To establish an adequate thermodynamic model for the mechanism of formation of hydrogen sulfide(H_2S) corrosion products,theoretical and experimental studies were combined in this work. The corrosion products of API X60 pipeline steel formed under different H_2S corrosion conditions were analyzed by scanning electron microscopy,energy-dispersive X-ray spectroscopy,and X-ray diffraction. A thermodynamic model was developed to clarify the environmental boundaries for the formation and transformation of different products. Presumably,a dividing line with a negative slope existed between mackinawite and pyrrhotite. Using experimental data presented in this study combined with previously published results,we validated the model to predict the formation of mackinawite and pyrrhotite on the basis of the laws of thermodynamics. The established relationship is expected to support the investigation of the H_2S corrosion mechanism in the oil and gas industry.
Lei ZhangHui-xin LiFeng-xian ShiJian-wei YangLi-hua HuMin-xu Lu
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.
Da-peng LiLei ZhangJian-wei YangMin-xu LuJin-hui DingMing-liang Liu
