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Corrosion Testing — Practice 151 • Water uptake and hydrolysis are greater than in the field. • A constant water film with high conductivity is present, which does not happen in the field. b. Elevated temperature • Water, oxygen, and ion transport are greater than in the field. • For some paints, the elevated temperature of the test comes close to the glass transition temperature of the binder. c. High chloride concentration (effect on corrosion depends on the type of protection the coating offers) • For sacrificial coatings, such as zinc-rich primers, the high chloride content together with the constant high humidity means that the zinc is not likely to form a passive film as it does in the field. • For inhibitive coatings, chlorides adsorb on the metal surface, where they prevent passivation. • For barrier coatings, the osmotic forces are much less than in the field; in fact, they may be reversed completely from that which is seen in reality. In the salt spray test, corrosion at a scribe or defect is exaggeratedly aggressive compared with a scribe under intact paint. Lyon, Thompson, and Johnson [56] point out that the high sodium chloride content of the salt spray test can result in corrosion morphologies and behaviors that are not representative of natural conditions. Harrison has pointed out that the test is inappropriate for use on zinc — galvanized substrates or primers with zinc phosphate pigments, for example — because, in the constant wetness of the salt spray test, zinc undergoes a corrosion mechanism that it would not undergo in real service [57]. This is a wellknown and well-documented phenomenon and is discussed in depth in chapter 7. 8.4.3 IMPORTANCE OF WET/DRY CYCLING Skerry, Alavi, and Lindgren have identified three factors of importance in the degradation and corrosion of painted steel that are not modeled by the salt spray test: wet/dry cycling, a suitable choice of electrolyte, and the effects of UV radiation (critical because of the breakdown of polymer bonds in the paint) [3]. Lyon, Thompson, and Johnson explain why wet/dry cycles are an important factor in an accelerated test method [56]: Many studies have shown the specific importance of wetting and drying on atmospheric corrosion... On a dry metal surface, as the relative humidity (RH) is increased, the corrosion rate initially rises, then decreases to a relatively constant value which becomes greater as the RH is increased. A similar effect is observed during physical wetting and drying of a surface. Thus, on initial wetting, the corrosion rate rises rapidly as accumulated surface salts first dissolve. The rate then decreases as the surface electrolyte dilutes with continued wetting. The corrosion rate also rises significantly during drying because of both the increasing ionic activity as the surface electrolyte concentrates © 2006 by Taylor & Francis Group, LLC
152 Corrosion Control Through Organic Coatings and the reduced diffusion layer thickness for oxygen as the condensed phase become thinner. However, eventually, when the ionic strength of the electrolyte layer becomes very high and salts begin to crystallize, the corrosion rate decreases. Simpson, Ray, and Skerry agree that ‘‘cyclic wetting and drying of electrolyte layers from the panel surface is thought to stress the coating in a more realistic manner than, for example, a continuous ASTM B-117 salt spray test, where panels are placed in a constant, high relative humidity (RH) environment” [58]. Several workers in this field have reported that cyclic testing with a significant amount of drying time yields more realistic results on zinc-coated substrates [59-61]. REFERENCES 1. Goldie, B., Prot. Coat. Eur., 1, 23, 1996. 2. Appelman, B., J. Coat. Technol., 62, 57, 1990. 3. Skerry, B.S., Alavi, A., and Lindgren, K.I. J. Coat. Technol., 60, 97, 1988. 4. Townsend, H., Development of an improved laboratory corrosion test by the automotive and steel industries, in Proc. 4th Annual ESD Advanced Coatings Conference, Dearborn, MI, 8-10 November 1994. Engineering Society of Detroit (ESD), 1994. 5. Prfung des korrosionsschutzes von kraftfahrzeuglackierungen bei zyklisch wechselnder beanspruchung, Std. VDA 621-415, German Association of the Automotive Industry, Frankfurt, Germany. 6. Accelerated corrosion test, Corporate Standard STD 423-0014, Issue 1, Volvo Group, Gothenburg, 2003. 7. Ström, M., in Proc. Conf. Automotive Corrosion and Prevention. Dearborn, MI, Dec 4-6, 1989, Society of Automotive Engineers, Warrendale, PA, 1989. 8. Ström, M. and Ström, G., Proc. Skan Zink ’91, Helsingör (Sweden), Sept. 1991. 9. Pletcher, D., A First Course in Electrode Processes, The Electrochemical Consultancy Ltd., Romsey, England, 1991, 229. 10. ISO 4628/3-1982-Designation of degree of rusting, International Organization for Standardization, Geneva, 1982. 11. Paul, S., (Ed.), Surface Coatings: Science & Technology, 2nd ed., John Wiley & Sons, Chichester, England. 1996. 12. Sacco, E.A. et al., Lat. Am. Appl. Res. 32, 4, 2002. 13. Walker, P., Paint Technol., 31, 22. 1967. 14. Özcan, M., Dehri, I. and Erbil, M., Prog. Org. Coat., 44, 279, 2002. 15. Lavaert, V. et al., Prog. Org. Coat., 38, 213, 2000. 16. Krolikowska, A., Prog. Org. Coat., 39, 37, 2000. 17. Sekine, I., Proc. 99th Symposium on Corrosion Protection, Tokyo, Japan, 1994, 51. 18. Sekine, I. and Yuasa, M., Proc. Annual Meeting of the Japan Society of Colour Material, Tokyo, 1995, 70. 19. Sekine, I., Prog. Org. Coat., 31, 73, 1997. 20. Kendig, M. and Scully, J. Corrosion, 46, 22, 1990. 21. Walter, G.W., Corros. Sci., 32, 1041, 1991. 22. Walter, G.W., Corros. Sci.,32, 1059, 1991. 23. Walter, G.W., Corros. Sci.,32, 1085, 1991. 24. M. Stratmann et al., Corros. Sci.,6, 715, 1990. 25. Forsgren, A. and Thierry, D., Corrosion properties of coil-coated galvanized steel, using field exposure and advanced electrochemical techniques, SCI Rapport 2001:4E. Swedish Corrosion Institute (SCI), Stockholm, 2001. © 2006 by Taylor & Francis Group, LLC
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© 2006 by Taylor & Francis Group,
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Corrosion of Ceramic and Composite
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Published in 2006 by CRC Press Tayl
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Preface This book has been written
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Author Amy Forsgren received her ch
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Contents Chapter 1 Introduction ...
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2.4.2 Reactive Reagents ...........
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6.2.4.1 Alkaline Blistering........
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1 Introduction This book is not abo
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Introduction 3 • A dissolution pr
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Introduction 5 1.2.2 ELECTROLYTIC R
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Introduction 7 to the metal is a ne
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Introduction 9 9. Thomas. N.L., Pro
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12 Corrosion Control Through Organi
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5 Abrasive Blasting and Heavy-Metal
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