Mo-based alloys are promising structural materials for high temperature processes due to their high melting points and high mechanical strength at elevated temperatures. However, at intermediate temperatures such alloys are prone to pesting, which leads to the total disintegration of the material as a result of oxidation. In order to improve the oxidation and corrosion resistance of Mo-based alloys, elements capable of forming protective oxide scales in this temperature range have to be introduced. This may be achieved by the application of Al- or Cr-rich protective coatings. In this study diffusion coatings were deposited on Mo and Mo-Si-Ti-alloys using pack cementation process, a type of chemical vapor deposition. During application the coating element (Al, Cr) is transported to the substrate surface by a halide activator where it diffuses into the substrate material or reacts with it and often forms intermetallic phases. Diffusion coatings are characterized by a good adhesion to the substrate as well as by a homogeneous composition and thickness.[1]
The influence of various pack cementation parameters, particularly the quantity of the halide activator (chloride, bromide), and the temperature, on the composition and microstructure of Cr- and Al-diffusion coatings on pure Mo was investigated (see Figure 1). The thicknesses of the deposited layers increase both with higher temperature and with increasing amounts of activator. Besides the added amount, the grain size and thus surface area of the activator strongly influences the appearance of the deposited layer. To mitigate chlorine corrosion a less reactive bromide activator can be used. The experimental development of the diffusion coatings was supported by thermodynamic calculations using the program FactSage 6.1. This helped to understand the experimental findings and was found to be in accordance with the theory of driving forces and phases formed.
After the fundamental work on pure Mo, the aforementioned coatings were optimized for more complex alloys (eutectic Mo-20.0Si-52.8Ti, eutectoid Mo-21.0-34.0Ti, and Mo-13.5Si-54.3Cr).[2,3] Al-coatings (up to 60 μm) and Cr-coatings (up to 20 μm) were successfully applied and analyzed using optical microscopy, XRD, SEM, EDX, and EPMA.

[1] A. S. Ulrich Oxidation of Metals, 2016, 86, 511-535.
[2] D. Schliephake Intermetallics, 2019, 104, 133-142.
[3] F. Hinrichs Corrosion Science, 2022, 207, 110566.