Transport Processes and the Mechanism of Pitting of Metals

A pit model was developed on the assumption that the metal ions hydrolyze inside the pits and that the corrosion products are transported by diffusion. Concentrations of Me²⁺, , and H⁺ ions, as a function of pit depth and current density, for Zn, Fe, Ni, Co, Al, and Cr were calculated. The main reason for passivity breakdown at the initial stages of pit growth, was found to be the localized acidification due to metal ions hydrolysis. Assuming a criticalpH value for pit initiation, the following experimental facts could be explained: (i) the effect of the external pH on the pitting potential of Fe and stainless steel; (ii) the effect of sodium borate concentration on the pitting potential of Zn; (iii) the effect of weak acid salts on the pitting potential of Al; (iv) the oscillations of the electrode potential of stainless steel and nickel in solutions of ions; (v) the existence of a pitting inhibition potential ; and (vi) the existence of a pitting protection potential. Through analysis of the transport processes inside a pit it was also concluded that the pitting potential of a metal should change with the Cl⁻ ion concentration according to the equation

being the slope of the curve at room temperature.