pressure. In any

water passage, when the stream flows through constrictions or abrupt changes in direction, or around immersed structures, the forceful reduction of pressure can lead to cavitation. Prevention

below the

is

a particularly

yield strength of the metal. Typically

stress-corrosion cracking consists of a localized

network of

fine cracks

with multiple branches, a

sort of crazing at the surface of the metal.

It is

detectable visibly or with the help of a low-

power magnifying

glass. Penetrating dyes also

are useful for detection.

The ing use

stress

can either be applied

— or be residual as

— that

a result of

is,

dur-

forming.

Stress-corrosion cracking involves a chemical-

mechanical interaction in which the electrochemical process is enhanced under the mechanical stress. Those areas in the part that are under stress become anodic with respect to surrounding areas. The electrochemical deterioration at the anodic areas combined with the tensile stress tends to pull the material apart.

The

result

is

cracking.

Stress-corrosion cracking frequently occurs in the crease in metal fittings that have been cold-

formed by bending, in such items as angle brackets, chain plates, and braces. It is wise to keep an eye on bent fittings when they are used in critical applications.

When opposed

failure occurs

due

to cyclical stress as

to constant stress, this

is

referred to as

corrosion fatigue, a mechanically assisted form of SCC.

CORROSION FATIGUE Corrosion fatigue

is

the premature fracture of a

metal under simultaneous conditions of an electrochemical corrosive action and recurring stress,

happening sooner than would normally occur were there no corrosive activity taking place.

consists primarily of avoiding these conditions

wherever possible.

This

lowever, cavitation

impellers and on

change

stress.

troublesome form of attack since the part can fail suddenly without any warning after years of satisfactory service and under stress levels

coarse.

normally think of cavitation only

the cracking

of a metal resulting from the combined action of

1

terminal corrosion of the inside surfaces.

is

Steel ter

is

subject to corrosion fatigue in seawa-

environments. Boat components and

fittings

— Chapters that are exposed to seawater

and are subject to

Electrochemical or Self-Corrosion

•

Also avoid unfavorable material combina-

— aluminum and stainless

continual flexing or vibration, such as chainplates, uncoated wire rope, or hull plating in

copper and cast iron. Increasing the clamping

close proximity to the propeller and shaft are

force (e.g., tightening of fastenings) to reduce

Cracks tend to originate at surface or near-surface sites and frequently, although not always, at corrosion

relative

likely to suffer corrosion fatigue.

many

pits. In

naked is

cases the cracks are visible to the

tions

but

it

provides

relief

but only temporarily. Sometimes

the use of sealants such as silicones or polysulfides are effective

The use of dye penetrants can

tors or separators.

also

brass or

movement seems logical and may help, can also make matters worse. Lubrication

eye, but an inexpensive magnifying glass

helpful.

steel,

because they serve as insula-

help reveal the extent of the cracking. Regular inspection, maintenance of protective coatings,

prompt attention

to the treatment of pitting

and

HYDROGEN DAMAGE

crevice corrosion damage, and, to the extent

Hydrogen damage

possible, elimination of stresses are principal

cludes several different forms of attack,

preventive measures.

volving hydrogen as a causative factor and,

most (but not gen

FRETTING CORROSION

—

in the

—

all)

is

a general

term that

cases, tensile stress.

in-

all in-

in

Hydro-

atomic state (H), not the molecular is a problem because the hydrogen

they are

(H 2 atom is extremely small. It is so small that it readily diffuses into and through the structure of

subjected to vibration or motion consisting of

the metal, concentrating in microstructural

The constant

voids and discontinuities. As the concentration

state

Fretting corrosion occurs on the load-bearing

surfaces between mating parts

when

very small amplitude oscillations.

abrasion between the metal surfaces causes minute particles to break away from the sur-

These particles corrode and may cause seizing or galling. This is a frequent problem in pumps, valves, fastenings, and bearings where the mating surfaces rub together without the faces.

faces

but

by as

fretting

much

as

It

to

and

contacts.

If

loss

of strength and ductility. So,

how

where does the hydrogen come from and

does

it

come

to be absorbed into the metal?

through contact with cleaning and maintenance

its

70 percent.

endurance In

life

limit

wire rope the

wire strands rub against each other as the rope flexes,

can be void growth, cracking, and a

integrity of the joint,

can reduce

50

result

the metal sur-

damage

also has a severe effect on the fatigue

of the metal.

The

is

Steel

—

and consequently the

it

created.

increases, a high internal pressure

can absorb hydrogen in several different ways during the manufacturing process, during heat treatment, during welding operations,

benefit of lubrication.

Not only does

)

chemicals (acids and

alkalis), as a result of cor-

rosion reactions, or due to excess hydrogen gen-

erated by cathodic protection systems.

problem

is

The

greatest in the hardened alloy steels,

fretting occurs at the wire-to-wire

including stainless steels and nickel-base alloys,

the fretting occurs in an area that

but

is

it

has also been

known

to occur in naval

subject to cyclic fatigue stresses, the wire rope

bronze under the influence of impressed-current

can

cathodic protection systems.

fail.

The

also occur in the noble metals

can be either internal that is, residual stresses that are inherent in most or extermaterials, especially hardened steels

metals. Since vibration

nal,

Fretting

where

it

is

elimination

is

not limited to ferrous metals,

accompanied by oxidation;

is

the

venting fretting

can

and even in nonthe primary cause, its

order of business

in pre-

— properly tuned standing

ging, stiffeners to plate areas,

first

is

it

dampen movement

rig-

of large

and proper tightening of fastenings.

tensile stresses

—

such as those that a structural component

normal function. Cleats, chainplates, turnbuckles, and similar structural deck and rigging fittings are likely candidates for hydrogen damage. might be expected to undergo

in

its

31

— 32

Chapters

The

Electrochemical or Self-Corrosion

•

several forms of

hydrogen

hydrogen damage are

of the stress on the metal, the temperature, and

hydrogen-assisted

the length of time of the exposure are factors

enibrittlefuent^

crackings and hydrogen blistering. ual stress or external loading

If

no

resid-

called hydrogen-assisted

is

cracking

(HAC)

or hydrogen stress cracking

(HSC).

there

active corrosion taking place,

If

is

typically crevice or pitting corrosion,

it is

called

stress-corrosion cracking.

Fortunately for us, steels below about 140,000

(9,843 kg/cm-) tensile strength, or 100,000

psi

psi (7,03

1

kg/cm“) yield strength, which includes

carbon and low-alloy

steels, are

not susceptible

to either stress-corrosion cracking or

hydrogen-

However, hydrogen blistering of ductility are frequently found in the

and accelerate the damage.

Great care should be taken

present, hy-

is

drogen embrittlement can manifest itself as blistering, internal cracking, and reduced ductility. If stress is present, it can result in crack growth and eventual fracture. If no corrosion is taking place, this

that facilitate

ing,

and welding

and

free of

in soldering, braz-

to be sure the parts are clean

contamination (zinc-pigmented paint,

scraps of zinc, lead,

etc.),

heating temperatures

no greater than necessary, joints are designed to minimize exposure, and the operation is carried out as quickly and as efficiently as possible. are

HOT-SHORT CRACKING At the

risk of giving the

thing to worry about,

novice welder one more I

must include another

welding-related type of degradation that can lead to stress-corrosion cracking, corrosion fatigue,

and crevice corrosion. Hot-short crack-

assisted cracking.

ing can occur during manufacturing or

and

metal

loss

low-strength steels

commonly used

construction. In low-strength steels psi yield strength or less

predominantly

in the

in vessel

— 100,000

— hydrogen damage

form of

is

blistering or in-

creased brittleness.

LIQUID METAL CRACKING Liquid metal cracking, while not strictly a cor-

phenomenon, is a potential problem for boat owner and repairer. Also known as liq-

rosion the

uid metal embrittlement., this brittle failure of a

is

the catastrophic

normally ductile metal.

It

re-

from the penetration of metals and alloys, including steels, by metals that have relatively low melting points, such as lead, zinc, copper, cadmium, and aluminum.

is

hot-worked or welded.

It

when

results

the presence of low-melting elements

—

a

from

— such as

copper at the grain boundaries in the alloy being welded. When the metal solidifies, the low-melting elements separate, causing minute cracks. Typically, these cracks show up within a matter of hours and are detectable with a dye penetrant. However, they may not show up for months or even vears. Silicon bronze screws, bolts, and threaded rod are used widely in the marine industry. Silicon bronze, an alloy of copper and to 3 per1

cent silicon,

is

subject to hot-short cracking after

welding, brazing, or hot-working.

sults

Liquid metal embrittlement occurs

low-melting-point metal

in

when

the

the molten state

such as during brazing, soldering, or welding operations is in intimate contact with a base

—

metal that

known

is

under

tensile stress.

It

also has been

to occur in galvanized steels that

been subjected to intense heat, as during a

and when welding

steel

have fire,

has been coated with

zinc-bearing paints. Fhe result

is

a serious degra-

dation of material properties, such as increased brittleness

and

loss of strength.

Fhe magnitude

CONCLUSIONS metal components,

and machinery on your boat are undergoing corrosion of one form or another. Underwater fastenings and fittings such as bronze through-hulls and raw water strainers are undergoing simple self-corrosion. However, good-quality bronze is highly All

fittings,

corrosion resistant, so corrosion

slow process

— so slow,

in fact,

is

an extremely

that you’d have

trouble detecting any loss of metal

in

the fitting

even after 25 or 30 years. Isolated above-the-waterline fittings lated

—

iso-

from each other and from metallic contact

Chapters with other dissimilar metals candidates air, salt

when exposed

— are also corrosion

to moisture-laden salt

Electrochemical or Self-Corrosion

•

of the fitting clean and by drying out trapped water.

The most

spray, or contact with a saltwater-satu-

practical

method of minimizing the

rated mass.

Deck hardware such as wire rigging, stanchions and bases, and cleats are all undergo-

consequences of corrosion is careful visual inspection. Use a bright light and a magnifying

ing basic electrochemical corrosion. Bronze deck

glass to take a

fittings

and

develop

last for

a corrosion-inhibiting

many

years.

Aluminum can

similar durability, especially

if it is

coating deliver

anodized.

we

good

close look at

all

discussed above. Wire rigging,

chainplates

those places

lifelines,

and

suddenly and at the worst posthere’s any doubt about any piece

fail

sible time. If

components develop a chromium oxide protective coating and also last very well, except for those areas where the coating fails and pitting takes place. Iron and mild steel parts will do as they always have done rust. Limiting self-corrosion basically comes down to two things protective coatings and metal selection (both covered in later chapters). You can

Even metallic underwater components such as through-hull fittings, rudder blades and posts, and props and shafts will develop protective coatings if they are isolated. But when the prop and shaft are of different metals, as they often are, then something more than simple electro-

also slow self-corrosion by keeping the surface

in

Stainless steel

—

—

of equipment, replace

chemical corrosion chapter 16

.

is

it.

going on.

We

discuss this

33

Chapter 6

Galvanic Corrosion

In chapter

process

4

we

talked about the basic corrosion

— what actually happens to the metals

involved and to the electrolyte in which they’re

immersed. Four things must be present for corrosion to take place: an anode, a cathode, a metallic path for the electrons, and a common electrolyte or electrolytic path for the ions. We

Second, galvanic corrosion is more serious and proceeds at a much more rapid rate than simple self-corrosion. Third, the chemical reactions that take place in a

galvanic

cell yield different results

increase the probability of

damage

to

that can

wood

in

the area of the cathode.

talked about the chemical reactions that take

In a galvanic cell, the difference in corrosion

place at the anode, where metal atoms ionize

two metals causes electrons to flow from the metal with the more negative potential (anode) toward the metal with the less

and dissolve into the electrolyte, and at the cathode, where the free electrons released by the anode combine with dissolved oxygen in the seawater to form hydroxyl ions. We see later how these hydroxyl ions can^ under certain conditions, combine with other elements in the seawater to form alkalis that can be very damaging to wood. In chapter 5 our focus was self-corrosion, corrosion involving local anodes and local cathodes present in a single piece of metal, and we talked briefly about what we could do about that. All right!

Now

corrosion that

is

WHAT

the

we’re getting to the kind of

main kind

you’ll encounter.

GALVANIC CORROSION?

15

most common form of corrosion in the marine environment and a major problem for boatowners. As in selfcorrosion, galvanic corrosion involves an anode and a cathode connected to each other by a metallic path for electrons and an electrolytic (ialvanic corrosion

is

the

potentials of the

negative potential (cathode). Electrons arriving

cathode increase

at the

its

negative potential.

This continues until the two metals are at the

same

potential, that

is,

until there

is

no voltage

difference between them. This occurs at a volt-

age level somewhere between their natural, or isolated, potentials.

Keep in mind that each of the metals was already undergoing simple self-corrosion as we saw in chapter 5. The reactions that take place at the local anodes of each metal, that is, the ionization of metal atoms into the seawater together with the release of free electrons into the

body of the metal, are balanced by the reactions that take place at their local cathodes, where dissolved oxygen in the seawater combines the excess electrons and water molecules to form hydroxyl ions.

When vanic

the

cell

two metals are connected,

the gal-

begins to function. F’lectrons flow

from the anode through the wire

path for ions. But galvanic corrosion differs from simple self-corrosion in some important

ode. These electrons are in addition to those

ways.

ready consumed

First,

more

galvanic corrosion involves two or

different metals.

Fach metal has

electromotive potential, as

its

in

the

commonly

corrosion potential, and

same

galvanic

electrolyte. This

cell.

all

is

a different

referred to

are

immersed

in

to the cathal-

the cathode reactions in

simple electrochemical corrosion. Therefore, more electrons must be produced by the anode, resulting in

more rapid corrosion of

the

anode

metal.

At the cathode the reaction rate must increase

referred to as a in

order to accommodate the extra electrons ar-

Chapter 6 riving there. at the

If

cathode

rate of the

number

the is

of electrons arriving

great enough, the self-corrosion

cathode metal

will decrease

hydroxide

film.

This

is

•

Galvanic Corrosion

the protective film dis-

cussed previously.

and may

In a galvanic cell, the

some distance from

anodic metal

may

be

stop altogether. In other words, the rate of cor-

quite

rosion of the anode metal increases while the

the probability of metal ions released at the

may

the cathodic metal, so

actions taking place at the local anodes and local

anode combining with the hydroxyl ions formed at the cathode is very small. Instead of combining into a protective film, the hydroxyl ions formed at the cathode stay in solution. If they are allowed to concentrate, the water becomes more alkaline and the wood in the area may be damaged. However, if the water is flowing and constantly changing, the hydroxyl ions are dispersed and the likelihood of alkali damage to

cathodes are extremely close to each other phys-

the

corrosion rate of the cathode metal less

he even

than was taking place under simple

corrosion

(fig.

6-1

self-

).

However, and this is one of the problems, the increase in the production of hydroxyl ions at the cathode can result, depending on other conditions, in

damage

to the

wood

in

contact with

or close to the cathode. In self-corrosion, the re-

wood is lessened. Any time a metal component

The hydroxyl ions formed at the local cathodes are close enough to the local anodes that they can combine with the metal ions re-

or touching another metal and both are im-

leased into the seawater there. This forms a

saturated

metal oxide that attaches to the metal as a metal

galvanic

ically.

Q

mersed

in

is

seawater or embedded

medium cell

connected to

in a saltwater-

—such as wood —they form

and are subject

to galvanic corro-

electron flow

©

electron

COH

hydroxyl ion (OH‘)

OO.

oxygen molecule (OJ sodium atom (Na) sodium ion

Oci

Galvanic corrosion. The steel

is

atom (Fe)

mild steel ion

(Fe'"'")

copper atom (Cu)

copper ion (Cu"")

predominantly anodic, whereas the copper is predominantly cathodic. Electrons move from the anode areas toward the cathode areas, leaving metallic ions that dissolve from the anode areas into the electrolyte. Sodium ions (Na^ may combine with hydroxyl ions to form sodium hydroxide, or caustic soda, which is damaging to wood. Fig. 6-1.

(Na'")

clorine ion (CI-)

mild Steel

0 @ ©

a

35

36

Chapter 6 sion.

