Corrosion of Carbon Steel in Carbon Dioxide-Saturated Solutions Containing Small Amounts of Hydrogen Sulfide

Abstract

Carbon steel specimens were exposed in carbon dioxide (CO2)-containing brine with different amounts of hydrogen sulfide (H2S) at 70°C and 80°C. The addition of 0.02 mmol/L sulfide, corresponding to 45 Pa (0.0065 psi) H2S partial pressure (PH2S), to brine with 70 kPa (10 psi) CO2 at pH 4.2 caused rapid formation of a protective corrosion film. After ~ 15 min, the anodic dissolution rate at a given potential was reduced two decades. The rate of the cathodic reactions was reduced only slightly. These changes in the electrochemical kinetics increased the corrosion potential to a region where localized breakdown of the protective film and pitting corrosion could occur. After a day's exposure, PH2S values in the range 14 Pa (0.002 psi) to 56 Pa (0.0081 psi) led to corrosion rates varying unsystematically between 0.2 mm/y and 3 mm/y. A pronounced decrease in the corrosion rate was observed frequently for low H2S contents. A PH2S value of 56 Pa (0.0081 psi) consistently resulted in an increase in the corrosion rate. For PH2S values between 15 Pa and 60 Pa (0.0022 psi and 0.0087 psi) and a CO2 partial pressure (PCO2) of 56 kPa (8.1 psi), the following relationship between corrosion rate and pH was observed: corrosion rate (mm/y) = 125 (1 – 034 x ΔpH).