Effect of Mo on pitting corrosion of ferritic steels in bromide and chloride solutions

A model for pitting corrosion of stainless steels, independent of changes of passive film properties, was tested using Fe-18%Cr-x%Mo alloys in bromide and chlorine solutions. In 1M LiCl the pitting potential improved from {minus}50 mV{sub sce} to about 1200 mV{sub sce} on increasing Mo from 2% to 10%. In 1M LiBr the pitting potential increased from 125 to only 560 mV{sub sce}. Active dissolution kinetics of these steels in saturated solutions in a simulated pit were measured. Tafel lines for dissolution moved to more noble potentials with increases in Mo, indicating Mo inhibited dissolution rates. The potential increases were found to be equal to the increases in pitting potential for both halides. Agreement was interpreted in terms maintaining high halide concentrations in the pit by high rates of active metal dissolution. Bromide was less effective suggesting it interacted with Mo adsorbed on the dissolving surface