Galvanic Corrosion

•

The extent

of the corrosion that takes place

is

a function of the following factors:

•

the

•

magnitude of the potential difference between the dissimilar metals and the nature of the electrical contact between them the relative areas of the two metal surfaces

•

the polarizdtion (see below) that takes place at

flow from the anode to the cathode has also been established. Since the magnitude of the current flow determines the amount of metal that will be removed from the anode, the rate at

which corrosion will take place is also estabassuming that protective films are not lished

—

formed.

each of the metals •

the nature of the environment in

which the

metals are immersed

CATHODE-TO-ANODE RATIO The

ratio of the area of the cathodic surface to

that of the anodic surface

POTENTIAL DIFFERENCE: THE GALVANIC SERIES

galvanic corrosion.

a critical factor in

is

Remember

we

that

said elec-

trons flow from the anodic surface area to the

cathodic surface area through the metal contact In chapter 4 a

list

we

introduced the galvanic series,

of metals and alloys ranked according to

their open-circuit potentials (relative to a silversilver chloride electrode).

metals are

in this list,

The

farther apart the

the faster the rate of gal-

vanic corrosion, so the galvanic series enables us to anticipate

how two

or connection.

The anodic metal

supplies elec-

trons by literally giving up pieces of

(metal

The anode must supply

ions) to the electrolyte.

enough electrons

itself

to maintain the cathode at

its

new, more negative potential. The greater the area of the cathode, the

metals will react to each

When

more

electrons required.

the area of cathodic surface (the one

when placed in contact. With this knowledge, we can try to avoid those combinations

than that of the active or anodic surface (the

likely to result in severe corrosion.

most

other

Although the seawater,

in a

series

is

based on immersion

way

general

most natural waters and

is

it

to the

in

applicable to

atmosphere, bar-

ring factors like acid rain or other contaminants

that significantly alter the linity)

pH

(acidity or alka-

of the water.

least likely to

likely to corrode),

to-anode ratio

is

is

significantly larger

an unfavorable cathode-

said to exist. Since the magni-

determined bv the area of the cathode, a larger cathodic surface can cause the current density at the anode to be tude of the current flow

excessively high (in

amps

is

per square inch). Be-

cause the amount of current determines the

amount

of anodic metal that will be dissolved,

severe galvanic corrosion

POLARIZATION When two

corrode)

is

likely.

The opposite

— where the anodic area larger than the cathodic area — provides favorable

condition

metals are electrically connected to

is

ratio.

a

example. Say we

form a galvanic cell, electron flow takes place between them, and they tend to come to a common potential somewhere between the isolated

unpainted steel-hull vessel with a small, unpainted copper-alloy through-hull fit-

potentials of the individual metals as listed in the

ting.

The potential move toward the

galvanic series. metal tends to

of the anodic potential of the

cathode, whereas the potential of the cathode tends to

move toward

shift in potentials

is

that of the anode. This

of the

cell will

the relative surface areas of the this

have

two

depend on

metals.

equilibrium corrosion potential has

been established, a rate at which electrons will

consider

surface area

I’he large steel

manded ode

a practical

a large,

and has no

is

the

anode

supplying the electrons deby the small surface area of the cathdifficulty

— the ct)pper-alloy through-hull — without

experiencing significant deterioration.

Now

referred to as polarization.

I'he resulting potential

Once

Let’s

suppose the

hull

bare, but the coating tiny pinholes or chips. site situation.

in

is

is

painted rather than

not perfect, exhibiting

Here we have the oppo-

The anodic

surface

is

just the steel

contact with the electrolyte, that exposed by

Chapter 6 the pinhole or chip, so

now

the anodic surface

compared to the area of the copperalloy fitting. The copper-alloy fitting is still the cathode and still requires the same number of electrons, but they must be supplied from an anode with a tiny surface area. This results in extremely high current densities and severe pitarea

is

tiny

ting corrosion.

Whenever

behooves us to maintain a favorable (low) cathode-to-anode ratio. We talk more about this when we discuss fastenings in

possible

it

chapter 16.

•

Galvanic Corrosion

on carbon steel in galvanized steel fastenings and fittings. Corrosion of the steel can occur at pores, chips, scratches, and the zinc coating

edges of the protective coatings. It is

the process of galvanic corrosion that

provides the basis for the cathodic protection

schemes we use to combat the corrosion process. We discuss those schemes in greater depth in chapter 13.

GALVANIC CORROSION SUSCEPTIBILITY The

FORMS OF GALVANIC CORROSION

ease with which a specific metal

ized has considerable influence

galvanic

Galvanic corrosion takes place at the anodic member of the galvanic cell. It can occur uni-

change

formly over the anodic surface or at discrete

its

on the anode, as

pitting, crevice corrosion,

stress-corrosion cracking.

Which form

it

sites

and

takes

depend on the specific nature of the galvanic cell, whether films are present, what types of films are found, and what metals are involved. Galvanic corrosion occurs most commonly where two or more dissimilar metals are in contact, such as prop and prop shaft, rudder and rudderpost, perhaps heat exchanger and engine. However, galvanic corrosion can also take place in situations where nonmetallic conductors act as cathodes in galvanic cells. An example is mill scale (magnetite, FeO), which forms on carbon will

steel.

Mill scale

a large area

is

a conductive film that acts as

cathode to the anodic substrate, the

steel.

A

less

commonly

recognized nonmetallic con-

carbon fiber composite used in masts and other spars and in rudders and rudderposts. Exposed carbon fibers are good conductors of electricity, and although the carbon fiber composite is not itself damaged by corrosion, it can ductor

is

function as the cathodic cell

when

in

contact with

such as fastenings and

member less

of a galvanic

noble components

fittings.

Another example of galvanic corrosion can occur when sacrificial metals are used as protective coatings on anodic metals, such as the

cell,

so

let’s

We

of polarizatio7i.

on

is

its

briefly review the

role in a

concept

said that polarization

in the metal’s

polar-

is

the

corrosion potential from

open-circuit value as the result of current

flow between the metals

in the

galvanic cou-

The potential of the anodic metal tends to become more noble and the potential of the cathodic metal tends to become more active. If the more noble metal is more easily polarized, its potential is shifted more toward the more active metal. The shift in potential of the more ple.

active metal in the direction of the cathode

is

therefore minimized so that accelerated gal-

vanic corrosion otherwise.

is

When

readily polarized, tive

metal shifts

not as great as

it

would be

more noble metal is not the potential of the more acfarther toward the cathode, the

and significant accelerated galvanic corrosion results.

Although we discuss the specifics of the metals commonly used in the marine environment in

now is a good time for some comments on common marine metals and

greater detail brief

later,

their susceptibility to galvanic corrosion.

Aluminum and

Its

Aluminum and

alloys are lightweight, fairly

its

Alloys

strong, and quite resistant both to atmospheric

corrosion and,

if

sion in seawater

of a suitable grade, to immer-

— assuming that the atmosphere

and the seawater are reasonably unpolluted.

37

38

Chapter 6

Galvanic Corrosion

•

Aluminum owes

this resistance to its ability to

rosion potential and the zinc at -0.98 to -1.03

we have

The

quickly form a protective oxide surface film. Aluminum alloys, however, are very active met-

volts,

can be seen from their position in the galvanic series (-0.76 to -1.00 volts; see table 4-1).

leaving a porous, spongy, brittle fitting. Brass

als, as

aluminum

In seawater,

alloys are susceptible to

localized galvanic corrosion such as pitting

and

and severe galvanic corrosion when aluminum alloys are coupled

zinc

is

wont

a strong galvanic

cell.

zinc, as

to do, sacrifices itself (dezincifies),

gate valves used in place of proper bronze sea-

cocks are

a classic

misuse of brass

example of the dangerous

fittings

below the waterline. This

discussed further in chapter 16.

crevice corrosion,

is

can result

Broadly speaking, copper alloys occupy the corrosion potential range between -0.18 to -0.42 volts, which places them between the ac-

with more noble metals. In freshwater or water with low chloride concentrations, aluminum

al-

loys are considerably less active because of the

greater stability of the protective oxide film,

hence galvanic effects are not as severe. Certain cial

aluminum

anodes

listed in the

in

alloys are used for sacrifi-

seawater.

The aluminum

alloys

galvanic series table do not include

Aluminum alloys used as sacrifianodes have a maximum corrosion potential

these alloys. cial

of -1. 100 volts.

Copper and In this

and passive positions for the stainless steels. Because they are not easily polarized in seawater, they can cause severe corrosion of more active metals such as aluminum and steel. tive

Its

and Steel

Iron and carbon and low-alloy steels corrode

when exposed

atmosphere, immersed, or in seawater. xAlthough they do develop oxide layers on the surface, these films are porous and loosely attached. They separate easily from the parent metal, re-exposing the surface and allowing the process to repeat. In other words, iron and steel rust. Even so, because of their low cost, ease of fabrication, and good strength, they are readily

Alloys

category we’re talking about the bronzes

and brasses. True bronzes are quite strong and ductile and are extremely resistant to corrosion, both atmospheric and immersed. Good-quality bronze is truly a superior marine metal. Nearly as strong as stainless steel, it is more ductile (an important safety feature) and is not subject to pitting corrosion.

Iron

It is

manand marine compo-

widely used

ner of fittings, fastenings,

nents. Silicon bronze (Hverdur)

is

in all

probably the

most widely used and the most satisfactory metal for marine screws and bolts. It is, to all intents and purposes, immune to self-corrosion either in the atmosphere or immersed in seawater. However, it is sid')ject to galvanic corrosion (as are all metals) when coupled with other more anodic metals. 1 he brasses, on the other hand, are frequently

used for decorative fittings and trim

in

benign

interior atmospheres, but they are not really sat-

where they can be exposed to salt air or spray or immersed in seawater. Most brasses are only 58 to 60 percent isfactory for outside use

copper and 39 percent zinc, and therein lies the rub. With the copper at -0.30 to -0.57 volts cor-

to the

widely used as structural materials. Iron and steel are quite active, with a range of

corrosion potentials of -0.60 to -0.71 volts.

Carbon

steel, also called

about the same does

in

mild

steel,

corrodes at

rate in aerated freshwater as

it

aerated seawater, but the higher electri-

cal conductivity of

seawater can result

in

more

severe pitting corrosion.

The

so-called alloy steels were developed to

provide added strength and toughness. Lowalloy steels typically contain the same basic constituent elements as the low-carbon grades but with small amounts of such alloying elements as niobium, vanadium, chrome, copper, or nickel

added. Several grades offer enhanced atmospheric corrosion resistance and are referred to as weathering steels because they develop a unique protective oxide layer

when exposed

atmospheric conditions. Cor-Ten is an example. These grades have found some use in steel vessel construction, but it should be noted that this proto

Chapter 6 rective layer does nor occur

immersed.

We

talk

when

more about

when

Galvanic Corrosion

•

more noble

the metal

is

steels

chapter

8.

metals such as the copper alloys. Also, galvanic

this in

they are in contact with

corrosion of carbon steels can be induced by

Stainless Steels

stainless steel in seawater, especially with unfa-

Stainless steels are high-alloy steels.

They have

better corrosion resistance than the

carbon and

low-alloy steels because they contain relatively large

amounts of chromium. The

three general

vorable cathode-to-anode ratios.

Lead and

Its

Lead finds limited application

classes of stainless steels are austenitic^ ferritic^

vironment;

and martensitic. Of these three, the austenitic stainless steels have the greatest resistance to corrosion and arc the most widely used in the marine environment for all manner of fittings, fastenings, and components. The basic composition of this group is commonly referred to as the 18-8 stainless steels, which refers to the percentages of chromium and nickel in the composition. The term 18-8 is imprecise and subject to various interpretations. talk more about this in

nal or internal

I

chapter

is

found

in several different

sitions in the galvanic series,

marine en-

used primarily as ballast

— and as solder Lead

—exter-

in electrical

and

noble metal with a corrosion potential range of -0.19 to -0.23 volts. The protective film that forms on lead

and

joints.

its

alloys

pheric corrosion

is

very effective, and atmos-

is

is

relatively slight.

being more noble than

in

in steel

Even in However,

essentially negligible.

seawater, corrosion

corrosion

a fairly

is

when

steel,

in

lead will accelerate

contact and immersed

seawater, hence the difficulty with steel keel

po-

depending on the

composition of the specific grade (that is, 304, 3 1 6, ASTM 564, ASTM 630) and on whether the steel is in the passive or the active state. The term passive here refers to a metal that has developed a protective oxide layer. The corrosion poten-

much more noble than

of passive grades are

same metal without For example. Type 304

the

its

protective oxide layer.

in

the passive state has a

corrosion potential range of-0.05 to -0. 1 0 volts,

whereas Type 304

in

the active (nonpassive) state

has a corrosion potential range of -0.46 to -0.58

A

plumbing

it is

in the

bolts in lead ballast keels.

8.

Stainless steel

tials

Alloys

Soldering Soldering utilizes lead-base or tin-base alloys

with melting points below 800°F (426. 7°C). Most solders in common use contain from 2 to 100 percent tin with the balance consisting of

The most common alloys are 50-50 and 40-60 tin-lead. These solders are close to coplead.

per and brass in the galvanic series.

Typically a flux

The function

is

used

in

making

the solder

remove oxides already present on the metal surface and to prejoint.

of the flux

vent the formation of

is

to

new oxide

films that might

found for Type 3 1 6, which has a corrosion potential range of 0.00 to -0. 0 volts in the passive state and -0.43 to -0.54

inhibit satisfactory wetting of the surfaces to be

volts in the active state.

natural resins.

volts.

similar difference

is

1

Stainless steels rely

on the protective oxide

coating for their resistance to corrosion.

When

joined. Mild fluxes, such as the resin fluxes

monly used

in electrical

They

solely of

and However, more power-

are essentially inactive

leave a harmless residue. ful

work, consist

com-

fluxes can contain chlorides and/or phos-

flowing water containing plenty of oxy-

phoric acid to remove more stubborn oxides.

gen, they are able to develop and maintain this

Residues from these fluxes can be harmful and

in clean,

oxide film. However,

when they

are deprived

of this fresh supply of oxygen, they lose their protective film, tible to serious

become

active,

corrosion.

and are suscep-

Oxygen deprivation

frequently occurs under barnacles,

in

stern

and under Gutless bearings. (lalvanically induced corrosion can take place in stainless

tubes,

must be removed due to their corrosive effects. Solder joints exposed to sea spray, trapped moisture, or flux residue can be susceptible to corrosion

— depending upon the particular solder

alloy used

mersed

in

and the metals joined. When imseawater, solder joints must be pro-

tected by coatings to prevent galvanic action.

39

40

Chapter 6

Galvanic Corrosion

•

Bnjzifig

Precious Metals

Brazing involves the use ot hard solders, typically

Gold

which have silver, copper, or nickel bases and melting points above 800°F (426. 7°C). These alloys make considerably stronger joints, and because they are higher in the galvanic sesilver solders,

ries

than the soft solders, they are more compati-

ble with the bronzes

Nickel and Nickel and

Its

its

alloys.

Alloys

more noble (less copper-base alloys. They range

alloys are even

active) than the in

and copper-nickel

corrosion potential from -0.05 to -0.17 volts.

They are very strong and highly resistant to corrosion, both in the atmosphere and immersed in seawater. However, most are not easily polarized and in galvanic couples will cause accelerated corrosion of

aluminum and

steel.

more

active metals like

Nickel-base alloys are used

Monel (-0.04 best known.

extensively in the marine industry. to -0.14 volts)

is

probably the

Two other well-known

nickel-base alloy trade

names. Inconel (alloy 600) and Hastelloy (alloy C-276), offer an attractive combination of passive surface and the high corrosion resistance of nickel-chromium alloy. They occupy a very high position in the galvanic series: -0. 14 to -0. 17 volts and -0.03 to +0.08 volts, respectively. Both

and therefore tend to minimize galvanic effects on less noble metals.

are easily polarized

Zinc and

Its

Alloys

on a wide variety of electrical and mechanical components. Gold is a good thermal and electrical conductor, and due use as a plated coating

to

its

very noble position

(0.00 to +0.200 volts),

and can therefore have

in

ceptible to galvanic corrosion

It is

Here again,

silver

widely used

in

is

marked

effect in gal-

not a marine metal, but

it is

brazing alloys, electrical contacts,

and printed circuit board fabrication. It is soft, ductile, and an excellent conductor of heat and electricity. Silver reacts with hydrogen sulfide in air to form a silver sulfide film (tarnish). Silver occupies a high position

the galvanic series (-0. 15

in

to +0.05 volts), close to the passivated less steels,

and

is

therefore relatively

atmospheric corrosion. Silver galvanic corrosion, but

in

it

is

1

8-8 stain-

immune

to

also resistant to

does not polarize read-

and can therefore have

ily

a significant influence

galvanic couples with other metals or alloys.

