Materials Aspects in Automotive Catalytic Converters [illustrated edition] 3527304916, 9783527304912

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Material Aspects in Automotive Catalytic Converters Edited by Hans Bode

Material Aspects in Automotive Catalytic Converters, Hans Bode Copyright © 2002 Wiley-VCH Verlag GmbH &Co. KGaA ISBN: 3-527-30491-6

Further Titles of Interest: B. Cornils, W. A. Herrmann, R. Schlägl, C.-H. Wong (Eds.) Catalysis from A-Z ISBN 3-527-29855-X S. Hagen, S. Hawkins Industrial Catalysis ISBN 3-527-29528-3 S. M. Thomas, W. J. Thomas Principles and Practice of Heterogenous Catalysis ISBN 0-471-29239-X G. Ertl, H. Knözinger, S. Weitkamp Handbook of Heterogenous Catalysis ISBN 0-471-29212-8

Material Aspects in Automotive Catalytic Converters

Edited by Hans Bode

Deutsche Gesellschaft für Materialkunde e.V.

Prof. Dr. Ing. Hans Bode Bergische Universität GH Wuppertal FG Werkstofftechnik Gaußstr. 20 D-42097 Wuppertal Germany

International Congress „Material Aspects in Automotive Catalytic Converters“, held from 03–04 October 2001 in Munich, Germany Organizer: DGM · Deutsche Gesellschaft für Materialkunde e.V.

This book was carefully produced. Nevertheless, authors, editor and publisher do not warrant the information contained therein to be free of errors. Readers are advised to keep in mind that statements, data, illustrations, procedural details or other items may inadvertently be inaccurate.

Library of Congress Card No. applied for. A catalogue record for this book is available from the British Library Deutsche Bibliothek Cataloguing-in-Publication Data: A catalogue record for this book is available from Die Deutsche Bibliothek ISBN 3-527-30491-6 © WILEY-VCH Verlag GmbH, Weinheim (Federal Republic of Germany), 2002 Printed on acid-free paper All rights reserved (including those of translation in other languages). No part of this book may be reproduced in any form – by photoprinting, microfilm, or any other means – nor transmitted or translated into machine language without written permission from the publishers. Registered names, trademarks, etc. used in this book, even when not specifically marked as such, are not to be considered unprotected by law.

Satz: W.G.V. Verlagsdienstleistungen GmbH, Weinheim Druck: betz-druck GmbH, Darmstadt Bindung: J. Schäffer GmbH + Co. KG, Grünstadt Printed in the Federal Republic of Germany

Preface Based on increased ecological demands, the car and car-supplying industries strive to meet the challenging requirements for higher performance and extended service life of future vehicle generations. Maintaining good performance is mandatory particularly in the view of thinner supports, higher cell densities and higher temperatures. Performance and service life predictions, based on tests or on modelling and simulation techniques, will depend on reliable materials data. Only very close cooperation between researchers and producers will help to meet these requirements. It was the aim of MACC, the second international conference on Materials Aspects in Automotive Catalytic Converters, to foster this cooperation. It refered to papers from both industry and research institutes which concentrate on the high-temperature mechanical and oxidation behaviour of both metal-supported and ceramic supported automotive catalysts. The metalsupported catalyst is based on a ferritic steel with 5–8% aluminum, 17–22% chromium and small additions of reactive elements. More than 11,000,000 units were produced in the year 2000. The ceramic-supported catalytic converter is based on corderite. The production rate is much higher. Both materials have specific advantages and disadvantages which determine the application for a given car model. In addition to these two basic groups of catalytic carriers, coating and canning aspects were also addressed by the conference programme. Especially the influence of coating thickness and composition is becoming more and more important when going to thinner supports and higher cell densities. I am very obliged to the authors for their valuable contribution to a comprehensive programme that covers the whole chain of product development and application, beginning with the melting process and ending with recycling aspects.

Munich, October 2001 Prof. Dr.-Ing. Hans Bode Conference Chairman

I Introduction Contribution of Automotive Catalytic Converters R. Searles, Association for Emissions Control by Catalyst, Brussels (B) ...................................3

II Metals Development Status of Metal Substrate Catalysts R. Brück, Emitec GmbH, Lohmar .............................................................................................19 Materials Issues Relevant to the Development of Future Metal Foil Automotive Catalytic Converters J. Nicholls, Cranfield University, Cranfield (GB); W. Quadakkers, Forschungszentrum Juelich (D) ................................................................................................................................31 High Temperature Corrosion of FeCrAlY / Aluchrom YHf in Environments Relevant to Exhaust Gas Systems A. Kolb-Telieps, Krupp VDM GmbH, Altena (D); R. Newton, Cranfield University, SIMS, Cranfield (GB); G. Strehl, TU Clausthal, Institut für Allgemeine Metallurgie Clausthal-Zellerfeld (D); D. Naumenko, W. Quadakkers, Forschungszentrum Jülich, IWV-2, Jülich (D) .....................................................................................................................49 Improved High Temperature Oxidation Resistance of REM Added Fe-20%Cr-5%Al Alloy by Pre-Annealing Treatment K. Fukuda, K. Takao, T. Hoshi, O. Furukimi, Technical Research Laboratories, Kawasaki Steel Corp., Chiba (Japan) ......................................................................................59 Oxidation Induced Length Change of Thin Gauge Fe-Cr-Al Alloys C. Chang, L. Chen, B. Jha, Engineered Materials Solutions, Inc., Attleboro (USA) ................69 Improvement in the Oxidation Resistance of Al-deposited Fe-Cr-Al Foil by Pre-oxidation S. Taniguchi, T. Shibata, Department of Materials Science and Processing, Graduate School of Engineering, Osaka University, Osaka (J); A. Andoh, Steel and Technology Development Laboratories, Nisshin Steel, Osaka (J) ...........................................83 Factors Affecting Oxide Growth Rates and Lifetime of FeCrAl Alloys W. Quadakkers, L. Singheiser, D. Naumenko, Forschungszentrum Jülich (D); J. Nicholls, J. Wilber, Cranfield University, School of Industrial and Manufacturing Science, Cranfield (GB) ...........................................................................................................93 On Deviations from Parabolic Growth Kinetics in High Temperature Oxidation G. Borchardt, G. Strehl, Institut für Metallurgie, TU Clausthal (D) ......................................106

VIII Effect of Reactive Elements and of Increased Aluminum Contents on the Oxide Scale Formation on Fe-Cr-Al Alloys V. Kolarik, M. del Mar Juez-Lorenzo, H. Fietzek, Fraunhofer-Institut für Chemische Technologie, Pfinztal (D); A. Kolb-Telieps, H. Hattendorf, R. Hojda, Krupp VDM GmbH, Werdohl (D) .....................................................................................................117 High Temperature Strength of Metal Foil Materials M. Cedergren, K. Göransson, R&D, AB Sandvik Steel, Sandviken (S) ..................................126 Lifetime Predictions of Uncoated Metal-Supported Catalysts via Modeling and Simulation, based on Reliable Material Data H. Bode, University of Wuppertal (D); C. Guist, BMW AG, Munich (D) ..............................134 Elastic-Plastic Thermal Stress Analysis for Metal Substrates for Catalytic Converters S. Konya, A. Kikuchi, Nippon Steel Corporation, Futtsu (J) ..................................................144 A New Type of Metallic Substrate R. Lylykangas, H. Tuomola, Kemira Metalkat Oy (SF) ..........................................................152

III Ceramics Development Status of Ceramic Supported Catalyst C. Vogt, E. Ohara, NGK Europe GmbH; M. Makino, NGK Insulators Ltd ...........................173 Evaluation of In-Service Properties and Life Time of Automotive Catalyst Support Materials U. Tröger, M. Lang, Zeuna Stärker GmbH & Co. KG, Augsburg (D) ...................................186 Loads, Design and Durability Evaluation of Mount Systems for Ceramic Monoliths G. Wirth, J. Eberspächer GmbH & Co., Esslingen (D) ..........................................................191 High Performance Packaging Materials M. Vermoehlen, D. Merry, S. Schmid, Corning GmbH, Wiesbaden (D) ................................202

IV Catalysts Three-way Catalyst Deactivation Associated With Oil-Derived Poisons J. Kubsh, Engelhard Corporation, Environmental Technologies Group Iselin (USA) ..........217 Catalytic Reduction of NOx in Oxygen-rich Gas Streams, Deactivation of NOx StorageRaduction Catalysts by Sulfur C. Sedlmair, K. Sehan, Technische Universität München, Institut für Technische Chemie II, Garching (D); J. Lercher, A. Jentys, University of Twente, Faculty of Chemical Technology, Enschede (NL) ...................................................................................223

IX Catalytic Reduction of NOx in Oxygen-rich Gas Streams: Progress and Challenges in Catalyst Development W. Grünert, Lehrstuhl Technische Chemie, Ruhr-Universität Bochum (D) ...........................229 Atomic Structure of Low-Index CeO2 Surfaces H. Nörenberg, University of Oxford, Department of Materials, Oxford (GB); J. Harding, University College London, Department of Physics and Astronomy, London (GB); S. Parker, University of Bath, Department of Chemistry, Bath (GB) .....................................237 Nanostructured Ceria-Zirconia as an Oxygen Storage Component in 3-way Catalytic Converters-Thermal Stability B. Djuricic, Austrian Research Centers, Seibersdorf (A), S. Pickering, Institute for Advanced Materials, Petten (NL) ...........................................................................................241

V Recycling Recycling Technology for Metallic Substrates: a Closed Cycle C. Hensel, Demet Deutsche Edelmetall Recycling AG & Co. KG, Alzenau (D) ....................251

VI Miscelleanous Hot-Corrosion of Metal and Ceramic Honeycombs by Alkaline Metals for NOx Adsorption M. Yamanaka, Nippon Steel Technoresearch, Futtsu (J); Y. Okazaki, Nippon Steel, Toukai, (J) ..............................................................................................................................263 The Effect of Trace Amounts of Mg in FeCrAl Alloys on the Microstructure of the Protective Alumina Surface Scales P. Untoro, M. Dani, National Nuclear Energy Agency, Kawasan PUSPIPTEK, Serpong (Ind); H. Klaar, J. Mayer, Gemeinschaftslabor für Elektronenmikroskopie, RWTH Aachen (D); D. Naumenko, J. Kuo, W. Quadakkers, Institut für Werkstoffe und Verfahren der Energietechnik (IWV-2), Forschungszentrum Jülich (D) ................................271

Subject Index*

A Adsorption, NOx 263 Al deposition 83 Alkaline metals 263 Alloys 271 Aluchrom YHf 49 Alumina surface 271 Aluminum content 117 Annealing 59 Atomic structure 237 Automotive catalysts 3, 31, 186 C Catalyst 134 - ceramic 173 - three-way 217, 241 - deactivation 217 - development 19, 229 - support materials 186 Catalytic converters 3, 31, 144, 241 Catalytic reduction 223, 229 CeO2 surfaces 237 Ceramic components 241 Ceramic honeycomb 263 Ceramic monoliths 191 Ceramic supported catalyst 173 Ceria-Zirconia 241 Corrosion 49, 263

Elements, reactive 117 Emission limits 152, 202, 251 Environmental protection 251 Exhaust gas systems 49, 152 F Fe-Cr-Al alloy 59, 69, 83, 93, 117, 271 FeCrAlY/Aluchrom YHf 49 Foil 31, 83, 126 G Gas flow 152 Gas streams 223, 229 Gas systems 49 Growth kinetics 106 H High temperature - corrosion 49 - oxidation 59, 106 - strength 126 Honeycomb 263 Hot-corrosion 263 I Increased aluminum content 117 In-service properties 186 K

D

Kinetics 106

Data, reliable 134 Deactivation 217, 223 Development 173 - catalysts 19, 229 - converters 31 Durability evaluation 191

L Length change 69 Lifetime 93, 186, 251 Lifetime predictions 134 Loads 191 Low-index CeO2 surfaces 237

E Elastic thermal stress 144 *

The page numbers refer to the first page of the article

Material Aspects in Automotive Catalytic Converters, Hans Bode Copyright © 2002 Wiley-VCH Verlag GmbH &Co. KGaA ISBN: 3-527-30491-6

