Surface Engineering for Corrosion and Wear Resistance (matsci)

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SURFACE ENGINEERING FOR CORROSION AND WEAR RESISTANCE

Edited by J.R. Davis Davis & Associates

I O M

heatio M iaclse InfoTrm nate Sro i ty Materials Park, OH 44073-0002 www. asminternational. org

Communications

IOM Communications is a wholly owned subsidiary of the Institute of Materials IOM Book No. B751

Copyright © 2001 by ASM International® All rights reserved No part of this book may be reproduced, stored in a retrieval system, or transmitted, in any form or by any means, electronic, mechanical, photocopying, recording, or otherwise, without the written permission of the copyright owner. First printing, March 2001

Great care is taken in the compilation and production of this Volume, but it should be made clear that NO WARRANTIES, EXPRESS OR IMPLIED, INCLUDING, WITHOUT LIMITATION, WARRANTIES OF MERCHANTABILITY OR FITNESS FOR A PARTICULAR PURPOSE, ARE GIVEN IN CONNECTION WITH THIS PUBLICATION. Although this information is believed to be accurate by ASM, ASM cannot guarantee that favorable results will be obtained from the use of this publication alone. This publication is intended for use by persons having technical skill, at their sole discretion and risk. Since the conditions of product or material use are outside of ASM's control, ASM assumes no liability or obligation in connection with any use of this information. No claim of any kind, whether as to products or information in this publication, and whether or not based on negligence, shall be greater in amount than the purchase price of this product or publication in respect of which damages are claimed. THE REMEDY HEREBY PROVIDED SHALL BE THE EXCLUSIVE AND SOLE REMEDY OF BUYER, AND IN NO EVENT SHALL EITHER PARTY BE LIABLE FOR SPECIAL, INDIRECT OR CONSEQUENTIAL DAMAGES WHETHER OR NOT CAUSED BY OR RESULTING FROM THE NEGLIGENCE OF SUCH PARTY. As with any material, evaluation of the material under end-use conditions prior to specification is essential. Therefore, specific testing under actual conditions is recommended. Nothing contained in this book shall be construed as a grant of any right of manufacture, sale, use, or reproduction, in connection with any method, process, apparatus, product, composition, or system, whether or not covered by letters patent, copyright, or trademark, and nothing contained in this book shall be construed as a defense against any alleged infringement of letters patent, copyright, or trademark, or as a defense against liability for such infringement. Comments, criticisms, and suggestions are invited, and should be forwarded to ASM International. ASM International staff who worked on this project include Scott Henry, Assistant Director of Reference Publications; Bonnie Sanders, Manager of Production; Nancy Hrivnak, Copy Editor; and Kathy Dragolich, Production Supervisor. Library of Congress Cataloging-in-Publication Data Surface engineering for corrosion and wear resistance / edited by J.R. Davis p. cm. Includes index. 1. Corrosion and anti-corrosives. 2. Mechanical wear. 3. Surfaces (Technology) I. Davis, J.R. (Joseph R.) TA462.S789 2001 620.1'1233—dc21 00-048537 ISBN: 0-87170-700-4 ASM International® Materials Park, OH 44073-0002 www. asminternational. org Printed in the United States of America

P r e f a c e

Corrosion, wear, or the combined effects of these destructive failure modes cost industrial economies hundreds of billions of dollars each year. One of the more effective means of mitigating damage due to corrosion and wear is to treat, or "engineer," the surface so that it can perform functions that are distinct from those functions required from the bulk of the material. For example, a gear must be tough and fatigue resistant yet have a surface that resists wear. For applications requiring only a moderate degree of impact strength, fatigue resistance, and wear resistance, a higher carbon through-hardening steel may be sufficient. For more severe conditions, however, a surface hardened steel may have to be used. What are the options? Should the gear be flame or induction hardened, carburized or nitrided, or would high-energy processes such as laser- or electron-beam hardening be more appropriate? As a second example, consider the use of steels for various outdoor structural applications. Steel is popular because it is inexpensive, strong, and easily fabricated. Unfortunately steel is highly susceptible to severe corrosion in many environments and must be coated to achieve a satisfactory service life. Once again there are a variety of options. Should the component be painted, hot dip galvanized or aluminized, electroplated, thermally sprayed, or clad with a more corrosion resistant material? For large steel components, such as bridge members, size, weight, and handling problems may limit the type of surface treatment considered. Finally, take into consideration parts that require wearresistant, thin-film coatings. Can more conventional chromium or hard nickel electroplating be used, or will harder coatings deposited by vapor deposition techniques or ion implantation be required? Will processing time or temperature be a factor in coating selection? From the above discussion, it is apparent that engineers are faced with a bewildering number of choices when selecting the appropriate surface engineering treatment for a specific corrosion and/or wear application. But where does one start? Where can a design engineer find practical guidelines to aid in the selection process? The answers to these questions

lie within Surface Engineering for Corrosion and Wear Resistance. In addition to devoting an entire chapter to process comparisons (see Chapter 7), this book contains dozens of useful tables and figures that compare surface treatment thickness and hardness ranges; abrasion and corrosion resistance; processing time, temperature, and pressure; costs; distortion tendencies; and other surface treatment characteristics that must be considered when choosing the right coating for the job. The starting point for this publication was an excellent overview published by the Institute of Materials (IOM) entitled "Surface Engineering to Combat Wear and Corrosion: A Design Guide," which was written by Keith Stevens (A.T. Poeton Ltd.). Assisting IOM in the project was AEA Technology pic. and their National Centre of Tribology located in Risley, United Kingdom. The IOM booklet presents a concise methodology for understanding corrosion and wear problems and the many factors that must be considered before selecting a surface treatment. Material from the IOM design guide can be found primarily in Chapter 7, "Process Comparisons," and Chapter 8, "Practical Design Guidelines for Surface Engineering." Special thanks are due to Stephen Harmer, the editor of the IOM "Design Guide" series, who also reviewed several key chapters, and Bill Jackson, Head of Publishing for IOM, who worked out the copublishing agreement with Scott Henry, Assistant Director of Reference Publications for ASM International. Other key contributions for this book originated from Volumes 4, Heat Treating, 5, Surface Engineering, 13, Corrosion, 18, Friction, Lubrication, and Wear Technology, and 20, Materials Selection and Design, of the ASM Handbook series and from the Metals Handbook Desk Edition, Second Edition. Of particular note are articles authored by Arnold R. Marder (Lehigh University) and Eric W. Brooman (Concurrent Technologies Corporation) originally published in Volume 20 of the ASM Handbook. These are acknowledged at the conclusions of Chapters 4, 5, 6, and 8. Tabular data comparing various surface engineering processes were also adapted from the ASM Materials Engineering Institute course "Surface Engineering Processes for Wear and Corrosion" developed by Ralph B. Alexander (R.B. Alexander & Associates). Joseph R. Davis Davis & Associates Chagrin Falls, Ohio

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E n g i n e e r i n g C o r r o s i o n

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R e s i s t a n c e

SURFACE ENGINEERING is a multidisciplinary activity intended to tailor the properties of the surfaces of engineering components so that their function and serviceability can be improved. The ASM Handbook defines surface engineering as "treatment of the surface and near-surface regions of a material to allow the surface to perform functions that are distinct from those functions demanded from the bulk of the material" (Ref 1). The desired properties or characteristics of surface-engineered components include: Improved corrosion resistance through barrier or sacrificial protection Improved oxidation and/or sulfidation resistance Improved wear resistance Reduced frictional energy losses Improved mechanical properties, for example, enhanced fatigue or toughness Improved electronic or electrical properties Improved thermal insulation Improved aesthetic appearance As indicated in Table 1, these properties can be enhanced metallurgically, mechanically, chemically, or by adding a coating. The bulk of the material or substrate cannot be considered totally independent of the surface treatment. Most surface processes are not limited to the immediate region of the surface, but can involve the substrate by

Table 1

Surface engineering options and property benefits

Surface treatment/coating type

Primary property benefits

Changing the surface metallurgy Localized surface hardening (flame, induction, laser, and electron-beam hardening) Laser melting Shot peening

Improved wear resistance through the development of a hard martensitic surface Improved wear resistance through grain refinement and the formation of fine dispersions of precipitates Improved fatigue strength due to compressive stresses induced on the exposed surface, also relieves tensile stresses that contribute to stress-corrosion cracking

Changing the surface chemistry Phosphate chemical conversion coatings Chromate chemical conversion coatings Black oxide chemical conversion coatings Anodizing (electrochemical conversion coating)

Steam treating Carburizing Nitriding Carbonitriding Ferritic nitrocarburizing Diffusion (pack cementation) chromizing Diffusion (pack cementation) aluminizing Diffusion (pack cementation) siliconizing Boronizing (bonding) Ion implantation Laser alloying

Used primarily on steels for enhanced corrosion resistance, increased plating or paint adhesion, and for lubricity (e.g., to increase the formability of sheet metals) Enhanced bare or painted corrosion resistance, improved adhesion of paint or other organic finishes, and provides the metallic surface with a decorative finish Used for decorative applications, e.g., the "bluing" on steel gun barrels Used primarily for aluminum for increased corrosion resistance, improved decorative appearance, increased abrasion resistance (hard anodizing), improved paint adhesion, and improved adhesive bonding (higher bond strength and durability) Used on ferrous powder metallurgy parts to increase wear resistance and transverse rupture strength Used primarily for steels for increased resistance to wear, bending fatigue, and rolling-contact fatigue Used primarily for steels for improved wear resistance, increased fatigue resistance, and improved corrosion resistance (except stainless steels) Used primarily for steels for improved wear resistance Improved antiscuffing characteristics of ferrous alloys Improved molten-salt hot corrosion Improved oxidation resistance, sulfidation resistance, and carburization resistance Improved oxidation resistance Improved wear resistance, oxidative wear, and surface fatigue Improved friction and wear resistance for a variety of substrates Improved wear resistance

Adding a surface layer or coating Organic coatings (paints and polymeric or elastomeric coatings and linings) Ceramic coatings (glass linings, cement linings, and porcelain enamels) Slip/sinter ceramic coatings Hot-dip galvanizing (zinc coatings) Hot-dip aluminizing Hot-dip lead-tin alloy-coatings (terne coatings) Tin plate (continuous electrodeposition) Zinc-nickel alloy plate (continuous electrodeposition) Electroplating

Electroless plating Mechanical plating Weld overlays Thermal spraying

Cladding (roll bonding, explosive bonding, hot isostatic pressing, etc.) Laser cladding Carbide (salt bath) diffusion Chemical vapor deposition (CVD) Physical vapor deposition (PVD)

Improved corrosion resistance, wear resistance, and aesthetic appearance Improved corrosion resistance Improved wear resistance and heat resistance Improved corrosion resistance via sacrificial protection of steel substrate Improved corrosion and oxidation resistance of steel substrate Improved corrosion resistance of steel substrate Improved corrosion resistance of steel substrate Improved corrosion resistance of steel substrate Depending on the metal or metals being electrodeposited, improved corrosion resistance (e.g., nickel-chromium multilayer coatings, and cadmium and zinc sacrificial coatings), wear resistance (e.g., hard chromium coatings), electrical properties (e.g., copper and silver), and aesthetic appearance (e.g., bright nickel or decorative chromium plating) Improved corrosion resistance (nickel-phosphorus) and wear resistance (nickel-phosphorus and nickel-boron) Improved corrosion resistance Improved wear resistance (hardfacing alloys) and corrosion resistance (stainless steel or nickel-base overlays) and dimensional restoration (buildup alloys) Primarily used for improved wear resistance (many coating systems including ceramics and cermets), but also used for improved corrosion resistance (aluminum, zinc, and their alloys) and oxidation resistance (e.g., MCrAlY), thermal barrier protection (partially stabilized zirconia), electrically conductive coatings (e.g., copper and silver), and dimensional restoration Improved corrosion resistance Improved wear resistance Used primarily for steels for improved wear resistance in tooling applications Improved wear (e.g., tools and dies), erosion, and corrosion resistance; also used for epitaxial growth of semiconductors Improved wear (e.g., tools and dies) and corrosion resistance, improved optical and electronic properties, and decorative applications

exposure to either a thermal cycle or a mechanical stress. For example, diffusion heat treatment coatings (e.g., carburizing/nitriding) often have high-temperature thermal cycles that may subject the substrate to temperatures that cause phase transformations and thus property changes, or shot-peening treatments that deliberately strain the substrate surface to induce improved fatigue properties. It is the purpose of this book, and in particular Chapters 4 to 6, to review information on surface treatments that improve service performance so that metallurgists, chemists, mechanical engineers, and design engineers may consider surface-engineered components as an alternative to more costly materials. Surface Engineering to C o m b a t Corrosion and W e a r The Economic Effects of Corrosion and Wear. The progressive deterioration, due to corrosion and wear, of metallic surfaces in use in major industrial plants ultimately leads to loss of plant efficiency and at worst a shutdown. Corrosion and wear damage to materials, both directly and indirectly, costs the United States hundreds of billions of dollars annually. For example, corrosion of metals costs the U.S. economy almost $300 billion per year at current prices. This amounts to about 4.2% of the gross national product. However, about 40% of the total cost could be avoided by proper corrosion prevention methods. Table 2 provides a breakdown of the cost of metallic corrosion in the United States. Similar studies on wear failures have shown that the wear of materials costs the U.S. economy about $20 billion per year (in 1978 dollars) compared to about $80 billion annually (see Table 2) for corrosion during the same period. Table 3 illustrates the extent of wear failures by various operations within specific industrial segments. Highway vehicles alone use annually 14,600 X 1012 Btu/ton of energy represented in lost weight of steel and 18.6% of this energy could be saved through effective wear-control measures. Table 2 Cost of metallic corrosion in the United States Billions of U.S. dollars Industry

All industries Total Avoidable Motor vehicles Total Avoidable Aircraft Total Avoidable Other industries Total Avoidable Source: Ref 2

1975

1995

82.0 33.0

296.0 104.0

31.4 23.1

94.0 65.0

3.0 0.6

13.0 3.0

47.6 9.3

189.0 36.0

Table 3 Industrial operations with significant annual wear economic consequences Industry

Utilities (28% total U.S. consumption)

Transportation (26% total U.S. consumption)

Mining

Agriculture

Primary metals

Operation

Loss mass (a), 10 12 Btu

Seate Accessories Bearings Reliability Total Brakes Valve trains Piston ring assemblies Transmission Bearings Gears Total Ore processing Surface mining Shaft mining Drilling Total Tillage Planting Total Hot rolling Cold rolling Total

185 120 55 145 505 (b) (b) (b) (b) (b) (b) (b) 22.80 13.26 10.70 5.58 52.34 16.85 2.47 19.32 14.30 0.14 14.44

(a) Assumes 19.2 X 106 Btu per ton of energy represented in lost weight of steel. (b) Lost mass not estimated. Source: Ref 3

Corrosive Wear. Complicating matters is the fact that the combined effects of wear and corrosion can result in total material losses that are much greater than the additive effects of each process taken alone, which indicates a synergism between the two processes. Although corrosion can often occur in the absence of mechanical wear, the opposite is rarely true. Corrosion accompanies the wear process to some extent in all environments, except in vacuum and inert atmospheres. Corrosion and wear often combine to cause aggressive damage in a number of industries, such as mining, mineral processing, chemical processing, pulp and paper production, and energy production. Corrosion and wear processes involve many mechanisms, the combined actions of which lead to the mutual reinforcement of their effectiveness. As listed in Table 4, 17 synergistic relationships among abrasion, impact, and corrosion that could significantly increase material degradation in wet and aqueous environments have been identified. The combined effects of corrosion and wear can also lead to galvanic corrosion in some applications, such as crushing and grinding (comminution) of mineral ores. Wear debris and corrosion products that are formed during comminution affect product quality and can adversely affect subsequent benefication by altering the chemical and electrochemical properties of the mineral system (Ref 5-8). Electrochemical interactions between minerals and grinding media can occur, causing galvanic coupling that leads to increased corrosion wear. More detailed information on galvanic corrosion can be found in Chapter 2.