Other Metals

and

is

is

tensively in aerospace

hardware, small hinges, and the

some

listed in the

the

most active of the

galvanic series.

fers real benefits,

emergency flares, marine indiistrv.

is

it

where

its

It is

light

used ex-

weight

of-

but with the exception of has

little

application

in

the

also used as

an inexpensive metal for small castings has also seen

magnesium

commonly

and electroplating, for expigment in some paint coatings.

Referred to as pot metals zinc

corrosion potential range of -1.60 to

metals

ing (in galvanizing

it

a

Silver

highly sus-

used as sacrificial anodes, as a sacrificial coat-

fortunately,

highly resistant to gal-

Mdgfiesiimi

the galvanic series.

ample), and as a

galvanic series

vanic couples with other metals or alloys.

volts,

magnesium

it is

in the

vanic corrosion. Gold does not polarize readily

-1.63

is

included here because

its

highly active, second only to

volts, zinc

is

of

a

-1.03

speaking, a marine metal

make you wonder), gold

With

a

strictly

(although the cost of some marine fittings might

corrosion potential range of -0.98 to

With

rative

Although not,

— deco-

like.

Un-

use as a casting

litaniiini

With

corrosion potential range of -0.05 to +0.06 volts, titanium is highly noble and tends a

metal for cleats, chocks, stanchion bases, and other deck fittings. Such castings are not nearly

t)n

as strong or corrosion resistant as steel, alu-

embrittlement.

minum, or bronze and

Titanium has the highest strength-to-weight ratio of any metal. It has long been used in air-

load applications.

are unsuitable for high-

to polarize easily,

other metals.

minimizing It is

its

galvanic effects

susceptible to hydrogen

Chapter 6 frame construction, bur high cost has restricted marine applications to specialized fittings on

Galvanic Corrosion

•

anode and the cathode members

to coat both the

of the galvanic couple (see chapter 15).

Another commonly

method

high-performance racing sailboats such as America’s Cup contenders, where light weight and

of nonmetallic insulators at the contact surfaces

high strength are

of the dissimilar metals to prevent the flow of

critical.

utilized

the use

is

current between them (see chapter 14).

Uranium

A method

Depleted uranium

rine

commonly

not

community but worthy

utilized in the

of mention

ma-

the use

waste by-product of the uranium-enrichment process. Because of its ex-

of a series of transition metals inserted between

tremely high density, depleted uranium was used

the contacting surfaces. In this

method

1980s for ballast

insert similar galvanically to the

anode metal but

is

a

for a short time in the late keels

on high-performance racing

sailboats.

It

is

a metal

closer to the cathode in corrosion potential

anode

was subsequently realized that exposure to depleted uranium is harmful to both humans and the environment, and so marine use was soon

the cathode potential,

discontinued.

with the

inserted at the

insert, similar to this

new

Another metal

interface.

new metal but closer is

then placed

metal, and so on.

is

The

in

still

to

contact

objective

is

form a gradual transition in corrosion potentials between the anode and the cathode so that no two metals are sufficiently far apart in corrosion potential to cause significant galvanic to

CONTROLLING GALVANIC CORROSION

corrosion.

Galvanic corrosion can be annoying, seriously damaging (and expensive), and even dangerous for the vessel

controlled

and the crew.

or, better yet,

there are a

number

of

may be accomplished. One such method is

It

can and should be

eliminated. Fortunately

methods by which

this

the selection of appro-

priate materials according to their position in the galvanic series

and

their relative ease of po-

larization (see chapter 14). is

A

related alternative

the use of metallic coatings to alter the differ-

ences

in

corrosion potentials of the

members

of

the galvanic couple (see chapter 15).

Altering or controlling the environment sur-

rounding the galvanic couple is another method. This involves the use of corrosion-inhibiting

An example

method is the use of corrosion-resistant enhanced ethylene glycol chemicals.

of this

engine coolant solutions (see chapter 17).

Applying organic harrier coatings (paint, plastic, etc.) to the surface of the anode and cathode to break the flow of current is an effective means

Cathodic protection (along with barrier coat-

by far the most common method utilized in the marine industry. Sacrificial anodes made of zinc (and, less frequently in the marine inings)

is

aluminum or magnesium) are made a part of the galvanic circuit. The most active dustry, of

member

of the circuit will corrode

first,

thus sac-

provide protection to the more

rificing itself to

noble members (see chapter

13).

Another cathodic protection method, similar to the use of sacrificial anodes,

is

the use of im-

pressed current systems. Here an externally generated current

is

used to provide the flow of

electrons to the cathode that the sacrificial

anode provides (see

in

the sacrificial

anode system

chapter 13). Impressed current systems are

not widely used

in

small craft because of their

expense and their requirement for relatively complex control systems. Finally, we can combat corrosion by paying attention to such design and construction considerations as the relative size of the

members

must be taken with these

of the galvanic couple (area ratios); avoiding

coatings, however, since scratches or punctures in

corners, cracks and crevices, and moist, stagnant

the anodic coatings can result in intensified cor-

pockets; and equipping limber holes with clear-

of control. Great care

rosive attack at these sites.

It is

usually advisable

ing chains (see chapter 14).

41

— Chapter 7

Electrolytic Corrosion

Thus

far

we’ve seen that simple self-corrosion

involves a single piece of metal

— pure or alloy

contact with an electrolyte.

in

proceeds con-

It

tinuously at an extremely slow rate, something

mm)

0.0002 inch (0.005

like

or less of surface

loss per year for a high-quality

The

water.

controlling self-corrosion

odizing

in sea-

method of reducing or

best practical

a protective coating

bronze

is

through the use of

—such as paint, epoxy, an-

— that bonds tightly to the metal’s sur-

and prevents the penetration or entrapment

face

of moisture. In chapter 6

rosion

it is

electrolyte.

is

When

galvanic corrosion

addition to simple electrochemical

in

more

still

takes place. Galvanic cor-

severe and can be very damaging.

The usual way

to deal with galvanic corrosion

through the use of sacrificial zinc anodes, which must be properly sized and positioned on the underbody of the boat. All fittings boat

is

that need to be protected should be connected to

the zinc anodes through a good, hefty

system odes is

the

(see

chapter

will not

18).

However,

bonding

sacrificial an-

prevent electrolytic corrosion. This

most aggressive form of corrosion and the

Electrolytic corrosion

is

the result of electri-

currents from within the boat, from a

nearby boat, or from

a shoreside outlet

stray currents, voltage can be as volts,

much

perhaps higher, so currents are

as

AC 120

likely to be

orders of magnitude greater than in galvanic corrosion.

How

happen? How does a current stray from its appointed rounds? Routes vary, but in all cases they result from faulty electrical does

this

wiring or equipment.

There was no room in the main battery box for the second battery, so we installed it a couple of feet away under a berth in the cabin. When the second battery went ashore for charging, separated the battery cables and tucked the terminal ends behind the bilge stringer well above the bilgewater. A few days later, preparing to reinstall the second battery back, we discovered that the positive battery cable had crawled out from behind the stringer and leaped into the teries.

1

ground

the seawater surrounding

was was

left

in a sort

of suicidal frenzy. All that

of the massive battery terminal clamp

a little

dollop of green paste. Once sub-

referred to as

merged in the bilgewater, the clamp had become an anode, and a corrosion cell was in place. Electrons and ions (and people) always seek the path of least resistance, so the most probable electrolytic path was through the bilgewater to

stray current corrosion. Hlectrolvtic corrosion

the saltwater saturated engine bed to the engine

can be extremely serious, destroying in a matter of weeks or in just hours

block and the

(in this case,

going to

the boat) through an underwater fitting. In

other words,

it

is

gone “astray,” and

electrical current that has it is

—

42

stray current corrosion can be

as high as the battery voltage. In the case of

bilgewater

subject of this chapter.

cal

DC

anode, a cathode, a metallic path for the electrons, and an electrolytic (or ionic) path for the ions. Let’s take an example from my own checkered past. We had rewired a boat for dual bat-

corrosion, which

a

involved in

of an amp). But the voltages

contact with one another and immersed

occurs,

on

amps (thousandths

in milli-

Remember, corrosion always requires an

same

the

in

that galvanic corrosion

range and produce currents measured

dissimilar metals in elec-

two or more

involves trical

we saw

sion are in the millivolt (thousandths of a volt)

commonly

a fitting in

severe

cases. 1 he voltages involved in galvanic corro-

common ground

metallic path for electrons tery cable itself

point.

was through the

The bat-

back to the positive battery post

— Chapter 7

•

Electrolytic Corrosion

to battery isolation switch

Cummins

diesel

1

//

//

/

to

common ground

point

u 4Tl

V

Ji

1

L-

Hot wire

7-1.

Fig.

in

the

The lead battery cable clamp on the positive cable fell into the bilge and dissolved in a matter of days. The boat used in the chapter diagrams is a 38 Jonesport redrawn from plans by Arno

n

bilge.

+J

'

bilgewater floors

(some

omitted for

floors

^

clarity)

battery

this battey

cable clamp

removed

Day in the collection of the Maine Maritime Museum,

^

Bath, Maine; engine outline

^e'd

courtesy of

Company,

Cummins Engine

Inc.

and from the negative battery post through the

metallic path for electron flow consists of the

cable to the engine block (see

through-hull and the wire to the positive side of

So rode

a

my

the battery, and, from the negative side of the

battery cable terminal did.

ground cable, engine block, shaft coupling, shaft, and prop. The prop becomes the cathode of the cell. The electrolytic path is from the prop through the seawater and back to the through-hull. Since we said that it is the anode

That’s the bad news.

The good news

as in the case of the battery sive action

is

7-1).

in the bilge will cor-

hot wire exposed

just as

fig.

self-limiting;

is

that, just

clamp, the corro-

when

the

anode

is

gone, the corrosion stops.

Now

let’s

say that a piece of metal immersed

—

seawater a through-hull, for example comes in contact with an electrically positive wire from the electrical system. The throughin

hull

becomes the anode of

a battery cell.

The

battery, the

of the cell that corrodes, the through-hull will suffer corrosion (see In

fig.

7-2).

another possible scenario, a wire from the

DC

system falls into the bilgewater, close to but not touching a positive side of the

electrical

throughbilgewater floors

(some

omitted for

Fig. 7-2.

Hot wire

in bilge

floors

hull

clarity)

contacting through-hull. The through-hull becomes an anode,

and an electrolytic path is established between the through-hull and the outside seawater to the prop and shaft.

battery cable ^

43

— 44

Chapter 7

•

Electrolytic Corrosion

Hot wire

near but not touching a through-hull with two ionic paths. The cable clamp becomes an anode; an electrolytic path is established between the cable clamp and the through-hull. The inboard end of the through-hull becomes a cathode, Fig. 7-3.

in bilge

and the outboard end becomes an anode. A secondary electrolytic path tween the through-hull and the prop and shaft.

immersed in the bilgewater. Here the corrosion circuit becomes a little more complicated. The positive wire becomes an anode with an ionic path through the water to the inboard end of the through-hull, which becomes a cathode. But the circuit doesn’t stop there. Because there is another ionic path through the seawater to the prop, the outboard end of the through-hull becomes an anode, and the prop becomes a cathode. The metallic path for electrons then goes from the prop through the shaft to the engine block and back to the battery (see fig. 7-3). The anodes the end of the wire and the outside end of the through-hidl will corrode. But here again, once the end of the wire in the water has dissolved, the circuit is broken and the corrosion will cease. through-hull that

is

—

The most common sources

of DC.' stray current

are faulty insulation, improper wiring, poor

grounds, shorts, and equipment leaks, them one by one.

l.et’s

take

Faulty Insulation don’t often

my own

fall

established be-

wire insulation and poorly insulated splices in the bilge are a fairly

common

time to do

Make up

right.

it

the splice in a dry

place using a properly sized waterproof butt

connector, and then encase

it

in a

into the bilge.

However, chafed

long piece of

adhesive-lined heat-shrink tubing.

Submersible bilge pumps seem to be likely candidates for stray currents since they are electrical

and

the bilge. However,

sit in

estly say I’ve

which the

come

across

pump was

bilge

many

rents can result

by sand or

grit,

if

1

can’t hon-

situations in

at fault.

The manu-

me

stray cur-

the shaft seal gets

damaged

facturer of a popular brand

tells

allowing water to get up into the

armature. The armature itself is well insulated, so the pump could continue to function for its

connection

is

established

through the seawater. Since the armature represents a fairly large mass, the stray current could continue for some time unless the wire connec-

armature dissolves, opening the circuit. Eventually the pump fails. You can avoid this by switching to a self-priming diaphragm tion to the

pump mounted up experience, loose wires

problem. You

should avoid wire splices near the bilge like the plague, but if there is no alternative, take the

some rime. Once the armature or becomes an anode, an ionic path

DC SYSTEMS

C.ontrary to

is

switch might if

still

the wire seals

out of the bilge, but the float be a source of the stray current

fail.

Chapter 7

Improper Wiring assume

Let’s

a

submersible electric bilge

pump

float switch

the float switch were wired into the nega-

tive side of the

DC

“hot,” and stray

motor would be current would flow regardless supply, the

of the float switch position (see figure 7-4B). Al-

ways

install switches, fuses,

ers in the

Anytime

a voltage difference exists

and

circuit break-

“hot” or positive side of a

circuit.

between two

points, there will be a tendency for current to

seek a path to the this will be a

more negative

point. Often

leakage path through water. The

greater the voltage difference, the stronger the

tendency for stray current to flow.

A common example SSB radio grounded

until the float switch closes. If

Electrolytic Corrosion

Faulty Grounds

wired as shown in the figure 7-4A. Notice that the float switch is in series with the “hot” or positive side of the power supply. This means that even if the shaft seal were damaged and water were to invade the motor, no voltage will be applied to the motor

and

•

side of the hull.

of faulty grounds

to a groundplate

The groundplate

ferent voltage than the

is

an

on the out-

will be at a dif-

common ground

point

on the engine block because of the resistance between the two. Current will flow through the seawater from the groundplate to any underwater fitting connected to the common ground point the prop via the shaft or a through-hull fitting via a bonding wire. From the ground point the current passes through the battery

—

+12 volts ,

GRND

cable to the negative battery terminal (see

fig.

7-5).

Equipment Leaks and electronic equipment contained in metal housings can be the source of stray curElectrical

rents.

A

frayed or bare wire touching the chas-

sis

energizes the housing, and

in

contact with, for example,

the housing

is

damp wood,

a

if

stray current flows to ground. This can be

Proper wiring, with the On/Off switch in the positive side. When the switch is in the Off position, the circuit is open and there is no voltage at Fig. 7-4A.

the load. + 12 volts

GRND

avoided by grounding all equipment cabinets and housings to a common ground point. Salt-laden moisture on insulators or terminal strips can also provide a path for leakage currents.

BONDING equipment housings should be connected to the same ground point by a heavygauge wire at least #8 AWG to ensure that no voltage differences exist. This includes all underwater metal fittings that come into contact All electrical

—

—

with the bilgewater. This will enable stray currents reaching these fittings to flow directly to

the

Improper wiring, with the On/Off switch in the negative side. The load is "hot" whether the switch is "On" or "Off." Fig. 7-4B.

common ground

point instead of seeking

other paths through metal fittings or fastenings

ground through the water. Bonding thus protects these fittings from internally generated 1)C^ to

45

— 46

Chapter 7

Fig. 7-5.

•

Electrolytic Corrosion

Faulty grounds.

