Abstract: Coated nickel, cobalt or nickel-cobalt superalloy bodies having increased resistance to oxidation, corrosion and thermal fatigue at high temperature are produced by applying a thin layer of a platinum-group metal, siliciding and heating to an elevated temperature to diffuse and integrate the silicided platinum-group metal into the surface of the superalloy body. Then the superalloy body is exposed to a diffusion powder composition containing sources of aluminum or aluminum/chromium metals and heated in a hydrogen or inert gas atmosphere to an elevated temperature to codeposit and diffuse aluminum or aluminum and chromium into the silicided platinum-group metal-treated surface layer.