281 M Mat, vermiculite 202 Material - data, reliable 134 - issues 31 - packaging 202 - strength 126 Metal foil 31, 126 Metal honeycomb 263 Metallic substrate 152, 251 Metals, alkaline 263 Metal substrate catalysts 19, 144 Metal-supported catalysts 134 Mg 271 Microstructure 271 Mixed gas flow 152 Modeling 134 Monoliths, ceramic 191 Mount systems 191 N Nanostructure 241 NOx - adsorption 263 - reduction 223, 229 - storage 223 O Oil-derived poisons 217 Oxidation 69, 106 Oxidation resistance 59, 83 Oxide growth rates 93 Oxide scale formation 117 Oxygen-rich gas streams 223, 229 Oxygen storage 241 P Packaging materials 202

Parabolic growth kinetics 106 Plastic thermal stress 144 Poisons, oil-derived 217 Pre-annealing treatment 59 Pre-oxidation 83 Protective surface 271 R Reactive elements 117 Recycling technology 251 Reliable material data 134 REM 59 S Separation process 251 Simulation 134 Storage-reduction catalysts 223 Stress analysis 144 Structure, atomic 237 Substrate 19, 144, 152 Sulfur, NOx reduction 223 Surfaces 237, 271 T Thermal stability 241 Thermal stress analysis 144 Thin alloys 69 Three-way catalyst 217, 241 Trace amounts, alkaline 271 U Uncoated metal-supported catalysts 134 V Vermiculite mat 202

I Introduction

Material Aspects in Automotive Catalytic Converters, Hans Bode Copyright © 2002 Wiley-VCH Verlag GmbH &Co. KGaA ISBN: 3-527-30491-6

Contribution of Automotive Catalytic Converters Robert A Searles Association for Emissions Control by Catalyst, Brussels, Belgium

1

Abstract

Catalyst-equipped cars were first introduced in the USA in 1974 but only appeared on European roads from 1985. In 1993 the European Union set new car emission standards that effectively mandated the installation of emission control catalysts on gasoline-fuelled cars. Now more than 300 million of the world’s over 500 million cars and over 85% of all new cars produced worldwide are equipped with autocatalysts. Catalytic converters are also increasingly fitted on heavy-duty vehicles, motorcycles and off-road engines and vehicles. The paper will review the technologies available to meet the exhaust emission regulations for cars, light-duty and heavy-duty vehicles and motorcycles adopted by the European Union for implementation during the new century. This includes low light-off catalysts, more thermally durable catalysts, improved substrate technology, hydrocarbon adsorbers, electrically heated catalysts, DeNOx catalysts and adsorbers, selective catalytic reduction and diesel particulate traps. The challenge is to abate the remaining pollutants emitted while enabling fuel-efficient engine technologies to flourish. This is paramount to the achievement of air quality and greenhouse gas targets given the large increase in the number of vehicles on European roads since 1970 and the projections for further increases in vehicle numbers and greater distances driven each year in future.

2

Introduction

AECC is an international association of European companies making the technologies for automobile exhaust emissions control: autocatalysts, ceramic and metallic substrates, specialty materials incorporated into the catalytic converter and catalyst, adsorber and filter based systems for the control of gaseous and particulate emissions from diesel and other lean burn engines. 2.1

European Emission and Fuel Legislation

The European Union (EU) emission limits for passenger cars set from 1993 have already been lowered from 1996 and again from 2000. For passenger cars and light commercial vehicles the emission standards and fuel composition, including sulfur levels, have been agreed for 2000 and 2005. [1] New test cycles (ESC and ETC) and tougher emission standards for heavy-duty diesel vehicles have been finalized for 2000 and 2005. The limits for Enhanced Environmentally.Friendly Vehicles (EEV) are set and can serve as a basis for fiscal incentives by EU Member States. A

Material Aspects in Automotive Catalytic Converters, Hans Bode Copyright © 2002 Wiley-VCH Verlag GmbH &Co. KgaA ISBN: 3-527-30491-6

4 further reduction in limit values for nitrogen oxides (NOx) in 2008 is subject to a review by the European Commission in 2002 on technical progress. [2] The Working Party on Pollution and Energy (GRPE), an expert group of the World Forum for Harmonization of Vehicle Regulations (WP.29) at UN-ECE in Geneva, is developing a Worldwide Harmonized Heavy Duty Certification procedure and is looking in new measurement protocols in order to ensure that ultra fine particles are controlled by future emission legislation to minimize the health effects of diesel particle emissions. A proposal by the European Commission to set tougher, catalyst-requiring emission limits for motorcycles is being ratified by the European Parliament and Council. Tighter emission limits from 2003 for new types of motorcycles are agreed and correspond to a reduction of 60% for hydrocarbons and carbon monoxide for four-stroke motorcycles, and 70% for hydrocarbons and 30% for carbon monoxide for two-stroke motorcycles. A second stage with new mandatory emission limits for 2006 are expected to be based on the new World Motorcycle Test Cycle (WMTC) which is also being developed by the UN-ECE in Geneva. In the final report of the European Auto Oil II Programme [3], it was concluded that some air quality problems, such as atmospheric levels of particulate matter and ozone, are not yet solved. The challenge is to abate the remaining pollutants emitted while enabling the development of fuelefficient engine technologies. This is paramount to the achievement of air quality and greenhouse gas targets given the large increase in the number of vehicles on European roads and the projections for further increases in vehicle numbers and greater distances driven each year in future. 2.2

Exhaust Emissions from Internal Combustion Engines

Exhaust emissions can be lowered by: · Reducing engine-out emissions by improving the combustion process and fuel management, or by changes to the type of fuel or its composition · Retrofitting catalytic converters and associated engine and fuel management systems if they are not original equipment · Decreasing the time required for the catalytic converter to reach its full efficiency · Increasing the conversion efficiency of catalysts · Storing pollutants during the cold start for release when the catalyst is working · Using catalysts and adsorbers to destroy nitrogen oxides under lean operation · Using particulate filters with efficient regeneration technology · Increasing the operating life of autocatalysts and supporting systems. This paper reviews all the above opportunities, except the first, from the standpoint of material requirements and will also look back into the history of the materials developed for catalytic converters with these requirements in mind.

3

A Brief History of Automotive Catalysts

The first reference to a catalytic converter known to AECC is a patent [4] published to a French chemist, Michel Frenkel, in 1909. The device uses a kaolin (china clay) “honeycomb” with 30 grams of platinum as the active catalytic material. (Figure 1) The patent describes

5 “deodorizing” the exhaust using air blown in by a fan. As far as is known the device was not put into commercial production at the time. No doubt the high loadings of platinum were a deterrent and there was no air pollution concerns in those early, carefree days of motoring.

Figure 1: 1909 Catalytic Converter invented by Michel Frenkel

Figure 2: Eugene Houdry in 1953 with a small prototype catalytic converter

The next report [5] of the concept of catalytic converters was in the 1920s. Another European invention, this time German, was taken to General Motors in the US and was described as a collection of wires and beads, again coated with platinum. The tests were at first a success with the device glowing red, but within seconds the catalyst had failed. This was because tetraethyl lead had been recently introduced in the US as an octane booster, but was at that time unknown in Europe. Lead poisons catalytic converters. The French engineer Eugene Houdry can be considered as the father of the modern catalytic converter. Born in France he moved to the US and invented a revolutionary method for cracking low-grade crude to high-octane gasoline – the “cat cracking” process. After the 1939–1945 war he set up the Oxy-Catalyst company and turned his attention to the health risks from the increasing volumes of automobile and industrial exhausts. In 1962, the year of his death, he patented the first modern catalytic converter (Figure 2). The modern history of the catalytic converter started with the developments that lead to the 1970 US Clean Air Act and the rate of invention has accelerated greatly. Excellent histories of the industry [6, 7, 8] have been published so only a summary will be covered here. The modern catalytic converter, based on platinum group metals deposited on a ceramic honeycomb base or monolith, was first patented in 1965 [9]. However the industrial use of catalysts was then dominated by catalysts deposited on pellet or bead supports. In the first years after 1974 when catalytic converters were used in the US and Japanese markets, both pellet (Figure 3) and monolithic converters (Figure 4) were used. The loss of catalyst material by attrition in pellet converters was largely overcome by reactor design. Early prototypes of ceramic honeycombs were made by two approaches: 1. Dipping paper in ceramic slurries, corrugating them and laying up a unitary structure, firing the composite and shaping 2. Calendaring a plastic material containing ceramic powders between grooved and plain rollers, rolling up into a unitary structure, firing the composite and shaping.

6

Figure 3: Schematic of pelletized catalytic con- Figure 4: “Cutaway” ceramic monolith catalytic converter verter

Both of these developments were ultimately replaced by extruded honeycomb substrates. These are based on cordierite (2MgO .2Al2O3 .5SiO2) and made from natural raw materials and a plastic material that is extruded to form a unitary structure with parallel fine channels and then fired to the final shape. These materials have high thermal shock resistance and high melting and softening points with higher attrition resistance and lower pressure losses than pellet converters, which they ultimately replaced. In the 1970s new ferritic steels became available that could be made into ultra thin foils, corrugated and then laid up to form a honeycomb structure. One such steel was developed at the Atomic Energy Research Establishment in Harwell, UK for “canning” Uranium 235 and was called Fecralloy. This name reflects the components of the alloy - Iron (Fe), Chromium (Cr), Aluminum (Al) and Yttrium (Y). The formation of a self-healing protective “skin” of alumina (Al2O3) allows the ultra-thin steels to withstand the high temperatures and corrosive conditions in auto exhausts. These materials also have high thermal shock resistance and high melting and softening points and facilitated the development of high cell densities with very low pressure losses.

a) b) Figure 5: a: Metallic substrate converter, b: Ceramic substrate converter

7 Further development of metallic and ceramic substrates (Figure 5a & 5b) is described in the next section.

4

Current Catalyst Technology for Emissions Control Autocatalysts

Oxidation catalysts convert carbon monoxide (CO) and hydrocarbons (HC) to carbon dioxide (CO2) and water and decrease the mass of diesel particulate emissions but have little effect on nitrogen oxides (NOx). Three-way catalysts (TWC) operate in a closed loop system including a lambda- or oxygen sensor to regulate the air-fuel ratio. The catalyst can then simultaneously oxidize CO and HC to CO2 and water while reducing NOx to nitrogen. These simultaneous oxidizing and reducing reactions have the highest efficiency in the small air-to-fuel ratio window around the stoichiometric value, when air and fuel are in chemical balance. 4.1

Fast light off catalysts

The catalytic converter needs to work as fast as possible by decreasing the exhaust temperature required for operation so that untreated exhaust is curtailed at the start of the legislated emissions tests and on short journeys in the real world. Changes to the type and composition of the precious metal catalyst (Figure 6) and to the thermal capacity of substrates (figure 7) have together effected big reductions in the required operating temperature and light off times have been reduced from one to two minutes down to less than 20 seconds. [10]

Figure 6: Effect of catalyst technology on light off temperature

Figure 7: Effect of substrate cell density on light off time

The introduction of the new generation platinum/rhodium (Pt/Rh) technology for current and future emission standards is a technically and commercially attractive alternative for current palladium (Pd) based technologies for high demanding applications in close-coupled and under floor positions using different cold start strategies. [11] 4.2

More thermally durable catalysts

Increased stability at high temperature allows the catalytic converter to be mounted closer to the engine and increases the life of the converter, particularly during demanding driving. Pre-

8 cious metal catalysts with stabilized crystallites and washcoat materials that maintain high surface area at temperatures around 1000°C are needed. Improved oxygen storage components stabilize the surface area of the washcoat, maximize the air-fuel “window” for three-way operation and indicate the “health” of the catalytic converter for On Board.Diagnostic (OBD) systems. Figure 8 shows the progress made with mixed cerium and zirconium oxides. [12]

Figure 8: Improvements to thermal stability and oxygen storage capacity (OSC)

