GROWTH OF CORROSION PITS ON STAINLESS-STEEL IN CHLORIDE SOLUTION CONTAINING DILUTE SULFATE

The effects of dilute sulphate on metastable and stable pitting of 304 stainless steel in chloride solution have been studied. The presence of sulphate causes the distribution of available pit sites to be shifted to a higher potential, implying that pit nucleation is inhibited. Pit propagation, in both the metastable and stable states. is also inhibited by the sulphate ion. The reduced pit propagation current densities are described quantitatively with respect to the effect of sulphate on the solubility of the metal cation in the pit anolyte. The results are consistent with the observation that metastable and stable pits grow under diffusion control, at a rate which is independent of electrode potential. Pit nucleation and propagation in stainless steel are two distinct processes, of which only the former is directly affected by the potential.