Methods to Control Corrosion. Owing to its many favorable characteristics, steel is well suited and widely used for a broad range of engineering applications and is referenced here to demonstrate the various corrosion-control steps that can be considered. Steel has a variety of excellent mechanical properties, such as strength, toughness, ductility, and dent resistance. Steel also offers good manufacturability, including formability, weldability, and paintability. Other positive factors include its availability, ferromagnetic properties, recyclability, and cost. Because steel is susceptible to corrosion in the presence of moisture, and to oxidation at elevated temperatures, successful use of these favorable characteristics generally requires some form of protection. Methods of corrosion protection employed to protect steel include: Altering the metal by alloying, that is, using a more highly alloyed and expensive stainless steel rather than a plain carbon or low-alloy steel Changing the environment by desiccation or the use of inhibitors Controlling the electrochemical potential by the application of cathodic or anodic currents, that is, cathodic and anodic protection Applying organic, metallic, or inorganic (glasses and ceramics) coatings Application of corrosion-resistant coatings is one of the most widely used means of protecting steel. As shown in Table 1, there are a wide variety of coatings to choose from, and proper selection is based on the component size and accessibility, the corrosive environment, the anticipated

Table 4 Synergistic relationships between wear and corrosion mechanisms Abrasion Removes protective oxidized metal and polarized coatings to expose unoxidized metal, in addition to removing metal particles. Forms microscopic grooves and dents for concentration cell corrosion. Increases microscopic surface area exposed to corrosion. Removes strain-hardened surface layers. Cracks brittle metal constituents forming sites for impact hydraulic splitting. Plastic deformation by high-stress metal-mineral contact causes strain hardening and susceptibility to chemical attack. Corrosion Produces pits that induce microcracking. Microcracks at pits invite hydraulic splitting during impact. Roughens surface, reducing energy needed to abrade away metal. May produce hydrogen with subsequent absorption and cracking in steel. Selectively attacks grain boundaries and less noble phases of multiphase microstructures, weakening adjacent metal. Impact Plastic deformation makes some constituents more susceptible to corrosion. Cracks brittle constituents, tears apart ductile constituents to form sites for crevice corrosion, hydraulic splitting. Supplies kinetic energy to drive abrasion mechanism. Pressurizes mill water to cause splitting, cavitation, and jet erosion of metal and protective oxidized material. Pressurizes mill water and gases to produce unknown temperatures, phase changes, and decomposition or reaction products from ore and water constituents. Heats ball metal, ore, fluids to increase corrosive effects. Source: Ref 4

Weld overlay Friction surfacing Thermal spraying Carburizing Carbonitriding Nitrocarburizing Nitriding Mechanical working Electrochemical plate + diffusion Transformation hardening Surface alloying—lasers Hot dipping (galvanizing and aluminum) Mechanical plating Electroless plating Electrolytic plating Chemical vapor deposition Physical vapor deposition Resin or laquer"bonding Ion implantation Thickness, mm Fig. 1 Approximate thickness of various surface engineering treatments

temperatures, component distortion, the coating thickness attainable (Fig. 1), and costs. Many of these selection criteria are addressed in Chapters 6 to 8 in this book. Painting is probably the most widely used engineering coating used to protect steel from corrosion. There are a wide variety of coating formulations that have been developed for outdoor exposure, marine atmospheres, water immersion, chemical fumes, extreme sunlight, high humidity, and moderately high temperatures (less than about 200 0 C, or 400 0 F). The most widely used corrosion-resistant metallic coatings are hotdipped zinc, zinc-aluminum, and aluminum coatings. These coatings exhibit excellent resistance to atmospheric corrosion and are widely used in the construction, automobile, utility, and appliance industries. Other important coating processes for steels include electroplating, electroless plating, thermal spraying, pack cementation aluminizing (for high-temperature oxidation resistance), and cladding (including weld cladding and roll-bonded claddings). Applications and corrosion performance of these coatings are described in Chapter 6 in this book. Methods to Control Wear. As is described in Chapter 3 in this book, there are many types of wear, but there are only four main types of wear systems (tribosystems) that produce wear and six basic wear control steps (Ref 9). The four basic tribosystems are:

Relatively smooth solids sliding on other smooth solids Hard, sharp substances sliding on softer surfaces Fatigue of surfaces by repeated stressing (usually compressive) Fluids with or without suspended solids in motion with respect to a solid surface As shown in Fig. 2, the wear that occurs in these tribosystems can be addressed by coatings or by modifications to the substrate metallurgy or chemistry. The six traditional techniques applied to materials to deal with wear produced in the preceding tribosystems include: Separate conforming surfaces with a lubricating film (see Chapter 3 in this book). Make the wearing surface hard through the use of hardfacing, diffusion heat treatments, hard chromium plating, or more recently developed vapor deposition techniques or high-energy processes (e.g., ion implantation). Make the wearing surface resistant to fracture. Many wear processes involve fracture of material from a surface; thus toughness and fracture resistance play a significant role in wear-resistant surfaces. The use of very hard materials such as ceramics, cemented carbides, and hard chromium can lead to fracture problems that nullify the benefits of the hard surface. Make the eroding surface resistant to corrosion. Examples include the use of cobalt-base hardfacing alloys to resist liquid erosion, cavitation, and slurry erosion; aluminum bronze hardfacing alloys to prevent cavitation damage on marine propellers or to repair props that have Wear-causing effects Coatings to reduce wear Polymers/elastomers Electrochemical (plating, etc.) Chemical (CVD, electroless plating) Thermal spraying Fusion welding Thin films (PVD, sputtering, ion plating) Wear tiles Cladding (cast, explosion, hot rolling) Lubricants

Substrate treatments to reduce wear Through hardening Surface hardening (flame, induction, EB, laser) Diffusion of a hardening species (carburizing, nitriding, etc.) Laser/EB alloying Ion implantation Work hardening

Tribosystem

Surface wear

FlC. 2 Surface engineering processes used to prevent wear. CVD, chemical vapor deposition; PVD, " physical vapor deposition; EB, electron beam

suffered cavitation damage; nickel-base hardfacing alloys to resist chemical attack; and epoxy-filled rebuilding cements used to resist slurry erosion in pumps. Choose material couples that are resistant to interaction in sliding (metal-to-metal wear resistance). Hardfacing alloys such as cobaltbase and nickel-chromium-boron alloys have been used for many years for applications involving metal-to-metal wear. Other surfaceengineering options include through-hardened tool steels, diffusion (case)-hardened surfaces, selective surface-hardened alloy steels, and some platings. Make the wearing surface fatigue resistant. Rolling-element bearings, gears, cams, and similar power-transmission devices often wear by a mechanism of surface fatigue. Repeated point or line contact stresses can lead to subsurface cracks that eventually grow to produce surface pits and eventual failure of the device. Prevention is possible through the use of through-hardened steels, heavy casehardened steels, and flame-, induction-, electron beam-, or laserhardened steels. More details on these surface-engineering techniques can be found in Chapters 5 through 8 in this book. Material/Process Selection (Ref 10). Faced with the wide range of possibilities indicated in Table 1 and the discussions on "Methods to Control Corrosion" and "Methods to Control Wear," selection of surface engiPredict working environment from consideration of design Proceed with one-piece construction (see note below)

Yes

Identify material requirements for structure and surface Consider one-piece construction

Analyze service failures to assist selection of better materials No

Select substrate material to suit strength, heat, and corrosion needs

Note: One-piece construction is often least Select surfacing material expensive for small parts as some surfacing alloys to suit requirements are available as castings machined to finished size or as powder metallurgical parts. Select from surfacing processes suitable for chosen material and job, Reconsider (must satisfy needs for coating density, materials thickness, dilution, etc.) Decide if chosen process suits substrate material Yes None and design (adhesion, Reconsider process HAZ, access, distortion, and/or material etc.) No Decide manufacturing Yes details, procedures, Identify quality assurance Finalize choice of health and safety and control needs materials and process requirements, etc. Fig, 3 Checklist for surface engineering material/process selection. HAZ, heat-affected zone

neering material and process may seem difficult, but it is normally straightforward. Often there are constraints placed on the choice because of availability (e.g., laser melting and/or alloying are not widely used, and these processes can only be obtained by a special arrangement with laser job shops). In many cases there is a precedent, but when considering a new problem it helps to follow a checklist of the type shown in Fig. 3. The sequence of decisions to be made covers several fundamental points. The first is the need to be clear about service conditions, based on experience or plant data. This is the key to material selection. The second decision is the choice of application process for the material. This involves the question of compatibility with the coating material; that is, not all materials can be applied by all processes. A further question of compatibility arises between both material and process with the substrate, for example, whether distortion from high-temperature processes be tolerated. All these issues are covered in subsequent chapters in this book (see, in particular, Chapters 7 and 8).

References 1. CM. Cotell and J.A. Sprague, Preface, Surface Engineering, VoI 5, ASM Handbook, ASM International, 1994, p v 2. Economic Effects of Metallic Corrosion in the United States, Battelle Columbus Laboratories and the National Institute of Standards and Technology, 1978 and Battelle updates in 1995 3. "Tribological Sinks in Six Major Industries," Report Number PNL5535, Sept 1985, Pacific Northwest Laboratory, Richland, WA, operated for the U.S. Department of Energy by Battelle Memorial Institute (NTIS No. DE86000841) 4. DJ. Dunn. Metal Removal Mechanisms Comprising Wear in Mineral Processing, Wear of Materials, K.C. Ludema, Ed., American Society of Mechanical Engineers, 1985, p 501-508 5. R.L. Pozzo and I. Iwasaki, Pyrite-Pyrrhotite Grinding Media Interactions and Their Effects on Media Wear and Flotation, /. Electrochem. Soc, VoI 136 (No. 6), 1989, p 1734-1740 6. R.L. Pozzo and I. Iwasaki, Effect of Pyrite and Pyrrhotite on the Corrosive Wear of Grinding Media, Miner. Metall. Process., Aug 1987, p 166-171 7. K.A Natarajan, S.C. Riemer, and I. Iwasaki, Influence of Pyrrhotite on the Corrosive Wear of Grinding Balls in Magnetite Ore Grinding, Int. J. Miner. Process., VoI 13 1984, p 73-81 8. R.L. Pozzo and I. Iwasaki, An Electro-chemical Study of PyrrhotiteGrinding Media Interaction Under Abrasive Conditions, Corrosion, VoI 43 (No. 3), 1987, p 159-169

9. K.G. Budinski, Surface Engineering for Wear Resistance, PrenticeHall, Inc., 1988, p 6-10 10. Engineering Coatings—Design and Application, 2nd ed., S. Grainger and J. Blunt, Ed., Woodhead Publishing Ltd., 1999, p 7

CHAPTER

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P r i n c i p l e s

o f

C o r r o s i o n

CORROSION of metal is a chemical or electrochemical process in which surface atoms of a solid metal react with a substance in contact with the exposed surface. Usually the corroding medium is a liquid substance, but gases and even solids can also act as corroding media. In some instances, the corrodent is a bulk fluid; in others, it is a film, droplets, or a substance adsorbed on or absorbed in another substance. All structural metals corrode to some extent in natural environments (e.g., the atmosphere, soil, or waters). Bronze, brass, most stainless steels, zinc, and pure aluminum corrode so slowly in service conditions that long service life is expected without protective coatings. Corrosion of structural grades of cast iron and steel, the 400 series stainless steels, and some aluminum alloys, however, proceeds rapidly unless the metal is protected against corrosion. As described in Chapter 1, corrosion of metals is of particular concern because annual losses in the United States attributed to corrosion amount to hundreds of billions of dollars. Although emphasis in this Chapter has been placed on irons and steels, the electrochemical corrosion basics and the forms of corrosion described are applicable to all metallic materials. For more detailed information on the corrosion resistance of various metals and their alloys, the reader should consult the selected references listed at the conclusion of this Chapter, as well as Corrosion, VoI 13, of the ASM Handbook or Corrosion: Understanding the Basics, published by ASM International in 2000.

Electrochemical Corrosion Basics Electrochemical corrosion in metals in a natural environment, whether atmosphere, in water, or underground, is caused by a flow of electricity from one metal to another, or from one part of a metal surface to another part of the same surface where conditions permit the flow of electricity.

Current flow in conductor Metal anode

Metallic conductor between the anode and the cathode Metal cathode Oxygen or other depolarizer in electrolyte

Oxidation reaction occurs at anode

Electrolyte, water containing conductive salts Reduction reaction occurs at cathode

Current flow through the electrolyte Fig. 1 Simple electrochemical cell showing the components necessary for corrosion

For the flow of energy to take place, either a moist conductor or an electrolyte must be present. An electrolyte is an electricity-conducting solution containing ions, which are atomic particles or radicals bearing an electrical charge. Charged ions are present in solutions of acids, alkalis, and salts. The presence of an electrolyte is necessary for corrosion to occur. Water, especially salt water, is an excellent electrolyte. Electricity passes from a negative area to a positive area through the electrolyte. For corrosion to occur in metals, one must have (a) an electrolyte, (b) an area or region on a metallic surface with a negative charge, (c) a second area with a positive charge, and (d) an electrically conductive path between (b) and (c). These components are arranged to form a closed electrical circuit. In the simplest case, the anode would be one metal, such as iron, the cathode another, perhaps copper, and the electrolyte might or might not have the same composition at both anode and cathode. The anode and cathode could be of the same metal under conditions described later in this article. The cell shown in Fig. 1 illustrates the corrosion process in its simplest form. This cell includes the following essential components: (a) a metal anode, (b) a metal cathode, (c) a metallic conductor between the anode and the cathode, and (d) an electrolyte in contact with the anode and the cathode. If the cell were constructed and allowed to function, an electrical current would flow through the metallic conductor and the electrolyte, and if the conductor were replaced by a voltmeter, a potential difference between the anode and the cathode could be measured. The anode would corrode. Chemically, this is an oxidation reaction. The formation of hydrated red iron rust by electrochemical reactions may be expressed as follows:

(EqI)

(Eq 2) During metallic corrosion, the rate of oxidation equals the rate of reduction. Thus, a nondestructive chemical reaction, reduction, would proceed simultaneously at the cathode. In most cases, hydrogen gas is produced on the cathode. When the gas layer insulates the cathode from the electrolyte, current flow stops, and the cell is polarized. However, oxygen or some other depolarizing agent is usually present to react with the hydrogen, which reduces this effect and allows the cell to continue to function. Contact between dissimilar metallic conductors or differences in the concentration of the solution cause the difference in potential that results in electrical current. Any lack of homogeneity on the metal surface or its environment may initiate attack by causing a difference in potential, and this results in localized corrosion. The metal undergoing electrochemical corrosion need not be immersed in a liquid but may be in contact with moist soil or may have moist areas on the metal surface.

