Corrosion resistance chemistry passive film

Data on the chemistry and structure of thin oxide layers (passive films) produced by anodic polarization of metallic electrodes are necessary to understand and predict the properties of these films, in particular their corrosion resistance. There are now many available data on the chemical composition of passive films formed on metals and alloys. Surfece chemical analysis techniques have been, and still are, very useful to obtain such data. In sharp contrast, there is a lack of data on the structure of passive films. This is in part due to the difficulty of any structural analysis of very thin films on... 

Thin anodic oxide overlayers (passive films) can be highly resistant against corrosion. They also play a major role in adhesion, tribology, catalysis and micro-electronics. A better understanding of the relationship of the chemistry and structure to the properties of metal surfaces covered with thin oxide overlayers is needed. 

Corrosion resistance of stainless steels results from formation of a passive oxide film which is stable in an oxidising enviromneait. Any physico-chemical instability of this oxide film either as a result of change in the chemistry of the environment or formation of cracks and/or scratches on the metal surface provides conditions for formation of an oxygen concentration cell which can result in localised corrosion. An example of chemical change of the environment leading into oxide film instability mentioned above is the effect of chloride ions. Chloride ions can locally damage the protective film on stainless steels [44]. 

The mechanism by which a pit is formed is always that of the film rupture/slip dissolution process, already mentioned. The passivizing oxide film that protects the metal surface, at least in corrosion resistant alloys where it is tight and stable, breaks somewhere. This can be the consequence of fatigue or a localized change of corrosion potential (local chemistry) that enters into the transpassive zone where pitting occurs, see Fig. 13.11. Pitting, in fact, is a form of extremely localized corrosion that leads to the creation of a small hole in the metal. The presence of... 

From the above results, we can conclude that nanocrystallization can significantly influence the chemistry or composition of the passive film, which influences the corrosion resistance. It can promote the emichment of passive elements such as Cr and Ti, and it can also decrease the adsorption of CT, both of which are due to the small grain size and the unique surface of NC coatings. 

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