Abstract: A process is provided for the pack diffusion coating of ferrous metal articles with aluminum, wherein a small but effective amount of an energizer, aluminum chloride, is employed associated with a small but effective amount of a halide selected from the group consisting of an ammonium halide and an anhydrous metal chloride mixed in a pack containing particulate aluminum metal mixed with an inert material, e.g. refractory oxide, in which the ferrous article to be coated is embedded, the process being characterized by the fact that substantial outgassing of aluminum chloride is inhibited during the coating process by controlling in combination the total amount of halides employed not to exceed about 1 - 1/2% of the pack.