Aluminizing UNS S30400: Thermodynamic and Kinetic Modeling

The deleterious effects of corrosion can be mitigated by the application of coatings that form a protective oxide layer. Halide Activated Pack Cementation (HAPC) is a coating method in which a halide vapor is generated within a pack and undergoes reactions that result in the coating element being deposited onto the substrate and diffusing into it. In this study, UNS S30400 was aluminized for various times (1 h, 4 h and 9 h) at 650°C. Coupon cross-sections were characterized using optical microscopy, scanning electron microscopy coupled with electron dispersive spectroscopy and electron probe micro analyzer to measure coating thicknesses and concentration profiles of coating element Al and substrate elements Fe, Cr and Ni. To understand the phases formed in the aluminizing of UNS S30400 the coatings, a study was performed using Thermo-Calc. An isopleth for the pseudo-binary system, Al-(Fe-Cr-Ni) was generated. In order to study the kinetics of the coating growth, diffusion coefficients were calculated using a diffusion tool from a Python library, pyDiffusion, where experimentally determined concentration profiles were provided as the input and inter-diffusion coefficients were calculated. The source code for calculation of diffusion coefficients for this system was validated by using concentration profiles as input from two different sources, (a) EDS data from aluminizing UNS S30400 (Cal Poly Pomona), and (b)aluminized Ni-Al alloy from external literature. There are several issues remaining in regard to the code and suggestions for future work have been provided.