Corrosion of Cobalt and Cobalt Alloys^★

Investigations are reviewed of the composition and structures of corrosion films and scales found on cobalt and on simple and complex cobalt-base industrial alloys. The corrosion resistance of these films and scales depends on metal and alloy composition, environment (an atmosphere containing one or more of CO₂, S, H₂S, O₂, H₂O, SO₂, V₂O₃, NaCl, etc.), pressure, temperature, and time. Additional factors, alone or in combination, such as surface preparation, stress, thermal shock, geometry of the part, velocity, density, and flow pattern of the corroding environment also have an effect. Data on oxidation, sulfidation, and hot corrosion (a combination of the first two) of cobalt and its alloys are summarized in some detail, as are the known characteristics of the films and scales that are formed. Only the work that used techniques permitting determination of composition and crystal structure of these films and scales was reviewed. The effect of impurities in metals and alloys may be as significant as that of added alloying elements; impurities may alter the scales' chemical and physical properties–increasing, decreasing, or neutralizing corrosion rates. Until data are available on the intereffect on diffusion of elements present in scales, films, and metal surfaces, the mechanisms of corrosion proposed are only hypotheses.