Depassivation protective oxide layers

Chloride is a depassivating ion. It reduces the corrosion resistance of many metals, especially those that naturally and strongly passivate to form a protective oxide layer, such as stainless steels and aluminum, by attacking the passivated film. 

In Chapter 2, we discussed the corrosion of steel in concrete and the effectiveness of the alkalinity in producing a passive layer of protective oxide on the steel surface which stops corrosion. In the previous section we observed that alkalinity is neutralized by carbonation. The depassivation mechanism for chloride attack is somewhat different. The chloride ion attacks the passive layer although in this case (unlike carbonation) there is no overall drop in pH. Chlorides act as catalysts to corrosion. They are not consumed in the process but help to break down the passive layer of oxide on the steel and allow the corrosion process to proceed quickly. This is illustrated in... 

Studies of the structure of passive layers are eventually of technological value only if they can substantially delay the breakdown of that passive layer which is so important to the stability of the metal it protects. As far as the all-important iron and its alloys are concerned, the polymeric oxide model, with the part played by water in putting together the polymer elements, seems to be the most consistent with the facts. In considering its breakdown, one generally discusses this in terms of the effects of Cl" adsorption, but there are other ions (T, Br, SO ) that also cause depassivation. 

Potential distribution at sites protected by a crystalline barrier-type oxide (a) at defect-free barrier layer, (b) at defect (grain boundary), and (c) at depassivated site (pit nucleus). (From Marcus, P. et al. Corrosion Sci., 50,2698,2008.)... 

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