Corrosive Conditions If oxygen and water are both present, corrosion will normally occur on iron and steel. Rapid corrosion may take place in water, the rate of corrosion being accelerated by several factors such as: (a) the velocity or the acidity of the water, (b) the motion of the metal, (c) an increase in temperature or aeration, and (d) the presence of certain bacteria. Corrosion can be retarded by protective layers or films consisting of corrosion products or adsorbed oxygen. High alkalinity of the water also retards the rate of corrosion on steel surfaces. Water and oxygen remain the essential factors, however, and the amount of corrosion is generally controlled by one or the other. For example, corrosion of steel does not occur in dry air and is negligible when the relative humidity of the air is below 30% at normal or lower temperatures. This is the basis for prevention of corrosion by dehumidification. Water can readily dissolve a small amount of oxygen from the atmosphere, thus becoming highly corrosive. When the free oxygen dissolved in water is removed, the water becomes practically noncorrosive unless it becomes acidic or anaerobic bacteria incite corrosion. If oxygen-free water is maintained at a neutral pH or at slight alkalinity, it is practically

noncorrosive to structural steel. Steam boilers and water supply systems are effectively protected by deaerating the water. Additional information on corrosion in water can be found in Ref 1. Soils. Dispersed metallic particles or bacteria pockets can provide a natural electrical pathway for buried metal. If an electrolyte is present and the soil has a negative charge in relation to the metal, an electrical path from the metal to the soil will occur, resulting in corrosion. Differences in soil conditions, such as moisture content and resistivity, are commonly responsible for creating anodic and cathodic areas (Fig. 2). Where a difference exists in the concentration of oxygen in the water or in moist soils in contact with metal at different areas, cathodes develop at points of relatively high-oxygen concentrations and anodes at points of low concentration. Further information on corrosion in soils is available in Ref 2. Chemicals. In an acid environment, even without the presence of oxygen, the metal at the anode is attacked at a rapid rate. At the cathode, atomic hydrogen is released continuously, to become hydrogen gas. Corrosion by an acid can result in the formation of a salt, which slows the reaction because the salt formation on the surface is then attacked. Corrosion by direct chemical attack is the single most destructive force against steel surfaces. Substances having chlorine or other halogens in their composition are particularly aggressive. Galvanized roofing has been known to corrode completely within six months of construction, the building being downwind of an aluminum ingot plant where fluorides were always present in the atmosphere. Consequently, galvanized steel should not have been specified. Selection of materials and evaluation of service conditions are extremely important in combating corrosion. The response of various materials to chemical environments is addressed in Ref 3 and 4. Atmospheric corrosion differs from the corrosion action that occurs in water or underground, because sufficient oxygen is always present. In at-

Cathodic area (steel at top of pipe) Buried pipe

Oxygen diffusing into earth from ground surface Electrolyte 1 (soil with ground water high in oxygen content) •Current flow

Anodic area (steel at bottom of pipe)

• Electrolyte 2 (soil with ground water deficient in oxygen content)

Fe2+ (rust) pjo 2 A metal pipe buried in moist soil forming a corrosion cell. A difference ^* in oxygen content at different levels in the electrolyte will produce a difference of potential. Anodic and cathodic areas will develop, and a corrosion cell, called a concentration cell, will form.

mospheric corrosion, the formation of insoluble films and the presence of moisture and deposits from the atmosphere control the rate of corrosion. Contaminants such as sulfur compounds and salt particles can accelerate the corrosion rate. Nevertheless, atmospheric corrosion occurs primarily through electrochemical means and is not directly caused by chemical attack. The anodic and cathodic areas are usually quite small and close together so that corrosion appears uniform, rather than in the form of severe pitting, which can occur in water or soil. A more detailed discussion on atmospheric corrosion can be found in Ref 5. Forms of Corrosion The differing forms of corrosion can be divided into the following eight categories based on the appearance of the corrosion damage or the mechanism of attack: Uniform or general corrosion Galvanic corrosion Pitting corrosion Crevice corrosion, including corrosion under tubercles or deposits, filiform corrosion, and poultice corrosion Erosion-corrosion, including cavitation erosion and fretting corrosion Intergranular corrosion, including sensitization and exfoliation Dealloying Environmentally assisted cracking, including stress-corrosion cracking (SCC), corrosion fatigue, and hydrogen damage (including hydrogen embrittlement, hydrogen-induced blistering, high-temperature hydrogen attack, and hydride formation) Figure 3 illustrates schematically some of the most common forms of corrosion. More detailed information pertaining to recognition and prevention of these forms of corrosion can be found in Ref 6 and 7. Uniform Corrosion General Description. Uniform or general corrosion, as the name implies, results in a fairly uniform penetration (or thinning) over the entire exposed metal surface. The general attack results from local corrosion-cell action; that is, multiple anodes and cathodes are operating on the metal surface at any given time. The location of the anodic and cathodic areas continues to move about on the surface, resulting in uniform corrosion. Uniform corrosion often results from atmospheric exposure (especially polluted industrial environments); exposure in fresh, brackish, and salt waters; or exposure in soils and chemicals.

More noble metal

No corrosion

Pitting

Uniform

Exfoliation

Galvanic

Dealloying

Flowing corrodent

Erosion

lntergranular

Cyclic movement

Metal or nonmetal

Fretting

Crevice

Tensile stress

Cyclic stress

Stress-corrosion cracking

Corrosion fatigue

Flg. 3 Schematics of the common forms of corrosion

Metals Affected. All metals are affected by uniform corrosion, although materials that form passive films, such as stainless steels or nickelchromium alloys, are normally subjected to localized forms of attack. The rusting of steel, the green patina formation on copper, and the tarnishing of silver are typical examples of uniform corrosion. In some metals, such as steel, uniform corrosion produces a somewhat rough surface by removing a substantial amount of metal, which either dissolves in the environment or reacts with it to produce a loosely adherent, porous coating of corrosion products. In such reactions as in the tarnishing of silver in air, the oxidation of aluminum in air, or attack on lead in sulfate-containing environments, thin, tightly adherent protective films are produced, and the metal surface remains smooth. Prevention. Uniform corrosion can be prevented or reduced by proper materials selection, the use of coatings or inhibitors, or cathodic protection. These corrosion prevention methods can be used individually or in combination. Galvanic Corrosion General Description. The potential available to promote the electrochemical corrosion reaction between dissimilar metals is suggested by the galvanic series, which lists a number of common metals and alloys arranged according to their tendency to corrode when in galvanic contact (Table 1). Metals close to one another on the table generally do not have a strong effect on each other, but the farther apart any two metals are separated, the stronger the corroding effect on the one higher in the list. It is possible for certain metals to reverse their positions in some environments, but the order given in Table 1 is maintained in natural waters and the atmosphere. The galvanic series should not be confused with the sim-

Table 1

Galvanic series in seawater at 25 0C (77 0F)

Corroded end (anodic, or least noble) Magnesium Magnesium alloys Zinc Galvanized steel or galvanized wrought iron Aluminum alloys 5052, 3004, 3003, 1100, 6053, in this order Cadmium Aluminum alloys 2117, 2017, 2024, in this order Low-carbon steel Wrought iron Cast iron Ni-Resist (high-nickel cast iron) Type 410 stainless steel (active) 50-50 lead-tin solder Type 304 stainless steel (active) Type 316 stainless steel (active) Lead Tin Copper alloy C28000 (Muntz metal, 60% Cu) Copper alloy C67500 (manganese bronze A) Copper alloys C46400, C46500, C46600, C46700 (naval brass) Nickel 200 (active) Inconel alloy 600 (active) Hastelloy alloy B Chlorimet 2 Copper alloy C27000 (yellow brass, 65% Cu) Copper alloys C44300, C44400, C44500 (admiralty brass) Copper albys C60800, C61400 (aluminum bronze) Copper alloy C23000 (red brass, 85% Cu) Copper C! 1000 (ETP copper) Copper alloys C65100, C65500 (silicon bronze) Copper alloy C71500 (copper nickel, 30% Ni) Copper alloy C92300, cast (leaded tin bronze G) Copper alloy C92200, cast (leaded tin bronze M) Nickel 200 (passive) Inconel alloy 600 (passive) Monel alloy 400 Type 410 stainless steel (passive) Type 304 stainless steel (passive) Type 316 stainless steel (passive) Incoloy alloy 825 Inconel alloy 625 Hastelloy alloy C Chlorimet 3 Silver Titanium Graphite Gold Platinum Protected end (cathodic, or most noble)

ilar electromotive force series, which shows exact potentials based on highly standardized conditions that rarely exist in nature. The three-layer iron oxide scale formed on steel during rolling varies with the operation performed and the rolling temperature. The dissimilarity of the metal and the scale can cause corrosion to occur, with the steel acting as the anode in this instance. Unfortunately, mill scale is cathodic to steel, and an electric current can easily be produced between the steel and the mill scale. This electrochemical action will corrode the steel without affecting the mill scale (Fig. 4). A galvanic couple may be the cause of premature failure in metal components of water-related structures or may be advantageously exploited.

Electrolyte (water) (rust)

Current flow

Cathode (broken mil scale)

Anode (steel) Fig, 4 Mill scale forming a corrosion cell on steel

Galvanizing iron sheet is an example of useful application of galvanic action or cathodic protection. Iron is the cathode and is protected against corrosion at the expense of the sacrificial zinc anode. Alternatively, a zinc or magnesium anode may be located in the electrolyte close to the structure and may be connected electrically to the iron or steel. This method is referred to as cathodic protection of the structure. Iron or steel can become the anode when in contact with copper, brass, or bronze; however, iron or steel corrode rapidly while protecting the latter metals. Also, weld metal may be anodic to the basis metal, creating a corrosion cell when immersed (Fig. 5). While the galvanic series (Table 1) represents the potential available to promote a corrosive reaction, the actual corrosion is difficult to predict. Electrolytes may be poor conductors, or long distances may introduce large resistance into the corrosion cell circuit. More frequently, scale formation forms a partially insulating layer over the anode. A cathode having a layer of adsorbed gas bubbles, as a consequence of the corrosion cell reaction, is polarized. The effect of such conditions is to reduce the theoretical consumption of metal by corrosion. The area relationship between the anode and cathode may also strongly affect the corrosion rate; a high ratio of cathode area to anode area produces more rapid corrosion. In the reverse case, the cathode polarizes, and the corrosion rate soon drops to a negligible level. The passivity of stainless steels is attributed to either the presence of a corrosion-resistant oxide film or an oxygen-caused polarizing effect,

Electrolyte (water)

(rust) Current flow Cathode (steel) Anode (weld metal) \

FlC, 5 Weld metal forming a corrosion cell on steel. Weld metal may be an^* odic to steel, creating a corrosion cell when immersed.

durable only as long as there is sufficient oxygen to maintain the effect, over the surfaces. In most natural environments, stainless steels will remain in a passive state and thus tend to be cathodic to ordinary iron and steel. Change to an active state usually occurs only where chloride concentrations are high, as in seawater or reducing solutions. Oxygen starvation also produces a change to an active state. This occurs where the oxygen supply is limited, as in crevices and beneath contamination on partially fouled surfaces. Prevention. Galvanic corrosion can be prevented or reduced by proper materials selection (i.e., select combinations of metals as close together as possible in the galvanic series), insulating dissimilar metals, applying a barrier coating to both the anodic (less noble) and cathodic (noble) metal, applying a sacrificial coating (aluminum, zinc, or cadmium) to the cathodic part, applying nonmetallic films (e.g., anodizing aluminum alloys), and by providing cathodic protection.

