Diffusion coating a substrate metal with a different metal, such as aluminum and zinc, that is then chemically removed from the coated substrate, provides the residual metal with a very desirable catalytic surface. At least about a third of the removable metal can be dissolved out. Platinum wire screens activated in this way make effective exhaust catalysts for automotive engines. Chromium-rich coating for protective purposes can be applied on a superalloy, diffusion coating in a pack that in addition to the chromium to be diffused, also 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. Pack aluminizing in the presence of chromium makes a very effective aluminum- and 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.