Corrosion of carbon steels in H₂S-containing alkaline brines

In order to study the effect of sour gas (H₂S) on the corrosion of high strength drilling steels, in-situ electrochemical measurements were conducted on ultra-high strength low alloy (UHSLA) carbon steels in alkaline brine conditions (pH 7.4 to 12.3) at 85 °C. Electrochemical methods included Linear Polarization Resistance (LPR), Electrochemical Frequency Modulation (EFM), and Electrochemical Impedance Spectroscopy (EIS). After the corrosion tests, the surfaces were analyzed by Scanning Electron Microscopy (SEM) with Energy-dispersive X-Ray Spectroscopy (EDS) to characterize the corrosion products. The speciation in H₂S-H₂CO₃-H₂O systems at different pH at 85°C was calculated. At P_H2S= 0.83 kPa (0.12 psi) and 69 kPa (10 psi), the corrosion rates measured at steady state increased from 0.05 mm/y to 0.2∼0.4 mm/y when pH increased from 7.4 to 10.5, and then decreased to 0.01 mm/y at pH 12.3. The highest corrosion rate occurring at pH 10.5 was suspected to be related with increased pitting corrosion that enlarged the corroded surface area. The lowest corrosion rate at pH 12.3 could be attributed to the suppressed cathodic reaction at high pH. EIS showed that the impedance generally increased with time then reached the steady state. This could be related to the formation of corrosion product layers.