Thermo-reactive diffusion (TRD) process is used to produce hard, wear-resistant, oxidation and corrosion-resistant coating layers on steel substrates. In this research, molten salt bath method was used for the formation of chromium coating in which, low- and high-ferrochromium powder was separately dissolved in molten borax as the source of chromium. The purpose of this research is to investigate the effect of the carbon content in the substrate and the salt bath on the formation of chromiumbased coating layers on sample surfaces. Coating thickness and microstructure formed in the baths were studied using scanning electron microscope (SEM) and optical microscope (OM). The coating thickness of the samples treated in low-carbon ferrochromium baths (38 ± 2 μm) was 5 times greater than those treated in the high-carbon ferrochromium baths (8 ± 2 μm). The sample surface, the final baths residue, and the initial ferrochromium powders were examined by X-ray diffractometer (XRD). Results showed that, no chromium carbide phases (Cr7C3 or Cr23C6) were formed on the pure iron samples which comes to the point that, the presence of carbon in the samples plays a major role on the formation of carbide coating layers. In comparison, the carbon content of bath has no positive effect on the chromium carbide formation and even reduces the chromium diffusion into the surface of the samples.

Keywords: Thermo-reactive diffusion process, Borax salt bath, Ferrochromium, Chromium carbide.