CHROMIZING-ALUMINIZING AND CHROMIZING-SILICONIZING COATING OF A FERRITIC STEEL

Simultaneous deposition of Cr-Al and Cr-Si as diffusion coatings for ferritic steel (Fe-12Cr) substrate have been carried out using the pack cementation method. A computer-assisted thermodynamic study of the equilibrium vapor pressures of volatile halide species formed by the pack components was performed. The simultaneous codeposition of Cr with Al is thermodynamically possible for chloride-activated packs when the activity of Al is about three orders of magnitude lower than the chromium activity ; for Si-Cr codeposition, the activity of Si must be approximately one order of magnitude lower than that for Cr. The experimental tests of the theory agree reasonably with the predictions, and the resulting coatings promise an improvement in resistance to oxidation and corrosion. The simultaneous addition of Ti as an active element to improve oxide-scale adhesion is demonstrated for a Ti-doped Cr-Al coating.