Effects of Adding CO2 to Low Level H2S Containing Aqueous Environments on the Corrosion and Hydrogen Penetration Behaviors of High-Strength Steel

The effects of adding CO2 to low level H2S containing aqueous environment on the corrosion and hydrogen penetration behaviors of high-strength steel were evaluated using a range of experimental and analytical methods. The corrosion rate of the steel sample exposed to a low level of H2S dissolved in an aqueous solution was comparatively higher than the one exposed to a mixture of low concentrations of H2S with CO2 dissolved in the aqueous solution. The higher corrosion resistance of the steel in the mixture of low concentrations of H2S and CO2 was attributed primarily to the three-layer structure of corrosion scale, comprised of an outer Fe-oxide, middle FeS1-X, and inner FeCO3, which formed on the steel sample. In particular, the formation of a thin FeCO3 layer with protective and non-conductive nature may serve as an effective barrier against the penetration of aggressive ionic species in solution, as well as hydrogen atoms formed by cathodic reduction or hydrolysis reactions. Consequently, the hydrogen permeation level, which was measured in a mixture of low-level H2S and CO2, was controlled to a comparatively lower value. Nevertheless, the higher level of hydrogen permeation in a mixture of low levels of H2S and CO2 at the early corrosion stage might increase the potential risk of pre-mature failure by hydrogen-assisted cracking.