Attempted explanations of passivity in metals and alloys since the early proposals of Faraday in 1836 fall into the categories: (1) metal modification, (2) reaction velocity, (3) oxide film, and (4) adsorption. Historically the subject proved to be unusually complex; general concurrence on any one viewpoint was not achieved despite considerable effort and thought by generations of able investigators. Notwithstanding, progress is being made as the search continues. The present survey emphasizes that two basic mechanisms of passivity prevail depending on the metal and its environment. The first depends on a diffusion barrier stoichiometric oxide or other type of reaction product film; the second depends on reduced reaction kinetics introduced by a surface film of chemisorbed oxygen ranging from less than a monolayer to the dimensions of a thin non-stoichiometric metal oxide of variable hydrogen content. The latter film in part impedes rate of metal ion hydration; it is a source of passivity mostly in the transition metals and their alloys. The film of chemisorbed oxygen is more strongly bonded to the base metal than is a metal oxide. This accounts for an observed better resistance to erosion and cavitation damage of metals passive because of chemisorbed oxygen (e.g. 18-8 stainless steel) compared to metals protected by stoichiometric oxides (e.g. iron, copper, cupro-nickels, lead). Since chemisorbed oxygen can be mechanically activated to form the metal oxide by pronounced impingement, abrasion and rubbing of the metal surface, any factor that resists such activation is beneficial. Alloyed cobalt apparently acts in this way, explaining why Co-base passive alloys are appreciably more resistant to erosion, cavitation damage and fretting corrosion than the Ni- or Fe-base passive alloys. © 1979 Pergamon Press Ltd.
