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

Wednesday, 27 May 2015: 10:25
PDR 2 (Hilton Chicago)
R. Feng, J. Beck, A. Buyuksagis, I. Wolfe, R. Cianni (The Pennsylvania State University), M. Ziomek-Moroz (National Energy Technology Laboratory), and S. N. Lvov (The Pennsylvania State University)
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, PH2S, 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 PH2S = 0 and 0.12 psi (0.83 kPa), the polarization resistance (Rpol) increased as pH increased from 7.5 to 12.3; however, Rpol at pH 10.5 were the lowest for PH2S 1.2 psi (8.3 kPa) and 10 psi (69 kPa). The lower and moderate H2S concentrations (PH2S 0.12 psi and 1.2 psi) were generally found to give larger Rpol values than without or high H2S levels (PH2S 0 psi and 10 psi) at each pH. The corrosion rates were inversely proportional to Rpol 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 Rpol. 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 Rpol 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.