Electrochemical Corrosion Behavior of High Strength Carbon Steel in H2S-Containing Alkaline Brines

In-situ electrochemical measurements were performed during the corrosion tests of high‑strength low-alloy steel, grade S-135, used as a drill pipe in oil and natural gas exploration, in 5 % mass NaCl(aq) solutions of pH 7.5, 10.5, and 12.3 at 85 °C. For each pH, four concentrations of H₂S were investigated which corresponded to one dose of H₂S gas at H₂S partial pressure, P_H2S, equal to 0, 0.12 psi (0.83 kPa), 1.2 psi (8.3 kPa), and 10 psi (69 kPa), respectively. It was found that the corrosion process generally reached a steady state by 60 hours in a stirred solution. For P_H2S = 0 and 0.12 psi (0.83 kPa), the polarization resistance (R_pol) increased as pH increased from 7.5 to 12.3; however, R_pol at pH 10.5 were the lowest for P_H2S 1.2 psi (8.3 kPa) and 10 psi (69 kPa). The lower and moderate H₂S concentrations (P_H2S 0.12 psi and 1.2 psi) were generally found to give larger R_pol values than without or high H₂S levels (P_H2S 0 psi and 10 psi) at each pH. The corrosion rates were inversely proportional to R_pol and ranged on the orders of 0.001 to 0.1 mm/year. The change of pH had a larger effect on the corrosion rate than the change of H₂S concentration. It is believed that higher pH suppressed the hydrogen reduction reaction by decreasing available proton sources and, therefore, resulted in higher R_pol. Besides pH, the corrosion rate was also affected by the nature of the corrosion product film on the surface. The corrosion films were predicted with Pourbaix diagrams and investigated with scanning electron microscopy (SEM) and energy‑dispersive X-ray spectroscopy (EDS). With the pH increase from 7.5 to 12.3, the major products in the corrosion film generally changed from iron sulfides to iron oxides, which agreed with the Pourbaix diagrams. The morphology of the corrosion products was also believed to have influenced the protectiveness of the films. The change in R_pol and corrosion rate was consistent with the change in the electrochemical impedance spectroscopy (EIS) behavior. Two time constants were generally observed in the Bode plots and the plots of pH 10.5 tended to have the time constants at higher frequencies than pH 7.5 and 12.3.