Thermodynamics and Kinetics of Borochromized Coatings Prepared on 5CrNiMo Steel by Thermo-Reactive Diffusion

Borochromized coatings on 5CrNiMo steel were prepared by thermo-reactive diffusion in a molten borax salt bath at 1173 K, 1223 K, and 1273 K for 0.5–6 h. The cross-sectional observation of optical microscopy (OM) revealed that the as-obtained coatings possessing a comb-like or needle-like microstructure with 21.4–84.7 µm thickness were smooth, compact, and homogeneous. X-ray diffractometry (XRD) results expressed that the borochromized coatings mainly consisted of FeB, Fe2B, Cr5B3, and a small amount of M7C3 (M–Cr, Fe) and α-Fe(Cr). B4C, as a boron-donating agent, first reacted with NaF (activator) to produce active boron atoms and then reduced Cr2O3 (chromium-donating agent) to generate active chromium atoms, whereas borax was only used as the base salt or heating medium in the present experiment. The activation energy of the as-prepared borochromized coating on 5CrNiMo steel was calculated as 161 kJ/mol.