Interdiffusion Behavior in Aluminide Coatings for Power Generation Applications

One of the critical issues for the application of iron aluminide coatings is the loss of Al from the coating into the Fe-base substrate alloys which do not contain aluminum. The interdiffusion behavior between chemical vapor deposited (CVD) aluminide coatings and ferritic and austenitic substrates is being studied for times up to 10,000h in the temperature range of 500-800oC. Coatings were synthesized using a laboratory-scale CVD reactor on representative commercial ferritic (Fe-9Cr-1Mo) and austenitic (type 304L stainless steel) alloys. The aluminide coatings on both alloys typically consisted of a relatively thin (20-25μm) Al-rich outer layer and a thicker (150250μm) inner layer with less Al. The composition profiles before and after interdiffusion testing were measured by electron probe microanalysis (EPMA). The decrease of the Al content at the coating surface was not significant after extended diffusion times (≤ 5000h) at temperatures ≤ 700oC. More interdiffusion occurred at 800oC in coatings on both Fe-9Cr-1Mo and 304L alloys. Particularly, a two-phase microstructure was formed in the outer coating layer on 304L after interdiffusion of 2000h at 800°C. The interdiffusion behavior also was simulated using a computer model COSIM (Coating Oxidation and Substrate Interdiffusion Model), which was originally developed for MCrAlY overlay coatings by NASA. Reasonable agreement was observed between the simulated and experimental composition profiles although more work is needed to confirm assumptions made in the model. INTRODUCTION Increasing the temperature capabilities of ferritic and austenitic alloys for advanced power generation applications has been of ongoing interest for many years due to potential gains in energy efficiency and concomitant decreases in emissions. These classes of alloys generally owe their oxidation resistance to the formation of Cr-containing oxides which, especially for the ferritic steels, become less protective at higher temperatures and in steam or exhaust environments so that protective coatings will have to be considered. Iron aluminides are well known to have excellent oxidation and sulfidation resistance due to the formation of an external, protective alumina scale. Thus, an aluminide coating could be a good candidate for improving the oxidation/corrosion resistance of ferritic and austenitic steels provided that it can be applied with the desired composition and microstructure and be mechanically sound (that is, minimal cracking and adherent to the substrate). Two lifetime issues of particular concern for application of iron aluminide coatings are (1) the loss of Al from the coating into the substrate alloys which do not contain any Al, and (2) possible compatibility problems between Fe-Al coatings and substrates which can have substantially different coefficients of thermal expansion (CTE). In the present work, the interdiffusion behavior between chemical vapor deposited (CVD) aluminide coatings and ferritic and austenitic substrates is being studied for times up to 10,000h in the temperature range of 500-800oC. Representative commercial ferritic (Fe-9Cr-1Mo) and austenitic (type 304L stainless steel) alloys were used as the substrate materials. Coatings were synthesized using a laboratory-scale CVD reactor in order to more rigorously