4.3

Substrate Technology

The technology of the substrates, on which the active catalyst is supported, has seen great progress. In 1974 ceramic substrates had a density of 200 cells per square inch of cross section (31 cells/square cm.) and a wall thickness of 0.012 inch or 12 mil (0.305 mm). By the end of the 1970’s the cell density had increased through 300 to 400 cpsi and wall thickness had been reduced by 50% to 6 mil. Now 400, 600, 900 and 1200 cpsi substrates are available and wall thickness can be reduced to 2 mil - almost 0.05 mm (Figure 9). [13, 14, 15, 16, 17] In the late 1970's substrates derived from ultra thin foils of corrosion resistant steels came onto the market. In the beginning the foils could be made from material only 0.05 mm thick allowing high cell densities to be achieved. Complex internal structures can be developed and today wall thickness is down to 0.025 mm and cell densities of 800, 1000 and 1200 cpsi are available (Figure 10). [18, 19] This progress in ceramic and metal substrate technology has major benefits. A larger catalyst surface area can be incorporated into a given converter volume and this allows better conversion efficiency and durability. The thin walls reduce thermal capacity and avoid the penalty of increased pressure losses. Alternatively the same performance can be incorporated into a smaller converter volume, making the catalyst easier to fit close to the engine, as cars get more compact. These improvements in substrate technology are now being applied in conjunction with heavy-duty diesel engines with catalysts placed as close as possible to the engine in order to.reduce the time to light off. To improve conversion behavior, catalysts are placed close to

9 the exhaust port before the turbocharger (Figure 11) and close-coupled catalysts using hybrid substrates are fitted (Figure 12). [20]

Figure 9: Progress in ceramic substrate design

Figure 10: Progress in metallic substrate design

Figure 11: Pre-turbo catalyst

Figure 12: Close-coupled hybrid catalyst

4.4

New Technology for Emissions Control Stoichiometric combustion

Conventional three-way catalysts are continually developed to improve high temperature stability and light off performance and to meet the demands of both the most challenging emission legislation in the world and new applications including motorcycles. Their performance can be further extended by the following additional technologies. 4.4.1 Hydrocarbon adsorbers Hydrocarbon adsorber systems incorporate special materials, such as zeolites, into or upstream of the catalyst. Hydrocarbon emissions are collected when exhaust temperatures are too low for effective catalyst operation. The hydrocarbons are then desorbed at higher temperatures when the catalyst has reached its operating temperature and is ready to receive and destroy the hydrocarbons. This technology has the potential to reduce hydrocarbons to less than half the levels emitted from a three-way catalytic converter (Figure 13). [21]

10

Figure 13: Influence of improved three-way catalyst and hydrocarbon adsorber on emissions (European cycle).

4.4.2 Electrically heated catalyst systems This uses a small catalyst ahead of the main catalyst. A metallic substrate, onto which the catalyst is deposited, allows an electric current to pass so it will heat up quickly. This brings the catalyst to its full operating temperature in a few seconds. [22] 4.5

Lean Combustion

With the development of lean burn direct injection gasoline engines and increased use of diesel engines, lean combustion is the challenge for automotive catalysis but is essential to reduce fuel consumption and limit CO2 emissions. New diesel technologies with electronic management and direct injection can achieve further fuel consumption improvements. Conventional three-way catalyst technology used on gasoline engines needs a richer environment with lower air-fuel ratios to reduce NOx so a radical new approach is required. DeNOx catalysts and NOx traps hold out the prospect of substantially reduced emissions of oxides of nitrogen. NOx conversion rates depend on exhaust temperature and availability of reducing agents. There are four systems under evaluation by industry: 1. Passive DeNOx Catalysts using reducing agents available in the exhaust stream 2. Active DeNOx Catalysts using added hydrocarbons as reducing agents 3. NOx traps or adsorbers used in conjunction with a three-way catalyst 4. Selective Catalytic Reduction using a selective reductant, such as ammonia from urea. Each of these systems offers different possibilities in the level of NOx control possible and the complexity of the system. Fuel parameters such as sulfur content can affect catalyst performance. 4.5.1 DeNOx (or Lean NOx) Catalysts DeNOx catalysts use advanced structural properties in the catalytic coating to create a rich "microclimate" where hydrocarbons from the exhaust can reduce the nitrogen oxides to nitrogen, while the overall exhaust remains lean. Further developments focus on increasing the operating temperature range and conversion efficiency. 4.5.2 NOx Adsorbers (or Lean NOx Traps) NOx traps are a promising development as results show that NOx adsorber systems are less constrained by operational temperatures than DeNOx catalysts. NOx traps adsorb and store NOx under lean conditions. A typical approach is to speed up the conversion of nitric oxide (NO) to nitrogen dioxide (NO2) using an oxidation or three-way catalyst mounted close to the

11 engine so that NO2 can be rapidly stored as nitrate. The function of the NOx storage element can be fulfilled by materials that are able to form sufficiently stable nitrates within the temperature range determined by lean operating points of a direct injection gasoline engine. Thus especially alkaline, alkaline earths and to a certain extent also rare-earth compounds can be used. When this storage media nears capacity it must be regenerated. This is accomplished in a NOx regeneration step. Unfortunately, alkaline and alkaline earth compounds have a strong affinity for sulfation. As a consequence alkaline and alkaline earth compounds are almost irreversibly poisoned by the sulfur contained in the fuel during the NOx storage operation mode, leading to a decrease in NOx adsorption efficiency during operation. The stored NOx is released by creating a rich atmosphere with injection of a small amount of fuel. The rich running portion is of short duration and can be accomplished in a number of.ways, but usually includes some combination of intake air throttling, exhaust gas recirculation, late ignition timing and post combustion fuel injection. The released NOx is quickly reduced to N2 by reaction with CO (the same reaction that occurs in the three-way catalyst for spark-ignited engines) on a rhodium catalyst site or another precious metal that is also incorporated into this unique single catalyst layer (Figure 14).

Figure 14: NOx adsorber working principle

Under oxygen rich conditions, the thermal dissociation of the alkaline and alkaline earth sulfates would require temperatures above 1000 °C. Such temperatures cannot be achieved under realistic driving conditions. However, it has been demonstrated in various publications [23, 24, 25] that it is in principle possible to decompose the corresponding alkaline earth sulfates under reducing exhaust gas conditions at elevated temperatures. In this way the NOx storage capacity can be restored. The heating of the catalyst, for example by late ignition timing, does however result in a considerable increase in fuel consumption, which is dependent upon the sulfur content. Therefore, reducing the sulfur concentration in the fuel must be regarded as the most effective way of using the full potential of modern direct injection gasoline engines with respect to fuel economy and CO2 reduction. One of the demands for a desulfation strategy must be to avoid any H2S emissions above the odor threshold during desulfation. [26, 27] Developments and optimization of NOx adsorber systems have been and are currently underway for diesel and gasoline engines. These technologies have demonstrated NOx conversion efficiencies ranging from 50 to in excess of 90 percent depending on the operating temperatures and system responsiveness, as well as fuel sulfur content. [28, 29] The system is in production with direct injection gasoline engines.

12 4.5.3 Selective Catalytic Reduction (SCR) SCR is a widespread technology to reduce nitrogen oxide emissions from coal, oil and gas fired power stations, marine vessels and stationary diesel engine applications. SCR technology has been used successfully for more than two decades. SCR technology for heavy-duty diesel vehicles has been developed to the commercialization stage and will be available as an option in the series production of several European truck-manufacturing companies in 2001. SCR technology permits the NOx reduction reaction to take place in an oxidizing atmosphere. It is called “selective” because the catalytic reduction of NOx with ammonia (NH3) as a reductant occurs preferentially to the oxidation of NH3 with oxygen. Several types of catalyst are used, the choice of which is determined by the temperature of the exhaust environment. For mobile source applications the reductant source is usually urea, which can be rapidly hydrolyzed to produce ammonia in the exhaust stream. SCR for heavy-duty vehicles reduces NOx emissions by circa 80%, HC emissions by circa 90% and PM emissions by circa 40% in the EU test cycles, using current diesel fuel ( is a probabilistic term that incorporates the fractional surface area that may spall during thermal cycling and any variance in the rate of oxidation (k) and its effect on the time to breakaway (tB/O). As spallation is rarely observed for foil components the term > takes values equal to, or close to, zero and component life is dominated by the second term on the left hand side (oxide growth) in equation (1). The first term on the left hand side of the equation accounts for oxide spallation, while the right hand side of the equation is a measure of the available aluminium reservoir, locally in the component (if V/A is the local volume/surface area ratio. This behaviour is illustrated in Figure 4, which provides model predictions (assuming parabolic kinetics) compared with breakaway oxidation lifetimes measured over a range of foil/alloy thicknesses from 70mm to 1.8mm (for rectangular geometry test pieces the V/A ratio is approximately half the sample thickness). The open symbols represent samples that have not yet gone into breakaway, while the filled symbols represent samples that show some evidence of breakaway corrosion, usually at one of the corners. Superimposed on this figure are the median predictions assuming a parabolic rate law and various failure criteria. The line marked ‘InCF’, with > = 0 and CB = 0wt%, corresponds to ‘Intrinsic Chemical Failure’. This is the most likely failure mode for foil material of low strength and results in the longest component lives at any foil thickness (V/A ratio). The lines marked ‘MICF’, with > = 0.1 or 1.0 and CB = 1.7 wt%, correspond to ‘Mechanical Induced Chemical Failure’; under these conditions either local spallation or tensile cracking of the alumina scale occurs, usually in areas of constraint, and the repair, cracking and spallation process rapidly depletes the available aluminium reservoir leading to early failure. The term ‘>’ is a measure of the extent of spallation, > = 1

37 being one limiting case whereby the alumina totally spalls at each shut down cycle. In the LEAFA experimental programme this was never observed to happen for the foil samples, but was approached for strong, thick alloy materials on rapid cooling. >=0.1

MICF

1.0E+04

>=1.0

Aluchrom Yhf - 1300C

1.0E+03 2

kp*tB [ (mm) ]

Aluchrom Yhf - 1300C (Not.) Kanthal AF - 1300C Kanthal AF - 1300C (Not.) Kanthal APM - 1300C Kanthal APM - 1300C (Not.) PM2000 - 1300C PM2000 - 1300C (Not.)

1.0E+02

1.0E+01 0.01

C0=5.5 wt.% CB=1.7 wt.% (MICF) CB=0.0 wt.% (InCF)

InCF >=0.0 0.10

1.00

10.00

Volume/Surface Area [mm]

Figure 4: Model of breakaway oxidation, based on parabolic rate law oxidation : the lines super-imposed on the figure correspond to median prediction.

It can be seen from Figure 4 that the parabolic based model predictions prove conservative (predict shorter lives) for all foil materials tested, whether weak (Aluchrom YHf, Kanthal AF) or strong (PM2000). This is because a-alumina growth is generally observed to follow subparabolic kinetics [18,19] with an exponent ‘n’ between 2.3 and 3.0; such behaviour would lower the rate of aluminium consumption and therefore lead to extended component lives. It can further be seen from Figure 4, that > (the propensity to spall) is much less than 0.1 for all foil materials in current usage. This means that the dominant foil failure mechanism is ‘Intrinsic Chemical Failure’ and thus CB (the aluminium content at onset of breakaway) can be expected to reduce to essentially zero. 4.2

The Influence of Component Geometry on Catalytic Converter Body Life

It is evident from the foregoing modelling work that specimen thickness is a significant factor in determining the life of the converter matrix. To be exact it is the local volume/surface area ratio that is critical in determining the onset of chemical failure, that is why most rectangular test samples fail at corners. This aspect has particularly been addressed by Strehl et al [22] where it is shown that material thickness, sample shape and local constraints resulting from component geometry may have significant effects on component life. It is also shown that the local oxygen pressure adjacent to the components surface can be reduced by unfavourable geometries, such as crevices, and this can trigger early breakaway corrosion. Probably the most significant contribution of this study is the recognition that is the ‘local’ volume/surface area ratio that controls component life. A simple analysis for plate material, which is obvious once demonstrated, shows that the volume/surface area ratio at a corner will be one third of that for a free surface, while for an edge the volume/surface area ratio is re-

