A novel approach for interdiffusion modeling of oxidation-resistant aluminide coatings on iron-base alloys

Loss of aluminum from diffusion aluminide coatings via formation of oxide scales and interdiffusion with the substrate is recognized as one of the important degradation mechanisms in their high temperature applications. A lifetime prediction model is essential for understanding the long term effect of high temperature oxidation on these coatings. Since lifetime of the coating is directly dependent on the ability of the coating to provide Al to form the protective alumina scale, the model needs to simulate the concentration profiles of major alloying elements in the coating, particular the Al profile, after extended oxidation exposures. A software program developed at NASA, COSIM (Coating Oxidation and Substrate Interdiffusion Model) is capable of modeling one-dimensional, diffusional transport associated with high temperature oxidation and interdiffusion in overlay coating systems. However, one of the major differences between overlay and diffusion aluminide coatings is the as-coated concentration profiles. In contrast to the overlay coatings that start with a constant composition, the diffusion aluminide coatings exhibit a gradient concentration profile, which makes it difficult to directly apply the existing COSIM program to aluminide coatings.;In this study, a novel approach was taken to modify the FORTRAN-based code of COSIM in order to receive the concentration profiles of as-coated experimental data as input and predict the concentration profiles after high temperature oxidation. Modeling work was focused on evaluation of interdiffusion performance of aluminide coatings on representative ferritic-martensitic alloy T91 (Fe-9Cr-1Mo) for power generation applications. Concentration profiles of aluminide coatings with different thicknesses were calculated after various exposures at 650-700°C. Good agreement was obtained between the calculated profiles and that experimentally determined by electron probe microanalysis (EPMA) for exposures up to 10,000h. In addition, the model was used to predict coating lifetime based on the criterion of a minimum Al content at the coating surface, below which protective oxide scales can no longer form. The lifetime predicted with 3.5 at. % as a failure criterion was above 500 kh for higher Al content coating and above 15 kh for lower Al content coating.