Performance of Al-rich oxidation resistant coatings for Fe-base alloys

The oxidation resistance of Al-rich coatings made by chemical vapor deposition and pack cementation was examined on representative ferritic-martensitic (FM, e.g. Grade 91, Fe-9Cr-1Mo) and austenitic steel substrates at 6508-800 degrees C. To evaluate the potential benefits and problems with these alumina-forming coatings, oxidation exposures were conducted in a humid air environment where the uncoated substrates experience rapid oxidation, similar to steam. Exposure temperatures were increased to accelerate failure by oxidation and interdiffusion of Al into the substrate. The difference in the coefficient of thermal expansion (CTE) between coating and substrate was found to cause cracking and coating failure during rapid thermal cycling on thicker coatings with Fe-Al intermetallic phases. Therefore, thinner coatings with less Al and a ferritic Fe(Al) structure were evaluated more extensively and tested to failure at 700 degrees and 800 degrees C on FM steels. The remaining Al content at failure was measured and used to improve a previously developed coating lifetime model. At 700 degrees and 800 degrees C, thin coated austenitic specimens continue to exhibit protective behavior at more than double the lifetime of a similar coating on FM steel. The longer lifetime was attributed to the ferritic coating-austenitic substrate phase boundary inhibiting Al interdiffusion.