Major corrosion problems cathodic reaction

Many surfaces will be homogeneous, i.e. metals or alloys in a uniform environment, and the anodic and cathodic reactions wilt occur randomly over the whole surface, although maybe at particular sites, e.g. lattice imperfections. Since metals are good electronic conductors, however, this need not be the only situation where corrosion occurs. For example, where two metals are in contact or one metal is in contact with two quite different environments, e.g. a teg of a North Sea oil platform passing through the sea water into the sea bed, there may be well defined anodic and cathodic areas and electrons pass through the metal from one to the other. Indeed, it will be seen that such situations often lead to the worst corrosion problems and are a major cause of component failure. 

Many semiconductor materials, by themselves, are not good electrocatalysts for Hz or Oz evolution, and in the photoanode reaction some dissolution or corrosion of the semiconductor itself is a common problem. It was proposed and shown by Bockris et al. (204) that major improvements in photoelectrolysis of water could be achieved, from the kinetic point of view, by preparing the semiconductors with thin coatings of electrocatalysts appropriate to the respective cathodic or anodic reactions involved such... 

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