The Effect of Valence of Sulfur on the Passivation of Alloys 600 and 690 at 25 and 95°C Available to Purchase

The effects of the valence of sulfur in sulfur oxyanions on the passivation behavior of Alloys 600 and 690 in various heat treatments was studied at 25°C and 95°C using the potentiodynamic polarization method; the effects of scanning rate of the potential, valence of sulfur oxyanions, pH, alloy composition and alloy heat treatment were investigated. In general, the 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 that the various heat treatments of Alloy 600. Examination of surfaces after polarization over the potential range showed that intergranular corrosion occurred mainly in solutions of $SO42−,SO32−,S4O62−$ and $S2O32−$ whereas S^2- produced pitting. The increase of $S2O32−$ concentration at pH 6 and 95°C accelerates anodic dissolution. The variation of pH from 3.5 to 8 in 10^-2 M $S2O32−$ solutions does not change the anodic current density significantly. The transient instability of protective films was assessed by taking the ratio, R_SR, of the current density from a rapid scan at 20 mV/s to a slow scan at 0.2 mV/s. These ratios were determined as a function of potential for the variables studied; in some cases ratios as high as 200 were observed.