A Competitive Adsorption Model for the Inhibition of Crevice Corrosion and Pitting

Similarities in the inhibition of crevice corrosion and pitting can be explained on the basis of competitive adsorption between inhibitive and aggressive ions. A mathematical model has been developed which takes into account competitive adsorption between aggressive ions (Cl − ) and inhibitive ions , with each ion adsorbing according to a Temkin isotherm. In the case of crevice corrosion of iron, this model explains the linear relationship which has been observed between the logarithm of the chloride ion activity and the logarithm of the minimum chromate activity required for protection against crevice corrosion. In the case of pitting, this same model describes the linear decrease in pitting potential with the logarithm of the Cl − activity at constant concentration. The mathematical model, in conjunction with electrochemical data and adsorption isotherms, allows calculation of a critical ratio, , of surface coverages of aggressive to inhibitive ion necessary to initiate localized corrosion. This critical ratio was computed to be approximately the same value (1.9) for the crevice corrosion and pitting of iron in chromate/chloride solutions. Nominal confirmation for this computed value of was provided by surface analyses using XPS.