Pitting General Description. Pitting is a type of localized cell corrosion. It is predominantly responsible for the functional failure of iron and steel water-related installations. Pitting may result in the perforation of water pipe, rendering it unserviceable, even though less than 5% of the total metal has been lost through rusting. Where confinement of water is not a factor, pitting causes structural failure from localized weakening while considerable sound metal still remains. Pitting develops when the anodic or corroding area is small in relation to the cathodic or protected area. For example, pitting can occur where large areas of the surface are covered by mill scale, applied coatings, or deposits of various kinds and where breaks exist in the continuity of the protective coating. Pitting may also develop on bare, clean metal surfaces because of irregularities in the physical or chemical structure of the metal. Localized, dissimilar soil conditions at the surface of steel can also create conditions that promote pitting. Electrical contact between dissimilar materials or concentration cells (areas of the same metal where oxygen or conductive salt concentrations in water differ) accelerates the rate of pitting. In closed-vessel structures, these couples cause a difference of potential that results in an electric current flowing through the water or across the moist steel from the metallic anode to a nearby cathode. The cathode may be copper, brass, mill scale, or any portion of a metal surface that is cathodic to the more active metal areas. In practice, mill scale is cathodic to steel and is found to be a common cause of pitting. The difference of potential generated between steel and mill scale often amounts to 0.2 to 0.3 V. This couple is nearly as powerful a generator of corrosion currents as is the copper-steel couple. However, when the anodic area is relatively large compared with the

cathodic area, the damage is spread out and usually negligible, but when the anode is relatively small, the metal loss is concentrated and may be very serious. On surfaces having some mill scale, the total metal loss is nearly constant as the anode is decreased, but the degree of penetration increases. Figure 4 shows how a pit forms where a break occurs in mill scale. When contact between dissimilar materials is unavoidable and the surface is painted, it is preferred to paint both materials. If only one surface is painted, it should be the cathode. If only the anode is coated, any weak points such as pinholes or holidays in the coating will probably result in intense pitting. As a pit, perhaps at a break in mill scale, becomes deeper, an oxygen concentration cell is started by depletion of oxygen in the pit. The rate of penetration by such pits is accelerated proportionately as the bottom of the pit becomes more anodic. Fabrication operations may crack mill scale and result in accelerated corrosion. Metals Affected. Pitting occurs in most commonly used metals and alloys. Iron buried in the soil corrodes with the formation of shallow pits, but carbon steels in contact with hydrochloric acid or stainless steels immersed in seawater characteristically corrode with the formation of deep pits. Aluminum tends to pit in waters containing chloride ions (for example, at stagnant areas), and aluminum brasses are subject to pitting in polluted waters. Despite their good resistance to general corrosion, stainless steels are more susceptible to pitting than many other metals. High-alloy stainless steels containing chromium, nickel, and molybdenum are also more resistant to pitting but are not immune under all service conditions. Pitting failures of corrosion-resistant alloys, such as Hastelloy C, Hastelloy G, and Incoloy 825, are relatively uncommon in solutions that do not contain halides, although any mechanism that permits the establishment of an electrolytic cell in which a small anode is in contact with a large cathodic area offers the opportunity for pitting attack. Prevention. Typical approaches to alleviating or minimizing pitting corrosion include the following: Use defect-free barrier coatings Reduce the aggressiveness of the environment, for example, chloride ion concentrations, temperature, acidity, and oxidizing agents Upgrade the materials of construction, for example, use molybdenumcontaining (4 to 6% Mo) stainless steels, molybdenum + tungsten nickel-base alloys, overalloy welds, and use corrosion-resistant alloy linings Modify the design of the system, for example, avoid crevices and the formation of deposits, circulate/stir to eliminate stagnant solutions, and ensure proper drainage

Crevice

Corrosion

General Description. Crevice corrosion is a form of localized attack that occurs at narrow openings or spaces (gaps) between metal-to-metal or nonmetal-to-metal components. This type of attack results from a concentration cell formed between the electrolyte within the crevice, which is oxygen starved, and the electrolyte outside the crevice, where oxygen is more plentiful. The material within the crevice acts as the anode, and the exterior material becomes the cathode. Crevices may be produced by design or accident. Crevices caused by design occur at gaskets, flanges, rubber O-rings, washers, bolt holes, rolled tube ends, threaded joints, riveted seams, overlapping screen wires, lap joints, beneath coatings (filiform corrosion) or insulation (poultice corrosion), and anywhere close-fitting surfaces are present. Figure 6 shows crevice corrosion in a riveted assembly caused by concentration cells. Occluded regions are also formed under tubercles (tuberculation), deposits (deposit corrosion), and below accumulations or biological materials (biologically influenced corrosion). Similarly, unintentional crevices such as cracks, seams, and other metallurgical defects could serve as sites for corrosion. Metals Affected. Resistance to crevice corrosion can vary from one alloy-environment system to another. Although crevice corrosion affects both active and passive metals, the attack is often more severe for passive alloys, particularly those in the stainless steel group. Breakdown of the passive film within a restricted geometry leads to rapid metal loss and penetration of the metal in that area.

Low metal ion concentration

Metal ion concentration cell

High metal ion concentration High oxygen concentration Oxygen concentration cell

Low oxygen concentration Fig. 6 Corrosion caused at crevices by concentration cells. Both types of concentration cells shown sometimes occur simultaneously as in a reentry angle in a riveted seam.

Prevention. Crevice corrosion can be prevented or reduced through improved design to avoid crevices, regular cleaning to remove deposits, by selecting a more corrosion-resistant material, and by coating carbon steel or cast iron components with epoxy or other field-applied or factoryapplied organic coatings. Erosion-Corrosion General Description. Erosion-corrosion is the acceleration or increase in the rate of deterioration or attack on a metal because of mechanical wear or abrasive contributions in combination with corrosion. The combination of wear or abrasion and corrosion results in more severe attack than would be realized with either mechanical or chemical corrosive action alone. Metal is removed from the surface as dissolved ions, as particles of solid corrosion products, or as elemental metal. The spectrum of erosioncorrosion ranges from primarily erosive attack, such as sandblasting, filing, or grinding of a metal surface, to primarily corrosion failures, where the contribution of mechanical action is quite small. All types of corrosive media generally can cause erosion-corrosion, including gases, aqueous solutions, organic systems, and liquid metals. For example, hot gases may oxidize a metal then at high velocity blow off an otherwise protective scale. Solids in suspension in liquids (slurries) are particularly destructive from the standpoint of erosion-corrosion. Erosion-corrosion is characterized in appearance by grooves, waves, rounded holes, and/or horseshoe-shaped grooves. Analysis of these marks can help determine the direction of flow. Affected areas are usually free of deposits and corrosion products, although corrosion products can sometimes be found if erosion-corrosion occurs intermittently and/or the liquid flow rate is relatively low. Metals Affected. Most metals are susceptible to erosion-corrosion under specific conditions. Metals that depend on a relatively thick protective coating of corrosion product for corrosion resistance are frequently subject to erosion-corrosion. This is due to the poor adhesion of these coatings relative to the thin films formed by the classical passive metals, such as stainless steels and titanium. Both stainless steels and titanium are relatively immune to erosion-corrosion in many environments. Metals that

Corrosion film

Water flow Impingement corrosion pits

Original metal surface

Metal tube wall Fig, 7 Schematic of erosion-corrosion of a condenser tube

are soft and readily damaged or worn mechanically, such as copper and lead, are quite susceptible to erosion-corrosion. Even the noble or precious metals, such silver, gold, and platinum, are subject to erosion-corrosion. Figure 7 shows a schematic of erosion-corrosion of a condenser tube wall. The direction of flow and the resulting attack where the protective film on the tube has broken down are indicated. Prevention. Erosion-corrosion can be prevented or reduced through improved design (e.g., increase pipe diameter and/or streamline bends to reduce impingement effects), by altering the environment (e.g., deaeration and the addition of inhibitors), and by applying hard, tough protective coatings. Cavitation General Description. Cavitation is a form of erosion-corrosion that is caused by the formation and collapse of vapor bubbles in a liquid against a metal surface. Cavitation occurs in hydraulic turbines, on pump impellers, on ship propellers, and on many surfaces in contact with high-velocity liquids subject to changes in pressure. The appearance of cavitation is similar to pitting except that surfaces in the pits are usually much rougher. The affected region is free of deposits and accumulated corrosion products if cavitation has been recent. Figure 8 is a simplified representation of the cavitation process. Figure 8(a) shows a vessel containing a liquid. The vessel is closed by an airtight plunger. When the plunger is withdrawn (Fig. 8b), a partial vacuum is created above the liquid, causing vapor bubbles to form and grow within Partial vacuum

Pressurized

Metal (a) Rest Quiescent liquid at standard temperature and pressure

(b) Expansion Liquid boiling at room temperature

(c) Compression Collapse of vapor bubbles

Metal oxide (d)

Approaching microjet torpedo

Destruction of metal oxide on impact

Repair of metal oxide at expense of metal

P J o - 8 Schematic representation of cavitation showing a cross section through a vessel and plunger enclosing a fluid. " (a) Plunger stationary, liquid at standard temperature and pressure, (b) Plunger withdrawn, liquid boils at room temperature, (c) Plunger advanced, bubbles collapse, (d) Disintegration of protective corrosion product by impacting microjet "torpedo." Source: Ref 8

the liquid. In essence, the liquid boils without a temperature increase. If the plunger is then driven toward the surface of the liquid (Fig. 8c), the pressure in the liquid increases, and the bubbles condense and collapse (implode). In a cavitating liquid, these three steps occur in a matter of milliseconds. As shown in Fig. 8(d), implosion of a vapor bubble creates a microscopic "torpedo" of water that is ejected from the collapsing bubble at velocities that may range from 100 to 500 m/s (330 to 1650 ft/s). When the torpedo impacts the metal surface, it dislodges protective surface films and/or locally deforms the metal itself. Thus, fresh surfaces are exposed to corrosion and the reformation of protective films, which is followed by more cavitation, and so on. Damage occurs when the cycle is allowed to repeat over and over again. Prevention. Cavitation can be controlled or minimized by improving design to minimize hydrodynamic pressure differences, employing stronger (harder) and more corrosion-resistant materials, specifying a smooth finish on all critical metal surfaces, and coating with resilient materials such as rubber and some plastics. Fretting Corrosion General Description. Fretting corrosion is a combined wear and corrosion process in which material is removed from contacting surfaces when motion between the surfaces is restricted to very small amplitude oscillations (often, the relative movement is barely discernible). Usually, the condition exists in machine components that are considered fixed and not expected to wear. Pressed-on wheels can often fret at the shaft/wheel hole interface. Oxidation is the most common element in the fretting process. In oxidizing systems, fine metal particles removed by adhesive wear are oxidized and trapped between the fretting surfaces (Fig. 9). The oxides act like an abrasive (such as lapping rouge) and increase the rate of material removal. This type of fretting in ferrous alloys is easily recognized by the red material oozing from between the contacting surfaces. Fretting corrosion takes the form of local surface dislocations and deep pits. These occur in regions where slight relative movements have occurred between mating, highly loaded surfaces.

Surface Oxide Bare Metal Metal and Oxide Debris Fig, 9 Schematic of the fretting process

Prevention. Fretting corrosion can be controlled by lubricating (e.g., low-viscosity oils) the faying surfaces, restricting the degree of movement, shot peening (rough surfaces are less prone to fretting damage), surface hardening (e.g., carburizing and nitriding), anodizing of aluminum alloys, phosphate conversion coating of steels, and by applying protective coatings by electrodeposition (e.g., gold or silver plating), plasma spraying, or vapor deposition (Ref 9). lntergranular Corrosion General Description. lntergranular corrosion is defined as the selective dissolution of grain boundaries, or closely adjacent regions, without appreciable attack of the grains themselves. This dissolution is caused by potential differences between the grain-boundary region and any precipitates, intermetallic phases, or impurities that form at the grain boundaries. The actual mechanism differs with each alloy system. Although a wide variety of alloy systems are susceptible to intergranular corrosion under very specific conditions, the majority of case histories reported in the literature have involved austenitic stainless steels and aluminum alloys and, to a lesser degree, some ferritic stainless steels and nickel-base alloys. Precipitates that form as a result of the exposure of metals at elevated temperatures (for example, during production, fabrication, and welding) often nucleate and grow preferentially at grain boundaries. If these precipitates are rich in alloying elements that are essential for corrosion resistance, the regions adjacent to the grain boundary are depleted of these elements. The metal is thus sensitized and is susceptible to intergranular attack in a corrosive environment. For example, in austenitic stainless steels such as AISI type 304, the cause of intergranular attack is the precipitation of chromium-rich carbides ((Cr5Fe)23C6) at grain boundaries. These chromium-rich precipitates are surrounded by metal that is depleted in chromium; therefore, they are more rapidly attacked at these zones than on undepleted metal surfaces. Impurities that segregate at grain boundaries may promote galvanic action in a corrosive environment by serving as anodic or cathodic sites. Therefore, this would affect the rate of the dissolution of the alloy matrix in the vicinity of the grain boundary. An example of this is found in aluminum alloys that contain intermetallic compounds, such as Mg5Al8 and CuAl2, at the grain boundaries. During exposures to chloride solutions, the galvanic couples formed between these precipitates and the alloy matrix can lead to severe intergranular attack. Susceptibility to intergranular attack depends on the corrosive solution and on the extent of intergranular precipitation, which is a function of alloy composition, fabrication, and heat treatment parameters. Prevention. Susceptibility to intergranular corrosion in austenitic stainless steels can be avoided by controlling their carbon contents or by

adding elements (titanium and niobium) whose carbides are more stable than those of chromium. For most austenitic stainless steels, restricting their carbon contents to 0.03% or less will prevent sensitization during welding and most heat treatment. Intergranular corrosion in aluminum alloys is controlled by material selection (e.g., the high-strength Ixxx and Ixxx alloys are the most susceptible) and by proper selection of thermal (tempering) treatments that can effect the amount, size, and distribution of second-phase intermetallic precipitates. Resistance to intergranular corrosion is obtained by the use of heat treatments that cause precipitation to be more general throughout the grain structure (Ref 10). Exfoliation General Description. Exfoliation is a form of macroscopic intergranular corrosion that primarily affects aluminum alloys in industrial or marine environments. Corrosion proceeds laterally from initiation sites on the surface and generally proceeds intergranularly along planes parallel to the surface. The corrosion products that form in the grain boundaries force metal away from the underlying base material, resulting in a layered or flakelike appearance (see, for example, the schematic shown in Fig. 3). Prevention. Resistance to exfoliation corrosion is attained through proper alloy and temper selection. The most susceptible alloys are the high-strength heat-treatable Ixxx and Ixxx alloys. Exfoliation corrosion in these alloys is usually confined to relatively thin sections of highly worked products. Guidelines for selecting proper heat treatment for these alloys can be found in Ref 10. Dealloying Corrosion General Description. Dealloying, also referred to as selective leaching or parting corrosion, is a corrosion process in which the more active metal is selectively removed from an alloy, leaving behind a porous weak deposit of the more noble metal. Specific categories of dealloying often carry the name of the dissolved element. For example, the preferential leaching of zinc from brass is called dezincification. If aluminum is removed, the process is called dealuminification, and so forth. In the case of gray iron, dealloying is called graphitic corrosion. In the dealloying process, typically one of two mechanisms occurs: alloy dissolution and replating of the cathodic element or selective dissolution of an anodic alloy constituent. In either case, the metal is left spongy and porous and loses much of its strength, hardness, and ductility. Table 2 lists some of the alloy-environment combinations for which dealloying has been reported. By far the two most common forms of dealloying are dezincification and graphitic corrosion. Copper-zinc alloys containing more than 15% zinc are susceptible to dezincification. In the dezincification of brass, selective removal of zinc

leaves a relatively porous and weak layer of copper and copper oxide. Corrosion of a similar nature continues beneath the primary corrosion layer, resulting in gradual replacement of sound brass by weak, porous copper. Graphitic corrosion is observed in gray cast irons in relatively mild environments in which selective leaching of iron leaves a graphite network. Selective leaching of the iron takes place because the graphite is cathodic to iron, and the gray iron structure establishes an excellent galvanic cell. Prevention. Dezincification can be prevented by alloy substitution. Brasses with copper contents of 85% or more resist dezincification. Some alloying elements also inhibit dezincification (e.g., brasses containing 1% tin). Where dezincification is a problem, red brass, commercial bronze, inhibited admiralty metal, and inhibited brass can be successfully used. Attack by graphitic corrosion is reduced by alloy substitution (e.g., use of a ductile or alloyed iron rather than gray iron), altering the environment (raise the water pH to neutral or slightly alkaline levels), the use of inhibitors, and avoiding stagnant water conditions. Stress-Corrosion Cracking General Description. Stress-corrosion cracking (SCC) is a cracking phenomenon that occurs in susceptible alloys and is caused by the conjoint action of a surface tensile stress and the presence of a specific corrosive environment. For SCC to occur on an engineering structure, three conditions must be met simultaneously, namely, a specific crack-promoting environment must be present, the metallurgy of the material must be susceptible to SCC, and the tensile stresses must be above some threshold value. Stresses required to cause SCC are small, usually below the macroscopic yield stress. The stresses can be externally applied, but residual stresses often cause SCC failures. This cracking phenomenon is of particular importance to users of potentially susceptible structural alloys because SCC occurs under service conditions that can result, often with no warning, in catastrophic failure. Failed specimens exhibit highly branched Table 2 Combinations of alloys and environments subject to dealloying and elements preferentially removed Alloy