The groundplate

for the SSB radio

is

at a different potential

than the common ground (engine block), creating potential for stray current. An electrolytic path is established between the groundplate and the prop and shaft, which are at

or

common ground

AC

should

potential.

stray-current corrosion. all

be connected to the

The following

common ground

all

the metallic particles dissolved into the elec-

trolyte

on the anodic half-cycle are recovered

on the cathodic half-cycle. The

point:

result

loss of metal particles, or corrosion, •

negative battery cable

•

ground return cable from the main main bonding conductor

•

radio groundplate

•

lightning groundplate

• •

a net

due to

AC

stray currents.

distribu-

tion panel •

is

much factual data available in the current literature. The most recent is a 1984 There

“Corrosion by Alternating Current” by Der-Tau Chen, which concludes that corrosion rates of aluminum, tin-lead solder, mild paper

grounding wire from shore power ground cable from the auxiliary generator

not

is

titled

and copper increase spectacularly with increasing current density. Even stainless steel steel,

(304) suffered pitting. Protecting metals required

AC SYSTEMS Now let’s take a

more cathodic current (from the absence of

look at stray alternating cur-

There is a school of thought that says stray AC cannot cause corrosion. The logic is that since alternating currents are equal and oppothat is, positive and negative half-cycles site any metal that was ionized during one half-cycle would he redeposited on the other half-cycle. This is a purely theoretical view and assumes 100 percent efficiency in the reactions during rent.

—

both half-cycles.

In the real

world, however, not

AC

zinc anodes) than in

currents. Given the high cur-

rents that can be involved, stray

may

damaging.

It

going to be

in the slip for just a

but

if

berth,

Some

can be very

not be a big deal

if

you’re

couple of nights,

your boat’s regular could be a serious problem.

this it

AC

condition exists

of the

in

more common causes of

stray-current corrosion are •

extension cords dropped into the bilge

•

improper wiring on the boat

AC

Chapter 7

Electrolytic Corrosion

•

•

improper wiring on the dock

equipment. Without the green wire, the hous-

•

polarity indicators

ing or cabinet

•

battery chargers

at

As

dropping extension cords into the bilge, the solution is simple don’t do that! For the rest of the causes, let’s take them one by one. for

—

Wiring on the Boat First of all, let’s

assume

for the

moment

that the

shoreside electrics are properly wired and

in

good condition (it ain’t necessarily sol). We’ll come back to this. To avoid stray current problems, AC wiring on the boat must be installed correctly and maintained in good condition. Leakage currents to ground can occur anywhere that bare wires or connections come into contact with a metal

enough

resist-

often high

to restrict the current to a level insuffi-

cient to activate circuit breakers or fuses, so the

stray current condition can persist for time.

Most

stray current conditions stem

some mistake made during

OK,

let’s

go

get confusing.

system sists

is

a little

The 120-volt

—

AC

this part

It

con-

of a hot (black) wire, a neutral (white) wire,

grounding (green or bare) wire. The black wire is an ungrounded current-

and

a

carrying wire. is

It

brings the current to the boat.

referred to as the

The white wire wire.

Its

purpose

ungrounded is is

It

side.

also a current-carrying to return the current to

—

ground at its power source where it wants very badly to go. The white wire is only connected to ground at its power source. So, the white wire is a grounded current-carrying wire and is referred to as the grounded side. The green wire is also grounded at the power source., the shoreside outlet in this case.

not normally carry current.

It is

It

is

does

referred to as

the grounding (with an i-n-g) conductor.

green wire

the

life

crew member. With the green, the short goes immediately to ground, probably tripping the breaker. In any case, it provides a lower resistance path to ground that protects the crew member. On the boat, the green wire is connected to outlet boxes, to equipment housings and cabinets, and to the boat’s common ground point. Note that the AC electrical system on a boat should be totally isolated from the DC electrical system. The only connection they should have is where the green wire connects to the boat’s common ground point. AC is brought aboard the boat through a special

marine-grade, three-wire shore power cable.

One end

From

can

shore-power

typically a three-wire system.

in

of the cable

is

it is

not possible to

connect to a 50-amp service with 30-amp cord.

installation.

slow here

moments

physically different so that

wood, condensation,

is

electrifying

from

saturated

ance of this short-circuit path

120 volts and thus a potential

would be source of some of the unwary is, it

some

an electrolyte (bilgewater, saltwater-

The

hot, that

plugged into a shore power outlet on the dock, and the other is plugged into an onboard receptacle. The plugs and receptacles are polarized and configured according to their voltage and current carrying capacities. The pin and slot arrangements are

fitting or

etc.).

would be

The

there in case of an electrical short

from the hot side of the circuit to a cabinet, frame, or housing of a piece of AC electrical

the

onboard receptacle, the black and

white wires should be wired to an

AC

distribu-

main shore The green wire

tion panel through a double-pole

power disconnect is

connected to a

circuit breaker.

common ground

point through

(more about this later). Note that in AC circuits, two-pole switches and circuit breakers should always be used, although the ABYC (American Boat and Yacht Council) does permit the use of single-pole devices in the branch circuits when a polarity-indicating device is installed. In other words, when the black hot wire is connected to a load, the white neutral must be available for the return path. Otha galvanic isolator

erwise the current will seek alternative or “stray

paths” to return to ground. Figure 7-6 shows our faithful old vessel

power

AC

so

all

gussied up with shore

we can watch

the soaps while

we’re cleaning up the boat.

more common mistakes made by residential electricians and home handymen in wiring boats for shore power is connecting both

One

of the

47

48

Chapter 7

Electrolytic Corrosion

•

Basic

Fig. 7-6.

AC shore power

circuit.

Basic shore

power systems generally have no

provision for stray current protection.

and the green grounding wire together on the boat. This is a very dangerous mistake. All of the current that comes aboard in the hot black wire must get hack to the shoreside ground. In a properly wired system, the only return path is the white neutral wire, as

To sum

the white neutral

shown by

the heavy lines in figure 7-7. But

•

allel

•

when

and the green grounding together on the boat, there are par-

return paths for this current.

the shoreside wire.

A

One

is

is

is

in

a •

third return path through the seawater sur-

shown by the heavy lines in figure 7-8. Not only do we now have a complete galvanic cell making corrosion likely, rounding the boat. This

is

this later,

—

1

The green wire will be grounded at the source (as above) and connected to the common

is

dangerous situation for people in the water around the boat. What’s worse, if the white wire should open for any reason, nil of the return current ct)uld be through the seawater. The danger for anyone in the water near the boat cannot be overstated! but this

More about

I

—

turn connected to metal fittings penis

circuit.

need to make the point here that improperly connecting the white and green wires can be lethal. If you not experienced with AC wiring systems and perhaps even if you are strongly urge you to engage the services of a qualified expert marine electrical technician, preferably one that is ABYC certified.

hack to shore side through the

etrating the hull to the water outside, there

onboard

connected to the hot

The white wire will be grounded only at the source of power (such as at the shoreside outlet), at an onboard generator, at an onboard inverter, or at the secondary winding of an but

hack to

green grounding wire. However, because the green wire is connected to the boat’s ground,

which

will be

isolation transformer.

ground through the white neutral

second

The black wire side of the

the white neutral wire

wire are tied

up, in a properly wired system,

•

a

ground on the boat. Outlet boxes and equipment cabinets and housings will be connected to the boat’s comnu)n ground.

•

A

properly wired bonding system will be

in

place.

Even when our

own

boat

faulty shoreside wiring or

is

properly wired,

improper wiring

Chapter 7

Fig. 7-7.

AC shore power

circuit. All of

the current that flows to the loads

black wire returns to ground at the source

Fig. 7-8.

AC shore power

circuit.

When

improperly grounded on the boat, heavy dark lines.

a

in

in

•

Electrolytic Corrosion

the hot

the grounded white wire.

the white wire

in

the

path for stray current

is

AC

shore power circuit

created, as

is

shown by the

49

50

Chapter?

Electrolytic Corrosion

•

shows the

AC

aboard boats nearby can cause us stray-current

ing wire. Figure 7-9

problems.

the polarity indicator installed.

Shoreside Wiring

been reversed in the shoreside outlet, the neutral will be hot and the lamp will light or the

There is

is

another concern

when

the white neutral

grounded on the boat. Should the black and

white wires be reversed, either at the shoreside

on the boat, then the white neutral wire becomes hot at 120 volts AC. If the white wire is grounded on the boat, everything on the boat that is also connected to ground will also outlet or

be hot. This

is

an extremely dangerous condition

and is, unfortunately, not as uncommon as you might think. There are some folks out there that are doing a good business with handy little devices whose purpose is to warn you when this situation exists.

These devices, called reverse polarity indicators^ are basically a lamp or a buzzer wired into the incoming side of the onboard AC circuit from the white neutral wire to the green ground-

Fig. 7-9.

AC shore power

lights

shoreside outlet.

polarity has

buzzer will sound.

ABYC

Standard E-8 recommends the installation of a polarity-indicating device if any AC equipment on the boat requires proper polarization

(i.e.,

black hot, white neutral) to func-

any branch circuit in the AC system is protected by a single-pole circuit breaker or a fuse in the hot side only. However, if the device is not of sufficiently high resistance, it can provide a path for stray current. It should have an impedance (total AC resistance at 120 volts, 60 Hz) of at least 25,000 ohms to avoid contributing to stray current corrosion. The ABYC does not require a polarity indicator if the system has an isolation transformer installed. More about isolation transformers later. (continued page 54) tion properly or

circuit with a polarity indicator.

The reverse

if

polarity indica-

between the white neutral conductor and the green grounding conductor up or sounds an audible alarm when the hot and neutral wires are reversed in the

tor installed

If

circuit with

Chapter 7

TESTING Every electrical circuit consists of a source of electric current that has a "hot" (ungrounded) side and a grounded side. A wire from the hot side of the source carries the current to the load. A second wire, the grounded return conductor, carries this same current back to the source. A third wire — the DC grounding conductor wire in DC circuits and the green grounding wire in AC circuits — is connected from non-current-carrying components (equipment cabinets, motor casings and frames, throughhulls, etc.) to the grounded side of thesource.Thiswirecarriesnocurrent except in the event of a short

ground fault resulting

Electrolytic Corrosion

AND TROUBLESHOOTING

bonding wire or the green grounding wire, we have a stray current problem. Thesecurrents can beappreciable, especially in the case of the i20-volt AC system; caution is required

when

the meter or even pose a serious threat to humans. So first we need totakesome preliminary measure-

mentstogetan

ments. a sim-

digital

mon ground

that's

point. Let's look at

the

DC system

DC

Electrical

meaning they auto-

matically select the appropriate

diagram of both an AC and a DC system. Remember, these are two separate systems. The only thing they have in common is that they share the same complified

howmuch

we may be dealing with. Many digital multimeters use

autoscaling,

The figure below shows

idea of

current

taking measure-

scale.

They

also select the correct

polarity automatically. As a result,

multimeters are not that easy to damage. This is not the case with analog meters, so if

what you're

using,

some

ad-

ditional steps are required to pro-

first.

tect the meter.

System

The best way to find

stray leakage

in

currents isto measure thecurrents.

leakage or stray currents, so if there is current flowing in the

However, these currents can be very high, high enough to damage

circuit or

•

Our first test is a voltage measurement to determine whether we have a leakage current problem. Make sure that all equipment in the DC system is switched off, using the On/Off switch on the equipment itself. (Don't forget Disconnect the positive cable from the battery and put the lights.)

Battery Isolation Switch

in

the

On

position for that battery bank. Set

your multimeterto read voltage and switch it to a scale greater than 12 volts. Place the positive (red) test probe from the meter on the positive terminal of the battery and the negative (black) test probe from the meter on the disconnected positive battery cable. The figure on page 52, top,

shows this condition. Since all equipment On/Off switches are open, that

is,

the Off position,

in

there should be no complete

cir-

back to the negative terminal of the battery, and the voltmeter should read zero volts. If the cuit

meter reads

12 volts, then either

some appliance Simplified wiring diagram of

AC and DC systems. The AC and DC

systems are separate but share gine block.

a

common ground

point on the en-

is

a

on or there leakage path. Recheck to be

sure that

all

is still

equipment On/Off

switches are Off.

.

(continued)

51

52

Chapter 7

•

Electrolytic Corrosion

Multitester on Voltage Scale

Conditions: Positive battery cable

1.

disconnected. 2.

3.

4.

Battery isolation switch

"On" position. Main circuit breaker in "On" position. All equipment On/Off switches

5.

in

Branch

"On"

in

"Off" position.

circuit breakers in

position.

Preliminary voltage test to check for presence of leakage current.

Let's say the

volts,

meter reads 12

confirming that

a

leakage

position, set the multimeter to

read

"Ohms" and switch

it

to the

path exists. We don't know how much current is flowing through

lowest scale, probably "Xi." Put the red test probe on the discon-

the leakage path; that depends on

nected positive battery cable and

how much

the black probe on the battery's negat/Ve terminal. (It doesn't mat-

resistance there

the path. The next thing to do

is

in

is

to

measure the resistance of the path. With the positive battery cable still disconnected and all equipment switches set to the Off

Multitester on

DC Amps

meter by mistake.) This in

out of the circuit. The meter is measuring the resistance of the leakage path in ohms. the battery

we

avoid

damaging the

in

amperes

is

equal to the voltage — 12 volts

get into the habit of observing poto

is

Since the current

when

larity

shown

the figure below. Notice that

which probe you use for resistance tests, but it's a good idea to

ter

is

the battery

is

connected-

divided by the resistance

can estimate

rent

is

in

ohms,

how much

cur-

be flowing through

likely to

Scale

Conditions: 1.

Positive battery cable

disconnected. 2.

Battery isolation switch

"On" 3.

4.

position.

Main circuit breaker in "On" position. All equipment On/Off switches

5.

in

Branch

in

"Off" position.

circuit breakers in

"On" position.

Resistance Check of Leakage Path. We measure the resistance of the leakage path to estimate how serious the problem is.

Chapter 7 Multitester on

DC Amps

Electrolytic Corrosion

•

Scale

Conciitions: 1.

Positive battery cable

disconnected. 2.

3.

4.

Battery isolation switch

"On" position. Main circuit breaker in "On" position. All equipment On/Off switches

5.

in

Branch

"On"

in

"Off" position

circuit breakers in

position.

Measuring current to isolate the leakage path. By monitoring the leakage current and systematically removing loads from the circuit we can isolate the faulty path. the leakage path. Assuming your

starter motor, bilge

pump, or

positive supply side to the equip-

multimeter has a lo-amp scale (most do), a resistance measurement above 1.2 ohms means you

high-water alarm. Disconnect each of these loads one at a time. If the reading drops to zero, that

ment housing. Depending on the type of load you haveon this side, you may want to enlist the serv-

can begin to isolate the leakage path using the multimeter to

is

the offending load.

icesof a qualified technician.

probe to the disconnected positive battery cable. Closethe Bat-

however, the reading dropped to zero when the Isolation Switch was opened, the leakage path isontheothersideofthe Isolation Switch. Leavethe meter connected and closethe Isolation Switch. At this point, all of the loads (appliances) on the side of the Isolation Switch away from the battery should have their On/Off switches in the Off posi-

tery Isolation Switch and read the

tion, butall circuit breakers should

measure current First,

Switch

directly.

placethe Battery Isolation

in

the Off position. Setthe

meter to measure DC amps and switch

it

to the highest

amp scale

on the meter, probably 10 amps. Fix the red test probe to the positive battery terminal and the black test

current on the meter, as

shown

the above figure.

is

If

there

in

littleor

no deflection of the needle, switch the metertoa lower scale until the needle readsabout in the middleof the scale. Leakagecurrents of less than 1 milliampare not considered significant.

Now, open the Battery tion Switch.

drops but

If

still

Isola-

the meter reading

shows

significant

current flow, this indicates a leak-

age path on the battery side of the Isolation Switch

— probably in

one of the "other loads" connected directly to the battery, such as the

If,

be closed or fuses

in

place.

Now,

open the circuit breaker or remove the fuse pro-

one

at a time,

tecting each branch circuit.

When

the current drops to zero, the fault is

on

this circuit.

