Corrosion Science Chemistry Research Article

The Effects and Economic Impact of Corrosion 5 • Appearance of the corroded metal: Corrosion is either uniform and the metal corrodes at the same rate over the entire surface, or it is lo- calized, in which case only small areas are affected. Classification by appearance, which is particularly useful in failure analysis, is based on identifying forms of corrosion by visual observa- tion with either the naked eye or magnification. The morphology of at- tack is the basis for classification. Figure 2 illustrates schematically some of the most common forms of corrosion. Eight forms of wet (or aqueous) corrosion can be identified based on appearance of the corroded metal. These are: • Uniform or general corrosion • Pitting corrosion • Crevice corrosion, including corrosion under tubercles or deposits, filiform corrosion, and poultice corrosion • Galvanic corrosion • Erosion-corrosion, including cavitation erosion and fretting corro- sion • Intergranular corrosion, including sensitization and exfoliation • Dealloying, including dezincification and graphitic corrosion • Environmentally assisted cracking, including stress-corrosion crack- ing, corrosion fatigue, and hydrogen damage In theory, the eight forms of corrosion are clearly distinct; in practice however, there are corrosion cases that fit in more than one category. Other corrosion cases do not appear to fit well in any of the eight catego- ries. Nevertheless, this classification system is quite helpful in the study Load More noble metal No corrosion Uniform Galvanic Flowing corrodent Cyclic movement Erosion Fretting Tensile stress Pitting Exfoliation Dealloying Intergranular Stress-corrosion cracking Fig. 2 Schematics of the common forms of corrosion Metal or nonmetal Crevice Cyclic stress Corrosion fatigue

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