Experimental Investigation on Corrosion Behavior of X80 Pipeline Steel under Carbon Dioxide Aqueous Conditions.

A combined steady-state and transient approach is employed to investigate the corrosion behavior of X80 pipeline steel in carbon dioxide-saturated brines. Continuous bubbling of carbon dioxide into a test vessel with 1 liter capacity is performed to simulate the flowing condition. The measurement of time-dependent open-circuit potential, polarization resistance, and electrochemical impedance spectroscopy (EIS) is conducted to interpret the evolution of dissolution processes at the corroding interface. Three distinguishing stages are observed at a temperature of 60 °C during a whole exposure of 144 h. Analyses mainly based on the consecutive mechanism show that after the first stage of the active-adsorption state, the anodic reaction is significantly retarded by the accumulation of (FeOH) on the iron surface, causing a sharp increase in the polarization resistance and the open-circuit potential, as well as the disappearance of the inductive loop in EIS. At the third stage, the formation of the corrosion product layer similarly reduces both the anodic and cathodic reactions, which arouses a linear increase in the polarization resistance with time and a capacitive loop in EIS but changes the open-circuit potential slightly. An increase in salinity in this study reduces the polarization resistance and enhances iron dissolution by promoting the formation and relaxation of (FeOH); however, it brings little change to the developing time of the three stages obviously. At a low temperature of 20 °C, a protective product layer is not observed in carbon dioxide-saturated brine, and the dissolution of iron is mainly under activation control during the whole exposure. A notable enlarged polarization resistance and different interfacial processes are observed in an alkaline solution compared with those in acidic environments, which is deduced to be resulted from an impedance in the relaxation of (FeOH) by increasing pH. The observations in this study support well that the iron dissolution reaction at the initial stage exposed in carbon dioxide aqueous environments is dominant by water adsorption on the iron surface, and further investigation should be performed on the role that carbon dioxide plays in the evolution of corrosion products and the formation of a protective film on the steel surface by taking into account local water chemistry.