Corrosion behavior of ultra-high strength drilling steel in alkaline brines containing hydrogen sulfide at high temperature

In-situ electrochemical measurements were used to study the sour corrosion of ultra-high strength low alloy carbon steel (UHSLA) in alkaline brines at 200 °C. The solutions were buffered with NaHC0₃ / Na₂C03/ NaOH to pH values calculated to be 8.1, 9.8, and 10.8 with the presence of H₂s gas at 200 °C. The partial pressure of H₂S was equivalent to 10 psi (69 kPa) at 85 °C. According to thermodynamic calculations, the dominant reactive ion changed from HC0₃"(aq) to OH(aq) with a comparable concentration of HS(aq) as pH increased. Measured polarization resistance values at 200 °C were one to two orders of magnitude lower than those at 85 °C, which corresponded to a drastic increase in corrosion rate. After 60 hours, the corrosion rates (CR) were 0.84 mm y1, 2.88 mm y1, and 1.83 mm y1 from pH 8.1 to 10.8. The CR calculated with commercial software was in reasonable agreement with the experimental CR. Limiting current was observed in the anodic region using linear sweep voltammetry (LSV) but not in the cathodic region. Even with the effect of limiting current, anodic Tafel slopes, ba, were able to be gathered from LSV using a new method derived from the generalized Butler-Volmer equation. 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. Cathodic Tafel slopes corresponded to a two-electron mechanism in the three conditions. Two layers were observed on the surface at pH 8.1 and 9.8, and the outer layer spalled off in some regions. As the pH increased, the major corrosion products transitioned from pyrrhotite/siderite to magnetite, and the S distribution moved outward from the inner layer.