The effect of dissolved O2 on the inhibition of CO2 corrosion was studied using carbon steel rotating cylinder electrodes (RCE). The test conditions were 40 °C, 1 bar CO2, pH 5.5, 1 wt% NaCI solution. The uninhibited CO2 corrosion rate was ca. 2 mm/y under these conditions. Three organic,
film-forming corrosion inhibitor formulations were tested in the
concentration range 20-50 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 RCE rotation rate (100-5000 RPM). In presence of 100 ppb O2 with 20-30 ppm inhibitor the corrosion rates depended on RCE rotation rate, the corrosion rate being under partial or full O2 diffusion control with some contribution of inhibited CO2 corrosion. The O2 corrosion was inhibited by increasing the inhibitor concentration. The consequences of the reported findings with respect to O2 concentration limits in CO2 corrosion
inhibitor tests are discussed. Furthermore, the permeability of O2 through corrosion inhibitor films is discussed by comparison to biological cell membranes.
Key words: corrosion, inhibitor, carbon dioxide, oxygen, carbon steel