The Sulfidation Behavior of an Fe--10 at.% Al Alloy in Ar--S sub 2 and H sub 2 --H sub 2 S Atmospheres at 1023K

The sulfidation behavior of Fe--10 at.% Al has been studied at 1023K in static Ar--S sub 2 gas for exposures up to 72 h and, for a shorter reaction time of 6 h, in flowing H sub 2 --H sub 2 S atmospheres. The sulfur pressure in the gas varied between 10 exp --3 -10 exp 4 Pa. Development of a triplex scale occurred, comprising an outer columnar-grained FeS layer, an intermediate Al sub 2 S sub 3 layer, and an inner FeS--Al sub 2 S sub 3 zone. Studies employing radioactive S demonstrated that growth of the inner FeS--Al sub 2 S sub 3 layer proceeded primarily by the ingress of ambient gas along fissures in the outer FeS and Al sub 2 S sub 3 layers. In the Ar--S sub 2 atmosphere, slow parabolic growth, at a rate approx one-twentieth of that for unalloyed Fe, was observed for reaction up to 40 h, following which mechanical failure in the scale led to an increased rate of reaction. Single-stage parabolic scaling behavior occurred in the H sub 2 --H sub 2 S atmospheres for the short periods studied. However, for the same S activity, the corrosion rat in H sub 2 --H sub 2 S was comparable to that for the accelerated-growth stage observed in the Ar--S sub 2 gas, in agreement with morphological observations which showed that the scale developed the "breakaway type" morphology in the H sub 2 --H sub 2 S atmospheres close to onset of exposure. 16 ref.--AA