Effects of the valence of sulfur on passivation of alloys 600, 690, and 800 at 25 C and 95 C
- Univ. of Minnesota, Minneapolis, MN (United States). Dept. of Chemical Engineering and Materials Science
Effects of the valence of sulfur, as sulfur oxyanions and sulfide (S{sup 2{minus}} or HS{sup {minus}}), on passivation of alloys 600, 690, and 800 (UNS N06600, NO6690, and NO8800) 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 {minus}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 (SO{sub 4}{sup 2{minus}}), sulfite (SO{sub 3}{sup 2{minus}}), tetrathionate (S{sub 4}O{sub 6}{sup 2{minus}}), and thiosulfate (S{sub 2}O{sub 3}{sup 2{minus}}); whereas S{sup 2{minus}} or HS{sup {minus}} produced pitting. The increase of S{sub 2}O{sub 3}{sup 2{minus}} concentration at pH 6 and 95 C accelerated anodic dissolution. Variation of pH from 3.5 to 8 in 10{sup {minus}2} M S{sub 2}O{sub 2}{sup 2{minus}} solutions did not change the anodic current density significantly. Reduction of S{sub 2}O{sub 3}{sup 2{minus}} and S{sub 4}O{sub 6}{sup 2{minus}} to species of lower valence was observed. The transient instability of protective films was assessed using a stress corrosion cracking (SCC) parameter (P{sub SCC}) that incorporates the ratio of current densities obtained in fast and slow potentiodynamic scans. The relationship of P{sub SCC} to the possible occurrence of SCC was discussed.
- Sponsoring Organization:
- USDOE
- OSTI ID:
- 349887
- Journal Information:
- Corrosion, Journal Name: Corrosion Journal Issue: 4 Vol. 55; ISSN 0010-9312; ISSN CORRAK
- Country of Publication:
- United States
- Language:
- English
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