The corrosion of alloy 4130 (UNS(1) G41300) steel has been investigated at temperatures up to 400°F in gaseous environments containing varying amounts of H2S and CO2 and in gas-saturated aqueous environments having dissolved chlorides. Thermogravimetric techniques have been used for establishing the kinetics of corrosion in the gaseous medium whereas electrochemical techniques have been applied in the case of the aqueous medium. Studies in the aqueous medium have been extended to pressures as high as 2000 psi.
In gaseous environments, the major corrosion product has been identified to be pyrrhotite, Fe1-xS, which is found to grow by the predominant outward migration of ferrous ions through the sulfide scale. The rate-limiting step, however, is found to be the dissociation of H2S at the outer surface of the sulfide scale. Additions of carbon dioxide to H2S have the effect of accelerating sulfide scale growth rate, presumably by increasing the dissociation rate of H2S on the iron sulfide scale.
In the aqueous medium having dissolved H2S in neutral and ionized forms, the corrosion process involves both the conversion of iron ions into iron sulfide at the sulfide scale/solution interface and the direct passage of iron ions through the scale into the solution. The corrosion rate is initially limited by ferrous ion transport through a scale that is predominantly pyrrhotite; at longer times, a steady state is reached where the growth rate of the scale equals its dissolution rate. A quantitative model is proposed to explain these results.
