Diffusion coatings for corrosion protection of high temperature structural materials

Diffusion coatings were produced using the pack cementation process on Ni-base materials as well as on austenitic and ferritic steels, which proved to enhance the corrosion resistance in reducing sulphidising atmospheres and under metal dusting conditions. Beside of simple Al-diffusion coatings advanced codiffusion coatings of the Al-Ti-, Al-Si- and Al-Si-Ti-type were investigated. Appropriate diffusion powder mixtures were identified by thermodynamical calculations. Especially the codiffusion coatings show a complex phase composition including the formation of different intermetallic phases. Contrary to Ni-base and austenitic alloys the formation of diffusion layers on ferritic steels is irregular. Possible explanations for the different diffusion behaviour depending on the type of substrate alloy are discussed.