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

Chemical vapor deposited (CVD) Fe-Al coatings are being investigated to address fundamental issues concerning aluminide coating performance and lifetime. By using a well-controlled laboratory CVD procedure, the coatings are uniform in composition, purity and microstructure. A typical ferritic steel, Fe-9Cr-1Mo, and an austenitic stainless steel, 304L (nominally Fe-18Cr-9Ni), were coated to examine differences in the two types of substrates. For both substrates, the as-deposited coating consisted of a thin (<5 μm), Al-rich layer above a thicker (30-50 μm), lower Al content layer. To follow-up on initial results, which showed good coating performance in air+10vol.%H₂O and H₂S-H₂O-H₂-Ar, cyclic tests were performed in both environments at 800°C and more detailed characterization of the isothermally exposed coatings was conducted. During 2-5, 25h cycles at 800°C in H₂S-H₂O-H₂-Ar, CVD coatings on both substrates showed progressively more attack during each cycle. However, in 1h cycles at 800°C in air + 10vol.%H₂O, the coatings showed excellent performance, similar to cast Fe-(15-20at.%)Al specimens. The uncoated alloys were significantly attacked during all of these tests. Thermal expansion measurements show Al additions up to 20at% have little effect on the mean expansion of ferritic alloys but the higher thermal expansion of austenitic steels may be a better match with Fe₃Al coatings.