Effects of the Valence of Sulfur on Passivation of Alloys 600, 690, and 800 at 25°C and 95°C

Effects of the valence of sulfur, as sulfur oxyanions and sulfide (S²⁻ or HS⁻), on passivation of alloys 600, 690, and 800 (UNS N06600, N06690, and N08800) in various heat treatments were studied at 25°C and 95°C using potentiodynamic polarization. Effects of scanning rate, sulfur valence, pH, alloy composition, and heat treatment were investigated. In general, stability of passive films was found to decrease with the decrease of sulfur valence from +6 to ∡2. Alloy 690 in the thermally treated condition tended to be more stable than the various heat treatments of alloys 600 and 800, with some exceptions. Examination of surfaces after polarization over the potential range showed intergranular corrosion occurred mainly in solutions of sulfate (SO₄²⁻), sulfite (SO₃²⁻), tetrathionate (S₄O₆²⁻), and thiosulfate (S₂O₃²⁻); whereas S²⁻ or HS⁻ produced pitting. The increase of S₂O₃²⁻ concentration at pH 6 and 95°C accelerated anodic dissolution. Variation of pH from 3.5 to 8 in 10⁻² M S₂O₃²⁻ solutions did not change the anodic current density significantly. Reduction of S₂O₃²⁻ and S₄O₆²⁻ to species of lower valence was observed. The transient instability of protective films was assessed using a stress corrosion cracking (SCC) parameter (P_SCC) that incorporates the ratio of current densities obtained in fast and slow potentiodynamic scans. The relationship of P_SCC to the possible occurrence of SCC was discussed.