THE CORROSION OF THE 9CR-1MO COMMERCIAL STEEL IN H2-H2S AND IN H2-H2S-CO2 MIXTURES AT 400-DEGREES-C-700-DEGREES-C

The corrosion behavior of a commercial steel containing nominally 9 wt% Cr and 1 wt% Mo was investigated at 400-700-degrees-C in H-2-H2S and in H-2-H2S-CO2 mixtures using two sulfur pressures at each temperature. The ternary mixtures had the same sulfur pressures as the binary gases and, in addition, a small partial pressure of oxygen, depending on temperature. The corrosion kinetics was generally irregular, presenting an initial stage of decreasing rate followed by a second stage of approximately constant rate. The scales were always composed of sulfides only. They had a typical duplex structure and showed a strong tendency to crack and spall off during cooling. The kinetic behavior observed is attributed to a change from an initial rate control by the reaction between the gas and the scale to a subsequent stage where the scales become very porous and the rate is essentially controlled by the transport of the gaseous reactants through the scale.