Brasses Gray iron Aluminum bronzes Silicon bronzes Tin bronzes Copper-gold single crystals Monels Gold alloys with copper or silver Tungsten carbide-cobalt High-nickel alloys Medium- and high-carbon steels Iron-chromium alloys Nickel-molybdenum alloys

Environment

Element removed

Many waters, especially under stagnant conditions Soils, many waters Hydrofluoric acid, acids containing chloride ions High-temperature steam and acidic species Hot brine or steam Ferric chloride Hydrofluoric and other acids Sulfide solutions, human saliva Deionized water Molten salts Oxidizing atmospheres, hydrogen at high temperatures High-temperature oxidizing atmospheres Oxygen at high temperature

Zinc (dezincification) Iron (graphitic corrosion) Aluminum (dealuminification) Silicon (desiliconification) Tin (destannification) Copper Copper in some acids, and nickel in others Copper, silver Cobalt Chromium, iron, molybdenum, and tungsten Carbon (decarburization) Chromium, which forms a protective film Molybdenum

Table 3

Some environment-alloy combinations known to result in stress-corrosion cracking (SCC) Alloy system Aluminum alloys

Environment

Carbon steels

Copper alloys

Nickel alloys

Austenitic

Stainless Steels Duplex Martensitic

Titanium alloys

Zirconium alloys

Amines, aqueous Ammonia, anhydrous Ammonia, aqueous Bromine Carbonates, aqueous Carbon monoxide, carbon dioxide, water mixture Chlorides, aqueous Chlorides, concentrated, boiling Chlorides, dry, hot Chlorinated solvents Cyanides, aqueous, acidified Fluorides, aqueous Hydrochloric acid Hydrofluoric acid Hydroxides, aqueous Hydroxides, concentrated, hot Methanol plus halides Nitrates, aqueous Nitric acid, concentrated Nitric acid, fuming Nitrites, aqueous Nitrogen tetroxide Polythionic acids Steam Sulfides plus chlorides, aqueous Sulfurous acid Water, high-purity, hot X, known to result in SCC

Stress-corrosion cracking control

Mechanical

Metallurgical

Environmental

Change alloy composition

Modify environment

Relieve fabrication stresses

Change alloy structure

Apply anodic or cathodic protection

Introduce surface compressfve stresses

Use metallic or conversion coating

Add inhibrtor

Avoid stress concentrators

Reduce operating stresses

Use organic coating

Nondestructive testing implications for design

Modify temperature

Fig. 1 0

M e t hods used to control SCC. Source: Ket I I

cracks (see Fig. 3) that propagate intergranularly and/or transgranularly, depending on the metal-environment combination. Table 3 lists some of the alloy-environment combinations that result in SCC. This table, as well as others published in the literature, should be used only as a guide for screening candidate materials prior to further indepth investigation, testing, and evaluation. Prevention. Figure 10 summarizes the various approaches to controlling SCC. Surface engineering treatments like shot peening, metallic coatings, and organic coatings play a key role in controlling SCC.

Corrosion Fatigue General Description. Corrosion fatigue is a term that is used to describe the phenomenon of cracking, including both initiation and propagation, in materials under the combined actions of a fluctuating or cyclic stress and a corrosive environment. Corrosion fatigue depends strongly on the interactions among the mechanical (loading), metallurgical, and environmental variables listed in Table 4. Corrosion fatigue produces fine-to-broad cracks with little or no branching (see Fig. 3); thus, they differ from SCC, which often exhibits considerable branching. They are typically filled with dense corrosion product. The cracks may occur singly but commonly appear as families or parallel cracks. They are frequently associated with pits, grooves, or some other form of stress concentrator. Transgranular fracture paths are more common than intergranular fractures.

Table 4 Mechanical, metallurgical, and environmental variables that influence corrosion fatigue behavior Variable Mechanical

Metallurgical

Environmental

Type Maximum stress or stress-intensity factor, a max or Kmax Cyclic stress or stress-intensity range, ACT or AK Stress ratio, R Cyclic loading frequency Cyclic load waveform (constant-amplitude loading) Load interactions in variable-amplitude loading State of stress Residual stress Crack size and shape, and their relation to component size and geometry Alloy composition Distribution of alloying elements and impurities Microstructure and crystal structure Heat treatment Mechanical working Preferred orientation of grains and grain boundaries (texture) Mechanical properties (strength, fracture toughness, etc.) Temperature Types of environments: gaseous, liquid, liquid metal, etc. Partial pressure of damaging species in gaseous environments Concentration of damaging species in aqueous or other liquid environments Electrical potential pH Viscosity of the environment Coatings, inhibitors, etc.

Prevention. All metals and alloys are susceptible to corrosion fatigue. Even some alloys that are immune to SCC, for example, ferritic stainless steels, are subject to failure by corrosion fatigue. Both temporary and permanent solutions for corrosion involve reducing or eliminating cyclic stresses, selecting a material or heat treatment with higher corrosion fatigue strengths, reducing or eliminating corrosion, or a combination of these procedures. These objectives are accomplished by changes in material, design, or environment and by the application of surface treatments. Shot peening, nitriding of steels, and organic coatings can successfully impede corrosion fatigue. Noble metal coatings (e.g., nickel) can be effective, but only if they remain unbroken and are of sufficient density and thickness. The relatively low corrosion-fatigue strength of carbon steel is reduced still further when local breaks in a coating occur. Hydrogen Damage General Description. The term hydrogen damage has been used to designate a number of processes in metals by which the load-carrying capacity of the metal is reduced due to the presence of hydrogen, often in combination with residual or applied tensile stresses. Although it occurs most frequently in carbon and low-alloy steels, many metals and alloys are susceptible to hydrogen damage. Hydrogen damage in one form or another can severely restrict the use of certain materials. Because hydrogen is one of the most abundant elements and is readily available during the production, processing, and service of metals, hydrogen damage can develop in a wide variety of environments and circumstances. The interaction between hydrogen and metals can result in the formation of solid solutions of hydrogen in metals, molecular hydrogen, gaseous products that are formed by reactions between hydrogen and elements constituting the alloy, and hydrides. Depending on the type of hydrogen/metal interaction, hydrogen damage of metal manifests itself in one of several ways. Specific types of hydrogen damage, some of which occur only in specific alloys under specific conditions include: Hydrogen embrittlement: Occurs most often in high-strength steels, primarily quenched-and-tempered and precipitation-hardened steels, with tensile strengths greater than about 1034 MPa (150 ksi). Hydrogen sulfide is the chief embrittling environment. Hydrogen-induced blistering: Also commonly referred to as hydrogen-induced cracking (HIC), it occurs in lower-strength (unhardened) steels, typically with tensile strengths less than about 550 MPa (80 ksi). Line pipe steels used in sour gas environments are susceptible to HIC.

Cracking from precipitation of internal hydrogen: Examples include shatter cracks, flakes, and fish eyes found in steel forgings, weldments, and castings. During cooling from the melt, hydrogen diffuses and precipitates in voids and discontinuities. Hydrogen attack: A high-pressure, high-temperature form of hydrogen damage. Commonly experienced in steels used in petrochemical plant equipment that often handles hydrogen and hydrogen-hydrocarbon streams at pressures as high as 21 MPa (3 ksi) and temperatures up to 540 0C (1000 0F) Hydride formation: Occurs when excess hydrogen is picked up during melting or welding of titanium, tantalum, zirconium, uranium, and thorium. Hydride particles cause significant loss in strength and large losses in ductility and toughness. Prevention. The primary factors controlling hydrogen damage are material, stress, and environment. Hydrogen damage can often be prevented by using more resistant material, changing the manufacturing processes, modifying the design to lower stresses, or changing the environment. Inhibitors and post-processing bake-out treatments can also be used. Baking of electroplated high-strength steel parts reduces the possibility of hydrogen embrittlement (see Chapter 8 for additional information).

Coatings and Corrosion Prevention As described in the previous section, surface treatments, and in particular protective coatings, are widely used to control corrosion in its varying forms. The problems of corrosion should be approached in the design stage, and the selection of a protective coating is important. Paint systems and lining materials exist that slow the corrosion rate of carbon steel surfaces. High-performance organic coatings such as epoxy, polyesters, polyurethanes, vinyl, or chlorinated rubber help to satisfy the need for corrosion prevention. Special primers are used to provide passivation, galvanic protection, corrosion inhibition, or mechanical or electrical barriers to corrosive action. Corrosion Inhibitors. A water-soluble corrosion inhibitor reduces galvanic action by making the metal passive or by providing an insulating film on the anode, the cathode, or both. A very small amount of chromate, polyphosphate, or silicate added to water creates a water-soluble inhibitor. A slightly soluble inhibitor incorporated into the prime coat of paint may also have a considerable protective influence. Inhibitive pigments in paint primers are successful inhibitors except when they dissolve sufficiently to leave holes in the paint film. Most paint primers contain a partially soluble inhibitive pigment such as zinc chromate, which reacts with the steel

substrate to form the iron salt. The presence of these salts slows corrosion of steel. Chromates, phosphates, molybdates, borates, silicates, and plumbates are commonly used for this purpose. Some pigments add alkalinity, slowing chemical attack on steel. Alkaline pigments, such as metaborates, cement, lime, or red lead, are effective, provided that the environment is not too aggressive. In addition, many new pigments have been introduced to the paint industry such as zinc phosphosilicate and zinc flake. Barrier coatings are used to prevent the electrolyte from reaching the component surface. Examples of barrier coatings include painted steel structures, steels lined with thick acid-proof brick, steels lined with rubberlike materials, or steels electroplated with a noble (see Table 1) metal (e.g., chromium, copper, or nickel). Protection is effective until the coating is penetrated, either by a pit, pore, crack, or by damage or wear. The substrate will then corrode preferentially to the coating (since it is anodic to the coating material), and corrosion products will lift off the coating and allow further attack (Fig. 11). Generally, electroplated coatings that are completely free of pores and other discontinuities are not commercially feasible. Pits eventually form at coating flaws, and the coating is penetrated. The resulting corrosion cell is shown in Fig. 12. The substrate exposed at the bottom of the resulting pit corrodes rapidly. A crater forms in the substrate, and because of the

Rust Paint Steel (a)

(b) p jo "I \ Illustration of the mechanism of corrosion for painted steel, (a) A void " in the paint results in rusting of the steel, which undercuts the paint coating and results in further coating degradation, (b) Photograph showing blistering and/or peeling (undercutting) of paint where exposed steel is rusting.

Moist air

Noble metal coating (cathode)

Steel substrate (anode) f\a 1 2 Crater formation in a steel substrate beneath a void in a noble metal " coating, for example, passive chromium or copper. Corrosion proceeds under the noble metal, the edges of which collapse into the corrosion pit. Water drop

Substrate (M3)

Coating (M1)

Coating (M2)

FlC, 1 3 Corrosion pit formation in a substrate beneath a void in a duplex ^* noble metal coating. The top coating layer (M1) is cathodic to the coating underlayer (M2), which is in turn cathodic to the substrate (M3). As in Fig. 12, the coating tends to collapse into the pit.

large area ratio between the more noble coating and the anodic crater, the crater becomes anodic, and high corrosion current density results. Electrons flow from the substrate to the coating as the steel dissolves. Hydrogen ions (H + ) in the moisture accept the electron and, with dissolved oxygen, form water at the noble metal surface near the void. Use of an intermediate coating that is less noble than a surface coating but more noble than the base metal can result in the mode of corrosion shown in Fig. 13. This would be typical of a costume jewelry item with a brass substrate, an intermediate nickel coating, and a tarnish-resistant gold top coat. It is also exemplified by nickel-chromium coating systems. Sacrificial coatings, which corrode preferentially to the substrate, include zinc, aluminum, cadmium, and zinc-rich paints. Initially these sacrificial coatings will corrode, but their corrosion products are protective and the coating acts as a barrier layer. If the coating is damaged or defective, it remains protective as it is the coating that suffers attack and not the substrate. Figure 14 shows the sacrificial (galvanic) protection offered by a zinc coating to a steel substrate. Cathodic protection involves the reversal of electric current flow within the corrosion cell. Cathodic protection can reduce or eliminate corrosion by connecting a more active metal to a metal that must be

Water drop

Zinc coating (anode)

steel substrate (cathode) FlC. 14 Principles and mechanism of galvanic protection of a substrate by a ^* coating. Galvanic protection of a steel substrate at a void in a zinc coating. Corrosion of the substrate is light and occurs at some distance from the zinc.

protected. The use of cathodic protection to reduce or eliminate corrosion is a successful technique of long-standing use in marine structures, pipelines, bridge decks, sheet piling, and equipment and tankage of all types, particularly below water or underground. Typically, zinc or magnesium anodes are used to protect steel in marine environments, and the anodes are replaced after they are consumed. Cathodic protection uses an impressed direct current (dc) supplied by any low output voltage source and a relatively inert anode. As is the case in all forms of cathodic activity, an electrolyte is needed for current flow. Cathodic protection and the use of protective coatings are most often employed jointly, especially in marine applications and on board ships where impressed current inputs do not usually exceed 1 V. Beyond 1 V, many coating systems tend to disbond. Current source for cathodic protection in soils is usually 1.5 to 2 V. Choice of anodes for buried steel pipe depends on soil conditions. Magnesium is most commonly used for galvanic anodes; however, zinc can also be used. Galvanic anodes are seldom used when the resistivity of the soil is over 30 fl • m (3000 ft • cm); impressed current is normally used for these conditions. Graphite, high-silicon cast iron, scrap iron, aluminum, and platinum are used as anodes with impressed current. The availability of low-cost power is often the deciding factor in choosing between galvanic or impressed current cathodic protection. Figure 15 illustrates both types of galvanic protection systems. Protective coatings are normally used in conjunction with cathodic protection and should not be disregarded where cathodic protection is contemplated in new construction. Because the cathodic protection current must protect only the bare or poorly insulated areas of the surface, coatings that are highly insulating, very durable, and free of discontinuities lower the current requirements and system costs. A good coating also enables a single-impressed current installation to protect many miles of piping. Coal-tar enamel, epoxy powder coatings, and vinyl resin are exam-

ac line Insulated copper wire

Rectifier. Insulated copper wire Soil

Soil Active metal anode

Pipeline

Pipeline

Current

Current

Backfill (a)

Anode

Backfil

(b)

Fig, 1 5 Cathodic protection for underground pipe, (a) Sacrificial or galvanic anode, (b) Impressed-cur^* rent anode, ac, alternating current

pies of coatings that are most suitable for use with cathodic protection. Certain other coatings may be incompatible, such as phenolic coatings, which may deteriorate rapidly in the alkaline environment created by the cathodic protection currents. Although cement mortar initially conducts the electrical current freely, polarization, the formation of an insulating film on the surface as a result of the protective current, is believed to reduce the current requirement moderately. Cathodic protection is used increasingly to protect buried or submerged metal structures in the oil, gas, and waterworks industries and can be used in specialized applications, such as for the interiors of water storage tanks. Pipelines are routinely designed to ensure the electrical continuity necessary for effective functioning of the cathodic protection system. Thus, electrical connections or bonds are required between pipe sections in lines using mechanically coupled joints, and insulating couplings may be employed at intervals to isolate some parts of the line electrically from other parts. Leads may be attached during construction to facilitate the cathodic protection installation when needed.

