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 carbon steels UD-165 and S-135 in alkaline brine conditions (pH 7.5 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 surface was analyzed by Scanning Electron Microscopy (SEM) and X-Ray Diffraction (XRD) to characterize the corrosion products. The corrosion rates measured after 60 hours increased from pH 7.5 to 10.5 then decreased at pH 12.3 at both PH2S=0.12 and 10 psia). EIS shows that impedance first increased with time then usually decreased a little before the steady state. This could be related with the formation of corrosion product layers and the emerging localized corrosion. The protectiveness of corrosion products determined the corrosion rate at steady state. The SEM images show that the corrosion product layer was more compact at pH 12.3 than the lower pH which tends to be more protective. The speciation and simulated corrosion rates were also calculated with commercial corrosion software. Both thermodynamic and kinetic factors were considered. Pourbaix diagrams suggest that corrosion products shift from iron sulfides to iron oxides with the pH increase.