Design of Al diffusion coatings for Fe-based and Ni-based alloys

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Components made from metallic materials that operate at high temperatures are susceptible to degradation from high temperature corrosion. Diffusion coatings aim to enhance the substrate surface with protective oxide-forming elements, such as aluminum. Aluminization of Fe- and Ni-based alloys results in aluminum diffusion coatings, which may consist of one or multiple phases depending on aluminum activity within the intermetallic phase, as indicated by the Fe-Al and Ni-Al phase diagrams. This work focuses on creating a predictive design procedure for manufacturing pack cementation aluminum coatings on austenitic steels and Ni-based alloys, grounded in thermodynamic and kinetic analyses of the pack cementation process. Thermodynamic calculations using FactSage® software were performed to assess aluminum activity (total partial pressure of aluminum-carrying halides) in the pack powder relative to process temperature and powder composition. Kinetic factors, such as diffusion coefficients influencing coating thickness, were determined through limited experiments for each alloy system, followed by Matano analysis. The developed coating design approach allows for quantitative predictions and adjustments of coating properties, including intermetallic phases and thickness, across various process parameters. This design concept has been successfully applied to a combustion chamber in a reformer system and is poised for further applicati