Materials Performance is the world's most widely circulated magazine dedicated to corrosion prevention and control. MP provides information about the latest corrosion control technologies and practical applications for every industry and environment.
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Understanding the basic principles and causes of corrosion Engineering Materials T he corrosion litera- ture is filled with data on the perfor- mance of various materials in myri- ad chemical environments. While modern electronic search techniques can pro- vide ready access to a wealth of constantly updated infor- mation, the sheer volume of data can be overwhelming. Engineers must constantly be on guard when consid- ering such information to be certain not only that the chemical environment is ade- Y]I\MTa LMÅVML J]\ IT[W \PI\ the particular alloy (including its heat treatment) and the character of attack are fully described and understood. For example, corrosion rates for "alumi- num" often are given without further alloy designation. There are approximately 200 commercial compositions of aluminum alloys available. Their level of corrosion resistance varies widely with composition; in fact, many of these alloys were developed _Q\P \PM [XMKQÅK X]ZXW[M WN QUXZW^QVO KWZ- rosion resistance in particular environments. The wrought alloys of the 1000, 3000, 5000, and 6000 series are roughly similar in their corrosion behavior and typically have far better corrosion resistance in most chemical environments than alloys of the heat-treat- 88 MATERIALS PERFORMANCE November 2012 able 2000 and 7000 series. The corrosion behavior of the 2000 and 7000 series alloys Q[ [\ZWVOTa QVÆ]MVKML Ja PMI\ \ZMI\UMV\ practices. For instance, merely varying the heat treatment can change the character of attack on the 7075 (UNS A97075) alloy in a 3.5% sodium chloride (NaCl) solution from intergranular to pitting. Stainless steels (SS) likewise tend to be treated in the literature as a class and not as individual alloys. Naturally, this W^MZ[QUXTQÅKI\QWV TMIL[ \W KWV[QLMZIJTM confusion and some misapplications. For example, their distinctive characteristics make it necessary to differentiate aus- tenitic steels from ferritic steels, as well I[ JM\_MMV [\IJQTQbML IVL VWV[\IJQTQbML grades of austenitic SS. The technical literature is far more reli- able in identifying which materials cannot be used than in deciding which can be used. Nevertheless, the literature can narrow the choice of materials to a manageable number WN IT\MZVI\Q^M[ NZWU _PQKP I ÅVIT KPWQKM KIV be made more quickly. It is no disgrace for an engineer to seek advice from experts who PI^M [XMKQITQbML SVW_TMLOM IJW]\ UI\MZQIT[ and applications. Major materials suppli- ers often maintain staffs of consultants to advise customers on the proper use of their materials. While the overall objective may be the sale of their products, these suppliers ZMKWOVQbM \PI\ Q\ UISM[ OWWL J][QVM[[ [MV[M to avoid an inappropriate application that could result in damaging publicity. It is perfectly fair to request case histories that demonstrate the suitability of materials for the project at hand. Failures of materials in service usually are traceable to misap- plications resulting from one or more of the following factors: Q Selection of the wrong materials Q Improper treatment or fabrication of the material Q An inadequately controlled or LMÅVML MV^QZWVUMV\ Q Improper design Materials Selection A large variety of materials, ranging from platinum to concrete, is used by the engineer to construct bridges, vehicles, process plant equipment, pipelines, and power plants. The properties of engineer- ing materials depend upon their physical structures as well as their basic chemical and metallurgical composition. Although the primary focus of a corrosion engineer is on the chemical stability and corrosion resistance of these materials, it is criti- cal to cooperate with other design team members familiar with the mechanical, physical, and other properties to ensure that the desired materials performance can be achieved. Throughout the process, it is essential that the materials under discussion are identified precisely to obtain accurate information and maintain consistency for comparisons. For most materials, there M`Q[\[ I [\IVLIZL \PI\ VWZUITTa QLMV\QÅM[ the analysis, form, properties, and other details of their characteristics. The use of []KP [XMKQÅKI\QWV[ XZW^QLM[ I ZMKWOVQbIJTM JI[Q[ NWZ \PM []XXTQMZ \W N]ZVQ[P \PM [XMKQÅK material desired. With few exceptions, the various fabrication and use codes are based on these standards. The use of these con- sensus standards whenever possible is sound engineering practice. This article is adapted by MP Editorial Advisory Board Member Norm Moriber from Corrosion Basics—An Introduction, Second Edition, Pierre R. Roberge, ed. (Houston, TX: NACE International, 2006), pp. 267-268. NACE International, Vol. 51, No. 11 Corrosion Basics