Pitting of Stainless Alloys in Sulfate Solutions Containing Thiosulfate Ions

A scratch test was used to study the pitting of an Fe-19Cr-10Ni alloy in a solution containing 0.25 molar (M) Na2SO4 and 0.025 M Na2S2O3. Pitting was confined to a range of potentials from − 325 to −100 mV (SCE). The maximum susceptibility was at −225 mV, where pitting was very rapid. In this system, pitting can occur only when the thiosulfate ion can be reduced to absorbed sulfur or sulfide, thereby activating anodic dissolution. If the surface was not scratched, pit initiation was very infrequent; because of the limited potential range of pitting, it would be unwise to use a potentiodynamic technique in this or a similar system. Pitting also occurred in a sodium perchlorate solution containing thiosulfate, confirming the essentially inert role of the sulfate ion.