38 duced by a factor of two. For foil samples, these geometry factors are even more critical; when the foil is thin the aluminium concentration within the foil is in equilibrium with two free surfaces (hence the assumption that for an infinite foil of thickness ‘d’ the volume/surface area ratio is d/2), however, at a corner the local volume/surface area ratio approaches d/4, half of that for the bulk of the sheet, while along edges the value approached d/3. Thus the local aluminium reservoir is significantly reduced at edges and corners because of the change in the local volume/surface area ratio in this region and this accounts for the onset of breakaway oxidation usually being noticed at corners first. This has major implications in manufacturing components and emphasises the need for good design particularly at corners, edges and fixings if premature breakaway is to be avoided. However, the local aluminium reservoir is not the sole controlling parameter; for example two small closely spaced holes could be expected to give rise to a geometry where the local volume/surface area ratio results in premature breakaway, however, this does not occur because additional aluminium can diffuse to this region from the bulk of the component. Thus it is the balance between the local aluminium reservoir, its rate of consumption through oxidation and the rate of supply by diffusion from the bulk of the component that determines whether breakaway will occur or not. Geometry also modifies the constraint that the oxidising surface sees and this can alter the oxidation rate. Thin unconstrained foils are free to expand as a result of growth and thermal stress (length increases of 10% have been measured in oxidation measurements of thin foil [21]). Constrained foils, and thick section components, can generate sufficient stress at surface imperfections that scale cracking, can occur locally [22,23] thus oxidation rates in areas of high constraint may differ from those of unconstrained surfaces. This effect of constraint is illustrated in Figure 5, which plots the mass gain per unit area for various geometry components, including a model catalytic converter body, manufactured from a 58mm Aluchrom YHf foil oxidised at 1000 °C for 700h. The lowest mass gain was observed for the rectangular specimen which was free to deform. Both the ring sample and model catalytic converter body showed an increased mass gain, both of similar magnitude. The additional mass gain was associated with cracks observed in the oxide scale on both the ring samples and catalytic converter body. This cracking has the apparent effect of increasing the rate constant by some 60%, during these discontinuous oxidation studies. In practice, this will significantly reduce component life as component failure switches from intrinsic chemical failure (InCF) to mechanically induced chemical failure (MICF) – see Table 2, and Figures 4 and 9. Table 2 provides predictions of the medium component lives for FeCrAl foil material with 5wt% aluminium, based on the LEAFA model, assuming parabolic kinetics, for the cases of intrinsic chemical failure and mechanically induced chemical failure. In calculating the values for mechanical induced chemical failure it is assumed that the catalytic body is constrained (kp is increased by a factor of 1.6) and the alloy concentration at which alumina can no longer reform (CB) is 1.7wt%.

39

Figure 5: Influence of component geometry on the oxidation of Aluchrom YHf: 58mm foil at 1100 °C [22].

One can see from Table 2 that reducing the foil thickness from 50mm to 30mm reduces the life of the component by a factor of 2.77. However, this reduction is not as great as that imposed by the constraint of the cylindrical geometry, which has the effect of reducing the life by a factor of 3.39 over the unconstrained foil. Table 2: Median predicted component lives for FeCrAl-RE foils as a function of foil thickness and degree of constraint

Component life measured as the kp.tB product [mm2] Foil thickness Unconstrained foil Catalytic converter body 0.27 0.90 20mm 0.60 2.03 30mm 1.66 5.63 50mm 3.26 11.03 70mm 4.3 Changes in Operating Temperature, Brought About by Catalytic Converter Positioning

Current underfloor catalytic converter bodies see peak temperatures of 910 °C, while those for close coupled configurations are expected to see 1000 °C. Assuming that a-alumina is the stable oxide formed on the FeCrAl-RE catalytic converter bodies (it is at more elevated temperatures > 1050 °C), then this temperature increase will raise the oxidation parabolic rate constant by a factor of 5.2, from 2.3 × 10–14 g2 cm–4 s–1 at 910 °C to 1.2 × 10–13 g2 cm–4 s—1 at 1000 °C as can be seen from Figure 6 [24]. In terms of numbers suitable to insert into the above mentioned life model the associated scaling rates are 2.16 × 10–2 mm2 h–1 at 910 °C and 1.13 × 10– 1 mm–2h–1 at 1000 °C. Thus assuming a foil thickness of 50 mm, under unconstrained conditions, then the hot exposure time would be reduced from 260 hours to some 50 hours before chemical

40 failure occurs under peak load conditions. Constraint or thinner foil sections would reduce these lives still further. One conclusion, therefore, is that catalytic converters must spend a considerable period of normal operation way below these peak load temperatures, for under such peak load conditions the foil components would not be able to sustain the warranted lives of the vehicle. To achieve the desired warranted life the mean operating temperature of a current, underfloor, catalytic body would have to 780 °C, based on the above FeCrAl-RE life model. These modelling assumptions are based on the rate of growth of a-alumina scales, however, at operating temperatures in the range 780 °C–910 °C it is more likely that transition aluminas will form. This aspect has been extensively studied. One elegant study by Molins et al [25] on a Ugine Savoie alloy examined the oxidation kinetics over the temperature range 850 °C– 1100 °C in flowing synthetic air. The samples were foils 45mm thick. The results obtained are reproduced in Figure 7, as relative mass gain against time (the norm is taken to be 168h at 950 °C).

Figure 6: Arrhenius plot of the parabolic rate constant as determined from TGA studies, for various FeCrAl-RE alloys, over the temperature range 750-1350 °C [24].

41

Figure 7: Oxidation kinetics for a Fe20Cr5.2Al0.01Ce alloy over the temperature range 850 °C–1100 °C [25]

This research shows that two domains exists, at low temperatures (T=0.01 5% 0.1% >=0.1 5% 0.1% >=1.0 5% 0.1%

Aluchrom Yhf - 1300C Aluchrom Yhf - 1300C Kanthal AF - 1300C Kanthal AF - 1300C Kanthal AF - 1200C Kanthal APM - 1300C

1.0E+03

Kanthal APM - 1300C Kanthal APM - 1200C

1.0E+02

PM2000 - 1300C PM2000 - 1300C

1.0E+01 InCF > = 0.0

1.0E+00 0.001

C0=5.5 wt.% CB=1.7 wt.% (MICF) CB=0.0 wt.% (InCF)

0.010

0.100

Volume/Surface Area [mm]

1.000

10.000

PM2000 - 1200C PM2000 - 1200C (Improve) Aluchrom Yhf 1200C (Iso) Kanthal APM 1200C (Iso) PM2000 1200C (Iso)

Figure 9: Stochastic life prediction model for the chemical failure of FeCrAl-RE, alumina forming alloys [4]

46 Thus, chemical failure reflects a balance between the available aluminium reservoir and its rate of consumption due to a complex interplay of oxidation parameters. Key among these is the oxide rate constant, which may follow parabolic or sub-parabolic kinetics, and for thicker sectioned components, or highly constrained geometries, the critical oxide thickness to spall. It has been proposed that the (k × tB) product is a temperature independent parameter, that defines the lifetimes of an alumina forming ferritic steels, whether a foil or sheet materials [4]. Figure 9 presents a plot of (kp × tB) against V/A ratio for a range of alumina forming ferritic steels over the temperature range 1050 °C–1400 °C, confirm the hypothesis that the (kp × tB) product may be used to provide a temperature independent estimate of component life. In Figure 9 alloy lives between a few tens of hours and 20,000 h are plotted, superimposed on the plot are statistical corrosion models, that define the risk of failure [4], two levels of risk are plotted: a 5% chance of failure and a 0.1% chance of failure assuming oxidation follows parabolic kinetics. All foil samples were observed to fail by intrinsic chemical failure (InCF), when unconstrained. A life model, based on intrinsic chemical failure (> = 0.0; CB = 0.0wt%) and parabolic kinetics, provides a conservative estimate of foil component life for a range of FeCrAl-RE materials over the temperature range 1100 °C–1400 °C.

6

References

[1] World Energy Outlook, 2000, International Energy Agency. [2] W. Maus ‘Mobility, Prosperity and Environment Protection – the Catalytic Converter is Indispensable’, in “Metal-Supported Automotive Catalytic Converters” (ed. H. Bode) p3– 13, Werkstoff-Informationsgesellschaft, Frankfurt, Germany (1997). [3] H. Bode (ed) “Metal-Supported Automotive Catalytic Converters” (ed. H. Bode) p3–13, Werkstoff-Informationsgesellschaft, Frankfurt, Germany (1997). [4] J. R. Nicholls, R. Newton, M. J. Bennett, H. E. Evans, H. Al-Badairy, G. Tatlock, D. Naumenko, W. J. Quadakkers, G. Strehl and G. Borchardt, ‘Development of a Life Prediction Model for the Chemical Failure of FeCrAlRE Alloys in Oxidising Environments, “Life Modelling of High Temperature Corrosion Processes’, (eds M. Schutze, W. JH. Quadakkers and J. R. Nicholls) EFC, Publication 28, IoM Communications 2001. [5] H. Bodes ‘Development Status of Materials for Metal Supported Automotive Catalysts’, in “Metal-Supported Automotive Catalytic Converters” (ed. H. Bode) p3–13, WerkstoffInformationsgesellschaft, Frankfurt, Germany (1997). [6] B. H. Engler “Katalysatoren fur den Umweltschutz”, Chem-Ing.Tech. 63, 298–312 (1991); cited in reference 5. [7] T. Nagel and W. Maus “Development of More Exacting Test Conditions for close Coupled Converter Applications”, in Metal-Supported Automotive Catalytic Converters” (ed. H. Bode) p107–126, Werkstoff-Informationsgesellschaft, Frankfurt, Germany (1997). [8] E. Lang (ed) “The Role of Active Elements in the Oxidation Behaviour of High Temperature Materials and Alloys” Elsevier Applied Science, London (1989). [9] M. J. Bennett and G. W. Lorimer (eds), “Microscopy of Oxidation”, Institute of Metals, London (1991). [10] S. B. Newcomb and M. J. Bennett (eds), “Microscopy of Oxidation-2” Institute of Materials, London, 1993.

47 [11] D. Coutsouradis et al (eds) Proc. “Materials for Advanced Power Engineering”, COST 501, Kluwer Academic Publishers, Dordrecht, Netherlands (1994). [12] S. B. Newcomb and J. A. Little (eds) “Microscopy of Oxidation-3”, Institute of Materials, London, 1997. [13] D. A. Shore, R. A. Rapp and P. Y. Hou (eds), Int. Conf. on “Fundamental Aspects of High Temperature Corrosion”, The Electrochemical Soc., USA, (1997). [14] J. Lecomte-Beckers et al (eds), Proc. “Materials for Advanced Power Engineering 1998”, Forschungszentrum, Julich, Germany (1998). [15] M. Schutze and W. J. Quadakkers (eds) “Cyclic Oxidation of High Temperature Materials”, EFC Publication 27, IoM Communications, London, (1999). [16] G. Tatlock and S. Newcomb (eds), Special Issue of Material at High Temperatures on “Microscopy of Oxidation-4” Vol. 17(1) (2000). [17] M. Schutze, W.J. Quadakkers and J. R. Nicholls (eds) “Life-time Modelling of High Temperature Corrosion Processes”, EFC Publication 28, IoM Communications, London (2001). [18] W. J. Quadakkers and K. Bongartz, Werst. U. Korros. 24, 232, (1994). [19] W. J. Quadakkers, K. Bongartz and F. Schutbert in Proc. “Materials for Advanced Power Engineering”, COST 501, (eds D. Coutsouradis et al) part II, p1533, Kluwer Academic Publishers (1994). [20] J. R. Nicholls and M. J. Bennett, “Cyclic Oxidation-guidelines for Test Standardisation aimed at the Assessment of Service Behaviour”, European Federation of Corrosion Publications Vol. 27, pp437–470 (1999). [21] R. Newton, M. J. Bennett, J. P. Wilber, J. R Nicholls, D. Naumenko, W. J. Quadakkers, H. Al-Badiary, G. Tatlock, G. Strehl, G. Borchardt, A. Kolb-Telieps, B. Jonsson, A. Westerlund, V. Guttmann, M. Maier and P. Beaven, “The Oxidation Lifetime of Commercial FeCrAlRE Alloys” in ‘Life Modelling of High Temperature Corrosion Processes’, (eds. M. Schutze, W. J. Quadakkers and J. R. Nicholls) EFC Publication 28, IoM Communications 2001. [22] G. Strehl, V. Guttmann, D. Naumenko, A. Kolb-Telieps, G. Borchardt, J. Klower, P. Beaven, J. R.Nicholls, “The Influences of Sample Geometry on the Oxidation and Chemical Failure of FeCrAl(RE) Alloys”, Life Modelling of High Temperature Corrosion Processes’, (eds M. Schutze, W. J. Quadakkers and J. R. Nicholls), EFC Publication 28, IoM Communications, 2001. [23] H. Al-Badairy, G. J. Tatlock and J. LeCoze, “An Auger Study of Thermally Spalled Oxides on Fe-20Cr-5Al Based Alloys” in “Microscopy of Oxidation-3” (eds S. Newcomb and J. A. Little) p105, Institute of Materials, London (1997). [24] M. Gobel, J. Schimmelpfennig, A. Glazkow, G. Borchardt, “Growth of a-alumina Scales on Fe-Cr-Al Alloys” in Metal-Supported Automotive Catalytic Converters” (ed. H. Bode) p191, Werkstoff-Informationsgesellschaft, Frankfurt, Germany (1997). [25] R. Molins, A. Germidis and E. Andrieu “Oxidation of Thin FeCrAl Strips: Kinetic and Microstructural Studies in “Microscopy of Oxidation-3”, (ed. S. B. Newcomb and J. A. Little, p3, Institute of Materials, London (1997). [26] A. Kolb-Telieps, U. Miller, H. Al-Badairy, G. Borchardt, G. Tatlock, D. Naumenko, W. J. Quadakkers, G. Strehl, R. Newton, J. R. Nicholls and V. Guttmann, “The Role of Bioxidant Corrodants on the Life Time Behaviour of FeCrAlRE Alloys” ‘Life Modelling of