If

there are any

loads on this side of the Isolation

Switch that don't have circuit breakers or fuses (shame on you — make a note to install protective devices on these circuits before putting the circuit back in service), disconnect their hot leads one at a time until the leakage path is found. Check the loadonthiscircuit for internal shorts from the

AC

Electrical

The AC

System

system is really not something that even talented amateurs should tackle. These AC voltages and currents can definitely be hazardous to your health, especially in the damp marine environment. Voltages can be 120 or maybe 240 volts, and currents are in the range of 15 to 50 amps, or more. A good marine electrician, if there happens to be one in your area, can run the checks for you and assist you in identifying the specific cause of electrical

the problem.

about corrosion, want to leave you with one thought. In a properly designed, installed, and maintained electrical system, stray-current corroAfter

all

this talk

I

sion

is

corrosion

eliminated, is

galvanic

controlled, and elec-

trochemical corrosion is not a problem. It's worth putting some effort and time into getting your electrical system right.

53

54

Chapter 7

•

Electrolytic Corrosion

Nearby Boats Figure 7-10 shows our boat in her slip plugged

shore power. In an adjacent slip, another boat has also plugged in. The dockside wiring is fine. Both boars are properly wired for AC shore power and have good bonding systems. Our boat in ro

is

properly fitted out with zincs (of course) to pro-

underwater hardware, but our neighbor hasn’t been as fastidious and has no zincs. What happens.^ As soon as the other boat operator plugs in to shore power, a big galvanic cell is created, as shown by the heavy dark lines in figure 7-10. The zincs on our boat become the anode, while the bronze underwater hardware on the other boat becomes the cathode. The metallic path for electrons is from our zincs through the wires to the common ground point, through the green grounding wire of the AC system to the shoreside source, then through the other boat’s green grounding wire to the common ground point on that boat to the bronze underwater hardware. The ionic path is from the zincs on our boat, through the seawater to the bronze fittings on the other boar. Our zincs

So what do we do about it.^ Some people have been known to cut the green grounding wire to prevent this problem. Don't do it! The dangers of electrical shock

we spoke

and very serious. This isolator comes in.

real

is

of above are very

where the galvanic

Galvanic Isolators

tect the

(dedicated

little

devils) will sacrifice themselves

underwater hardware, so we’ll be eating up zincs like peanuts! And the green grounding wire makes this possible. to protect both boats’

Fig. 7-10.

Corrosion due to nearby boats.

A

galvanic isolator

is

a device that

is

installed

with the green grounding conductor. The isolator blocks stray direct current (DC) flow while permitting AC to pass, thus maintaining an unrestricted path for ground-fault in series

currents. Galvanic isolators are available at

most chandleries and through marine catalogs. To install a galvanic isolator, you simply mount it near the shore power receptacle on the boat, break the green ground wire, and connect the (now two) wires to the two terminals on the isolator. Figure 7-1 shows the AC wiring system 1

with an isolator installed.

Isolation

A

Transformers

galvanic isolator will not block stray

do that

A one

you’ll

AC. To

need an isolation transformer.

transformer transfers electrical energy from circuit to

another with no direct electrical

connection between the two.

It

does

this

bv *

elec-

Chapter 7

Fig. 7-11.

AC shore power system

stalled in series

allowing

AC

•

with galvanic isolator. The galvanic isolator

with the green grounding wire to block

DC

Electrolytic Corrosion

is

in-

galvanic current flow while

currents to flow as necessary.

hot wire to the secondary winding,

tromagnetic induction: current flowing through

black

the primary winding in the transformer induces a

through the winding to the white ground wire and to the boat’s ground. The circuit is thus complete. Second, since there is now no DC path back to shoreside ground, there is no path for galvanic currents. With an isolation transformer installed, a galvanic isolator is unneces-

current to flow through the secondary winding.

Transformers are

classified

according to their

Sometimes the transfer is made to effect a change in voltage or current step-down or step-up transformers. The isolation transformer makes the transfer with no change to either use.

—

voltage or current. late the

secondary

Its

function

side, that

is,

boat, from the primary side

dockside wiring. This

is

is

simply to

the wiring

—

shown

on the

in this case, in figure

iso-

transformers tend he somewhat

bulky, heavy, and a hit pricey hut writer’s opinion

—worth the trouble.

—

in

this

the

7-12.

When

an isolation transformer is used, the secondary side of the transformer becomes the

power source. Consequently, this means that both the white neutral wire and the green grounding conductor are grounded on the boat. This does a couple of good things for us. First, the AC current on the boat now finds a comboat’s

plete path

sary. Isolation

from the appliance (load) through the

Ground-Fault Circuit Interrupter

One

other thing

we should

talk

about

is

the

ground-fault circuit interrupter, or (T'CI. This

is

a device, often incorporated into a circuit breaker,

that senses any difference in the

amount

rent flowing in the hot conductor

and the neutral

conductor.

Remember

that

we

said

rent in the black hot conductor in

all

of cur-

of the cur-

must he returned

the white neutral conductor, so they should he

55

56

Chapter 7

Electrolytic Corrosion

•

AC shore power system with isolation transformer. The the shoreside AC circuit from the onboard AC system.

Fig. 7-12.

isolates

the same.

If

there

is

two

a difference in the

amount

some current

of

going someplace we don’t want it to go. It is either going through the green grounding wire, in which case it can cause stray current corrosion, or it is going through someone, which is not good either. When

current

in

the

wires,

is

type. is

a

that

isolation transformer

As long

GFCI same

as the first receptacle

type,

all

downstream

on the

circuit

receptacles on

circuit are protected.

Battery Chargers

and instantaneously opens the circuit. Figure 7-13 shows the AC^ system with the CiFCd included. In this instance, however, the

Automotive battery chargers used aboard a boat are one of the more common causes of stray current corrosion. If you want a battery charger aboard, it must be designed for marine use. The one in your garage is not suitable for use aboard

CjFCd provides ground-fault protection only for

the boat.

the primary side of the transformer. Fo provide

Marine battery chargers are designed to have separate AC and DC ground points. All of the current brought to the charger from the AC source through the hot wire is returned to the source through the neutral wire. The cabinet is separately grounded through the green grounding wire. Most automotive chargers, however, use an aiitotransformer in which one side of the primary and of the secondary windings is com-

the CiFCd senses a difference,

it

acts like a breaker

protection to equipment and personnel on the

secondary side of the transformer, that is, on the boat, a CiFCd circuit breaker could be installed at the distribution panel.

An

alternative

use of CiFCd-type receptacles.

mends

that

if

is

the

ABYC> recom-

convenience outlets are installed

in

the head, in the galley, in machinery spaces, or

on

a

weather deck, thev should be of the CiFCI

— Chapter 7

AC shore power system

Fig. 7-13.

•

Electrolytic Corrosion

with GFCI. The ground-fault circuit interrupter

both the hot wire and the white neutral wire. These two currents should be equal. If the current in the white wire drops, indicating that some current has found an alternative path, the GFCI shuts off all current on the circuit in something like 0.02 seconds. (GFCI) monitors the current

mon.

If

in

the charger has a two-prong appliance

plug and

it

is

the incoming

inserted the

AC

wrong way,

or

if

neutral and hot leads get

crossed, the entire negative side of the boat’s

charging. But keep an eye on those disconnected battery cables. Loose battery cables can’t he trusted! They’ve been

known

to leap into the

bilge water.

DC

system can he at 120 volts AC. This constitutes a very severe shock hazard for people

on the boat and for swimmers near the boat! This type of charger should never be used to charge batteries aboard the boat. If you have a dead battery in the boat, and you don’t have a properly installed marine-grade battery charger,

and you

can’t start the engine to charge the bat-

tery, don’t

bring a portable automotive-type

charger aboard. The safest course to follow to

remove the battery and take

it

is

ashore for

SUAAAAARY What can we conclude?

AC

If

you’re going to have

aboard your boat, the circuit in figure 7-13 makes the most sense from all points of view from a corrosion point of view, from a safety point of view, and, lest we forget, from an insurance point of view. Insurance companies aren’t known for their willingness to pay off if the system isn’t up to standard.

57

'^»^Chapter 8

and Iron Alloys

Iron

Only

few metals, such as copper, gold, and platinum, occur naturally in their elemental a

Most metals occur in nature as oxides in ores, combined with some worthless junk like clay and silica. The ores must be processed to forms.

many

contain as ilar

different processes for this pur-

and

much

as 4 percent

carbon and

percentage of other elements. brittle.

Through

It

It is

may

a sim-

very hard

a succession of refinement

pose as there are metals. The process, as well as

processes, pig iron can be converted into other

the elements present, greatly influences the

more usable forms, such

properties of the metal.

of metals

istic

is

An important

character-

the extremely significant effect

amounts of other elements can have upon their properties. The huge difference in properties resulting from a small amount of

that very small

carbon alloyed with iron to make ample of this.

steel

After processing ores to reduce

elemental form,

we

is

them

an ex-

then further process them to

achieve the properties necessary or desirable for

some

particular application or class of applica-

tions.

Then we go

to great lengths to keep

them

—

from deteriorating that is, corroding. In effect, we have converted oxides to usable metals, and now we’re trying to keep them from reverting back to the oxide form. Chemically, a metal is an element that tends to form positive ions in solution and whose oxides form hydroxides rather than acids with water.

We saw

the significance of this

when we

talked about basic corrosion in chapter 4. Physically, a

trons,

metal

is

a material containing free elec-

which give

it

certain

characteristic

properties such as high thermal and electrical conductivity.

number of ways, consider them according

Alloys can be classified

in a

to their

book we’ll dominant elemental metal. Thus,

chapter

is

but

in this

iron.

use of pure iron in

this

about alloys that are predominantly

is

as iron

iron,

and

and

Most wrought

very limited.

processed forms such as

steel. is

The

utilized

iron, cast

steel.

WROUGHT Wrought

to their

combine them with other substances

to

58

contains a high percentage of impurities.

and there are

get the pure metals out of them,

nearly as

There are a wide variety of iron alloys, each produced from ore by various refinement processes. Pig iron is the least refined form and

iron

IRON is

purified iron with between

1

and 3 percent silica slag. It is tough, malleable, ductile, and easily welded and forged. It is used primarily to make ornamental ironwork, rivets, bolts, pipes, chains, and anchors. In its forged condition

it

has the reputation of being strong

and surprisingly

deep rust and corrosion. Wrought iron has been largely displaced in most of its applications by carbon steel, which is less expensive to produce and of more uniform quality. resistant to

CAST IRONS C.ast iron refers to a large family of ferrous alloys.

They

consist primarily of iron

and con-

tain 2 to 4.5 percent carbon, 0.5 to 3 percent

sil-

and lesser amounts of sulfur, manganese, and phosphorus. By varying the relative percentages of carbon and silicon and by altering the melting, casting, and heat-treatment processes, a large group of materials with a broad range of properties can be produced. Adding icon,

various alloying elements

in

appropriate pro-

Chapter 8 portions extends this range of properties

still

ron and Iron Alloys

tion of the white iron surface.

Such surfaces are

further.

referred to as chilled cast iron.

Cast irons, so-called because they are cast to shape while molten, are perhaps the least ex-

Malleable

pensive of the structural engineering materials.

Malleable cast iron contains comparatively

They offer high strength, ease of manufacturing, and excellent corrosion resistance. A comprehensive discussion is beyond the scope of this book. However, we discuss some of the types of

carbon and

interest to mariners.

For our purposes, cast irons can be divided into two broad categories, unalloyed cast irons and allov cast irons.

little

heat treated after casting, during

is

which the carbon precipitates out as small rounded nodules rather than flakes. The result is a metal of great strength and toughness that is resistant to impact, ductile, machinable, and relatively corrosion resistant.

Ductile Ductile cast irons are another widely used type

Unalloyed Unalloyed cast irons are by far the largest category of cast irons. These typically have corrosion resistance equal to or slightly better than that of unalloyed steels, but they are not as cor-

rosion resistant as the alloyed cast irons. There are four basic types of unalloyed cast irons: gray,

white, malleable, and ductile.

Gray Gray

The addition of small amounts of alloying elements together with some changes

of cast iron.

cast irons are the

most

common form

of

unalloyed cast iron. Because the casting is allowed to cool slowly, carbon precipitates out as flakes of graphite,

which give the iron the gray-

heat treatment process produces a ball-

in the

shaped graphite structure and results in a material with high strength, ductility, castability, machinability, and corrosion resistance. Its ability to withstand impact (toughness) is somewhere between that of most cast irons and steel, and it can be welded and brazed. Ductile cast irons are used in engine crankshafts, pistons, and cylinder heads.

Alloy Alloy cast irons are cast irons to which alloying

elements have been specifically added or

ish color seen in the fracture surfaces.

in-

creased to alter the properties of the material.

White

The

produced by rapidly cooling a low-silicon, high-manganese gray cast iron. Because the carbon is not allowed to precipitate as flakes but is retained in the form of iron carbides,

White

cast iron

is

and extremely difficult to machine. It has extremely high compressive strength typically greater than 200,000 psi. white iron

is

hard,

brittle,

—

When

may

objective

be to increase strength, wear

or abrasion resistance, heat resistance, or corrosion resistance. Typical alloying elements are

chromium, copper, molybdenum, and manganese and, less commonly, vanadium and titanium. silicon, nickel,

—

Silicon

ance.

used to increase corrosion

is

When

resist-

used specifically as an alloying

ele-

sil-

ment, the silicon content varies from 3 to 14

very white appearance from which the iron gets

percent. This increases the corrosion resistance

its

fractured, the fracture surfaces have a

name. White iron

is

used primarily

in

the pro-

duction of malleable iron castings by annealing or graphitization.

It is

also sometimes used in ap-

wear resistance. Ciray iron castings sometimes have a wearresisting surface of white cast iron. These surfaces are produced in a mold that has provision for rapid cooling, which results in the forma-

plications requiring high

of the alloy substantially but reduces strength

and

The

silicon facilitates the develop-

a tightly

adherent protective surface

ductility.

ment of

film over the first several days of exposure. Initial

corrosion rates can be quite high,

range of

1

I

8 to

196

in

the

mil/yr. (3 to 5 mm/yr.),

and

then decreasing sharply to (I

mm/yr.).

less

than 39 mil/yr.

59

60

Chapters Nickel

•

is

Iron

and Iron Alloys

used to improve strength and hard-

ness and also contributes to improved corrosion

forms a protective oxide film on the surface of the metal. Nickel is typically added in combination with chromium to inresistance. Nickel

high-chromium cast irons, IGA is not commonly encountered in cast irons. Also, since ironcasting design specifically limits stresses in the casting, stress-corrosion cracking

is

also rela-

tively rare.

crease both strength and corrosion resistance.

The

increase in hardness

is

particularly useful

erosion corrosion.

in increasing its resistance to

Nickel content

in these alloys

is

normally 12

percent or greater.

Chromium

in

com-

bination with nickel, primarily to improve cor-

rosion resistance

in

seawater.

The chromium

also forms tightly adherent protective oxide film

on the surface. Chromium content can varv from 15 to 30 percent, but high chromium content will cause a reduction in ductility of the alloy.

Copper may

added

also be

in

amounts up

to

10 percent to increase corrosion resistance.

Molybdenum

is

added primarily

to increase

strength but also contributes to corrosion resistance.

Atmospheric corrosion

Molybdenum

contents are typically

less

rates for the unalloyed

low

cast irons are typically quite mil/yr. (0. 13

added, either alone or

is

Atmospheric

mm/yr.)

— which

is

—

less

than 5

superior to steel

under the same conditions. However, these rates are dependent on the nature of the atmosphere.

High humidity, the presence of sulfur or other pollutants, and chlorides found in many industrial and marine atmospheres can significantly

When

exposed to moist air, unalloyed cast iron, like mild steel, forms a flaky, reddish-brown hydrated ferric oxide called rust. This rusting process is accelerated in seawater. It proceeds even faster where rust is allowed to accumulate and the surface of the metal becomes pitted. Atmospheric corrosion is nor a significant problem for the alloy cast irons. increase corrosion rates.

than 4 percent. Grapljitic

Most

of the cast irons encountered on or around

boats will be unalloyed, and most applications

Ciraphitic corrosion It is

unique to gray cast iron.

is

similar to dezincification

and

is

sometime

compo-

referred to as degraphitizutiou. In gray cast

find gray iron used for engine

irons, the graphite that precipitates out as flakes

blocks and cylinder heads, transmission hous-

are cathodic to the iron, thus forming a galvanic

will be in the

nents.

engine and

You can

ings, oil

its

auxiliary

pumps, and exhaust manifold, piping,

and elbows. Malleable (nodular) or ductile cast iron is used in crankshafts and camshafts and for cleats and propeller- and ruddershaft flanges. Ductile iron is also used for pistons and piston rings, for tiller arms, oars, and oarlocks, and for engine-leveling shims.

cast iron ballast keels

is

The

usual material

in

gray iron.

