Evaluation of iron-aluminide CVD coatings for high temperature corrosion protection

Chemical vapor deposited (CVD) Fe-Al coatings are being investigated to add ress fundamental issues concerning aluminide coating performance and lifeti me. By using a well-controlled laboratory CVD procedure, the coatings are u niform in composition, purity and microstructure. A typical ferritic steel, Fe-9Cr-1Mo, and an austenitic stainless steel, 304L (nominally Fe-18Cr-9M) , were coated to examine differences in the two types of substrates. For bo th substrates, the as-deposited coating consisted of a thin (<5 mum), Al-ri ch layer above a thicker (30-50 mum), lower Al content layer. To follow-up on initial results, which showed good coating performance in air+10vol.%H2O and H2S-H(2)0-H-2-Ar, cyclic tests were performed in both environments at 800 degreesC and more detailed characterization of the isothermally exposed coatings was conducted. During 2-5, 25h cycles at 800 degreesC in H2S-H2O- H-2-Ar, CVD coatings on both substrates showed progressively more attack du ring each cycle. However, in lh cycles at 800 degreesC in air + 10vol.%H2O, the coatings showed excellent performance, similar to cast Fe-(15-20at.%)A l specimens. The uncoated alloys were significantly attacked during all of these tests. Thermal expansion measurements show Al additions up to 20at% h ave little effect on the mean expansion of ferritic alloys but the higher t hermal expansion of austenitic steels may be a better match with Fe3Al coat ings.