Corrosion Basics: Engineering Materials

There are approximately 200 commercial compositions of aluminum alloys available, and their level of corrosion resistance varies widely with composition. Photo by Getty Images.

The corrosion literature is filled with data on the performance of various materials in myriads of chemical environments. While modern electronic search techniques can provide ready access to a wealth of constantly updated information, the sheer volume of data can be overwhelming. 

Engineers must constantly be on guard when considering such information to be certain not only that the chemical environment is adequately defined, but also that the particular alloy (including its heat treatment) and the character of attack are fully described and understood.

For example, corrosion rates for “aluminum” 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 with the specific purpose of improving corrosion resistance to 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 have alloys of the heat-treatable 2000 and 7000 series. 

The corrosion behavior of the 2000 and 7000 series alloys is strongly influenced by heat-treatment practices. For instance, merely varying the heat treatment can change the character of attack on the 7075 (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 oversimplification leads to considerable confusion and some misapplications. For example, their distinctive characteristics make it necessary to differentiate austenitic steels from ferritic steels, as well as between stabilized and nonstabilized grades of austenitic SS.

The technical literature is far more reliable 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 of alternatives from which a final choice can be made more quickly. 

It is no disgrace for an engineer to seek advice from experts who have specialized knowledge about materials and applications. Major materials suppliers 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 recognize that it makes good business sense 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 misapplications resulting from one or more of the following factors:

  • Selection of the wrong materials
  • Improper treatment or fabrication of the material
  • An inadequately controlled or defined environment
  • 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 engineering 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 critical 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 exists a standard that normally identifies the analysis, form, properties, and other details of their characteristics. The use of such specifications provides a recognizable basis for the supplier to furnish the specific material desired. 

With few exceptions, the various fabrication and use codes are based on these standards. The use of these consensus standards whenever possible is sound engineering practice.

This article is adapted from Corrosion Basics—An Introduction, Second Edition, Pierre R. Roberge, ed. (Houston, TX: NACE International, 2006), pp. 267–268.

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