48 High Temperature Corrosion Processes’, (eds M. Schutze, W. J. Quadakkers and J. R. Nicholls), EFC Publication 28, IoM Communications, 2001. [27] G. J. Tatlock and H. Al-Badairy “The Oxidation of Thin Foils of FeCrAl-RE Alloys in Moist Air”, accepted for publication in Materials at High Temperatures (2001). [28] M. J. Bennett, R. Perkins, J. B. Price and F. Starr in Proc. “Materials for Advanced Power Engineering”, COST 501 (eds D. Coutsouradis et al), p1553, Kluwer Academic Publishers (1994). [29] J. Klower and A. Kolb-Telieps, “Effect of Aluminium and Reactive Elements on the Oxidation Behaviour of Thin FeCrAl Foils in “Metal-Supported Automotive Catalytic Converters” (eds H. Bode), p33, Werkstoff-Informationsgesellschaft, Frankfurt, Germany, (1997). [30] J. Klower, Materials and Corrosion, 49, 758 (1998). [31] A. B. Smith, A. Kempster and J. Smith, “Characterisation of Aluminide Coatings formed on Nickel Based Superalloys by Vapour Aluminising” in “High Temperature Surface Engineering” (eds J. R. Nicholls and D. S. Rickerby) p13, IoM Communications, London (2000). [32] L. Vandenbulcke, G. Leprince and B. Nciri, “Low Pressure Gas-Phase Pack Cementation Coating of Complex-Shaped Alloy Surfaces”, Materials Science and Engineering, A121, 379 ff, (1989). [33] W. J. Quadakkers, T. Malkow, H. Nickel and a. Czyrska-Filemonowics in Proc. 2nd Int. Conf on “Heat Resisting Materials”, Gatlinburg USA, p91, ASM International, Ohio, USA (1999). [34] J. G. Smeggil, “Some Contents on the Role of Y in Protective Oxide Scale Adherence”, Materials Science and Eng. 87, 261 (1987). [35] J. L. Smialek, “Sulphur Impurities and the Microstructure of Alumina Scales” in “Microscopy of Oxidation-3” (eds S. B. Newcomb and J. A. Little), p127, Institute of Materials, London, (1997). [36] B. Pint, Oxid. Met. 45, 1 (1996). [37] J. Klower and J. G. Li, Materials and Corrosion 47, 545 (1996). [38] P. A. Van Manen, E. W. A. Young, D. Schlakoord, C. J. an der Wekken and J. H. W. de Wit, Surface and Interface Analysis 12, 391 (1988). [39] W. J. Quadakkers and L. Singheiser, “Practical Aspects of the Reactive Element Effect”, in “High Temperature Corrosion”, Les Embiez, France, (May 2000). [40] D. Naumenko, W. J. Quadakkers, P. Beaven, H. Al-Badairy, G. Tatlock, R. Newton, J. R. Nicholls, G. Strehl, G. Borchardt, J. Le Coze, B. Jonsson, A. Westerlund, “Critical Role of Minor Elemental Constituents on the Lifetime Oxidation Behaviour of FeCrAl-RE Alloys”, “Life Modelling of High Temperature Corrosion Processes”, (Eds. M. Schutze, W. J. Quadakkers and J. R. Nicholls) EFC Publication 28, IoM Communications 2001. [41] H. Al-Badairy, G. Tatlock, H. E. Evans, G. Strehl, G. Borchardt, R. Newton, M. J. Bennett, J. R. Nicholls, D. Naumenko and W. J. Quadakkers, “Mechanistic Understanding of Chemical Failure for FeCrAl-RE Alloys in Oxidising Environments” in “Lifetime Modelling of High Temperature Corrosion Processes”, (eds M. Schutze, W. J. Quadakkers and J. R. Nicholls) EFC publication 28, IoM Communications, London (2001).

High Temperature Corrosion of FeCrAlY/Aluchrom YHf in Environments Relevant to Exhaust Gas Systems Angelika Kolb-Telieps1), Gernot Strehl2) , Dmitry Naumenko3), Willem. J. Quadakkers3) , Rachel Newton4) 1)

Krupp VDM GmbH, Kleffstr. 23, 58762 Altena, Germany TU Clausthal, Institut f³r Allgemeine Metallurgie, Robert-Koch-Str. 42, 38678 Clausthal-Zellerfeld, Germany 3) Forschungszentrum Juelich, IWV-2, 52425 J³lich, Germany 4) Cranfield University, SIMS, Cranfield, Bedfordshire, MK43 0AL, UK 2)

1

Abstract

Fe-20Cr-5.5Al-Y/Aluchrom YHf foils were exposed in air, N2 + 5000vppm NO, a simulated fuel-rich exhaust gas, air + SO2 and air + 50vppm HCl at 1200°C. N2 + NO and the exhaust gas act as shielding gases, a behaviour which probably can be explained by the higher oxygen partial pressure in air compared to that of the mixed gas. Air + 0.3% SO2 leads to earlier breakaway, which is supposed to be induced by internal sulphidation. Air + 50vppm HCl seems to result in the formation of volatile species and active oxidation.

2

Introduction

Assessments of different drive concepts with regard to emissions and weight/cost ratios show that the spark-ignition engine will remain the preferred drive concept during the next years [1]. The necessity of further improving the cold-start efficiency, which is highly dependent on the cell density of the catalyst substrate, leads to the objective to provide the largest possible catalytic surface with the lowest possible heat capacity, as can be seen in Figure 1.

Figure 1: Dependence of cold-start factor (catalytic surface/heat capacity) on cell density and foil thickness [1]

Material Aspects in Automotive Catalytic Converters, Hans Bode Copyright © 2002 Wiley-VCH Verlag GmbH &Co. KGaA ISBN: 3-527-30491-6

50 Increasing the cell density also means to reduce the thickness of the foil of the catalyst substrate, for which iron chromium aluminium alloys with additions of reactive elements proved to be an excellent solution. The chemical composition of the foil has been optimised with respect to its oxidation resistance, especially with respect to thickness [2]. However, for a thickness of 50 Ám or less environmental influences have to be considered in more detail than for the thicker foils. Therefore this paper will compare results gained in air to data obtained in atmospheres relevant to exhaust gas systems.

3

Experimental

The tests were performed on Aluchrom YHf, an FeCrAl alloy with additions of reactive elements. The chemical composition is given in Table 1. 0.05mm × 20mm × 10mm coupons were cut from the foil and exposed to multicomponent corrodants at 1200 °C. Table 1: Chemical compositions (mass%) element Cr (wt.-%) Al (Wt.-%) Y (ppm) C (ppm) S (ppm) O (ppm) N (ppm) P (ppm) Zr (ppm) V (ppm) Ti (ppm) Cu (ppm) Ca (ppm) Hf (ppm) Mn (ppm) Si (ppm) Nb (ppm) Mg (ppm) Mo (ppm)

Aluchrom YHf 19.7 5.5 460 2100 1.3 < 10 40 130 540 860 98 110 12 310 1800 2900 < 50 78 100

The atmospheres (in vol.% res. vppm) were the following: 1. N2 + 5000 vppm NO, which simulated the major NOx components in exhaust gas. The only important NOx compound NO was chosen, since engine temperatures reach values from 900 °C to 1300 °C, where the equilibrium partial pressure of NO is most relevant at levels between 500 ppm – 3000 ppm, as can be seen in Figure 2. The cycle time was 20 hours. 2. A simulated fuel-rich exhaust gas (N2 + 12% CO2 + 2% CO + 10% H2O). The oxygen partial pressure at this temperature was calculated to be about 10 Pa–15 Pa. The cycle time was again 20 hours.

51 3. Foils were exposed in air + 0.3 vol.% SO2. Although a lower SO2 concentration, e.g. of 10 vppm, is more relevant in automotive exhaust systems, air + 0.3 % SO2 was chosen as worst case condition. Figure 3 shows the equilibrium partial pressures for both concentrations. From these calculations an influence of the SO2 enrichment at 1200 °C should not be visible for the 10 vppm. The cycle time was 20 hours. 4. 0.05mm and 1 mm thick samples were discontinuously exposed in air + 50 vppm HCl with 100 hours cycles.

Figure 2: Equilibrium composition of air with 5000 ppm NO in the temperature range 0 °C to 1400 °C, calculated with ChemSage [3]

a) Air + 10 vppm SO2

b) Air + 0.3 vol.% SO2

Figure 3: Equilibrium composition of air with (a) 10 vppm resp. (b) 0.3 vol.% SO2 in the temperature range 0 °C to 1400 °C at 1.013 bar, calculated with FactSage

After every cycle the furnace was cooled down to room temperature and the mass changes of the species were measured. No differences between net and gross mass changes were found in

52 N2 + 5000 vppm NO, N2 + 12% CO2 + 2% CO + 10% H2O and air + SO2. In air + 50 vppm HCl the gross mass change could not be determined due to volatile species. The surface coloration was evaluated and a grey colour was associated with (-alumina, a green colour with chromia and red and black with iron oxides. The microstructure has been characterised by optical and scanning electron microscopy. The test results were compared to those gained in parallel tests performed in laboratory air.

4

Results

4.1

N2+5000 vppm NO

As shown in Figure 4, the oxidation rate in the N2-NO mixture is always smaller than in air but shows a similar development. This is also true for the coloration changing from metallic to grey and then to green before breakaway occurs. No evidence of internal nitridation has emerged.

Figure 4: Comparison of the net mass gain of 50 mm foils after cyclic oxidation in N2 with 5000 vppm NO and in laboratory air at 1200 °C [3]

4.2

Fuel-rich exhaust gas (N2 + 12% CO2 + 2% CO + 10% H2O)

Specimen mass gains are shown in figure 5. The mass increase of the foil was slower in the fuel-rich exhaust gas (N2 + 12% CO2 + 2% CO + 10% H2O) than in air.

53

Figure 5: Specimen mass change in artificial exhaust gas at 1200 °C [3]

4.3

Air + 0.3 vol.% SO2

Figure 6 indicates little effect on growth kinetics when adding 0.3 vol.% SO2 to the air environment. But the onset of breakaway at 1200 °C occurred after 150 hours in air in comparison to 120 hours in the sulphur dioxide containing environment. Breakaway occurred with the formation of chromia underlying the alumina scale, and later non-protective iron oxide formation, which resulted in a rapid increase of mass gain.

Figure 6: Comparison between the gross mass gain in air + 0.3% SO2 with that in laboratory air at 1200 °C [3]

4.4

Air + 50 vppm HCl

As can be seen from Figure 7a, the net mass gain at 1200 °C is lower in the HCl containing gas than in air. In contrast to the tests performed in laboratory air, HCl induces spallation in all samples (see Figure 7b). Small red and black spots can be recognised on the surfaces after relatively short exposure times in the HCl containing gas.

