Effect of Oxygen Contamination on Inhibition Studies in Carbon Dioxide Corrosion

The effect of oxygen (O2) contamination on the inhibition of carbon dioxide (CO2) corrosion was studied using carbon steel rotating cylinder electrodes (RCE). The test conditions were 40°C, 1 bar CO2, pH 5.5, and 1 wt% sodium chloride (NaCl) solution. The uninhibited CO2 corrosion rate was 1.5 mm/y to 2 mm/y under these conditions. Three organic, film-forming corrosion inhibitor formulations were tested in the concentration range from 20 ppm to 50 ppm. With less than 1 ppb of dissolved O2 in the test brine, the inhibitors reduced the corrosion rate by two orders of magnitude, and the inhibited corrosion rates were insensitive to the RCE rotation rate (100 rpm to 5,000 rpm). In the presence of 100 ppb O2 with 20 ppm to 30 ppm inhibitor, the corrosion rates depended on the RCE rotation rate, the corrosion rate being under partial or full O2 diffusion control with some contribution of CO2 corrosion. The O2 corrosion rate was reduced by increasing the inhibitor concentration. The consequences of the reported findings with respect to O2 concentration limits in CO2 corrosion inhibitor tests are discussed.