Performance of Al-Rich Oxidation Resistant Coatings for Fe-Base Alloys

This multi-layer program has examined the oxidation resistance of Al-rich coatings made by chemical vapor deposition and pack cementation on ferritic-martensitic (e.g. T91, Fe-9Cr-1Mo) and austenitic (Type 304L, Fe-18Cr-8Ni) substrates at 650°-800°C. The main goal of this work was to demonstrate the potential benefits and problems with alumina-forming coatings. To evaluate their performance, 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 final results focused on thinner coatings with less Al and a ferritic Fe(Al) structure which have a lower thermal expansion than intermetallic phases. To improve the previously developed coating lifetime model, a final series of exposures were conducted to determine the effect of substrate composition (e.g. Cr content using Fe-12Cr and Fe-9Cr-2W substrates) and exposure temperature on the critical Al content for coating failure. For the coated Fe-(9-12)Cr specimens, there was little effect of Cr on lifetime at 800°C. At 700° and 800°C, thin coated austenitic specimens (304L and 316) continue to be protective at up to double the lifetime of a similar coating on T91. This increase could be attributed to the higher Cr content or the slower interdiffusion in austenitic substrates which is illustrated with electron microprobe measurements from thicker coatings stopped after 10-20kh.