Corrosion Resistance

als

when

in

contact with other met-

such as passive stainless

steels,

bronze, and

the nickel-copper alloys. While there have been a

few instances of intergranular attack

in

some

which the iron

trolyte. Just as

is

dissolved into the elec-

with dezincification, graphitic

corrosion can result

in

serious loss of strength,

hardness, and ductility.

Cast irons require protection for satisfactory long-term service. Sacrificial zinc anodes can be

and are used

many

components, but the principal means of protecting most cast iron components is by the use of protective coatings.

Unalloyed cast irons are, in general, similar to ordinary steels in resistance to corrosion, they are subject to pitting and crevice corrosion in stagnant seawater and, of course, subject to galvanic corrosion

cell in

to protect

iron

Four general types of coatings are commonly

used: metallic, organic, conversion,

and enamel,

d’hese are discussed in detail in chapter 15.

STEEL term that describes a large family of iron-carbon alloys. Raw iron ore is converted in a blast furnace into pig iron. The Steel

is

a generic

Chapters greatest bulk of pig iron produced, cent,

processed to

is

make

steel.

some 95

per-

The steel-making

Iron

•

and Iron Alloys

Engineers (SAE) and the American Iron and Steel Institute (AISI).

These systems have been

process consists of refining pig iron by removing

incorporated into the Unified Numbering Sys-

undesirable elements from the melt and then

tem (UNS). In this system each steel is assigned a unique alphanumeric designation consisting of a letter, usually “G” for carbon and low-alloy steels or “K” for miscellaneous steels and ferrous alloys, followed by a five-digit number. The first two digits denote the major alloy additions. The next two digits indicate the carbon content in hundredths of a percent. The last digit represents any special requirements. However, whenever possible, the numbers in the UNS system use numbering sequences taken directly from

adding other elements

in specific

amounts

to ob-

tain the desired properties in the final product.

The wrought

can then be cast to shape or reheated and hot-worked by rolling, forging, or extruding into what are called wrought mill steel

shapes, such as plate, rod, and bar.

The percentage of carbon in the various grades varies from a few hundredths of a cent to about

steel

per-

percent. All steels also contain

1

varying amounts of other elements, principally

manganese, but silicon, phosphorus, and sulfur are also always present to some extent. The alloy

may

contain other elements, either as a

sult of their

added

presence

in the

re-

raw materials or

to achieve specific properties in the final

other existing systems to facilitate their identification. Eor example, a plain

Carbon

is

the

principal

strengthening element in

steel,

hardening and but no single ele-

SAE

percent carbon such as

steel

of 0.20

(or AISI)

1020

would be designated G10200. AISTSAE grades 1005 through 1030 are

commonly

product.

carbon

They

referred to as the low-carbon group.

where cold formability and weldability are primary considerations. As the carbon content is allowed to increase, strength and are used

ment controls the steel’s characteristics. It is the combined effect of all the elements that determines how the final product behaves in terms

hardness also increase, but at the cost of forma-

mi-

bon content greater than about 0.25 percent are

crostructure, corrosion resistance, and forma-

not favored for small vessel construction be-

For our purposes, we consider three carbon or mild steels, alloy principal groups steels, and high-alloy steels.

cause of the greater difficulty

of heat treatment,

hardness, strength,

bility.

—

Carbon Steel More than 90

percent of

are carbon

Carbon steels convarying amounts of carbon and not more

steels, also called

tain

all steels

mild

steels.

than 1.65 percent manganese, 0.60 percent silicon, and 0.60 percent copper. Total alloy conusually less than about 2 percent. The carbon percentage is varied to control the tent

is

strength and optimize

No

cations.

its

use for various appli-

other alloying elements are added

intentionally, but they

may

be present as trace

elements. These steels are very ductile

drawn, molded, or shaped, not

easily

This

is

brittle.

the type of steel generally used in ship

and boat

The

— they are

hulls.

numbering systems for steel were developed by the Society of Automotive original

bility

— the ease of bending. Grades with in

a car-

bending and

forming.

Most

steel vessels are built

using ordinary

low-carbon steels, the kind commonly available at any steel distribution center. Typically, these are general-purpose steels suitable for a wide variety of applications. AISI grades 1005 through

1023 are

typical. Boatbuilders like these grades

because of their bending capabilities and ease of welding (see table A7page 281). Slight variations in the carbon content cause 1 ,

significant changes in the basic microstructure

of the steel. Varying the rate at which the steel

cooled also affects the microstructure. The hardness, mechanical properties of the steel is

strength, toughness, etc. final ties

—

microstructure of the

may

—

all

depend on the

steel.

These proper-

be further modified by heat treating,

cold-working, or adding other alloying elements

A7-2, page 282). The difference between the tensile strength and the yield strength (see table

61

62

Chapters is

a

and Iron Alloys

Iron

•

measure of the metal’s

ductility, that

is, its

tendency to stretch or dent rather than rupture. Two specific types of carbon steel are favored for boat construction

—

A

of Shipping) grade

ABS (American Bureau steel and ASTM A36.

These are preferred by many builders of steel boats because of their consistent quality and formability. Chemical properties are shown in table 8-1.

Obviously, boats can be built with just about

any grade of

steel.

People have been

build steel boats out of

unknown

pedigree at

bargain prices from salvage yards. question of what will work, but the

work required

in

to

kinds of raw steel

all

stock, including plate of

known

not a

It’s

amount

of

forming and welding and the

longevity of the result.

to

The medium carbon content steels (AlSl 1030 1053) have between 0.31 and 0.55 percent

carbon. High-carbon steels (AlSl 1055 to 1095)

have between 0.56 percent and approximately percent carbon. These have greater strength and hardness but are more difficult to form and weld and thus are not really suitable for use in 1

small craft. Steel

is,

Alloy steels

rusting.

make up

a class of steels that has a

higher percentage of alloying elements than

carbon steels, including elements not found in carbon steels purposely added to produce a formulation optimized for specific uses. They contain small amounts of such other elements as and larger nickel, molybdenum, and chrome amounts of manganese than plain carbon steels.

—

The combined percentage of in

low-alloy steels

cent; for the

alloying elements

limited to about 4 to 5 per-

is

most part thermal treatments are

used to achieve specific mechanical properties. Steels containing

more than

5 percent alloying

elements are generally considered high-alloy High-alloy steels contain more nickel,

steels.

chrome, and molybdenum. These include the stainless steels, which we cover in a separate section.

Alloy steels are not typically used in hull fabrication.

They may, however,

find

some use

in

high-strength fittings or for structures on the

boat that carry heavy loads. Generally, these steels are

may

of course, subject to corrosion, that

However, the advent of advanced epoxy coatings has gone a long way toward eliminating the constant chipping, scraping, and is,

Low-Alloy Steels

supplied

in

the afuiealed condition but

normalized condition, in which tensile and yield strengths and hardness are considerably higher (see tables A7-3 and A7-4, page 282). This can have significance for be obtained

in the

us in certain types of corrosion.

painting necessary to avoid the seemingly everpresent rust streaks.

We

talk

more about

this in

chapter 15.

High-Strength, Low-Alloy (H5LA) Steels High-strength, low-alloy steels were developed to provide greater strength

Table 8-i

and toughness than

the low-carbon grades. Typically they contain be-

Chemical Contents

tween

Element

ABS Grade A

ASTM A36

carbon

0.21% (0.23 max.)

0.25%

manganese

2.5

phosphorus

0.035%

rnax.

0.040

sulfur

0.035%

rnax.

0.050

silicon

0.50% max.

copper*

= or
Vi inches Twenty penny equals 4 inches Twenty-five penny equals 4A inches Thirty penny equals 4‘/2 inches Forty penny equals 5 inches Fifty penny equals 5Vi inches Sixty penny equals 6 inches polyethylene terephthalate (PET) A dimensionally stable thermoplastic with superior machining characteristics compared to acetal.

A colorless

petrolatum

to amber-colored semisolid mix-

ture of hydrocarbons obtained

from petroleum. Also

called petroleum jelly.

A

phenolic

thermosetting family of plastics with very

low thermal expansion, high compressive strength, excellent wear and abrasion resistance, and a low coefficient of friction. Used for bearing applications and

molded pine tar

parts.

A

thick, sticky

brownish-black substance pro-

duced by the distillation of pine wood. Used in roofing compositions; formerly was used extensively in boatbuilding and maintaining. Pine tar was widely used in all sorts of coating applications and as a corrosion inhibitor on mooring and anchor shackles, turnbuckles, and all manner of screw threads. No longer

commonly

pitting corrosion

available.

A form

of localized corrosion

the surface of the metal

is

in

which

attacked in small localized

sometimes resulting in very deep cavities. plastics A broad class of synthetic organic materials that are readily molded and shaped under heat and pressure; the shape is retained upon removal of the heat and pressure. Plastics are made up of long chainlike areas,

molecules (polymers) that

may

be natural materials,

such as cellulose, or synthetic resins. The two principal classes of plastics are thermoplastic types,

which

can be resoftened and reshaped repeatedly with the application of heat and pressure, and thermosetting

which due to cross-linking of the molecular chains, cannot be resoftened and reshaped after they plastics,

are kirmed.

polar bonds

Covalent bonds with a large electronegativity difference but not large enough for the formation of an ionic bond. Since the electronegativity difference is unequal, the electrons spend more time around the more electronegative atom. As a result, the molecule has a slight polarity, which causes polar bonds to resemble weak ionic bonds. polarization The change in potential of a metal as a result of current flow.

The

potential of an anodic metal

becomes more noble and that of a cathodic metal becomes more active than their normal open-circuit potentials.

Appendix 8 A

polycarbonate

transparent thermoplastic with high-

impact strength, excellent chemical resistance and electrical properties, and good dimensional stability, polyester resin A thermosetting resin. Cure is effected through catalysts and promoters, or heat, to accelerate the reaction. These are the most commonly used resins in fiberglass boat production. Polyester resins

are economical, easy to use,

and provide good mold

do not present the health hazards associated with the epoxy resins, polymer A chemical compound consisting of a number release; they

of

monomers

linked together by covalent bonds. Ex-

amples of natural polymers are cellulose, natural rubber, and silk. Synthetic polymers are the basis for plastics, synthetic fibers, and synthetic rubber, polypropylene (PP) A lightweight thermoplastic, a poly-

mer of propylene.

It is less

dense than water, has ex-

and has good resistance to water absorption, oils, and solvents. It is used to make a wide variety of products, including packaging materials, textiles, luggage, and ropes that float, polysulfide (PS) A popular adhesive sealant. It bonds well to most surfaces but will chemically attack plastics and should not be used on them. Teak must be primed before this sealant is applied. Both one-part and two-part sulfides are available, and the properties after curing are essentially the same. One-part polycellent electrical properties,

trical

A

durable thermoplastic having good elec-

properties and temperature capabilities over the

range -150°F to 300°F (-101. 1°C to 148. 9°C).

polyurethane (PU)

A

thermoplastic polymer used for

padding and insulation in furniture, clothing, and packaging and in the manufacture of resins for adhesives, elastomers, and fillers. Polyurethanes have excellent impact and abrasion resistance and good resistance to sunlight and weathering. These adhesives are recommended for permanent bonding, such as hull-to-deck joints, because of their enormous adhesive strength. Polyurethanes will attack ABS and Fexan Corrosion that occurs where

dirt, de-

and other matter are continuously kept moist and in contact with metal. The term is used bris, insulation,

in

a

is

PTFF

load applications.

plastic.

A

polyvinyl chloride (PVC)

good

light

thermoplastic.

resistance to corrosive solutions

and

PVC

has

used

in a

is

plumbing applications, such as hose, pipe, fittings. It also has good electrical insulation

variety of

and

rigid

properties.

A subatomic

proton

particle having a single positive

Fwery atomic nucleus contains one or more protons. The mass of the proton is about electrical charge.

1,840 times the mass of the electron and slightly

less

than the mass of the neutron,

A

polyvinylidenefluoride (PVDF)

thermoplastic; out-

standing chemical resistance; excellent substitute for

PVC or

PVDFs have good mechanical

polypropylene.

strength and dielectric properties,

The water

raw water

whether

ated,

A

reactant

in

which the boat

is

being oper-

fresh, salt, or brackish,

substance involved

in a

chemical reaction,

es-

pecially a directly reacting substance present at the ini-

tiation of the reaction.

A change

reaction

or transformation

in

which

a sub-

stance decomposes, combines with other substances,

or interchanges constituents with other substances,

The addition of electrons

reduction

of atoms, resulting

A broad

resin

in a

to an

atom or group

decrease of valence,

family of synthetic polymers, certain of

boatbuilding as the binder that encapsulates the fiber-reinforcing material. Typically these are polyester, vinylester, and epoxy, all of which are thermosetting resins. Polyesters and vinylesters require a catalyst, whereas the

epoxy

in

resins require a hardener to facilitate the cur-

ing process at normal ambient temperatures, resin content

The amount of

resin in laminate expressed

as either a percent of total weight or total

RMS

value

Fiterally root

effective value.

It is

mean

volume.

square; also called the

equal to 0.707213 times the peak

value of the current or voltage, rosin

A

translucent yellowish to dark

brown

resin de-

from the sap of various pine trees and used to increase sliding friction, as on the bows of certain stringed instruments, and to manufacture a wide varived

plastic.

poultice corrosion

Teflon

used mostly for

is

which are commonly used

sulfides are easier to use but slower to cure,

polysulfone

capacity and

Glossary

•

the automotive industry to describe the corrosion

of vehicle body parts due to the collection of road salts

and debris on ledges and in pockets that are kept moist hy weather and washing, polytetrafluoroethylene (PTFF) A thermoplastic that has an extremely low coefficient of friction, withstands temperatures up to 500°F (260°C), is inert to chemicals and solvents, is self-lubricating, and has a low thermal-expansion rate. PTFF has excellent impact and abrasion resistance and resists sunlight and weathering. It is lightweight, tough, flexible, and unaffected by acids and alkalis except if highly concentrated. It is an electrical insulatf)r. It has low load

products, including soldering fluxes,

riety of

roving

A

term used to designate a collection of bundles

of continuous filaments, either as untwisted strands or as twisted yarns. Roving for filament

may

be lightly twisted, but

winding the strands are generally wound

bands or tapes with as little twist as possible. Cilass rovings are predominantly used in filament winding, salt A chemical compound formed by replacing all or part of the hydrogen ions of an acid with metal ions as

or electropositive radicals.

seacock

A

sea-valve with a flanged base that allows the

valve body to be

sea-valve

Any

mounted

directly to the hull,

positive closure device specifically de-

signed for marine use.

299

300

Appendix 8

Glossary

•

A

sea-valve, in-line

mounted on

sea-valve

a

through-

A complex

seawater

sodium

mixture of inorganic

salts

(mainly

chloride), dissolved gases (mainly oxygen),

various suspended solids, organic matter, and living

marine organisms. However, the term also includes a number of different types, including open-ocean water, coastal waters, brackish estuarial waters, and bottom-sediment waters. Largely because of the presence of living organisms that influence the corrosion

ma-

process, testing of the corrosion susceptibility of

simulated seawater has proven unreliable,

terials in

The increased

sensitization

corrosion

in the

area immediately adjacent to a It

occurs primarily

in

austenitic stainless steels at the grain boundaries

the metal

is

heated to temperatures

in the

the

when

range of

1,000°F to 1,550°F (550°C to 850°C). Chromium combines with carbon and precipitates as chromium carbides, leaving the grain boundaries depleted of chromium and thus susceptible to corrosive attack in this area.

sewage

terms of marine sanitation, as human body wastes and waste from toilet and other receptacles intended to receive or retain body waste. Thus, sewage does not include garbage, trash, bath Defined,

water, food,

A

sheave

in

oil,

gasoline, or any other waste,

pulley.