Corrosion Testing Many tests exist for establishing the reliability of protective coatings on metal substrates. Existing tests and standards are under continuous development, and new tests are being designed. Organizations active in the development and standardization of corrosion tests for coatings include ASTM, NACE International, the Society of Automotive Engineers (SAE), the National Coil Coaters Association (NCCA), the International Standards Organization (ISO), international systems (e.g., DIN), and commercial (e.g., automotive, architectural, electronics), proprietary, and

military organizations. This section provides a brief review of the most widely used test methods including: Field tests Simulated service tests Laboratory (accelerated) tests (e.g., salt spray tests, humidity tests, and electrochemical tests) Table 5 lists selected tests used for determining the effectiveness of protective coatings in corrosive environments. More detailed information on testing of coated specimens can be found in several excellent sources. Gaynes (Ref 13) and Munger (Ref 14) give descriptions and the framework for effective use of tests and standards. Gaynes provides detailed descriptions including photographs, cross-listing ASTM to federal tests and a broader perspective encompassing the federal standard, miscellaneous tests, and some caveats of traditional testing. Munger offers practical material directed toward large structures and provides a listing based on ASTM standards. Altmayer (Ref 15) compiled a table of 13 applicable corrosion tests for 30 metallic, inorganic, and organic coating/substrate combinations. Other useful sources of information can be found in review articles by Simpson and Townsend (Ref 16) and Granata (Ref 12), which describe tests for metallic coatings and nonmetallic coatings, respectively.

Field Tests The most reliable performance data are obtained by field tests/surveys. One example would be to monitor and test the corrosion of autobody panels that sit in junkyards. Another example of in-service testing would be to monitor the behavior of the materials in a fleet of captive vehicles. This enables better control and recording of the exposure and driving conditions. The use of fleet vehicles also makes it possible to test coupons representing a larger database of materials.

Simulated Service Tests The most widely used simulated service test for static atmospheric testing is described in ASTM G 50, "Practice for Conducting Atmospheric Corrosion Tests on Metals." It is used to test coated sheet steels for a variety of outdoor applications. Test materials, which are in the form of flat test panels mounted in a test rack (Fig. 16), are subjected to the cyclic effects of the weather, geographical influences, and bacteriological factors that cannot be realistically duplicated in the laboratory. Test durations can last from several months up to many years. Some zinc-coated steel specimens have undergone testing for more than 30 years.

Table 5

Widely used tests for determining the corrosion resistance of protective coatings

Test Salt spray (ASTM B 117)

100% relative humidity (ASTM D 2247) Acetic acid-salt spray ASTM G 85, Al (formerly ASTM B 287) Sulfur dioxide-salt spray (ASTM G 85, A 4) Copper-accelerated salt spray, or CASS (ASTM B 368) FACT (formerly ASTM B 538) Accelerated weathering

Lactic acid

Acidified synthetic seawater testing or SWAAT (ASTM G 85, A3; formerly ASTM G 43)

Electrographic and chemical porosity tests

Adhesion (ASTM D 3359-90)

T-bend adhesion (ASTM D 4145)

Scab test Exterior exposure (ASTM D 1014) Service test data

Description and remarks

Most widely specified test. Atomized 5% sodium chloride (NaCl), neutral pH, 35°C (95 0F) (a), follow details of ASTM B 117, Appendix Xl. Emphasizes wet surfaces (nondrying), high oxygen availability, neutral pH, and warm conditions. Control of comparative specimens should be run simultaneously. Corrosivity consistency should be checked as described in ASTM B 117, Appendix X3. Notes: May be the most widely misused test. Requires correlation to service tests for useful results. Do not assume correlation exists. Widely used test. Condensing humidity, 100% RH, 38 0 C (100 0 F). Emphasizes sensitivity to water exposure Widely used test. Atomized 5% NaCl, pH 3.2 using acetic acid, 35 0C (95 0 F). More severe than ASTM B 117. The lower pH and the presence of acetate affect the solubility of corrosion products on and under the protective coatings. Atomized 5% NaCl, collected solution pH = 2.5-3.2, 35 0 C (95 0 F), SO2 metered (60 min • 35 cm3/min per m 3 cabinet volume) 4 times per day Atomized 5% NaCl, pH 3.2 with acetic acid, 0.025% cupric chloride-dihydrate, 35 0C (95°F). Galvanic coupling due to copper salt reduction to copper metal. More severe than ASTM B 117 Testing anodized aluminum specimens. Electrolyte as in salt spray or CASS test. Specimen is made the cathode to generate high pH at defects. Exposure of coated specimens to effects of ultraviolet radiation experienced in outdoor sunlight conditions, which may be combined with other exposures such as moisture and erosion. Exposure cabinets use carbon arc (ASTM D 822), xenon lamp (ASTM G 26), or fluorescent lamp (ASTM G 53). On substrates of brass and copper alloys, determines coatings porosity and resistance to handling (perspiration). Consists of immersion in 85% lactic acid solution, drying, and incubating above acetic acid vapors for 20 h to reveal discoloration spots at failure points or delaminations Atomized synthetic seawater (ASTM D 1141) with 10 mL glacial acetic acid per L of solution, pH 2.8 to 3.0, 35 0 C (95 0 F). More severe than ASTM B 117. The lower pH and the presence of acetate affect the solubility of corrosion products on and under the protective coatings. Pores and active defects in nonmetallic coatings can be revealed by color indication or deposit formation. On nickel substrates, dimethylglyoxime, or steel, potassium ferricyanide (ferroxyl test) indicator can be applied to surface on filter paper while substrate is made the anode. Alternatively, a substrate immersed in acidic copper sulfate can be made the cathode to form copper nodules at conductive coatings defects. Knife and fingernail test consists of cutting through the coating with knife or awl and dislodging coating with thumbnail or fingernail (pass/fail). The ASTM D 3359 test consists of "X" scribes or parallel cross-hatches followed by adhesive tape stripping of loosened coating. Combined flexibility and adhesion test consists of clamping end of coated flat metal panel in vise or similar tool bending (convex) through 90°, reclamping to bend through 180° to give "071" bend (where T is panel thickness and the numeral (0, 1, 2,...) is the number of panel thicknesses). Rebending over the 180° bend gives a IT bend. Adhesive tape is pressed down along edge of bend and any loose coating stripped off.

Cyclic testing consisting of short salt exposure, short drying period, and long period of high humidity. Undercutting from scribe is measured. Method for conducting exterior exposure tests of paints on steel. Well-defined exposure setup, not necessarily equivalent to service tests Performance data of coatings systems under use conditions. Slowest evaluation method; provides tangible results

FACT, Ford anodized aluminum corrosion test, (a) Note that dissolved CO 2 concentration at 0 0C (32 0F) is three times that of concentration at 35 0 C (95 0F) and can affect corrosion. Source: Ref 12

Flg. 1 6 Atmospheric corrosion test rack Salt Spray Tests

As indicated in Table 6, salt spray testing is the most popular form of testing for protective coatings. These tests have been used for more than 90 years as accelerated tests in order to determine the degree of protection afforded by both inorganic and organic coatings on a metallic base. Table 5 lists several widely used salt spray tests. The neutral salt-spray (fog) test (ASTM B 117—Method 811.1 of Federal Test Method 151b) is perhaps the most commonly used salt spray test in existence for testing inorganic and organic coatings, especially where such tests are used for material or product specifications. The duration of this test can range from 8 to 3000 h, depending on the product type of coating. A 5% sodium chloride (NaCl) solution that does not contain more than 200 ppm total solids and with a pH range of 6.5 to 7.2 when atomized is used. The temperature of the salt spray cabinet is controlled to maintain 35 + 1.1 or -1.7 0C (95 + 2 or - 3 0F) within the exposure zone of the closed cabinet. The acetic acid-salt spray (fog) test (ASTM G 85, Annex Al; Former Method B 287) is also used for testing inorganic and organic coatings but is particularly applicable to the study or testing of decorative chromium Table 6 Results of a survey to determine the most widely used tests for protective coatings Test Salt spray Immersion Outdoor Ultraviolet/condensation Accelerated/weathering Humidity/condensation Cathodic disbondment Adhesion Atlas cell test (NACE TMO174) Other physical tests Other chemical tests Flexibility

% respondents(a) 52 24 22 20 14 10 7 7 4 4 3 2

(a) Multiple tests used (total greater than 100%). Source: Ref 12

plate (nickel-chromium or copper-nickel-chromium) plating and cadmium plating on steel or zinc die castings and for the evaluation of the quality of a product. This test can be as brief as 16 h, although it normally ranges from 144 to 240 h or more. As in the neutral salt spray test, a 5% NaCl solution is used, but the solution is adjusted to a pH range of 3.1 to 3.3 by the addition of acetic acid, and again, the temperature of the salt spray cabinet is controlled to maintain 35 + 1.1 or -1.7 0C (95 + 2 or - 3 0F) within the exposure zone of the closed cabinet. The copper-accelerated acetic acid-salt spray (fog) test (CASS test), which is covered in ASTM B 368, is primarily used for the rapid testing of decorative copper-nickel-chromium or nickel-chromium plating on steel and zinc die castings. It is also useful in the testing of anodized, chromated, or phosphated aluminum. The duration of this test ranges from 6 to 720 h. A 5% NaCl solution is used, with 1 g of copper II chloride (CuCl2-2H2O) added to each 3.8 L of salt solution. The solution is then adjusted to a pH range of 3.1 to 3.3 by adding acetic acid. The temperature of the CASS cabinet is controlled to maintain 4 9 + 1 . 1 or —1.7 0C (120 + 2 or —3 0F) within the exposure zone of the closed cabinet. Humidity Cabinet Tests In a humidity cabinet the humidity is raised to a value chosen as appropriate to the material under test. The temperature is generally cycled, so that the specimen is exposed to alternating humid air and condensation. The apparatus is automated to ensure that conditions are controlled within narrow limits. Other corrodent materials, such as sulfur dioxide, may also be introduced. Examples of humidity cabinet tests include ASTM D 2247 and ASTM G 85 listed in Table 5. Electrochemical Tests Corrosion of metallic substances is an electrochemical process. An alternate approach to field or other accelerated tests in understanding and predicting metallic corrosion is the use of electrochemical parameters/ tests. Electrochemical tests often complement other test methods by providing kinetic and mechanistic data that would be otherwise difficult to obtain. Electrochemical tests are typically grouped as direct current (dc) or alternating current (ac) methods based on the type of perturbation signal that is applied in making the measurements. A number of investigators have used dc and ac electrochemical methods to study the performance and the quality of protective coatings, including passive films on metallic substrates, and to evaluate the effectiveness of various surface pretreatments. Several are discussed below.

Anodized Aluminum Corrosion Test. One such method is the Ford anodized aluminum corrosion test (FACT) listed in Table 5. This test involves the cathodic polarization of the anodized aluminum surface by using a small cylindrical glass clamp-on cell and a special 5% NaCl solution containing cupric chloride (CuCl2) acidified with acetic acid. A large voltage is applied across the cell by using a platinum auxiliary electrode. The alkaline conditions created by the cathodic polarization promote dissolution at small defects in the anodized aluminum. The coating resistance is decreased, more current begins to flow, and the voltage decreases. The cell voltage (auxiliary electrode to test specimen voltage) is monitored for 3 min, and the parameter cell voltage multiplied by time is recorded. A similar test, known as the cathodic breakdown test, involves cathodic polarization to -1.6 V (versus saturated calomel electrode, SCE) for a period of 3 min in acidified NaCl. Again, the test was designed for anodized aluminum alloys because the alkali created at the large applied currents will promote the formation of corroded spots at defects in the anodized film. The electrolytic corrosion test was designed for electrodeposits of principally nickel and chromium on less noble metals, such as zinc or steel. Special solutions are used, and the metal is polarized to +0.3 V versus the SCE. The metal is taken through cycles of 1 min anodically polarized and 2 min unpolarized. An indicator solution is then used to detect the presence of pits that penetrate to the substrate. Each exposure cycle simulates 1 year of exposure under atmospheric-corrosion conditions. The ASTM standard B 627 describes the method in greater detail. The paint adhesion on a scribed surface (PASS) test involves the cathodic polarization of a small portion of painted metal. The area exposed contains a scribed line that exposes a line of underlying bare metal. The sample is cathodically polarized for 15 min in 5% NaCl. At the end of this period, the amount of delaminated coating is determined from an adhesive tape pulling procedure. The impedance test for anodized aluminum (ASTM B 457) is used to study the seal performance of anodized aluminum. In this sense, the test is similar to the FACT test, except that this method uses a 1 V root mean square 1 kHz signal source from an impedance bridge to determine the sealed anodized aluminum impedance. The test area is again defined with a portable cell, and a platinum or stainless steel auxiliary electrode is typically used. The sample is immersed in 3.5% NaCl. The impedance is determined in ohms X 103. In contrast to the methods discussed previously, this test is essentially nondestructive and does not accelerate the corrosion process. Electrochemical impedance spectroscopy (EIS) offers an advanced method of evaluating the performance of metallic coatings (passive film forming or otherwise) and organic barrier coatings. The method does not accelerate the corrosion reaction and is nondestructive. The technique is

quite sensitive to changes in the resistive-capacitive nature of coatings. The technique has been used to evaluate phosphate coverage/stability on galvanneal, painted cold-rolled steel, electrogalvanized steel, and electrogal vannealed steel (Ref 16). It is also possible to monitor the corrosion rate with this technique. In this respect, the electrochemical impedance technique offers several advantages over dc electrochemical techniques in that the polarization resistance related to the corrosion rate can be separated from the high dc resistance of the dielectric coating. This is not possible with the dc methods.

