Interdiffusional degradation of oxidation-resistant aluminide coatings on Fe-base alloys

One of the potential degradation modes of oxidation-resistant iron aluminide coatings is the loss of Al from the coatings into Fe-base substrate alloys that typically contain no Al. To address this issue, interdiffusion between aluminide coatings and steel substrates was studied for times up to 10,000 h in the temperature range of 500-800°C. Coatings were synthesized in a laboratory chemical vapor deposition (CVD) reactor on representative commercial ferritic alloy Fe-9Cr-lMo and type 304L austenitic stainless steel. The microstructural and compositional changes after diffu-sion anneals were examined in detail. An initial attempt to model the interdiffusion behavior was carried out by applying an existing software program COSIM (coating oxidation and substrate inter-diffusion model). Complementary work was conducted using a simple mathematic model developed by Hechel et al. Reasonable agreement was observed between the simulated and experimental composition profiles for the aluminide coatings on ferritic alloys. Model results were then applied to predict coating lifetime.