Influence of temperature and thiosulfate on the corrosion behavior of steel in chloride solutions saturated in CO2

The corrosion resistance of mild steel in environments of 1 M NaCl solutions saturated in CO2 was investigated in the absence and presence of different concentrations of thiosulfate (0.01–1 M Na2S2O3), one of the main sulfide oxidation product, at 22 and 50 °C. Polarization measurements and immersion tests revealed remarkable increase in the chloride corrosion rate in the presence of dissolved CO2 species and/or thiosulfate ions. Solutions containing Cl−–S2O32--dissolved CO2 species exhibited higher corrosion attack than those conducted in Cl−–S2O32- or Cl−-dissolved CO2 solutions and is increased with increasing thiosulfate concentration. The room temperature results were compared to those measured at 50 °C. Similar polarization type behavior was obtained with higher corrosion rates for the higher solution temperature. Although the presence of thiosulfate and/or dissolved CO2 species activated the steel anodic/cathodic polarization curves in chloride solutions, this effect was more pronounced on the cathodic side and in the presence of dissolved CO2 species. The cathodic current density showed substantial displacement to more active values in the presence of thiosulfate and/or dissolved CO2 and increased with increasing thiosulfate concentration.