Commercial alloy tube components from a biomass gasifier were removed and
characterized to quantify the extent of corrosion. The Fe- and Ni-base outer bed tubes exposed
to the fluidized bed of the gasifier had a metal temperature of ~700°C. Oxalic acid etching
suggested that all of the bed tubes were internally carburized. However, evidence of grain
boundary carbides or sigma phase was not apparent using electron microprobe. Both chromiaand
alumina-forming shield tubes were placed inside the bed tubes to shield the combustion
flame from overheating the bed tubes. These tubes were exposed to a much higher
temperature oxidizing environment. As expected, the alumina-forming shield tube material
showed less degradation. However, the Al depletion profiles were much higher than typically
observed in laboratory tests. Compared to laboratory exposures of the chromia-forming shield
tube alloy, the Cr depletion profiles measured in the shield tube suggest that the metal
temperature was much lower than 1100°C.
Keywords: FeCrNi and NiCr alloys, high temperature oxidation, aluminide coatings, biomass