Effect of H₂S concentration on the corrosion behavior of pipeline steel under the coexistence of H₂S and CO₂

The effect of H₂S concentration on H₂S/CO₂ corrosion of API-X60 steel was studied by scanning electron microscopy, a weight-loss method, potentiodynamic polarization tests, and the electrochemical impedance spectroscopy technique. It is found that the corrosion process of the steel in an environment where H₂S and CO₂ coexist at different H₂S concentrations is related to the morphological structure and stability of the corrosion product film. With the addition of a small amount of H₂S, the size of the anode reaction region is decreased due to constant adsorption and separation of more FeS sediment or more FeHS⁺ ions on the surface of the steel. Meanwhile, the double-layer capacitance is diminished with increasing anion adsorption capacity. Therefore, the corrosion process is inhibited. The general corrosion rate of the steel rapidly decreases after the addition of a small amount of H₂S under the coexistence of H₂S and CO₂. With a further increase in H₂S concentration, certain parts of the corrosion product film become loose and even fall off. Thus, the protection provided by the corrosion product film worsens, and the corrosion rate tends to increase.