Aggression-inhibiting properties

The reasons why some anions exhibit strong inhibitive properties while others exhibit strong aggressive properties are not entirely clear. The principal distinction seems to be that inhibitive anions are generally anions of weak acids whereas aggressive anions are anions of strong acids. Due to hydrolysis, solutions of inhibitive anions have rather alkaline pH values and buffer capacities to resist pH displacement to more acid values. As discussed... 

The corrosion of iron (or steel) can be inhibited by the anions of most weak acids under suitable conditions " . However, other anions, particularly those of strong acids, tend to prevent the action of inhibitive anions and stimulate breakdown of the protective oxide film. Examples of such aggressive anions are the halides, sulphate, nitrate, etc. Brasher has shown that, in general, most anions exhibit some inhibitive and some aggressive behaviour towards iron. The balance between the inhibitive and aggressive properties of a specific anion depends on the following main factors (which are themselves interdependent). 

In a similar way, the concentration of aggressive anions, such as chloride, sulfate, or nitrate, affects the adsorption or film-forming properties of the inhibitor. It is emphasized that the pH as well as inhibitor and anion concentrations at the corroding metal surface rather than in the bulk of the solution are relevant for the inhibition efficiency, which is especially important in the case of localized corrosion. For example, a linear... 

Stabilization of the oxide films by repassivation is also important (function 2). The plugging of pores by formation of insoluble compounds (function 3) does not appear to be an essential fiinction but is valuable in extending the range of conditions under which inhibition can be achieved. The suppression of the adsorption of aggressive anions (function 4) by participation in a dynamic reversible competitive adsorption equilibrium at the metal surface appears to be related to the general adsorption behavior of anions rather than to a specific property of inhibitive anions. 

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. ... 

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