Design of Al Diffusion Coatings for Fe-based and Ni-based Alloys

Components, which consist of metallic materials and operate at high temperatures, can degrade due to high temperature corrosion attack. The concept of the diffusion coatings is to enrich the substrate surface with one or more protective oxide forming elements (e.g. Al). Aluminization of Fe- and Ni-based alloys leads to Al diffusion coatings, which can consist of one phase or several stacked phases, depending on the Al activity within the intermetallic phase, according to the Fe-Al and Ni-Al phase diagrams. In this work, the aim was to develop a predictive design procedure for the manufacturing of pack cementation Al coatings on austenitic steels and Ni-based alloys, which is based on thermodynamic and kinetic considerations of the pack cementation process. Thermodynamic considerations were conducted by calculations with the thermodynamic software FactSage®, to determine the Al activity (total partial pressure of Al carrying halides) within the pack powder as a function of process temperature and powder composition. Kinetic values, as the diffusion coefficient, which affect the resulting coating thickness, have been determined via a limited amount of experiments for each alloy system, followed by Matano analysis. It was shown that the coating design approach developed enables a quantitative prediction and adjustment of the resulting coating properties (intermetallic phases and coating thickness) for a wide range of process parameters. The design concept has successfully been applied to a combustion chamber in a reformer system and is available for further use in coating technology.