54

a)

b)

Figure 7: a) Net mass gain in air and air+50 vppm HCl at 1200 °C [3]; b) cross section of 1 mm thick Aluchrom YHf after 100 hours

5

Discussion

The nitrogen-oxygen-bioxidant and the synthetic exhaust gas retard the breakaway in comparison to exposures in air. In these cases the alumina scale, which could grow due to sufficiently high oxygen levels, protects the alloys against nitridation [4]. So these environments actually can be used as shielding gases for FeCrAlRE alloys. Similar results in exhaust gas have been found for thicker species by Sigler [5] and have been attributed to different oxide morphologies. The oxidation mechanisms are the same in these environments and in air. The formation and almost parabolic growth of alumina is followed by that of chromia and subsequently iron oxides, which then leads to chemical breakaway failure. The experiments show that the growth rates of the aluminium oxide in the two low- pO2 test gases (N2/NOx and exhaust gas) are substantially slower than in air. Two-stage oxidation studies using 18O-Tracer [7] have shown that gas tight alumina scales on FeCrAl alloys containing reactive elements grow by oxygen grain boundary diffusion. For scales exhibiting this growth mechanism, the relation between scale thickness x and oxidation time t is given by:

x2 = k p × t

(1)

The parabolic rate constant kp has been calculated in /8,9/ to be

kp =

4@ DB æ DmO2 ö ç ÷ r è RT ø

(2)

55 with DB the oxygen grain boundary diffusion coefficient along grain boundaries with a width @and a grain diameter of r , DmO2 the chemical potential gradient across the oxide scale, R the gas constant and T the temperature. Previous experiments [7,8] have shown that in an alumina scale with optimum protective properties the grains develop a columnar type structure whereby the grain size tends to increase in growth direction. Thus, the grain size at the scale-metal interface increases with increasing scale thickness, i.e. in the above equation r = r(x). Based on theoretical considerations and experimental observations it was previously shown [7,8] that this increase in grain size can with reasonable accuracy be described by the relation r = r0 × x p

(3)

whereby p is close to one and r0 is the initial grain size, when diffusion along grain boundaries becomes the rate determining process. The scale thickness can easily be transformed into æ Dm ö mass gain ç ÷ using a factor f, because this is the common way to measure oxidation kiè A ø netics. x = 0.53374

mm Dm Dm = f g / m2 A A

(4)

The equations (1) to (4) can now be combined to achieve a comprehensive understanding of the oxidation process.

x2 = k p × t x2 =

4d DB æ DmO2 ç r è RT

Û ö æ DmO2 ÷×t = ç ø è RT

ö 4d DB ÷ ×t × p ø r0 × x

æ 4d DB ö x2+ p = ç ÷ DmO2 t è r0 RT ø

Û Û

1/ 2 + p

x= f

Dm æ 4d DB ö =ç ÷ A è r0 RT ø

Using the definitions

Dm 1/ n =  kct  . A

1/ 2 + p

 Dm t  O2

DmO2 4d DB 1 1 the growth law simplifies to: = and kc = f n r0 RT n 2+ p (5)

For the tested foils p equals one. This reveals a cubic time dependence for the scale thickening and thus a scale thickness dependence on the oxygen partial pressure

x : DmO1/2 2 + p = DmO1/23

(6)

The chemical potential gradient across the scale is proportional to the difference between the logarithm of the oxygen activities at the scale/alloy and the scale/oxide interface, respectively.

56 DmO2 = RT  ln aO2 |surface - ln aO2 |int erface 

(7)

The activity of oxygen at the surface is given by the oxygen partial pressure in the surrounding atmosphere. The oxygen activity at the scale/alloy interface has been calculated from thermodynamic data [10] and activity coefficients measured in the LEAFA [3] project to be 2.305 × 10–25 at 1200 °C. Table 2 gives the oxygen partial pressures for the tested atmospheres and the expected retardation in the oxide growth. Table 2: Oxygen partial pressures for the tested atmospheres and the expected retardation in the oxide growth

atmosphere air N2-NO exhaust gas

aO2 0.21 2.5 × 10–3 1 × 10–10

,mO2 / kJmol–1 675.5 621.6 412.8

retardation 1 0.9 0.6

The retardation factor is calculated assuming that breakaway oxidation in air and in the reduced oxygen partial pressure atmosphere occurs for the same mass gain. 1/ n Dm air air 1/ n  kc tB  =  kcatmospheretBatmosphere  Û kcairtBair = kcatmospheretBatmosphere A

retardation =

t Bair t Batmosphere

(8)

atmosphere

=

kcatmosphere DmO2 = kcair DmO2

air

(9)

In Table 3 the calculated data are compared to the measured data. They are the same for the exhaust gas but differ for N2+NO. However, this difference might be explained by the cycle time. The data gained in air were measured with 20 hours cycles whereas those measured in the bioxidant refer to 100 hours cycles. Table 3: Comparison of calculated and measured retardation factors after 100 and 150 hours exposure

atmosphere N2-NO exhaust gas

calculated retardation 0.9 0.6

measured retardation after 100 and 150 hours 0.8 0.6

Results gained for PM 2000, a Plansee FeCrAl alloy, support this idea. Isothermal thermogravimetric tests were performed in Ar + 20% O2 and in Ar + 4% H2 + 7% H2O at 1200 °C. Figure 8 shows the retarded gross mass gain in Ar + 4% H2 + 7% H2O, the atmosphere with the lower oxygen partial pressure. The exponential fits of these data resulted in k values of 0.31 mg2cm4/h for Ar + O2 and 0.19 mg2cm4/h for Ar + H2 + H2O. This method which allows to predict results from retardation factors is not applicable for atmosphere where volatile species or inclusions occur, like in air + HCl or air + SO2. A similar oxidation mechanism is found in air and in air + SO2, but 0.3% SO2 induce earlier breakaway. The SO2 bearing environment affects the oxidation behaviour dramatically by internal sulphidation. The sulphidation can also be explained by the oxygen partial pressures of

57 SO2 and SO2 at 1200 °C, which are shown in Figure 3. Since SO2 contents more realistic in automotive exhaust systems are in the range of 10 vppm, the conclusion is that SO2 has not to be considered for oxidation processes in vehicles with spark-ignition engines.

Figure 8: Gross mass gain of PM 2000 in atmospheres with different oxgen partial pressures gained at 1200 °C

In the above mentioned environments the species fail due to chemical breakaway when most of the aluminium is consumed and other oxides types form. Therefore for the species with 5.5 mass% Al, the foil thickness is critical. Spallation was only noticed on thicker coupons. However, the growth of alumina in the HCl containing atmosphere is not parabolic. Even for the foils the alumina scale spalled. The mechanistic understanding for the behaviour of species in HCl containing atmospheres is based on the observation, that an influence is visible before the first cracking or spalling of the alumina scale occurs. Furthermore very fine spall was found early on and also the scale adherence in some species seems to be weaker. Red and black spots on top of the alumina indicate that iron is involved in the process, but not chromium, because green spots are missing. Together with the oxygen HCl can react at 1200 °C to H2O and Cl2. Similar reactions have already been found at 600 °C [6]. This might promote the formation of volatile chlorides. These compounds deteriorate the alumina scale, which in turn allows access of the gaseous species O2, HCl, H2O and Cl2 to the metal oxide interface. Thermodynamical calculations revealed that the chlorides with the highest fugacity at 1200 °C are aluminium chlorides. These ideas are confirmed by the observation that the processes accelerate as soon as the first cracks appear in the alumina.

6

Conclusions

For wrought Aluchrom YHf N2 + 5000 vppm NO and N2 + 12% CO2 + 2% CO + 10% H2O act as shielding gases at 1200 °C. An explanation for this behaviour is thought to be the higher oxygen partial pressure in air compared to that of the mixed gas.

58 Air + 0.3 vol.% SO2 leads to earlier breakaway due to sulphidation. Air + 50 vppm HCl seems to result in the formation of volatile aluminium chlorides and active oxidation at 1200 °C.

7

Acknowledgements

We are grateful to the European Commission for financial support under the LEAFA project no. BRPR-CT97-0562 and to our partners for the supply of the alloys tested, for the chemical analysis of alloys and for their contribution to scientific input in discussing these results. We also want to thank Dr. K. Hack, Gesellschaft für Technische Thermochemie und -physik, Herzogenrath, for the calculations of figure 3.

8 [1] [2]

References

W. Maus, R. Brück, G. Holy, Int. Congress in Graz, Sept. 2–3, 1999 J. Klüwer, A. Kolb-Telieps, M. Brede, Int. Conf. MACC ’97 in Wuppertal, Oct. 27–28, 1997 [3] BRITE-EURAM project, Contract no. BRPR-CT97-0562 [4] M.J. Bennett, R. Newton, J.R. Nicholls, Eurocorr 2000, available as CD [5] D.R. Sigler, Oxidation of Metals, Vol. 40, No.5/6, 1991, p. 555–583 [6] A. Zahs, M. Spiegel, H.J. Grabke, Materials and Corrosion 50, 1999, p. 561–578 [7] W.J. Quadakkers, H. Holzbrecher, K.G. Briefs, H. Beske, Oxidation of Metals 32, (1/2) (1989), 67–88 [8] K. Bongartz, W.J. Quadakkers, J.P. Pfeifer, J.S. Becker, Surface Science 292 (1993) 196–208 [9] S.N. Basu, J.W. Halloran, Oxidation of Metals 27 (1987) 143 [10] Ihsan Barin, Thermochemical Data of Pure Substances, Weinheim, Basel, Cambridge, New York, VCH, 1989

Improved High Temperature Oxidation Resistance of REM Added Fe-20%Cr-5%Al Alloy by Pre-Annealing Treatment K. Fukuda, K. Takao, T. Hoshi and O. Furukimi Technical Research Laboratories, Kawasaki Steel Corp., Chiba (Japan)

1

Introduction

Fe-Cr-Al alloys exhibit outstanding oxidation resistance at high temperatures because a protective α-Al2O3 scale forms on their surface. As a practical application of this good oxidation resistance, Fe-20mass%Cr-5mass%Al alloy foils have recently been used as a catalytic converter substrate for automobiles, in which the material is exposed to high temperature exhaust gas. (1)-(3) The thickness of these foils is usually limited to 30 µm–50 µm so as not to increase the back pressure in the exhaust system. Since the surface-to-volume ratio of foils is higher than that of thicker alloy sheets, the Al in the foils is consumed as Al2O3 in a shorter time than in sheets. Therefore, reducing the growth rate of Al2O3 scale is important for this application. It is widely known that the addition of reactive elements such as rare earth metals (REM), Ti, Zr, and Hf to these alloys improves their oxidation resistance by preventing spalling of the Al2O3 scale.(4)-(7) Considered these studies, Fe-20Cr-5Al-La-Zr alloy foils have been widely used for catalytic converter substrates. Foils for this application are usually supplied as-cold rolled or after pre-annealing in a reducing atmosphere. However, the effect of pre-annealing conditions on the growth rate of Al2O3 scale formed on Fe-20Cr-5Al with the addition of reactive elements such as rare earth metals (REM), Zr, and Hf alloys had not been clarified. Therefore, in this study, the effect of pre-annealing in a hydrogen atmosphere on improvement of high temperature oxidation resistance in Fe-20mass%Cr-5mass%Al alloy foils with a small content of La-Zr or La-Hf was investigated.

