In the pack diffusion coating of chromium into the surface of a superalloy, the formation of undesirable oxide inclusion is reduced when the diffusion coating pack contains at least about 3% Ni.sub.3 Al. Also the formation of alpha-chromium is reduced when the pack diffusion is carried out in a retort effectively not over five inches in height. On the other hand an alpha-chromium-rich layer at least 0.2 mil thick can be deliberately formed and the superalloy thus coated subjected to an aluminizing treatment to convert the alpha-chromium into islands that provide very high sulfidation resistance. Pack aluminizing in the presence of chromium makes a very effective aluminum- or chromium-containing top coating over platinum plated or platinum coated nickel-base superalloys. Depletion of diffusible material from workpieces heated in a powder-pack can also provide a surface on which aluminizing produces a highly impact-resistant coating.