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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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— Of
who have contributed to the writing of this hook, those who bound by ties of family and friendship
those
all
—
there are
had no choice but nonetheless made Their contributions were
fully.
their contributions cheer-
made
in
terms of nights,
weekends, and vacations shared with “the book.”
and Greg
— wife and sons,
in that
order
—
building and sailing buddies in any order. that this
book
is
Betty, Scott,
friends, partners, boatIt
to these contributors
dedicated with thanks.
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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.