Effect of oxygen and hydrogen sulfide on carbon dioxide corrosion of welded structures of oil and gas installations

It is proved that CO2 corrosion of carbon steel increases by 1.5–2 times with increase of H2S content in the mixture (pH2S<0.5 MPa) in the temperature range 20–80°C. Further increase in H2S content (pH2S≥0.5–1.5 MPa) weakens corrosion, especially in the temperature range 100–250°C, because of influence of FeS and FeCO3 on corrosion.It is shown that with increase of O2 concentration in the mixture (pO2=0–1.5 MPa) the CO2 corrosion rate increases 3–4 times in the temperature range 40–80°C. As the temperature rises to 100–250°C, oxygen hardly exerts any effect on the corrosion rate.A scheme is proposed to classify processes of CO2 corrosion of carbon steel into three types based on the type of corrosion products formed, which enables one to evaluate the effect of combined influence of H2S, O2, and temperature on the nature of CO2 corrosion.The thermodynamic potential of formation of CO2 corrosion products (FeS and FeCO3) upon interaction between the mixture containing CO2 and H2S and the carbon steel sample surface has been calculated; the equilibrium conditions for the formation of metastable FeS and its dissolution in the main corrosion product, viz., FeCO3, have been determined.It has been shown that the combined effect of H2S and O2 on the CO2 corrosion process, consists in acceleration of cathodic reaction of hydrogen ion reduction.