Chromate as a pitting corrosion inhibitor: Stochastic study

The pit generation rate gEwas measured on a stainless steel by a statistical method. Several amounts of chloride and chromate ions are used. The decrease in the pit generation rate, due to the presence of inhibitor in the chloride solution, allows the calculation of the pitting inhibition efficiency ξ for conventional conditions. For a 304 stainless steel with chromate ions added in chloride solutions, two domains are shown, corresponding to pitting (ξ < 99%) and inhibition (ξ > 99%). The study of the variation of ξ with chloride and chromate contents gives information on the process occurring in the pitting domain. Three processes are discussed: competitive adsorption between ions, displacement of chemical equilibrium and diffusion of the species. The limiting stage of the kinetics is competitive adsorption in the pitting domain, and diffusion in the inhibition domain where all active sites are covered by the inhibitor.