Corrosion Inhibition Mechanism of Chromates

Chromates are very effective inhibitors of Fe, Al, Cu, Zn corrosion. The unique chemical and electronic properties of the oxo-compounds of chromium give rise to a unique ability to inhibit corrosion in ferrous and nonferrous materials. They are both anodic and cathodic inhibitors due to their abilities to form precipitates with the dissolving metal ions 

Specifically for aluminum corrosion released Cr oxoanions inhibit pit initiation by adsorbing onto aluminum oxides, thereby discouraging adsorption of anions such as chloride and sulfate, which promote dissolution and destabilization of the protective oxides. Thus, competitive adsorption of chromates with regard to aggressive anions such as chloride and sulfate appears as another major property of chromate conversion coatings.  

Along with nitrites, chromates passivate independent of dissolved oxygen in contrast to molybdates and vanadates, which require the presence of dissolved oxygen as a primary passivator.  

The hydrolysis reactions generate which are consumed by redox reactions. In alkaline conditions,  

In case of iron corrosion in near neutral conditions,  

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Given some basic information about the corrosion inhibition mechanisms of chromates, many studies have been conducted for chromate replacements. For effective replacement of hexavalent Cr, however, an inhibitor has to inhibit the oxygen reduction reaction as well as anodic dissolution/pitting, and several studies indicate that hybrid formulations seem to be the best way to do just that. Typically, in these hybrid formulations an organic oxygen reduction reaction inhibitor is included with environmentally benign anodic inhibiting anions. 

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