A

process for the coating of steel with a thin

cladding of zinc by heating them

and powdered

in a

mixture of sand

zinc.

The oxidized form

of silicon, Si02. Silica

is

gen-

form of its prepared white powder, primarily in the manufacture of various types of glass, ceramics, and abrasives. It is also called silicon erally used in the

dioxide.

A

silicone

is

family of polymers containing alternate

trical insulation. Silicones are

chemical resistant, and

sili-

very elastic, are highly

makegood

between dissimilar metals.

insulation barriers

They are not as strong

in

adhesive strength as polysulfide or polyurethane but are compatible with plastics,

sodium chloride (NaCl) Ciommon salt; a chemical compound containing equal numbers of positively charged sodium and negatively charged chlorine ions. When dissolved in water, the ions move about freely and conduct electricity. Salt makes up nearly 80 percent of the dissolved material

in

production technique

used as the processing tool.

in

which

a spray

gun

In reinforced plastics,

example, fibrous glass and resin can be simultaneously sprayed into a mold. In essence, roving is fed through a chopper and injected into a resin stream that is directed at the mold by the spray system. In foamed plastics, very fast-reacting urethane foams or epoxy foams are fed in liquid streams to the gun and for

sprayed on the surface.

An

and carbon with small quantities of other elements. Steelmaking involves the removal of iron’s impurities and the addition of desirable alloying elements. Steel is often classified by its carbon content: high-carbon steel is hard and brittle; low- or medium-carbon steel can be welded and tooled. Alloy steels, the most widely used, contain one or more elements that give them special properties. Aluminum steel is smooth and has a high tensile strength. Chromium steel is used in automobile and airplane parts because of its hardness, strength, and elasticity. Stainless steel has a high tensile strength and resists abrasion and corrosion because of its high chromium alloy of iron

content.

Corrosion that results when

stray-current corrosion

source causes a metal

seawater,

sodium hydroxide (NaC)H) A strong alkaline compound used in the manufacture of soaps and oven and drain cleaners; also called caustic soda and lye. soft water Water that contains little or no dissolved salts of calcium or magnesium, especially water containing than 85.5 parts per million of calcium carbonate.

in

a

DC

contact with an electrolyte to

become anodic w'ith respect to another metal in contact with the same electrolyte. stress-corrosion cracking (SCC) A corrosion-induced failure mode resulting from the simultaneous influence of static tensile stresses and a corrosive environment. The stresses may be internal, such as those caused by cold-work, welding, and heat treatment, or

may set

con and oxygen atoms as (Si-O-Si-O). They are used as adhesives, lubricants, and hydraulic oils and in elec-

less

in the

current from a battery or other external electrical

wheel or disk with a grooved rim, used as a

sherardize

A

spray-up

steel

susceptibility to intergranu-

weld (heat-affected zone).

silica

more commonly used

is

Scandinavian countries.

hull.

lar

Zinc; the term

spelter

be external forces caused by mechanical stresses

up by assembly

practices.

sulfate-reducing bacteria (SR.B) corrosion

A form

of

anaerobic (oxygen-deficient) corrosion; the corrosive action of the sulfate-reducing bacteria in anaerobic en-

vironments. The shape and form of the attack

most always localized and

t)ften

looks

is

much

al-

like

Because anaerobic microenvironments can exist under barnacles, marine growth, crevices, and Haws in the coating system, anaerobic corrosion by SKB can take place in nominally aerated environments. Anaerobic corrosion of iron and steel has been encountered in waterlogged soils and bottom mud of rivers, lakes, marshes, and estuaries, especially when they contain decaying organic material. pitting.

superaustenitic alloys

A

new group of alloys or more molybdenum

relatively

with approximately 6 percent (Mo) content and having significantly improved sistance to chloride-pitting corrosion

and

re-

stress-

corrosion cracking. These alloys have very low carbon

content and significant percentages of both nickel (Ni)

and chromium

(Cr).

Some

of these alloys have “S”

Appendix 8

UNS

type

254-SMO (UNS

designations, such as

S3 1254) and 654-SMO (UNS S32654), and some have “N” type designations, such as AL-6XN (UNS

N08367) and alloy 926 (UNS N08926). surfacing mat A very thin mat, usually 7

to

20 mils

thick, of highly filamentized fiberglass used primarily

to

produce

smooth surface on

a

a reinforced plastic

decrease

A hinged deck-mounted mast

step that al-

lows the raising and lowering of the mast; typically used on boats of less than 30 feet. Tedlar A polyvinyl fluoride film (PVF); a family of films

in

temperature. Thermoplastic polymers

consist of long polymer chains that are not joined to

each other (cross-linked). Polyethylene, polypropylene, polystyrene, polyester, polyvinyl chloride,

spandex-type polyurethanes, polyamides, polycarbonates, fluorocarbons, and celluacrylics, nylons,

common types of thermoplastics, A plastic that can not be softened on

losics are

thermoset

laminate.

tabernacle

a

Glossary

•

In a

heating.

thermosetting plastic, the long polymer chains are

joined to each other (cross-linked) during fabrication

produced by DuPont and having excellent resistance to weathering, excellent electrical characteristics, low

through the use of chemicals, heat, or radiation; this process is called cnrifig or vulcanization. Alkyds, phenolics, ureas, melamines, epoxies, polyesters, silicones, rubbers, and polyurethanes are examples of thermoset

moisture absorption

plastics.

than 0.5 percent), excellent resistance to chemicals and solvents. It has been used (less

on some Dacron sails as border of roller-furling

UV

protective film for the

sails.

However, there have

a

been reports that these applications have not been entirely successful, that the

from the

Tedlar tape has pulled away

cloth due to

sail

UV

embrittlement, leaving

the sail unprotected. Better success relatively light sails,

Tefgel

A

weighing

five

is

experienced on

ounces or

recommended

bricant

for use

lu-

on screw-type fastenings

Teflon See polytetrafluoroethyleue (PTFE). Teflon produced by DuPont.

A measure

is

of the load a material can

more less indefinitely. This property can be changed radically hy cold-work, heat treatment, or both, depending upon the nature of the material. It is or

sustain

expressed as

A

polyester fiber

by the Imperial Chemical Industries under

this trade

name. The product is marketed in the United States by DuPont under the trade name Dacron. thermal conductivity A measure of the capacity of a material to conduct or transfer heat. It is of importance primarily in heat exchanger design. Thermal conductivity

is

greater

in

thermal expansion

60-40

brasses; also called the naval

brasses. Their principal marine use

is

in

hardware

items such decorative fittings, turnbuckles, fastenings,

toughness general

shafts.

The ability to withstand impact; way to ductility.

A

related in a

thermoplastic. Tufnol and nylon are hygro-

scopic and swell

when

saturated with water,

A

yellow or brownish oil extracted from the seeds of the tung tree and used as a drying agent in varnishes and paints and for waterproofing; also

tung

oil

called

Chinawood

oil.

ultrahigh molecular weight polyethylene

(UHMWPE) A

specially formulated thermoplastic with zero water

psi.

made from terephthalic acid and ethylene glycol. Terylene was first developed in 1941 in the United Kingdom, where it was marketed

Terylene

Tobin bronze

Tufnol

to prevent “freezing.”

tensile strength

time.

and prop

less.

proprietary anticorrosion and antiseizing

The property of materials to be gel-like at rest but fluid when agitated; having high static shear strength and low dynamic shear strength at the same

thixotropy

pure metals than

in their alloys.

Hxpressed as a decimal part of an

absorption, a very low coefficient of friction, and high

compressive strength.

It is

an excellent material for

bearing applications.

uniform corrosion

See general corrosion.

The ultimate or

ultimate tensile strength

tained by a specimen in a tension

moment

test;

final stress sus-

the stress at the

of rupture.

Waste matter

95 percent water, in which urea, uric acid, mineral salts, toxins, and other waste

urine

that

products are dissolved. valence

is

It is

highly corrosive to metals,

The combining capacity

of an atom, represented

or negative integer. Valence

inch per inch (or millimeter per millimeter) per unit of

by

temperature. The practical significance of thermal ex-

mined by the number of electrons that the atom

pansion

lose,

lies in

the stresses caused by the difference in

expansion between different components of equipment or between a substrate and its coating. thermal spray coatings A group of coating or welding prf)cesses in which finely divided metallic or nonmetallic materials are deposited in a molten or semimr)lten ct>ndition to form a coating. The coating material may be in the form of powder, ceramic, rod, wire, or molten materials.

A

a positive

add, or share

vented loop

when

A U-shaped

top that admits

air,

it

is

deterwill

reacts with other atoms,

tube with a simple valve at the

thus preventing the formation of

vacuum and the resulting siphon, vinyl (Temical compounds containing a

(CH 2 CH),

the vinyl radical

derived from ethylene and typically highly

reactive, easily polymerized, als for plastics.

vinylester resin

and used as basic materi-

Vinyls are tough, flexible plastics,

A

thermosetting resin used

in

the con-

capable of being repeatedly soft-

struction of fiberglass boats to encapsulate and bind

ened by an increase of temperature and hardened by

the fiberglass-reinforcing material. Other resins used

thermoplastic

plastic

301

302

Appendix 8 for this

Glossary

•

purpose are polyester

both are thermosetting better

mold

resin

and epoxy

resins. Vinylester resins

release than the epoxies

pensive. Also, they

resins;

and are

have

less ex-

do not present the health hazards

are laid in the grooves {worming), going with the lay of

the wire rope.

The

splice

is

then spirally wrapped

{parceled) with a strip of burlap or tarred canvas. After

the

worming and

parceling have been completed, the

then served over the parceling with spun

associated with the epoxies. Vinylester resins have less

entire splice

water absorption and lower density than the poly-

yarn or three-strand tarred nylon.

esters but are

The

viscosity

weight

A

somewhat more expensive,

facilitate

wrought

resistance of a fluid to flow,

body in weight of a body

measure of the force of gravity on

a

pounds or grams. Unlike muss, the depends on its location in the gravitational field of Earth or of some other astronomical body. Weight is a measure of the force with which a body is attracted to Earth. Weight is equal to the product of the body’s mass and the acceleration due to gravity, weld decay More properly called intergranular corrosion; occurs as the result of sensitization in the heat-

affected zone (EIAZ), principally in stainless steels

and

certain nickel-base alloys during welding,

The drum on an anchor windlass that takes the chain, as compared to the gypsy, which is the wheel

wildcat

that hauls the rope.

worming

Refers to a procedure with wire rope. After a

splice has

been

made

in

wire rope, strands of spun yarn

is

Worming

is

done

to

handling and prevent water ingress,

Something that has been produced through the

effort or activity of a

same

person or a thing.

It is

essentially

worked, as in rolled, drawn, extruded, machined, forged, and so on. yield strength The stress at which a material exhibits a specified limiting deviation from the proportionality of stress to strain; the lowest strain at which a material undergoes plastic deformation. Below this stress, the material is elastic; above it, viscous. Zamak A zinc-aluminum alloy, consisting of 95 to 96 percent zinc, 3.5 to 4.3 percent aluminum, and trace amounts of copper, magnesium, and other metals. It has moderate strength and hardness and is an excellent, economical die-casting metal. Zamak fittings have a highly polished look when new but only a modest amount of corrosion resistance. They are only the

as

suitable for freshwater applications.

.

Bibliography

Ahrens, David. “Engine Advice: What Causes Pitting in Cylinder Liners?” Marine Performance and Fisheries

Biewenga,

Products Neivs (spring 1998): 27. Akashi, Massy. “Stress-Corrosion Cracking of Stainless

Blevins, Steve.

Steels.” wwwA7ig.0r.jp/massy/C0rrFun/ssscc/intro.

html

(

1

March

1

998).

Allenbach, Christian

P.

{[email protected]).

Carbonitridation. [email protected] (21 Aug. 1997).

Ament,

Peter.

“Corrosion Fatigue of Structural

Steel in

Seawater.” http://dutsm.tudelft.nl/tmc/people/ament/ project.html (1999).

American Boat and Yacht Council. “Installation of Exhaust Systems For Propulsion and Auxiliary Engines.” Standards and Recommended Practices for Small Craft. Edgewater MD: American Boat and Yacht Council,

998, P-1 American Boat and Yacht Council. Standards and Rec1

ommended (DC)

Practices for Small Craft. E-9 Direct Cur-

Systems on Boats, 1998; E-4 Lightning Protection, 1996. Edgewater MD: American Boat and Yacht Council, 1998. American Bureau of Ships. Rule Requirements for Materials and Welding 1996. Part 2, ABS Grade A Steel. Houston: American Bureau of Ships (April 1997). American Society for Testing and Materials. Standard Specification for Carbon Structural Steel: Designation: A 36/ A 36M-96. West Conshohocken PA: American Society for Testing and Materials, 1997. Armco Advanced Materials. “Armco AQUAMET Boat Shafting,” Product Data Bulletin no. S-26. Middlerent

Electrical

town OH: Armco Advanced Materials, 985. Armco Advanced .Materials. Development of the Stainless Steels. Butler PA: Armco Advanced Materials, 1

ASM

Handbook, vol. 13, Corrosion, by Lawrence J. Korb and David L. Olson. Materials Park OH: AS.M 1

“It’s the

End of

the l.ine.” Cruising

(April 1999): 77.

bulletin

“Carbon Fiber Performance.” Electronic hoard www.tivanet.com (1997). Available

from http :/home/prindle/messages/225. html. Booth, W. D. Upgrading and Refurbishing the Older Fiberglass Boat. Centerville MD: Cornell Maritime Press, 1985.

Brown, David G. “Don’t Let Corrosion Steal Your Radio Voice.” Offshore (March 1997): 75. Brunswick Corporation. Quicksilver: Everything You Need to Know about Marine Corrosion, publication no. 90-809844-95. Fond du Lac WI: 1995. Bryson, James H. “Corrosion of Carbon Steels.” In A.5M Handbook, vol. 13, Corrosion, 509-15. Buchanan, George. The Boat Repair Manual. New York: Arco, 1985. Buehler, George. 1991. “Thoughts on the Inexpensive Rig.” Boathuilder (July-Aug. 1991): 32.

Burkstrand, Beth. “Titanium Metals

Boom Has

the

Com-

pany Flying High: As Aerospace Industry Gains, Timet Pursues More Down-to-Earth l.ines.” Wall Aug. 1997): B4. Cadwalader, George. “A Cautionary Tale.” WoodenStreet Journal (20

Boat, no. 41 (July-Aug. 1981): 85.

Cadwalader, George. “A Cautionary Tale Continued.” WoodenBoat, no. 70 (May-June 1986): 23.

“Cadmium

Plating Alternatives.” 1994. National Defense

Center for Faivironmental FNcellence. Technology Abstract.

www.ndcee.ctc.com/Core/ ah_cdplt.htm (May

1998).

Calder, Nigel. Boatowner's Mechanical and FJectrical

Manual:

How to

Maintain, Repair, and Improve Your

Boat's Fissential Systems. 2nd ed.

Camden MFk

Inter-

In

ASM

Naval Architecture for Non-Naval ArJersey City Nj: Society of Naval Architects

Benft)rd, Harry.

and .Marine Engineers, 1991.

Ocean Nav-

igator {]pper alloy), 9

1

branch circuits, 14 hose (polyethvlene, cross-linked),

parallel

See marine sanitation devices sheathing, 171

Muntz metal

paints: antifouling, 148; chlorinated

rubber, 124; vinyl, 124; volatile

rudders, 141

MSDs.

oxygen: argon-oxygen-decarburization (.AOD) process, 66; dissolved, 26

1

500, 83, 84, 86;

mooring

139

t)utside ballast,

(nickel-base alU>y), 83-84; fresh-

mufflers,

marine metals, 97-10 marine sanitation devices (MSDs), 186; I'ype and 2, 188; Type 3, 188 marine seacocks and through-hulls, 137-38

gasoline-engine, 155

once-through cooling systems, 156

water tanks,

in

63

moisture collection, 108-9

Monel

aluminum, 77; nickelaluminum-manganese bronzes, 93; in stainless steel, 65 manganese bronze (copper alloy), 92 marine-grade aluminum alh)ys, 77-78 marine-grade crimp connectors, 72 manganese:

steel,

1

niobium, 68 nitrogen, 65

1 ;

4

1

211,216

28; stainless steel, 74

76

86-87; zinc-nickel coatings,

nickel silvers, 95

metallic elements, 7

sition,

176-77

of, 87;

106-7

uring current to isolate leakage path,

clad,

1

and erosion

1

metals: anodic,

lifeline

cavitation

metal rudders, 141-4?

