Effect of nitrate and chromate on the localized corrosion of iron and nickel

Abstract

The combined effect of some inorganic inhibitors - chromate and nitrate - on the rate of localized corrosion of iron and nickel was investigated. The efficiency of these inhibitors and the mechanism of their effect were found to depend on the potential. If the breakdown potential is lower than the second passivation potential, the corrosion rate decreases owing to the competitive adsorption of inhibitor and of aggresive ions and owing to a change in the activation energy of the metal dissolution. Substances that are strongly adsorbed on the electrode act in this region as strong inhibitors. In the range of the second passivation potential there operates another mechanism consisting in a favourable effect of the water molecules in the inhibitor hydration sphere on the rate of the passivation reaction. In this range, even such substances that are only weakly adsorbed on the electrode can act as strong inhibitors.