References 1. Corrosion of Steels in Waters, ASM Specialty Handbook: Carbon and Alloys Steels, J.R. Davis, Ed., ASM International, 1996, p 408-429 2. Corrosion of Steels in Soils, ASM Specialty Handbook: Carbon and Alloys Steels, J.R. Davis, Ed., ASM International, 1996, p 430-438 3. Corrosion of Steels in Chemical Environments, ASM Specialty Handbook: Carbon and Alloys Steels, J.R. Davis, Ed., ASM International, 1996, p 439-151 4. Types of Corrosive Environments, Corrosion: Understanding the Basics, J.R. Davis, Ed., ASM International, 2000, p 193-236 5. Atmospheric Corrosion of Steels, ASM Specialty Handbook: Carbon and Alloys Steels, J.R. Davis, Ed., ASM International, 1996, p 393^07 6. Forms of Corrosion: Recognition and Prevention, Corrosion: Understanding the Basics, J.R. Davis, Ed., ASM International, 2000, p 99-192 7. Corrosion Control by Proper Design, Corrosion: Understanding the Basics, J.R. Davis, Ed., ASM International, 2000, p 301-362 8. H.M. Herro and R.D. Port, Cavitation Damage, The Nalco Guide to Cooling Water System Failure Analysis, McGraw-Hill, Inc., 1993, p 270-271 9. R.B. Waterhouse, Fretting Wear, Friction, Lubrication, and Wear Technology, VoI 18, ASM Handbook, ASM International, 1992, p 242-256 10. Intergranular and Exfoliation Corrosion, Corrosion of Aluminum and Aluminum Alloys, J.R. Davis, Ed., ASM International, 1999, p 63-74 11. R.N. Parkins, An Overview—Prevention and Control of StressCorrosion Cracking, Mater. Perform., VoI 24, 1995, p 9-20 12. R.D. Granata, Nonmetallic Coatings, Corrosion Tests and Standards: Application and Interpretation, R. Baboian, Ed., ASTM, 1995, p 525-530 13. N.I. Gaynes, Testing of Organic Coatings, Noyes Data Corp., 1977

14. CG. Munger, Corrosion Prevention by Protective Coatings, National Association of Corrosion Engineers, 1984, Chapter 12 15. F. Altmayer, "Choosing an Accelerated Corrosion Test," Met. Finish., 61st Guidebook and Directory Issue, VoI 91 (No. IA), Jan 1993, p 483 16. T.C. Simpson and H.E. Townsend, Metallic Coatings, Corrosion Tests and Standards: Application and Interpretation, R. Baboian, Ed., ASTM, 1995, p 513-524 Selected References Corrosion, VoI 13, ASM Handbook, ASM International, 1987 Corrosion Basics—An Introduction, L.S. Van Delinder, Ed., NACE International, 1984 Corrosion: Understanding the Basics, J.R. Davis, Ed., ASM International, 2000 M.G. Fontana, Corrosion Engineering, 3rd ed., McGraw-Hill, 1986 H.H. Uhlig and R.W. Revie, Corrosion and Corrosion Control, 3rd ed., John Wiley & Sons, 1985

CHAPTER

4 J

P r i n c i p l e s a n d

o f W

F r i c t i o n e

a

r

FRICTION, WEAR, AND LUBRICATION are complex, interwoven subjects that may all affect the service life of a component or the efficient operation of a machine. While all three are important factors, the major emphasis in this Chapter will be on wear and the various methods used to reduce or prevent it, including the application of surface engineering treatments. More detailed information on the science and technology of friction, wear, and lubrication—known as tribology—can be found in Friction, Lubrication, and Wear Technology, Volume 18 of the ASM Handbook Friction Friction is the resistance to motion when two bodies in contact are forced to move relative to each other. It is closely associated with any wear mechanisms that may be operating and with any lubricant and/or surface films that may be present, as well as the surface topographies. The heat generated as a result of the dissipation of frictional interaction may affect the performance of lubricants, may change the properties of the contacting materials and/or their surface films, and, in some cases, may change the properties of the product being processed. Any of these results of frictional heating can cause severe safety problems because of the danger of mechanical failure of components due to structural weakening, severe wear (for example, seizure), or fire and explosion. In moving machinery, friction is responsible for dissipation and loss of much energy. It has been estimated, for example, that 10% of oil consumption in the United States is used simply to overcome friction. The energy lost to friction is an energy input that must continually be provided in order to maintain the sliding motion. This energy is dissipated in the

system, primarily as heat—which may have to be removed by cooling to avoid damage and may limit the conditions under which the machinery can be operated. Some of the energy is dissipated in various deformation processes, which result in wear of the sliding surfaces and their eventual degradation to the point where replacement of whole components becomes necessary. Wear of sliding surfaces adds another, very large component to the economic importance of friction, because without sliding friction, these surfaces would not wear. The need to control friction is the driving force behind its study. In many cases low friction is desired (bearings, gears, materials processing operations), and sometimes, high friction is the goal (brakes, clutches, screw threads, road surfaces). In all of these cases, constant, reproducible, and predictable friction values are necessary for the design of components and machines that will function efficiently and reliably. Important Terms and Concepts. It is useful to clearly separate the various terms and concepts associated with friction, such as "friction force," "friction coefficient," "frictional energy," and "frictional heating." These terms are defined subsequently in the context of solid friction, which can be defined as "the resistance to movement of one solid body over another." The movement may be by sliding or by rolling. The friction force is the tangential force that must be overcome in order for one solid contacting body to slide over another. It acts in the plane of the surfaces and is usually proportional to the force normal to the surfaces, N, or: (EqI) The proportionality constant is generally designated |JL or/and is termed the friction coefficient; which is the ratio between the friction force, F, and the load, N: (Eq 2) The friction coefficient typically ranges from 0.03 for a very well lubricated bearing, to 0.5 to 0.7 for dry sliding, and even >5 for clean metal surfaces in a vacuum. A JUL-value of 0.2 to 0.3 allows for comfortable walking; however, walking on ice is very difficult because the JUL-value for the ice/shoe pair may be 5.00 3.43 1.15 0.32 1.38 0.38 0.12 0.21 0.36

Manufacturing method(a)

Composition (manufacturer)(b)

Metals Ti-6A1-4V Haynes 93 25Cr iron Stellite 6K Stellite 3 Stellite 6B Type 304 stainless steel Type 316 stainless steel Haynes 188 Haynes 25 Type 430 stainless steel HK-40 Inconel 600 RA 330 Incoloy 800H Beta III Ti Incoloy 800 RA 333 Inconel 671 Mild steel Molybdenum Tungsten

W C C W C W W W W W W C W W W W W W W W W W

17Cr-16Mo-6.3Co-3C-bal Fe (Stellite) 25Cr-2Ni-2Mn-0.5Si-3.5C-bal Fe (OGC) 30Cr-4.5W-1.5Mo-1.7C-bal Co (Stellite) 31Cr-12.5W-2.4C-balCo 30Cr-4.5W-1.5Mo-1.2C-bal Co (Stellite) 17Cr-9Ni-2Mn-lSi-balFe 17Cr-12Ni-2Mn-lSi-2.5Mo-bal Fe 22Cr-14.5W-22Ni-0.15C-bal Co (Stellite) 22Cr-15W-10Ni-1.5Mn-0.15C-bal Co (Stellite) 17Cr-lMn-lSi-0.1C-balFe 26Cr-20Ni-0.4C-bal Fe 76Ni-15.5Cr-8Fe (HA) 19Cr-35Ni-1.5Mn-1.3Si-bal Fe (RA) 32.5Ni-21Cr-0.07C-46Fe (HA) 11.5Mo-6Zr-4.5Sn-balTi 32.5Ni-46Fe-21Cr (HA) 25Cr-1.5Mn-1.3Si-3Co-3Mo-3W-18Fe-bal Ni (RA) 50Ni-48Cr-0.4Ti (HA) 0.15C-balFe

0.17

Ceramics ZRBSC-M Chromite Refrax20C HD 435 Carbofrax D HD 430 Si3N4 Norbide BT-9

HP PS PS PS HP HP PS

(continued) (a) W, wrought; C, cast; HP, hot pressed; PS, pressed and sintered, (b) Manufacturer: BW, Babcock and Wilcox; Carbor, Carborundum Co.; GE, General Electric Co.; HA, Huntington Alloy Products; N, Norton Co.; OGC, Oregon Graduate Center; RA, Rolled Alloys Corp.; Stellite, Stellite Div., Cabot Corp.; UCAR, Union Carbide Corp. (c) REF = Volume loss material/volume loss Stellite 6B

Table 6

(continued) Relative erosion factor (REF)(c)

Material BT-12 BT-Il ZRBSC-D BT-24 BT-IO Noroc 33 TiC-Al2O3 SiC CBN GE diamond

Manufacturing method(a) PS PS HP PS PS HP PS HP

Composition (manufacturer)(b) 1.5MgO-49TiB2-3.5WC-bal Al 2 O 3 (OGC) 1.7MgO-38TiB2-3.5WC-bal Al 2 O 3 (OGC) ZrB2-SiC (N) 2MgO-30TiB2-3.5WC-bal Al 2 O 3 (OGC) 2MgO-30TiB2-3.5WC-bal Al 2 O 3 (OGC) Si3N4-SiC (N) (BW) (N) (GE) (GE)

20 0 C (700F)

700 0 C (1290 0F)

0.35 0.33 0.32 0.32 0.30 0.20 0.19 0.12 0 0

0.16 0.26 0.07 0.20 0.25 0.42 0.30 0.02 0 0

(a) W, wrought; C, cast; HP, hot pressed; PS, pressed and sintered, (b) Manufacturer: BW, Babcock and Wilcox; Carbor, Carborundum Co.; GE, General Electric Co.; HA, Huntington Alloy Products; N, Norton Co.; OGC, Oregon Graduate Center; RA, Rolled Alloys Corp.; Stellite, Stellite Div., Cabot Corp.; UCAR, Union Carbide Corp. (c) REF = Volume loss material/volume loss Stellite 6B

metals and ceramics, respectively. Tungsten carbide-cobalt (WC-Co) cermets gave REFs from about 0.1 to 1.6, and REF was found to increase with binder content. The REFs of most metals were similar at 20 and 700 0 C (68 and 1300 0 F), typically within about 20% of unity (Fig. 10). The three lowest room-temperature REFs for metals were for tungsten (0.48), molybdenum (0.52), and 1015 steel (0.76), and the highest was for Ti6A1-4V (1.26). The 700 0C (1300 0F) erosion rate of the standard (Stellite 6B) was 20% higher than the room-temperature value, so that 7000 C (1300 0F) REF values greater than 0.8 represent increases of erosion rate with temperature for a given material. These results illustrate the unfortunate fact that alloy-strengthening mechanisms such as solution or precipitation hardening that increase hardness do not significantly improve erosion resistance. According to Hansen (Ref 5), if service experience reveals an erosion problem for a metallic component, substitution of another metallic alloy will generally provide little improvement. Most ceramics tested had REF values in the range 0.3 to 0.6, although a few were much higher, and a few were nearly zero. It is important to note here (as discussed later) that for erodent particles of lower hardness than Al 2 O 3 (used in Hansen's study), significant improvements of erosion resistance can be obtained when the ratio of particle to target hardness, HJHV is less than 1. Prevention. Various design solutions have been developed in which high erosion rates are avoided by reconfiguring the system—such as the blocked tee configuration, in which a tee joint with one end closed is used in place of a gradual bend in a pipeline to prevent low-angle impingement. A good example of the variety of engineering solutions to SPE is provided by the case of power-generating steam turbines, in which exfoliation of iron oxide scale formed on steel heater tubes generates large pieces of scale that are fragmented into approximately 100 |xm particles, causing erosion of turbine blades, shrouds, valves, rivets, and other components. Liquid droplet erosion is also present. Solid particle erosion solutions include minimizing of scale formation by using austenitic steels or chromiz-

Ti-6AI-4V Haynes 93 25Cr iron HaynesStellite6K Haynes Stellite 3 Haynes Stellite 6B Type 304 stainless steel Type 316 stainless steel Haynes 188 Haynes 25 Type 430 stainless steel HK-40

Fiff. 1 0 R e ' a t ' v e erosion factors for *** selected commercially available metals at an impingement angle of 90°. Stellite 6B cobalt-base alloy was used as the reference material. Source: Ref5

lnconel 600 RA-330 lncoloy 800H BetaHTi lncoloy 800 RA 333 lnconel 671 Mild steel Molybdenum Tungsten

ZRBSC-M

Specimen perforated

Chromite Refrax20C HD 435 Carbofrax D HD 430 Si3N4 Norbide BT-9

p j a -| I Relative erosion factors for ^* selected ceramics at an impingement angle of 90°. Ratings based on using Stellite 6B cobalt-base alloy as the reference material. Source: Ref 5

BT-12 BT-11 ZRBSC-D BT-24 BT-10 Noroc 33 TiC-AI2O3 SiC Cubic boron nitride Diamond Relative erosion factor (REF)

ing treatments, particle removal with cyclones or screens, application of plasma-sprayed or diffusion coatings to blades, and redesign of turbine configurations. Liquid Erosion General Description. Erosion of a solid surface can take place in a liquid medium even without the presence of solid abrasive particles in that medium. Cavitation, one mechanism of liquid erosion, involves the formation and subsequent collapse of bubbles within the liquid. The process by which material is removed from a surface is called cavitation erosion, and the resulting damage is termed cavitation damage. The collision at high speed of liquid droplets with a solid surface results in a form of liquid erosion called: liquid impingement erosion. Cavitation damage has been observed on ship propellers and hydrofoils; on dams, spillways, gates, tunnels, and other hydraulic structures; and in hydraulic pumps and turbines. High-speed flow of liquid in these devices causes local hydrodynamic pressures to vary widely and rapidly. In mechanical devices, severe restrictions in fluid passages have produced cavitation damage downstream of orifices and in valves, seals, bearings, heatexchanger tubes, and Venturis. Cavitation erosion has also damaged water-cooled diesel-engine cylinder liners. Liquid impingement erosion has been observed on many components exposed to high-velocity steam containing moisture droplets, such as blades in the low-pressure end of large steam turbines. Rain erosion, one form of liquid-impingement erosion, frequently damages the aerodynamic surfaces of aircraft and missiles when they fly through rainstorms at high subsonic or supersonic speeds. Liquid impingement and cavitation erosion are of concern in nuclear power systems, which operate at lower steam quality than conventional steam systems, and in systems using liquid metals as the working fluid, where the corrosiveness of the liquid metal can promote rapid erosion of components. Basic Mechanisms. Liquid erosion involves the progressive removal of material from a surface by repeated impulse loading at microscopically small areas. Liquid dynamics is of major importance in producing damage, although corrosion also plays a role in the damage process, at least with certain fluid-material combinations. The process of liquid erosion is not as well understood as most other wear processes. It is difficult to define the hydrodynamic conditions that produce erosion and the metallurgical processes by which particles are detached from the surface. Evidently, both cavitation and liquid impingement exert similar hydrodynamic forces on a solid surface. In any event, the appearance of damaged surfaces (Fig. 12) and the relative resistance of materials to damage are similar for both liquid impingement and cavitation erosion. Additional information on the mechanism of material removal during cavitation can be found in Chapter 2, "Principles of Corrosion."

pja 1 2 A cast steel feedwater-pump impeller severely damaged by cavitation. Note how ^" damage is confined to the outer edges of the impeller where vane speed was maximum.

