Effects of H2s and pH on Tafel Slopes in Electrochemical Corrosion of High Strength Carbon Steel S-135 in Alkaline Brines

Effects of H2S concentration and pH on electrochemical corrosion of high-strength low-alloy steel, Grade S-135, in NaCl-based solutions containing H2S were investigated using linear sweep voltammetry (LSV). S-135 is often used as drill pipe in oil and natural gas exploration. LSV was conducted at a scan rate of 5 mV/s in 5 %wt. NaCl solutions of pH 7.9, 10.7, and 12.4 and with four H2S partial pressures (P H2S) ranging from 0 to 69 kPa at 85 oC after >60 hours of corrosion. Limiting current density was observed in the anodic LSV plots at lower P H2S (0 and 0.83 kPa) but disappeared at higher P H2S (8.3 and 69 kPa). The cathodic LSV plots did not show limiting current densities, suggesting that H2O was probably the main reactant for hydrogen evolution reaction at high pH. However, the cathodic current density increased in the presence of H2S. A modified Tafel analysis method was employed to obtain the Tafel slopes when the LSV plots were influenced by limiting current. Electrochemical reaction mechanisms have been discussed based on the anodic and cathodic Tafel slopes. Also, the corrosion rate values have been calculated using the Tafel slopes obtained from LSV and compared to those calculated with the Tafel slopes from electrochemical frequency modulation.