17. Corrosion

Very few events, if any, have such a devastating effect from the point of view of structural, economical and human casualties as corrosion. Virtually any metallic material is subjected to corrosion even in a clean air environment. Structures are continuously attacked and sometime even completely demolished by corrosion. The damage caused in the world to metallic structures by corrosion can be assessed in terms of billions of Euro every year. Corrosion is certainly a very complex and many-sided phenomenon hardly tied to a single parameter theory. A first, generic classification divides corrosion into generalized or uniform and localized corrosion. Both are fundamentally electrochemical processes, but while the first affects almost uniformly the entire surface exposed to the corrosive agent, the second attacks the metal locally and selectively. Generalized corrosion is very common in carbon steel where it results in the formation of the so called rust which advances very slowly so that it is effectively dangerous only in very thin materials or very long terms. Localized corrosion, instead, initiates locally, unexpected and once started it proceeds very rapidly along intergranular or transgranular paths to go through the thickness of a work piece in matters of minutes or days, at most. Initiation may last years, but growth is fast. Localized corrosion is a subtle and continuous process in which stresses play a role, which is referred to as stress corrosion or SCC (stress corrosion cracking) or fatigue.