Simultaneous chromizing-aluminizing diffusion coating of austenitic stainless steel by a two-step CVD process

Chromium and aluminum were simultaneously deposited by diffusion into austenitic stainless steel substrates by the pack-cementation process. Diffusion was carried out in two different thermal runs (one- and two-step processes) and the results are presented. The effects of various pack chemistries on the coating characteristics of 310S and 316 stainless steel were also investigated. The major controlling factor to achieve the desired coatings was the composition of the masteralloy, particularly, the aluminum activity in the pack. A more uniform coating was achieved on 316 than on 310S for the same coating conditions due to the austenite-stabilization effect of Ni, which is less abundant in 316. The best coating condition in the range examined was with a pack composed of 80Cr-20Al masteralloy and 2wt.% NH4Cl activator by the two-step process.