Diffusion Chromising of Steel by CYD Using Chloride Compounds: 3 Thermodynamic and Kinetic Aspects

Abstract

Chemical vapour deposition of chromium using CrCl₂ produced by the reaction of FeCl₃ with 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 FeCl₃ and chromium occurs in two stages. Initially FeCl₂ and CrCl₂ are formed above 200°C and the FeCl₂ formed 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.