Prevention. Damage from liquid erosion can be prevented or minimized by reducing the intensity of cavitation or liquid impingement through design, using erosion-resistant materials, for example, cobaltbase alloys and tool steels including weld overlays of these materials, or, under certain conditions, using elastomeric coatings. Slurry Erosion General Description. Slurry erosion is progressive loss of material from a solid surface by the action of a mixture of solid particles in a liquid (slurry) in motion with respect to the solid surface. If the solid surface is capable of corroding in the fluid portion of the slurry, the slurry erosion will contain a corrosion component. Figure 13 shows an example of slurry erosion. A slurry by definition is a physical mixture of solid particles and a liquid (usually water) of such a consistency that it can be pumped. The particles must be in suspension in the liquid, and most pumpable slurries contain at least 10% solids. Apparent Abrasivity. Typical pumpable slurries possess inherent "apparent abrasivity," which must be determined by testing to enable cost predictions for pump replacement parts or other equipment used for slurries. Apparent abrasivity, without inhibition, is the complex synergistic reaction of many factors (Fig. 14). This reaction, known as the MorrisonMiller effect (Ref 6), is such that the wear response of a given material in

Fig. 1 3 Schematic of slurry erosion.

Resistance of protective film of corrosion products to abrasivity of slurry

Corrosive liquid

Dissolved air (oxygen or environment)

Specimen

Liquid

Lap

Galvanic corrosion (if two metals involved)

Released corrosive connate water from ore particles

True abrasivity of solids (particle hardness, size, shape, and concentration)

Soluble elements in solids forming corrosive solution

Fig. 1 4 Synergistic effects of seven factors in slurry abrasivity

a certain slurry does not indicate how that material would respond to another slurry. Similarly, the effect of a certain slurry on one material does not indicate how it would affect another material. Other modes of wear are also encountered when handling slurries. As shown in Fig. 15, these include abrasion-corrosion (the most severe wear mode), scouring wear, abrasive metal-to-metal wear (crushing and grinding), high-velocity erosion, low-velocity erosion, saltation wear (rapid wear caused when particles are moved forward in a series of short intermittent bounces from a bottom surface), and cavitation.

Polymer with embedded abrasive particles

(b)

(a)

Velocity profile

(O

Pipe wall (e)

(d) Pipe wall

Collapsing vapor bubbles

Large tumbling rocks U)

(g)

Fig. 1 5 blurry erosion wear modes, (a) Abrasion-corrosion, (b) Scouring wear, with wear areas equal (left) and unequal (center and right), (c) Crushing and grinding, (d) High-velocity erosion. (e) Low-velocity erosion, (f) Saltation erosion, (g) Cavitation

Miller numbers are used to determine the abrasivity of slurries, based on the rate of metal loss from a standard 27% chrome-iron wear block that reciprocates through any slurry, on a rubber lap, with an imposed load of 22.2 N (5 lbf) placed on the wearing block. The higher the number, the greater the aggressive effect of the slurry on part life. The additional effect of corrosion (usually present in liquid slurries, even those mixed with distilled water) was slow to be recognized. This was because such chrome-iron is rather corrosion resistant and the original test actually fulfilled its objective to reveal the "true abrasivity" of the dry particles. The effects of both abrasion and corrosion must be recognized in the operation of any slurry-handling system. Table 7 lists typical Miller numbers for selected slurry materials. The wide variation in Miller numbers for some materials is due to the inclusion of varying amounts of "tramp" materials that usually occur with the basic mineral. Information about the factors that contribute to Miller number abrasivity can be found in the ASTM G 75 standard. Prevention of slurry erosion is accomplished through design changes, for example, lessening the severity of pipe bends or using replaceable

Table 7

Typical Miller numbers for selected slurry materials

Material

Alundum (400 mesh) Alundum (200 mesh) Aragonite Ash Ash, fly Bauxite Calcium carbonate Carbon Carborundum (220 mesh) Clay Coal Copper concentrate Detergent Dust, blast furnace Gilsonite Gypsum Iron ore (or concentrate) Kaolin Lignite Limestone Limonite Magnesium hydrate Magnetite Microsphorite Mud, drilling Nickel Phosphate Potash Pyrite Quartzite Rutile Salt brine Sand and sand fill Sea bottom Shale Serpentine Sewage, digested Sewage, raw Sodium sulfate Soda ash tailings Sulfur Tailings (all types) Tar sand Waste, nickel Waste, coal

Miller number(s)

241 1058 7 127 83, 14 9, 33, 50, 76, 134 14 14, 16 1284 34,36 6,10,21,28,47,57 19, 37, 58, 68, 111, 128 6,8 57 10 41 28, 37, 64, 79, 122, 157, 234 7,30 14 22, 30, 39, 43, 46 113 4 64, 71, 134 76 10 31 68, 74, 84, 134 1,2 194 99 10 11 51, 68, 85, 116, 138, 149, 246 11 53,59 134 15 25 4 27 1 24, 61, 91, 159, 217, 480, 644 70 53 22, 28

wear backs on 90° elbows in high-velocity slurry pipelines and protective coatings. These include hardfacing alloys (e.g., cobalt alloys), plasmasprayed ceramics and cermets, hard platings, ceramic and carbide wear tiles, ceramic-filled repair cements, chromized steels, cast cylinder liners, plastic-lined pipe, and basalt-lined pipe (Ref 2). Adhesive Wear General Description. Adhesive wear is defined as wear by transference of material from one surface to another during relative motion under load due to a process of solid-state welding (Fig. 16); particles that are removed from one surface are either permanently or temporarily attached to the other surface. Adhesive wear may be between metallic materials, ceramics, or polymers, or combinations of these. It is dependent on adhesion between the

material, and that, in turn, depends on surface films like oxides or lubricants, as well as the mutual affinity of one material for another. If loads are light and the natural spontaneous oxidation of a metal can keep up with the rate of its removal by wear, then that wear rate will be relatively low (the oxide acting as a lubricant). This is called mild wear. If loads are high and the protective oxide is continually disrupted to allow intimate metal to metal contact and adhesion, then the wear rate will be high. This is called severe wear. Theory. Solid surfaces are almost never perfectly smooth but rather consist of microscopic or macroscopic asperities of various shapes. When two such surfaces are brought into contact under a load normal to the general planes of the surfaces, the asperities come into contact and elastically or plastically deform until the real area of contact is sufficient to carry the load. A bond may then occur between the two surfaces that is stronger than the intrinsic strength of the weaker of the two materials in contact. When relative motion between the two surfaces occurs, the weaker of the two materials fails, and material is transferred to the contacting surface. In subsequent interactions, this transferred material may be retransferred to the original surface (probably at a different location) or may become totally separated as a wear debris particle of an irregular morphology (Ref 7). Formulas that have been proposed (Ref 8, 9), to describe this phenomenon are of the form: (Eq 5) where Vis the wear scar volume, S is the distance of sliding, L is the load, H is the indentation yield strength (hardness) of the softer surface, and k is a probability factor that a given area contact will fracture within the weaker material rather than at the original interface. Formulations similar to Eq 5 have been shown to describe adhesive wear over fairly wide ranges of sliding distances, under a variety of conditions, over limited ranges of load, and over limited ranges of hardness when the same classes of material were compared (Ref 10, 11). While initial theoretical considerations assumed bare metal-to-metal contact, later Load Bodyl

Motion

Flg. 1 6 Schematic of adhesive wear

work assumed that oxide films, adsorbed films, and/or lubricant effects could be accounted for by changing k or by using more complex formulations (Ref 12). It has also been proposed that true metal-to-metal adhesive wear occurs at some time after motion is initiated when surface films or contaminants are worn away. Presumably, therefore, more than one adhesive wear mechanism could be operating at any given time, depending upon the presence or absence of various surface films in local areas. Changes in the apparent value of k or klH as a function of load may be the result of penetration of such films at sufficiently high load or the generation of new films as a result of frictional heating. The wear coefficient, k, has been determined experimentally for a large number of materials couples under various test conditions and geometries. The values found range from about 10~3 to 10~8 (Ref 7). For example, representative values of k for the end of a cylinder sliding against the flat surface of a ring at 1.8 m/s (6 ft/s) under a 400 g load are given for various combinations of cylinder and ring materials in Table 8. In many laboratory experiments, a stationary specimen with a small surface area rubs against a moving specimen with a large area. This frequently leads to a much higher wear rate on the smaller specimen than on the larger because of the constant contact and associated heating of the smaller specimen. This relative area effect may influence the wear mechanisms operating and may not be representative of field use. For most practical applications, volume loss, as predicted by Eq 5, must be converted to a linear value representing penetration or decrease in length, for example, increase in diameter of a journal bearing bushing, reduction in shaft diameter, or reduction in the length of brush in an electric motor. Primary Material Parameters. Materials selection for adhesion resistance requires careful consideration of the operating environment of the workpiece in addition to the total functional performance required of the workpiece itself. Wear properties of the steels vary widely with processing and heat treatment. Polymers are selected for sliding contact applications because of inherent properties such as inertness to many chemicals, relatively low galling tendency, and self-lubricating properties. Ceramics Table 8 Wear coefficients for various combinations of materials under conditions of dry sliding Sliding combination Cylinder material

Ring material

Low-carbon steel 60-40 brass PTFE Bakelite Beryllium copper Tool steel Stellite Tungsten carbide Tungsten carbide

Low-carbon steel Hardened steel Hardened steel Hardened steel Hardened steel Hardened steel Hardened steel Low-carbon steel Tungsten carbide

Wear coefficient, k 7.0 6.0 2.5 7.5 3.7 1.3 5.5 4.0 1.0

X X X X X X X X X

10~ 3 10~ 4 10~ 5 10~ 6 10~ 5 10~ 4 10~ 5 10~ 6 10" 6

Hardness of softer member, 106 g/cm2

18.6 9.5 0.5 2.5 21.0 85.0 69.0 18.6 130.0

Wear coefficients given are for the end of a cylinder sliding against the flat surface of a ring at 1.8 m/s (6 ft/s) under a 400 g load.

are used where extreme resistance to high-temperature oxidation or resistance to highly corrosive materials or gases is required. Prevention. The following guidelines are recommendations to prevent adhesive wear in metals, polymers, and ceramics: Avoid sliding similar materials together, particularly metals. If fatigue due to repeated high-contact pressure is not likely to be a problem, then high hardness is a desired property. However, avoid sliding hard metals against hard metals in lubricated systems to avoid scuffing and to accommodate debris. Consider the effect of relative hardness of phases in materials. For example, a high-chromium cast iron may have a hardness of 400 HB, which is moderate. However, that cast iron may contain Cr7C3, which has a hardness of about four times that of 400 HB and will damage the countersurface considerably. The same applies to polymers, which seem rather soft relative to metals. However, wear-resisting polymers often contain glass or some other hard filler that wears metal counterfaces rather severely. Hard phases in one body may fragment and become embedded in the counterface, which causes abrasion if the fragments extend above the surface. Even if done inadequately, lubrication will reduce wear. Some lubrication can be applied by providing an atmosphere that is corrosive in order to form surface films, many of which produce lower friction than if that film were not present.

Galling General Description. Galling can be considered a severe form of adhesive wear. With high loads and poor lubrication, surface damage can occur on sliding metal components. The damage is characterized by localized macroscopic material transfer, that is, large fragments or surface protrusions that are easily visible on either or both surfaces. This gross damage is usually referred to as galling, and it can occur after just a few cycles of movement between the mating surfaces. Severe galling can result in seizure of the metal surfaces. The terms scuffing and scoring are also used to describe similar surface damage under lubricated conditions. Scuffing is the preferred term when the damage occurs at lubricated surfaces, such as the piston ring-cylinder wall contact. Scoring typically describes damage that takes the form of relatively long grooves. Primary Material Parameters. Materials that have limited ductility are less prone to galling, because under high loads surface asperities will tend to fracture when interlocked. Small fragments of material may be lost, but the resultant damage will be more similar to scoring than to galling. For highly ductile materials, asperities tend to plastically

deform, thereby increasing the contact area of mated surfaces; eventually, galling occurs. Another key material behavior during plastic deformation is the ease with which dislocations cross slip over more than one plane. In face centered cubic (fee) dislocations easily cross slip. The rate of cross slip for a given alloy or element is usually indicated by its stacking-fault energy. Dislocation cross slip is hindered by the presence of stacking faults, and a high stacking-fault energy indicates a low number of impeding stacking faults and an increased tendency to cross slip and, hence, gall. Table 9 lists the stacking-fault energies of four fee elements. Nickel and aluminum have poor galling resistance, whereas gold and copper have good galling resistance. Materials that have a hexagonal close-packed (hep) structure with a high cla ratio have a low dislocation cross slip rate and are less prone to galling. This explains why cobalt-base alloys and cadmium-plated alloys resist galling while titanium alloys tend to gall. Prevention of galling is accomplished through proper design, for example, parts should have sufficient clearance, because tightly fitted parts are more prone to galling. Adequate lubrication and various hard surface coatings also can help prevent galling. Control of surface roughness is another important factor. Highly polished surfaces (1.5 |ixm, or 60 |min.) increase the tendency for wear and galling. It is theorized that very smooth surfaces lack the ability to store wear debris because of the absence of valleys between asperities, which means the asperities will have greater interaction. Also, lubricants will tend to wipe off the smoother surface. Too rough a finish results in interlocking asperities, which promote severe tearing and galling.

Fretting General Description. Fretting is a wear phenomenon that occurs between two mating surfaces; initially, it is adhesive in nature, and vibration or small-amplitude oscillation is an essential causative factor. Fretting is frequently accompanied by corrosion. In general, fretting occurs between two tight-fitting surfaces that are subjected to a cyclic, relative motion of extremely small amplitude. Fretting generally occurs at contacting surfaces that are intended to be fixed in relation to each other but that actually undergo minute alternating Table 9 Metals Gold Copper Nickel Aluminum Source: Ref 13

Stacking-fault energies of some common metals Stacking-fault energy, eVgs/cm2

30 40 80 200

Next Page

relative motion that is usually produced by vibration. The relative displacements between bodies are quite small (