2

Experimental Procedure

2.1

Specimen Preparation

The chemical compositions of the alloys used in this study are shown in Table 1. The basic composition was Fe-20mass%Cr-5mass%Al with a small addition of La, La-Zr, or La-Hf. All the alloys were melted in a vacuum induction furnace and cast as 10kg ingots, and were then hot-rolled to 3mm thick plate. These plates were annealing and cold-rolled to foils with a thickness of 50 mm or 300 mm. Some of the foils were annealed at 1223K and polished with #600 SiC paper, and some of the polished foils were also pre-annealed in hydrogen gas or air at 1223K for 60 seconds. These foil specimens were cut into coupons with a size of 20–30mm, which were degreased in acetone and in alcohol before oxidation. Alloy specimens with a

Material Aspects in Automotive Catalytic Converters, Hans Bode Copyright © 2002 Wiley-VCH Verlag GmbH &Co. KGaA ISBN: 3-527-30491-6

60 thickness of 300 mm were used for observation by scanning electron microscope (SEM) and transmission electron microscope (TEM). Table 1: Chemical compositions of specimens Alloy A B C D E F 2.2

Cr (mass%) 19.8 20.0 20.1 20.1 20.1 20.6

Al (mass%) 5.5 5.7 5.6 5.6 5.7 5.6

La (mass%) 0.024 0.045 0.082 0.098 0.088 0.072

Zr (mass%)

Hf (mass%)

0.029 0.032

Oxidation tests

Oxidation tests were carried out in air at 1373 K or 1423 K using an electric furnace. The mass change was measured by weighing the specimens at certain time intervals after cooling to room temperature. 2.3

Analysis method

The surfaces of the foils after polishing, pre-annealing in hydrogen, and oxidation were investigated by secondary ion mass spectroscopy (SIMS). Mass filtered O2+ primary ions (acceleration voltage: 15 kV) were rastered over areas of 100 × 100, 150 × 150, and 200 × 200 µm on the targets. Specimens of the 300 µm thick sheets of the alloys after continuous oxidation at 1373 K for 172.8 ks were cracked by immersion in liquid nitrogen to observe the cross-section of the scale by SEM. The Al2O3 scale which formed on the specimen foils after continuous oxidation at 1373 K for 172.8 ks was examined with a field emission type TEM equipped with an energy-dispersive X-ray spectrometer (EDX). Specimens of the foils were prepared by mechanical grinding of the alloy substrate and subsequent ion-milling with an Ar gun, aiming at the mid-thickness region of the scale. Specimens of the 300 mm thick sheets of the alloys were prepared by a focused ion beam (FIB) system using Ga-ions to observe the cross-section of the scale.

3

Results and Discussion

3.1

Effect of pre-annealing on oxidation rate

Figure 1 shows the mass change curves of 300 µm thick foil samples of alloy C during oxidation at 1423 K. No spalling of the scale was observed with any of the specimens, and a protective Al2O3 layer formed on all the specimens after the oxidation test. The mass gain results were virtually the same with the as-cold rolled, polished, and pre-annealed in air specimens. The mass gain of the specimen which was pre-annealed in hydrogen gas was lower than that of the other specimens. Figure 2 shows the oxidation gain after oxidation in air at 1423K for 86.4ks with 300 µm thick foil samples of alloys A, B, C, and D with several La contents in the as-cold rolled con-

61 dition and with pre-annealing in hydrogen. The oxidation gain of the as-cold rolled specimens decreased as the La content increased up to about 0.04 wt%, and remained approximately constant 5.0 (g/m2) above about 0.04 wt%. In contrast, the oxidation gain of the samples which were pre-annealed in hydrogen continued to decrease as the La content increased even at values of more than 0.04 wt%. This demonstrates that pre-annealing treatment in hydrogen gas is more effective for decreasing the oxidation rate as the La content increases.

Mass gain (g/m2)

20.0

15.0

as rolled annealed in air polished after annealing annealed in hydrogen

10.0

5.0

0 0

200

400

600

800

Oxidation time (ks) Figure 1: Effect of annealing treatment on oxidation behavior of 300 µm thick sheets of alloy C at 1423K in air

Mass gain (g/m2)

10 as rolled annealed in hydrogen

8 6 4 2 0 0.00

0.02

0.04

0.06

0.08

0.10

La content (mass%) Figure 2: Effect of La content and hydrogen annealing on mass gain after oxidation in air at 1423K for 86.4ks

Figure 3 shows the mass change curves during oxidation at 1373K with 50 µm thick foil samples of alloys C, E, and F in the as-cold rolled condition and with pre-annealing in hydrogen. The oxidation gain results of the as-cold rolled alloys E (La + Zr) and F (La + Hf) were smaller than that of alloy C, which contained only La. The oxidation gain of these La + Zr and La + Hf co-added alloys was also smaller when the samples were given pre-annealing treatment in hydrogen.

62

5

100 (b)

Alloy C as rolled Alloy E as rolled Alloy F as rolled Alloy C annealed in hydrogen Alloy E annealed in hydrogen Alloy F annealed in hydrogen

Mass gain (g/m2)

10

(a)

2

Mass gain (g/m2)

15

Alloy C as rolled Alloy E as rolled Alloy F as rolled Alloy C annealed in hydrogen Alloy E annealed in hydrogen Alloy F annealed in hydrogen

10

0.4 0 0

200

400 Time(ks)

600

800

1 10

100 Time (ks)

1000

Figure 3: Effect of hydrogen pre-annealing annealing on oxidation behavior of 50 µm thick foils of alloy C, E, and F at 1373K in air.

The mass change curves of the 50 mm thick alloys during oxidation at 1373 K are expressed in a log-log plot in Figure 3(b) in order to understand the kinetics. The slope of each sample is about 0.4. This means that the oxidation curve basically conforms to the parabolic rate law. 3.2

SIMS analysis

Figure 4 shows the depth profiles of 50 µm thick foils of alloy E in the as-cold rolled condition and with pre-annealing in hydrogen as obtained by SIMS. No apparent peak was detected near the surface of the as-cold rolled foil. However, with the pre-annealed foil, the intensities of Al, La, and Zr secondary ions near the surface of the foil were stronger than that inside the substrate. This indicates that a thin layer of Al2O3 which contained La and Zr had formed on the surface as a result of the pre-annealing treatment. Figure 5 shows the depth profiles after oxidation at 1373 K for 10.8 ks for the 50 µm thick foils of alloy E in the as-cold rolled condition and with pre-annealing in hydrogen as obtained by SIMS. With the as-cold rolled sample, the intensities of Cr and Fe were high at the outer side of the oxide layer. This indicates that the Al2O3 which formed on the as-cold rolled foil contained Fe and Cr oxides. However, with the foils which were pre-annealed in hydrogen, no apparent peaks of Fe or Cr were detected. This means that the Al2O3 which formed on the alloy pre-annealed in hydrogen contained little Fe or Cr.

63 109

(b) annealed in hydrogen

(a) as cold rolled

Secondary ion intensity (counts)

108

27Al

27Al

53Cr

53Cr

107 106 105 104

57Fe

57Fe

103

16O

102

139La

16O 139La

101 100

90Zr 0

500

1000 Time(s)

1500

2000 0

500

1000

1500

90Zr 2000

Time(s)

Figure 4: Depth profiles of 50 µm thick foils of alloy E as obtained by SIMS. (a) As-cold rolled (b) annealed in hydrogen at 1223 K for 60 s in hydrogen; contined litle Fe or CR

109

(a) as cold rolled

27Al

27Al

108 Se co nd ary ion int ens ity (co unt s)

(b) annealed in hydrogen

107 106 105 104

53Cr

53Cr

57Fe

16O

16O 139La

103 102

57Fe 139La

90Zr 90Zr

101 100 0

2000

4000 Time(s)

6000

8000 0

2000

4000 Time(s)

6000

8000

Figure 5: Depth profiles of 50 µm thick foils of alloy E after oxidation at 1373 K for 10.8 ks as obtained by SIMS. (a) As-cold rolled (b) annealed in hydrogen at 1223 K for 60 s before oxidizing

64 3.3

SEM observation

Figure 6 shows cross-sectional SEM images of the Al2O3 scale which formed on the specimens of alloys E and F in the as-cold rolled condition and with pre-annealing in hydrogen after oxidation at 1373 K for 172.8 ks in air. The alloy/scale interface was smooth with no voids at the interface. The Al2O3 scale which formed on the cold rolled foil consisted of two layers. The outer layer was about 0.5 µm in thickness and showed a morphology characterized by equiaxed grains. The inner layer was about 2.0 µm in thickness and had a columnar grain morphology, which is the same morphology as that reported by Golightly et al. (8) In as-cold rolled foils, Al2O3 scale forms in equiaxed grains during the initial oxidation period and then grows by forming columnar grains. On the other hand, the Al2O3 scale which formed on foil that had been pre-annealed in hydrogen consisted of only one layer, which was approximately 1.5um in thickness and had a columnar grain morphology. (a)

(b)

2.0um (c)

2.0um (d)

2.0um

2.0um

Figure 6: Cross-sectional SEM images of Al2O3 scale formed on alloy E and F after oxidation at 1373 K for 86.4 ks in air; (a) and (b) as-cold rolled , (c) and (d) annealed in hydrogen at 1223 K for 60 s before oxidizing

3.4

TEM observation

Figure 7 shows the results of TEM observation of the Al2O3 scale which formed on alloys E and F in the as-cold rolled condition and with pre-annealing in hydrogen after the specimens were oxidized at 1373K for 86.4ks in air. No secondary phases were apparent at the grain boundaries, as shown in Figure 7. As can be seen in this figure, the Al2O3 scale which formed on the as-cold rolled foil consisted of two layers, an outer layer of equiaxed grains and an inner layer of columnar grains. In contrast, the Al2O3 scale which formed on the foil that was preannealed in hydrogen consisted of only one layer and had a columnar grain morphology. Moreover, the size of the columnar grains of Al2O3 which formed on the pre-annealed foil was slightly larger than that of the Al2O3 grains which formed on the cold rolled foil.

65

(a)

(b)

0.5um (c)

0.5um (d)

0.5um

0.5um

Intensity (counts)

Figure 7: Cross-sectional TEM images of Al2O3 scale formed on alloy E and F after oxidation at 1373 K for 86.4 ks in air; (a) and (b) as-cold rolled, (c) and (d) annealed in hydrogen at 1223 K for 60 s before oxidizing

X-ray intensity ratio La-L/Al-K, Zr-L/Al-K

(a)

0.06 0.05

(c)

0.04 0.03

50 (b) 40 Al-K 30 Fe-K 20 O-K Cr-K 10 Zr-L La-L 0 0 2 4 6 8 10 X-ray energy, E (keV)

La/Al Zr/Al

0.02 0.01 0

40 30 20 10 0

10 20 30 40

Distance from grain boundary, d / nm Figure 8: (a) TEM image of grain boundary in parallel section of Al2O3 scale formed on alloy E after oxidation at 1373 K for 86.4 ks in air; (b) EDX spectrum from grain boundary in parallel section of Al2O3 scale formed on alloy E and (c) X-ray intensity ratios of La-L/Al-K and Zr-L/Al-K in EDX spectrum across grain boundary

66 One of the authors has reported previously that the segregation of La, Zr, and Hf at grain boundaries in A l 2 O 3 scale suppresses oxygen diffusion along the Al2O3 grain boundaries, resulting in a decrease in the growth rate of the Al2O3 scale. (9) In the present study, a distinct La-L intensity and Zr-L peak were also detected at each grain boundary by EDX analysis, as shown in Figure8 (b). In Figure 8(c), the X-ray intensity ratios of La-L to Al-K and Zr-L to AlK were taken from the points indicated by the dots in Figure 8(a) and plotted against the distance from one grain boundary. The results revealed that La and Zr had segregated to the grain boundary. Figure 9(a) and 9(b) show TEM images of the grain boundary in a parallel section of the Al2O3 scale formed on alloy F in the as-cold rolled condition and with pre-annealing in hydrogen after the specimens were oxidized at 1373 K for 86.4 ks. In the EDX spectrum from the columnar grain boundary of the Al2O3 scale on the as-cold rolled specimen of alloy F, a distinct Fe-L intensity and Cr-L intensity peak were detected, as shown in Figure 9(c). Moreover, from the equiaxed grain boundary of the Al2O3 scale on alloy F with pre-annealing in hydrogen, LaL intensity and an Hf-L intensity peak were detected, but an Fe-L intensity and Cr-L intensity peak were not detected, as shown in Figure 9(d). These results were in good agreement with the results obtained by SIMS. (b)

(a)

0.5um Intensity (counts)

Intensity (counts)

0.5um 50 (c) Al-K 40 Cr-K 30 O-K Fe-K 20 10 0 0 2 4 6 8 10 X-ray energy, E/keV

50 (d) Al-K 40 30 Cr-K 20 O-K La-L Fe-K Hf-L 10 00 2 4 6 8 10 X-ray energy, E/keV

Figure 9: (a) TEM image of grain boundary in parallel section of Al2O3 scale formed on as-cold rolled alloy F; (b) TEM image of grain boundary in parallel section of Al2O3 scale formed on alloy F annealed in hydrogen; (c) EDX spectrum from grain boundary of columnar grain in parallel section of Al2O3 scale formed on as-cold rolled alloy F; (d) EDX spectrum from grain boundary of equiaxed grain in parallel section of Al2O3 scale formed on alloy F annealed in hydrogen

