High-Temperature Sulfidation of Fe₃Al Thermal Spray Coatings at 600°C

Sulfidation behavior of Fe₃Al thermal spray coatings was studied inAr-3.5% H₂-0.1% hydrogen sulfide (H₂S) at 600°C for 500 h. Coatings were processed from the same lot of gas atomized Fe₃Al powder using a high-velocity oxygen fuel (HVOF) process and an air plasma spray (APS) process. In general, the Fe₃Al-type composition displayed excellent resistance to sulfidation corrosion at 600°C, which correlated with the reported literature on wrought Fe₃Al alloys. However, the method of processing affected the corrosion response. Particle degradation and porosity were two important factors that affected corrosion resistance. HVOF processing did not degrade significantly the composition of the powder and produced coatings with low porosity, low oxide content, high sulfidation resistance, and high resistance to sulfur penetration. HVOF coatings produced from finer sized powders exhibited slightly more corrosion damage because a greater percentage of the consumable was degraded. In contrast, APS processing caused significant degradation to the consumable and created coatings with a significant quantity of alloy-depleted regions, high oxide content, and high porosity. As a result, sulfur attacked alloy-depleted regions within the “splats” and permeated through the porous splat boundaries to the coating-substrate interface.