Electrochemical corrosion of ultra-high strength carbon steel UD-165 was investigated using in-situ electrochemical techniques in H2S-containing alkaline brines (5 wt% NaCl; pH 7.9, 10.7, and 12.4) with four H2S partial pressures (p(H2S)) from 0 to 69 kPa at 85 degrees C. The effects of pH and p(H2S) were studied in terms of corrosion rate (CR), reaction mechanism, and corrosion products considering the Fe dissolution reactions with HCO3 (aq), OH (aq), and HS (aq). While CR generally decreased at higher pH and increased at higher p(H2S), both accelerating and inhibiting effects of H2S on CR were observed. High CR at pH 10.7 correlated with a high ratio of HS (aq) to the total concentration of reactive anions. The corrosion products changed from iron carbonate and sulfides to iron oxides as the pH increased. Sulfur was detected mostly in the inner layer of corrosion products at pH 7.9. (C) 2016 Elsevier Ltd. All rights reserved.