Chromized ferrous metal sheet material is prepared by a novel open pack chromizing process. In practicing the process, an adhesive composition, an adherent particulate coating of a source of chromium, and dispersed adherent spacing particles are applied on at least one side of the ferrous metal sheet material. The spacing particles are substantially uniform in size and larger than the chromium particles. The adhesive composition has an adhesive characteristic under the conditions of the process whereby an adherent coating containing the inert particles and chromium particles is formed in the absence of compaction. A plurality of layers of the coated sheet material are assembled into a pack, and the resulting open pack is subjected to an elevated chromizing temperature in a protective atmosphere and in the presence of a halogen-containing energizer to produce chromized ferrous metal sheet material. The adhesive for the spacing particles and chromium particles preferably includes the halogen-containing energizer. The invention further provides composite ferrous metal sheet material coated with the adhesive, the chromium particles and the spacing particles which is especially useful for preparing open packs for chromizing. The invention is also useful when practicing other processes wherein ferrous metal or other metal sheet material is heat treated, such as carburizing, decarburizing, nitriding and diffusion processes in general wherein a metallic or a non-metallic substance is intimately contacted with the ferrous metal or other metal sheet material and diffused into the surface thereof.