An Experimental Investigation on the Effect of Salt Concentration on Uniform CO₂ Corrosion

Hydrocarbon extraction from underground reservoirs is typically accompanied by the production of water as a by-product. Dissolved salts are a general characteristic of produced water. Salt concentration (salinity) in produced water can vary from few milligrams per liter of produced water to about 25 wt.%. Uniform CO₂ corrosion of carbon steel facilities is often a major problem when handling produced water in the oil and gas fields. A very limited amount of research has been conducted on the effect of salt concentration on uniform CO₂ corrosion and those studies did not investigate the subject mechanistically over a large enough range of salt concentration. In this study, the effect of salt concentration on the rate and mechanism of uniform CO₂ corrosion of carbon steel was investigated in a wide range of NaCl concentration from zero to 20 wt.%. Weight loss and electrochemical experiments were performed in a solution saturated with CO₂ (autogenous pH) at 30^oC and 1 bar total pressure. The results of weight loss, linear polarization resistance (LPR), and electrochemical impedance spectroscopy (EIS) showed that the corrosion rate decreased with increasing NaCl concentration. Both anodic and cathodic reactions were retarded by higher NaCl concentrations. However, the decrease in the mass transfer of H⁺ from the bulk to the surface was identified as the main influential parameter on the decreasing rate of the corrosion process. EIS data indicated that the charge transfer resistance increased and the capacitance of the double layer at the metal/solution interface decreased with increasing NaCl concentration.