K-Monel, 84

lead,

205

203

step,

metallic coatings,

139-40

lag bolts, 13

or

metal grains, 28-29

keel bands, 141

keel coolers, 107,

luff

nickel-aluminum bronzes, 93 nickel-aluminum-manganese bronzes, 93 nickel and nickel-base alloys, 60, 83-85; atmospheric corrosion of, 85;

53 mechanical coating, 1 14 mechanical steering systems, 142-43

139

keel, galleried ballast,

spreaders

and fastenings,

205-6; heel corrosion, 200;

M-bronze, 94 measurements:

181

keel bolts,

on, 202-3;

sion,

iron-manganese bacteria {Siderocapsa),

booms and

masts, 199-202;

'l

1

1

in stainless steel,

1

piping: avoiding corrosion

1

in,

109-10;

types of, 164, 182-84 nails,

133-35

pitting corrosion,

National Hlectrical Gode, 171 National hire Protection Association

pitting index, 7

PL-259 connectors, 206 and avoiding corrosion. 111;

(NFPA), 171 National Science Foundation (NSF),

27-28, 71, 152

plastics: 1

80

fiberglass-reinforced. 111, 146; flexi-

naval brass (copper alloy), 9

ble

negative atoms, 3

183

opaque,

1

83-84;

rigid

opaque,

8 62

8

1

1

;

3

1

;

;

;

Index plates 1

and

35,

1

plating: backing plates,

restoration, 17

92-93;chainplates, 2 5-16; 1

1 ;

electro-

4; ion plating, 1 1 6; stepped stanchion baseplates, 193-94

plating,

1

1

plugs, 137

plumbing systems: freshwater, 180-86; wastewater, 186-88 polar bonds, 7-8 polarity indicators, 50 polarization, 36, 37 polyamide-cured epoxies, 125 polyamine-cured epoxies, 125 polybutylene piping, 183 polyester urethanes, 126 polyethylene: PEX hose (polyethylene, cross-linked), 83; water tanks, 182 polyol, 126 1

polysulfide sealants,

12

1

polyurethane coatings, 126 polyurethane sealants, 112

pop

1

1

siphon breaks, 163-64, 188 6

and galvanic

series,

36

rod rigging, 207, 2 10-1

rubber paints, chlorinated, 1 24 rubber plugs, all bronze, 37

running rigging, 2

1

7

rust, 9, 60,

spelter fittings,

rust neutralizers, 128

spelter (zinc) solders, 98

rust passivators, 128

splash zone, 25

126-27

spraying, thermal, spreaders,

rust staining, 71

sputtering (vapor deposition),

anodes, 4 1 99, 03-5 anode cathodic protection ,

1

wood

hulls,

printed circuit boards, propellers

and

26-27;

1

in

shafts,

1

12

protection against corrosion: active, 02; cathodic systems, 02-5; for electrical and electronic systems, 1

1

1

77-79; impressed current systems,

41,1 02-3; passive, anodes, 41,1 03-4

1

02; sacrificial

protection against lightning, 1

75,

76-77

protective films,

pumps,

on

steel,

63-64

84 pyrophoric (carbon), 161 1

aluminum

rivet),

1

1,

56-57

12; circuits,

68; measuring, 16; parallel, 14 resistance check, 52 1

resistance to corrosion: of

aluminum

of aluminum-zinc

coatings,

1

1

9;

coatings,

1

1

9-20; of cast irons, 60;

of nickel and nickel-base alloys,

85-87; of zinc coatings, 117-18 Resource Cionservation and Recovery Act, 120

1 ;

galling, 2

performance nickel-rich

1

5;

alloys,

71; hose clamps, 188; hot water

85; immersion corrosion of, 72; intergranular corrosion of,

tanks,

ically

1

induced corrosion

of, 74;

molybdenum-bearing grades, 68; Ni-

197-98

shafts. See propellers and shafts

tronic 50, 69; passive, 27, 39, 64,

shapes, 108-9

33; pitting corrosion of, 7 rudders, 141; rust staining, 7 1

sheathing: copper, 149; protective, for wiring, 171; wood veneer, 150 1

1

80-8

73-74; marine-grade, 65, 67-68; martensitic, 39, 65, 66; microbiolog-

36 77

1

alloys,

1

galvanic corrosion of, 39, 72; highperformance, 65, 68-70; high-

series-parallel circuit, 14

mandrel (blind

66-67, 69; classes

corrosion characteristics,

water tanks,

1

sherardizing (coating process),

resistance, electrical,

12;

sheaves, 2 7-1

raw-water cooling systems, red brass (copper alloy), 90

1

affect, 67; ferritic, 39, 65, 66; fresh1

rosion

shackles, 1

active, 39,

1

1

1

series

68

71-74; corrosion fatigue, 72-73; deck gear, 89; duplex grades, 70; 1 8-8 grades, 67-68; exhaust system piping, 164-65; factors that

seawater corrosion, resistance of coatings to, 17-18, 19-20 self-corrosion. See electrochemical cor-

7000

15

154

64-65;

stainless steels,

of, 39;

74-75 screws, 131-33 seacocks, 37-38 sealants: and avoiding corrosion, and coatings, sea-valves, 137

setting

stainless steel grades,

stabilizer additives,

70austenitic, 39, 65,

1

proprietary sealant formulations, propulsion systems, 151-66 propylene glycol antifreeze, 158

1

33; Aquamet 22, 69; atmospheric corrosion of, 7 1 -72;

1

74 165-66 1

202-3

64-65,

162 sail track, 205 salinity, 25 salt(NaCl),26 1

145-46

14-15

1

rust removers, 128

salvaging electronic equipment,

3-28; rust preventives,

213-14

rust converters, 128

sacrificial

97

97-98; splices, 171-72 solid rivets, 135 solvent cutbacks, 124-25 spade rudders, 141 spars, 199

rudders, 141-42

pressurized water systems, 181 for,

78

solders,

1

sailboats,

prevention of corrosion: coating

alloys,

62

systems, 103-5

1

aluminum

soldering, 39; hard, 97-98; soft,

28,8 1-82 power distribution and control, 70-73 power generation and storage, 167-70 precious metals, 40

1

series

Society of Automotive Engineers (SAE'),

207-10

roves, 134

sacrificial

00

poultice corrosion,

1

Mo alloys, 209

6000

69stabilized

potential difference

1

solders, 98

electrochemical corrosion

1

positive atoms, 3

pot metal,

1 ;

single-metal corrosion. See

1

1

rope, wire,

galvanic corrosion susceptibility,

40; nickel silvers, 95; resistivity of,

1

rust preventives,

135-36

rivets,

50 rigging, 99; rod, 207, 210-1 running, 2 6; standing, 207rigid opaque plastic piping, 83 ring nails, annular, 134-35 rivet body (blind rivet), 136 rivets, 133, 135-36 reverse polarity indicators,

1

chromium plating, 20-2

silver:

shore-power systems:

1

14

AC circuits, 47,

1

1

superaustenitics, stabilized,

70-7

shoreside wiring, 50, 54

Siderocapsa (iron-manganese bacteria),

64-65;

waterlift,

1

aluminum, 77;

164

in cast iron,

59;

bronze(copper alloy), 93-94; fetter rings, 34-35; screws, 132

silicon

1

silicone sealants,

1

1

;

titanium-

68

1

93-94

standing rigging, 207-1

181 silicon: in

1

Sta-Eok terminal fittings, 2 1 stanchion baseplates, stepped: bronze,

48,49,50,55-57

silencers,

1

33; stabilized grades, 68; stress-corrosion cracking of, 74;

screws,

1

36 aluminum-coated, 18; carbon, 61-62, 180, 185; chain, 196-97; chemical contents, 62; Cor-Ten, 38, 62, 63; galvanic corrosion of, 38-39;

staples,

1

steel:

1

90; high-alloy, 64-7 high-strength, low-alloy (1 ESI. A),

galvanized,

1

1

313

1

1

314

11

1

1

7

1

11

Index 164 systems: drinking water, 158; 62-63; hulls, 147; low-alloy, 6 shwater cooling, 156, 157-59, masts, 20 1-2; mild, 38; mill sea 8; freshwater plumbing, 180-86; 63; protective films, 63-64; UnifJ pressurized, 181; raw-water cooling, Numbering System (UNS), 6 1; tinplate, 97 _ ^ *U CT tllG 156-57; wastewater, 186-88 weathering, 38-39, 62; zones of cor-^Qit|Qjl!^^0p-^\3l|DrCpOS rosion for pilings, 25. See also stainwater tanks: freshwater, 180-82; hot timniuimstabiU^ed^suunle^S:^j4:e.e^^^ ipropeH^f /• water tanks, 184-86 less steels T©0i§ii^-4l) 8T(ili£ei,S3; {continued)

steel

rlift

141-43 (Hastelloy), 84

steering systems, Stellite

stepped stanchion baseplates, 193-94 stone boats, 149 storage, power,

167-70 1

8,

26-27, 42;

AC sources, 46-47; DC sources, 44. See also electrolytic corrosion strength: creep, 99; high-strength, low''

steels,

62-63;

tensile,

79

30

stress, cyclical,

alloys, 8

(TBTO), 148 troubleshooting electrical circuits, tributyltinoxide

51-53 109-10; thin-wall heat-shrink, 110-11

tubing: avoiding corrosion

1 ;

in,

of nickel and

and U-shaped members,

two-stroke gasoline-engine tions, 155

2000 series aluminum T\V sheathing, 7 Type MSDs, 188 Tvpe 2 MSDs, 88

designa-

oil

77

alloys,

surface-tolerant coatings, 128

Numbering System (UNS), 6 uranium, 41, 100-101 urethanes: acrylic, 126; polyester, 126 U-shaped structural members, 108, 109

swage terminals, 212 Swedish iron, 132

valence, 2

symbols: atomic, 4; chemistry, 4

valence number, 7, 9

surfaces,

70-7

108-9

vapor deposition (coating process):

tanks, 109-10; freshwater, 180-82;

chemical,

1;

DC systems, 5 1; electrical,

veneer sheathing, 150 working load limit (WIT.) of chain, 196

wrought aluminum, 75, 77-78; designations, 75-76 wrought iron, 58

16; physical,

1

1

ing, 171

15

yellow brass (copper alloy), 90-9

Zamak

terminal fittings, 21 1-14 terminals, 21 1-15 terminology, 4

viscosity index improvers, 153

zinc

ing,

51-53

compounds, 124 1

2; alternative

1

5;

16;

measur-

across parallel branch

voltage

1

1

1

23

voltaic

1

1

8,

2

1

,

34

waisting (keel bolt thinning), 140, 144 wasp-waisting, 144

I

thimbles, 197-98

wastewater systems,

thin-wall heat-shrink tubing,

1

10-1

1

threaded inserts (fasteners), 136

aluminum

1

1

1

8;

1

1

sacrificial anodes, 04-5; sealants and coatings, solders, 98 1

1

;

zinc-nickel coatings,

1

18

zinc-rich coatings, 126

1

alloys,

water: as electrolyte,

86-88 9; and immersion 1

corrosion, 25-26, 67; molecular shape, 8; types, 22-23, 17

77

1

bilge,

36-38; hot wire 43-44; wood hull, 145

IWN

(thermoplastic) sheathing, 171

1

t)r

1

cell,

9

16-18, 9-20; dezincification, 29; galvanic corrosion of, 40; galvanized coatings, 99-100; 99,

52

galvanic

1

and zinc alloys, 99, 191; in aluminum, 77; castings, 100;

resistivity of,

1

thermosetting acrylics, 123 thermosetting plastics,

through-hulls,

test,

deck gear,

(zinc alloy)

coatings,

voltage drop, 12

14-15

thermoplastics,

Tl

1

circuits, 14

thermoplastic acrylics,

series

0,

1

magnitude versus time,

thermal spray (coating process),

3000

1

voltage, 10, 11,

AC

systems, 53; electrical circuits,

1

1

volatile organic

and troubleshooting:

marine-grade, 171;

wood

vinyl coatings, 124

testing

1;

wooden cheek blocks, 2 wood-epoxy composite hulls, 146-47 wood hulls, 143 —16 wood masts, 200-20

vented loops, 163-64, 188 ventilation,

back panel, 170; boat,

tems, 5

47-50;

holding, 88; hot water, 184-86 temperature, 26 tempering, 76 1

AC sys-

171;

XHHW (cross-linked polvmer) sheath-

valves, 137

Talurit terminal fittings, 21 1,216

ABYC standards,

1

Unified

SLiperaustenitic stainless steels,

wiring:

power systems, 47

1

unalloyed cast irons, 59, 60

65

sulfur-reducing bacteria, 28

windlass, 195-96

shoreside, 50, 54; three-wire shore-

submerged engines, 154 sulfur, in stainless steel,

winding: primary, 55; secondary, 55

45, 17

1

1

Typed MSDs, 188

108, 109

steels, 62 weld decay, 28-29; of nickel and nickel-base alloys, 85-86; of stainless steel, 73 wet cylinder liner, 15 1 wet exhaust systems, 161-62 white cast iron, 59 white metal (tin-base alloy), 97 winches, 191-92

wire rope, 207-10, 208 wire-terminal fittings, 21 1-14

1

turnbuckles, 21 1-15, 2 14

nickel-base alloys, 86-87; of rigging, 209; of stainless steel, 74, 186 structures, 108-9; internal metal support structure for fiberglass rudder, 142; L-

54-55, 56

tributyltin (TBT), 148

tungsten,

stress-corrosion cracking, 30, 32; of

aluminum

transformers: autotransformers, 56-57; transition metals, 4

stray-current corrosion,

(HSLA)

toxic ethylene glycol, 186 isolation transformers,

strain-hardening, 76

allov

the

Sale of

silencer,

in

water-base acrylics, 123 water-jacketed exhaust riser and elbow, 163

zone of lightning protection, 176, 177 zones t)f corrosion, 25

1

>4

I

i

»

I '

-

f

/.

*

^

•

BOATING

,

iT'.'X-:;

all-too-common electrical panel rat's nest, all the usual

— undersized bus

I

improperly sized and untinned

no labeling of wires, inadequate overcurrent protection, and corrosion are visible.

—

.

^

bars,

cables,

..

fj^orrosion

In this

suspects

;

is

a constant, often expensive,

and sometimes

dangerous problem for boaters. Moisture,

salt, electrical

I

currents,

and chemicals

create a potent

combination that

can attack the metallic (and sometimes nonmetallic) parts of

your boat. Everett

Collier,

an expert in marine technology,

— including simple galvanic, electrochemical, and electrolytic — and explains how to identify, details all the types of corrosion

combat, and prevent them.

The most comprehensive book on this Boatowner's Guide to Corrosion shows you how to:

|(jRi'event corrosion with proper grounding, cathodic One way to get a plastic-coated lifeline

that can be checked for

have the lifelines made up with uncoated wire and then slip a PVC chafe guard corrosion

is

to

protection, protective coatings,

^ -

The

subject.

m

and careful selection

and matching of metal parts Protect your boat’s hull, deck gear, masts,

over the top.

as well as

its

and

rigging,

propulsion, electrical, plumbing, and

steering systems

k

|ecognize and cure developing corrosion before

it

can damage your boat

Everett Collier

who A stainless steel seacock mounted to

a bronze through-hull with

bronze tailpiece

— an invitation

an

electrical

and consults

engineer and amateur boatbuilder

advanced manufacturing technology management. An avid cruising sailor, he also serves as an adjunct faculty member at Northern Essex Community College and writes for

for galvanic corrosion.

is

lectures

in

marine industry magazines including National Fisherman.