Electrochemical corrosion of carbon steels in H2S-containing brines.

摘要

The electrochemical corrosion behaviors of high strength low alloy carbon steel, Grade S-135, and ultra-high strength low alloy carbon steel, Grade UD-165, were investigated in alkaline brines at pH of 7.9, 10.7, and 12.4 and four H2S partial pressures (P H2S) from 0 to 69 kPa at 85 °C using in situ electrochemical measurements, ex situ surface analyses, and software modeling. HS-(aq) was calculated to be the dominant sulfide species from pH 7.9 to 12.4. After 60 hours, polarization resistance (Rpol) of S-135 and UD-165 generally increased as pH increased at lower PH2S (0 and 0.83 kPa), whereas Rpol decreased and then increased as pH increased at higher PH2S (8.3 and 69 kPa). At each pH, the lower PH2S increased Rpol or did not significantly change R pol, whereas the higher PH2S decreased Rpol. Two opposite effects from H2S were proposed, an accelerating effect due to H2S(aq) and HS-(aq) facilitating the Faradaic reactions and the localized corrosion, and an inhibiting effect due to the protectiveness of the corrosion products. The inhibiting effect was often observed at relatively low H2S concentrations at pH 7.9 and 12.4.;The electrochemical impedance did not change significantly at different stir rates for both S-135 and UD-165, indicating that mass transport in the bulk solution was not the rate determining step. The corrosion products generally changed from iron carbonate and sulfides to iron oxide as pH increased, which agreed with the Pourbaix diagrams.;Solution pH in the three solutions were calculated to be 8.1, 9.8, and 10.8 at 200 °C, respectively. Polarization resistance values of UD-165 at 200 °C were consistently one to two orders of magnitude lower than at 85 °C, which corresponded to a drastic increase in corrosion rate at elevated temperature. At 200 °C, Rpol at 9.8 was the smallest after 60 hours among the three solutions. The modeled results were in reasonable agreement with the experimental CR values within a factor of 4.;A new method derived from the generalized Butler-Volmer equation allowed to obtain anodic Tafel slopes from LSV even with the effect of limiting current. At 200 °C, the ba values indicated that the anodic reactions followed the Bockris mechanism at pH 8.1 and a two-electron mechanism at pH 9.8 and 10.8. As pH increased, the major corrosion products changed from pyrrhotite/siderite to magnetite although the corrosion products were a mixture of iron carbonate, sulfide and oxide.;At 4 °C and 10 MPa total pressure, experimental results for two drill steels, Grade S-135 and Grade UD-165, showed a distinct drop in corrosion rate when transitioning from sweet (CO2) corrosion to sour (H 2S) corrosion in 5% wt. NaCl brine. It was found that a CO2:H 2S ratio as high as 1000 was still enough for sour corrosion to dominate the conditions tested. The CO2+H2S condition and H 2S condition had the similar anodic and cathodic Tafel slopes, which were smaller than the CO2 condition. The presence of H2S could have caused the formation of FeS(s) at the steel surface where the pH and Fe2+(aq) concentration might be higher than in the bulk solution.