The electrochemical corrosion behavior of high strength low alloy carbon steel API S-135 was investigated using in-situ electrochemical measurements, ex situ surface analyses, and software modeling in the alkaline brines at pH of 7.9, 10.7, and 12.4 and at four different PH2S equal to 0 kPa, 0.83 kPa, 8.3 kPa, and 69 kPa at 85 °C. Linear polarization resistance (Rpol) at steady state increased as pH increased from 7.9 to 12.4 at lower PH2S whereas Rpol decreased and then increased at higher PH2S. The fastest corrosion rate (CR) was 0.26 mm yr-1 at pH 10.7 with PH2S 69 kPa, and the slowest CR was 0.0053 mm yr-1 at pH 12.4 with PH2S 0.83 kPa. The correlation between the primary species, corrosion products, localized corrosion and Rpol were discussed. The experimental CR and corrosion products were compared with the modeling CR and Pourbaix diagram.