Figure 10 is a plot of the X-ray intensity ratios of La-L, Zr-L, and Hf-L to Al-K at the Al2O3 grain boundary, against the distance from the alloy/scale interface. These results show that the degree of La, Zr, and Hf segregation at the Al2O3 grain boundaries was higher at the columnar grain boundaries than at the equiaxed grain boundaries. In addition, the intensity ratios of La-

67 L, Zr-L, and Hf-L at the Al2O3 c o l u mn a r grain boundaries were stronger with the preannealed alloys than with the as-cold rolled alloys. Based on the results of SEM observation, Golightly has reported that the Al2O3 scale on FeCr-Al alloys with small contents of rare earth metals forms at the alloy/scale interface mainly by inward diffusion of oxygen through the scale. (7) Similarly, based on experiments using an O18 tracer, Reddy has reported that the Al2O3 scale on an Fe-Cr-Al alloy grew by inward diffusion of oxygen through the grain boundary of the Al2O3. (10) In this study, in Fe-Cr-Al alloys with small contents of La-Zr or La-Hf, the alloy/scale interface was smooth with no voids at the interface, as shown in Figure 5. This means that outward diffusion of Al was suppressed, and the Al2O3 scale grew by inward diffusion of oxygen through the Al2O3 scale. Considering the fact that the oxidation curves of the alloys which were pre-annealed in hydrogen basically conformed to the parabolic rate law, it was inferred that the pre-annealing treatment did not change the mechanism of oxidation, but rather suppressed the inward diffusion of oxygen through the Al2O3 scale.

as cold rolled annealed in hydrogen

0.01

00.0 0.5 1.0 1.5 2.0 Distance from interface (um) (b) 0.02 as cold rolled annealed in hydrogen 0.01

0 0.0

0.5

1.0

1.5

2.0

Distance from interface (um)

X-ray intensity ratio , La-L/Al-K

(c)

X-ray intensity ratio , Hf-L/Al-K

X-ray intensity ratio , Zr-L/Al-K

X-ray intensity ratio , La-L/Al-K

(a) 0.02

0.02

as cold rolled annealed in hydrogen

0.01

0

0.0 0.5 1.0 1.5 2.0 Distance from interface (um) (d)

0.02

as cold rolled annealed in hydrogen

0.01

0 0.0

0.5

1.0

1.5

2.0

Distance from interface (um)

Figure 10: X-ray intensity ratios, (a) La-L/Al-K, (b) Zr-L/Al-K, (c) La-L/Al-K, (d) Hf-L/Al-K in EDX spectra from grain boundaries in columnar grain and equiaxed grain oxide layers of Al2O3 scale on alloys E and F

As with the cold rolled alloy specimens, Fe and Cr were oxidized together with Al during the initial oxidation period because the partial pressure of oxygen at the surface was high. The large amount of Fe and Cr oxides in the Al2O3 prevented the Al2O3 from growing by forming columnar grains. Instead, Al2O3 grew by forming small equiaxed grains during the initial oxidation period. The growth of these equiaxed grains then reduced the partial pressure of oxygen at the alloy/scale interface, and as a result, only Al was oxidized. After this point, Al2O3 grew by forming columnar grains at the alloy/scale interface, resulting in a two layer structure, and La, Zr, and/or Hf segregated at the grain boundary.

68 However, when pre-annealing in hydrogen was performed, Fe and Cr were not oxidized in this reducing atmosphere. The Al2O3 oxide, including a small amount of La, Zr, and/or Hf, grew by forming large columnar grains beginning in the initial oxidation period. It may be inferred that the low density of grain boundaries in the Al2O3 and the high segregation of La, Zr, and/or Hf at the Al2O3 grain boundaries suppressed oxygen diffusion along these grain boundaries. For this reason, the pre-annealing treatment in hydrogen reduced the growth rate of the Al2O3 scale.

4

Conclusion

The effect of pre-annealing on the oxidation behavior of Fe-20mass%Cr-5mass%Al alloy foils containing a small amount of La-Zr or La-Hf was examined in a cyclic oxidation test at 1373 K in air. 1. The oxidation rate of these alloys was reduced by pre-annealing in hydrogen. 2. In the Al2O3 scale which formed on the pre-annealed alloys, the outer equiaxed grain layer was thinner and the grain size of the inner columnar grain layer was larger than in the scale which formed on as-cold rolled alloys. 3. The segregation of La, Zr, and/or Hf at the columnar grain boundaries of the Al2O3 scale was higher with the pre-annealed alloys than with as-cold rolled alloys. It may be inferred that the low density of grain boundaries in the Al2O3 and high segregation of La, Zr, and/or Hf in the Al2O3 grain boundaries suppressed oxygen diffusion along these grain boundaries, reducing the growth rate of the Al2O3 scale.

5 [1] [2] [3] [4] [5] [6]

References

S. Isobe: Denki-seikoh, 58 (1987), 104. D. R. Sigler: Oxid. Met, 32 (1989), 337. D. R. Sigler: Oxid. Met, 40 (1993), 555. . A. Golightly, F. H. Stott and G. C. Wood: Oxid. Met, 10 (1976), 163. K. Ishii and T. Kawasaki: J. Japan Inst. Metals, 56(1992), 854. H. Hindam and D. P. Whittle: Proc. 3rd JIM Int. Symp. on High Temperature Corrosion of Metals and Alloys, The Japan Institute of Metals, Supplement to Trans. JIM, 24 (1983), 261. [7] F. A. Golightly, F. H. Stott and G. C. Wood: J. Electrochem. Soc., 126 (1979), 1035. [8] T. A. Ramanarayanan, M. Raghavan and R. Petkovic-Luton: J. Electrochem. Soc., 131(1984), 923. [9] K. Fukuda, K. Ishii, M. Kohno and S. Satoh: Proc. Int. Symp. on High-Temperature Corrosion and Protection 2000, (2000) Hokkaido ISIJ, p. 309. [10] K. P. R. Reddy, J. L. Smialek and A. R. Cooper: Oxid. Metals, 17(1982), 429.

Oxidation Induced Length Change of Thin Gauge Fe-Cr-Al Alloys C. Steve Chang, Leigh Chen, and Bijendra Jha Engineered Materials Solutions, Inc., Attleboro, MA 02703 USA

1

Abstract

Alloys of Fe-20Cr-5Al have been used extensively as the material of choice in metallic catalytic converter substrates. The alloy chemistry has been developed through the last decade to provide the oxidation weight gain resistance that was thought to be adequate. Mainly by the addition of rare earth and active elements, the cyclic oxidation behavior has been improved to the point to meet the regulation requirements on the durability of catalytic converters. The major uncertainty on the understanding of oxidation behaviors of thin gauge Fe-Cr-Al foil is the cause of foil length change. It has been suggested that this length change is one of the causes of buckled honeycomb observed in the fractured substrate. Analysis on the stress and strain of thin gauge Fe-Cr-Al foil under the oxidation condition has been published to account for the length changes due to the oxidation. However, the potential metallurgical factors that control the length change has not been rationalized yet. In this paper we will present (1) the oxidation test results on Fe-Cr-Al foils with different gauge and chemistry, (2) the microstructure evolution and (3) oxide scale development during the oxidation test. The oxidation failure mechanisms will be demonstrated. A phenomenological model to incorporate the alloy chemistry, microstructure and oxide scale will be described to account for the oxidation behaviors of these Fe-Cr-Al alloys.

2.

Introduction

Catalytic converters have become the universal solution for automobiles to meet the emission regulations. The catalytic converters with metallic type substrates have seen ever-wider acceptance because of several advantages over conventional ceramic-based converters[1]. One of the advantages of metallic substrate is the thinner substrate wall (30 to 50 microns) which provides lower backpressure and smaller package. However, the desirable thin gauge of alloy foils limit the usable life of the substrates. This limit is due to the fact that the metallic alloys require the formation of stable, protective scales such as aluminum and chromium oxides to slow down the oxidation of the alloy. The scale acts as a barrier to slow the diffusion of oxidizing agents such as oxygen to reach the alloy. However, the continuous oxidation of Al or Cr to form scale is like a reservoir being continuously drained and eventually the protective elements will be exhausted. At this point, oxygen will be able to react with the rest of the alloys to form the non-protective oxides, which causes the alloy to gain weight rapidly. The rapid and catastrophic oxidation weight gain is usually referred to as breakaway oxidation [2].

Material Aspects in Automotive Catalytic Converters, Hans Bode Copyright © 2002 Wiley-VCH Verlag GmbH &Co. KGaA ISBN: 3-527-30491-6

70 The alloy of choice for the metallic converter substrate has been the ferritic stainless steel with a nominal composition of 20wt% Cr, 5wt% Al and the balance of Fe. The addition of 5wt% Al provides the stable scale for the alloy to be used above 1100°C while the 20wt% Cr provides the oxidation and corrosion resistance from the ambient to where the Al oxidation becomes significant. The cyclic oxidation resistance is improved by the addition of rare earth elements such as Y, La and Ce. The addition of active elements such as Zr, Hf provides further improvement on the oxidation resistance [3]. The oxidation resistance is commonly measured by the amount of sample weight gain due to the scale formation. The oxidation weight gain is important, as it is an indication of the effectiveness of protective scale. The oxidation weight gain for the Fe-20Cr-5Al alloy has seen significant improvement and known to be one of the most oxidation resistant materials. Nonetheless, Fe-Cr-Al foils for the catalytic converter applications have to be dimensionally stable to avoid rupture of the substrate during the service [4]. The source of the dimensional instability has been attributed to the stress between scale and ally, which causes the creep deformation of substrate. Heats of Fe-Cr-Al alloys having identical oxidation weight gain behaviors have shown drastically different dimension changes. This paper summarizes the effort to rationalize the oxidation length change mechanism and attempts to draw a guideline to prevent dimensional instability. On the practical aspect of producing Fe-Cr-Al foils, it has been known that the conversion of Fe-20Cr-5Al alloy to thin gauge has been difficult and contributed to the high cost of the materials. A commercially feasible process [5,6] to produce thin gauge Fe-Cr-Al foils has been developed to address the issue. The process starts out with roll bonding the Fe-Cr alloys (AISI 4xx type ferritic stainless steels) to proper amount of Al alloys to form a three layer composites. The composite coil, which was roll bonded with proper attention so it can be cold rolled to an intermediate thickness. The intermediate thickness was selected to allow a homogenization heat treatment to be conducted at the temperature and time, which are commercially acceptable. After the heat treatment, the strip is cold rolled to provide the desirable temper and finish. Obviously, the advantage of alloying the Al to Fe-Cr by the roll bonding process is to circumvent the limits of Al content and the conversion difficulties[7]. In this study, oxidation tests on the oxidation weight gain and dimension stability were conducted on materials taken from roll bonding produced Fe-Cr-Al alloys. The effects of chemistry and heat treatment on the oxidation behaviors, in particular the dimensional stability is rationalized with a phenomenological model developed from examining the length change behaviors of hundreds of samples. This oxidation length change model will attempt to show that from the synergetic effects of physical (e.g. density and thermal expansion) and metallurgical (e.g. scale adhesion) changes, the various length change behaviors can be accounted for.

3

Experimental Procedure

3.1

Materials

The Fe-Cr-Al alloy foils were produced via the roll bonding process. In brief, Fe-Cr alloy (stainless steel) strips were clad with Al strips on both sides by feeding the strips into a fourhigh rolling mill. The cladding process was developed to apply sufficient reduction to form a well adhere three layer (Al/SS/Al) composite. The roll-bonded composites were cold rolled to

71 an intermediate thickness followed by heat treatment to homogenize the Al layers with the FeCr alloy. A finial cold rolling was applied to reduce the alloyed strip to the finish foil thickness. Fe-Cr-Al alloys with Al content range between 5 to 8 wt% are readily produced by this roll bonding process. Rare earth addition in the manner of La+Ce was accomplished by casting the Fe-Cr alloy with misch-metals. Typical alloy chemistry is shown in Table 1 in weight %. Table 1: Nominal composition of Fe-Cr-Al alloys in wt% C Mn P S Si Cr Ni Al N O La Ce 0.02 0.2 0.02