1143 Metallurgical Effects

As previously discussed corrosion processes take place at the metal-electrolyte interface with specific reactions occurring at the metal surface. It seems quite reasonable, therefore, that the nature of the metal, its composition, and metallurgical structure will determine its corrosion resistance. These are in turn influenced by the manufacturing history and any thermal treatments, that is, heat treatment or welding.

Whether or not a metal will corrode uniformly or at specific sites (locally) will be dependent upon the relative stability of the components of the alloy, the presence of metalloids such as carbides, and local compositions within a single phase. Nonmetallic inclusions such as oxides and sulfides, for example, play an important role in the development of pitting. Corrosion may occur because of the difference in electrochemical potential between the matrix (grain) and grain boundary. Such "intergranular" corrosion is favored by alloys that exhibit an active-passive behavior, for example, stainless steels. A further description of intergranular corrosion is given in Section 11.5.

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