AbstractChemical vapour deposition of chromium using CrCl2produced by the reaction of FeCl3with metallic chromium has been examined both thermodynamically and kinetically. The chemical processes occurring during coating have been examined by thermodynamic calculations and verified by comparison with experimental results. Kinetic measurements have been made on chromium diffusion in titanium alloyed mild steel. The diffusion promoting effect of titanium is explained thermodynamically. Reaction of FeCl3and chromium occurs in two stages. Initially FeCl2and CrCl2are formed above 200°C and the FeCl2formed reacts above 500°C, forming metallic iron and CrCl2. The beginning of the coating process, which is mainly diffusion controlled, is determined by the evaporation of CrCl2. The interdiffusion coefficient of chromium in the steel has been determined by various methods. Enhanced diffusion wasfound below 860°C, but the diffusion rate is too low to allow these temperatures to be used in practice. A strong dependence of the interdiffusion coefficient on chromium concentration was found in the temperature range 860–980°C.