The corrosion at 600°C of four 8-12 Wt% Cr ferritic steels in chemically cycling oxidizing-sulfidizing gases starting with sulfidation

Abstract

The corrosion of four ferritic steels containing 8-12 wt% Cr was studied at 600°C in gas mixtures whose composition changed cyclically with time from sulfidizing to oxidizing conditions and vice versa. The initial cycle was sulfidiziag, while each cycle lasted 24 hr and the maximum number of cycles was four. Oxidation after the first sulfidizing cycle produced a layer of iron oxides growing over the initial surfide layer at rates much higher than for the pure oxidation of the same materials at the same temperature. The second sulfidizing stage produced thin sulfide scales over the oxide layer, growing much more slowly than during the initial sulfidation. Finally, the second oxidizing stage produced porous external oxide layers growing mainly directly in contact with those formed during the first oxidizing stage, while the second sulfide layer tended to disappear. The corrosion behavior of the steels and in particular the growth of the complex, multilayered scales observed are a direct consequence of the change in the gas composition at the end of each cycle.