Corrosion of X65 steel in CO2-saturated oilfield formation water in the absence and presence of acetic acid

Electrochemical corrosion behavior of X65 steel in CO2-saturated formation water in the absence and presence of acetic acid was studied by electrochemical measurements, scanning vibrating micro-electrode (SVME), localized electrochemical impedance spectroscope (LEIS) and surface analysis techniques. It is found that, when steel is immersed in formation water, the dissolution of Fe dominates the anodic process and the steel is in active dissolution state. Adsorption of intermediate product on the electrode surface results in generation of an inductive loop in the low frequency range of EIS plot. As corrosion proceeds, the concentration of Fe2+ in the solution increases. When the product of [Fe2+] × [CO32-] exceeds solubility product of FeCO3, FeCO3 will deposit on the electrode surface, and protects the steel substrate from further corrosion. The steel is in a “passive” state. When the electrode surface is completely covered with FeCO3 film, the inductive loop in the low frequency range disappears. In the presence of acetic acid in formation water, the cathodic reaction will be enhanced due to the direct reduction of undissociated acetic acid. Addition of acetic acid degrades the protectiveness of corrosion scale, and thus, enhances corrosion of steel by decreasing the FeCO3 supersaturation in solution.