Effects of the valence of sulfur on passivation of alloys 600, 690, and 800 at 25°C and 95°C

R. W. Staehle

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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 potenliodynamic 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 (Pscc that incorporates the ratio of current densities obtained in fast and slow potentiodynamic scans. The relationship of Pscc to the possible occurrence of SCC was discussed.

Original languageEnglish (US)
Pages (from-to)355-379
Number of pages25
Issue number4
StatePublished - 1999


  • Alloy 600
  • Alloy 690
  • Alloy 800
  • Heat treatment
  • Kinetics
  • Passivation
  • Ph
  • Pitting
  • Potentiodynamic polarization
  • Scanning rate
  • Stress corrosion cracking
  • Sulfur
  • Transient current
  • Valence


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