Effects of Sulfur Compounds on the Pitting Behavior of Type 304 Stainless Steel in Near-Neutral Chloride Solutions Available to Purchase

Pitting potentials have been measured potentiodynamically for Type 304 stainless steel in neutral or mildly acidic solutions containing 0.25M NaCl and various concentrations of sulfur species from 0 to 2 molar. Additions of 0.01 to 0.02M Na₂S₂O₃ lowered the pitting potential by more than 300 mV, while additions of more than 0.5M Na₂S₂O₃ inhibited pitting. KSCN showed similar but less marked effects, while increasing Na₂S additions up to 0.1 molar (giving H₂S and HS⁻ at neutral pH) caused an increasing reduction in the pitting potential. Additions of Na₂S₄O₆ up to 0.05M promoted pitting, but addition of 0.01M Na₂SO₃ had no effect. Parallel pitting potential measurements using a scratch technique gave lower values for the sulfur containing solutions but a higher value for the plain chloride solution; this effect of surface deformation was also reflected in the results obtained potentiodynamically on abraded and electropolished surfaces.