The high temperature electrochemical behavior of carbon steel in alkaline sulfide solutions

The high temperature, high pressure electrochemical behavior of A516 Gr. 70 carbon steel in aqueous alkaline sulfide solutions was studied by means of polarization tests and cyclic volt-ammetry. The effects of variation of temperature (90-150°C), sulfide concentration (0-3 m), scan rate (1-50 mV/s) and scan range, and the effects of stirring and polarization at the switching potentials between scans were investigated. Passivation was consistent with formation of a protective Fe₂O₃ film. An understanding of the electrochemical behavior of iron in the alkaline sulfide solutions was facilitated by the construction of E-pH diagrams for S-H₂0 and Fe-S-H₂O systems at 25, 100 and 150°C. Sulfide, S²-, currently considered to be stable only at extremely high pH, was excluded from the E-pH diagrams. Reference electrodes, compatible with sulfide solution, were designed for use with an autoclave. The response of the Ag/Ag₂S electrode (SSSE) to variation of temperature, sulfide and hydroxide concentration, and chloride addition was studied. Liquid junction potential and thermal liquid junction potential corrections were applied. The SSSE was not reliably predictable thermodynamically, but was stable and was proven effective in the polarization study. The electrochemical potentials of inert metal reference electrodes, Hg and Pt, were found to be consistent with the mixed potential between polysulfide Sײ⁻, and thiosulfate, S₂O₃²⁻. The Tafel slopes from anodic polarization curves of Pt in alkaline sulfide solutions were consistent with oxidation of HS⁻ to S₂0₃²⁻ at the mixed potential. At more noble potentials, oxidation to polysulfide occurred. Reaction path mechanisms were proposed. Understanding of sulfide oxidation aided in interpretation of the electrochemical behavior of steel in the alkaline sulfide solutions.Applied Science, Faculty ofMining Engineering, Keevil Institute ofGraduat