Abstract
Effect of sulfate and other several oxoacid salts on the crevice corrosion behavior of type 304 stainless steel in chloride solutions has been investigated at 423K. In chloride solutions at 423K, passive films typically broke in pitting mode within crevices. In the existence of oxoacid salts, crevice corrosion potential shifted to nobler values, which was accompanied by the change in corrosion mode within a crevice from pitting to general dissolution. The composition of solution at which crevice corrosion was completely inhibited was expressed by the empirical equation:
log [Cl-]=a log [oxoacid]+b.
The value of a was almost same as obtained for free surface while b decreased significantly both in case of sulfate and phosphate. Effect of sulfate on crevice corrosion was discussed quantitatively in terms of dissociation equilibrium of hydrogen sulfate ion which buffers a decrease in pH on hydrolysis of chromium (III). Effect of oxoacid salts on the repassivation potential (ER) was also examined. Nitrate and phosphate shifted ER to nobler values, while molybdate worked the other way about. Sulfate did not change ER.
