Investigating Growth of Iron Borides with the Formation of Monolithic Fe₂B Layer on AISI 304 Stainless Steel via Cathodic Reduction and Thermal Diffusion-Based Boriding

This study is focused on the determination of the effects of processing time and temperature on the thickness, morphology, and hardness of boride layers grown on AISI 304L stainless steels. For boriding, a new molten salt electrolysis method called as CRTD-Bor (Cathodic Reduction and Thermal Diffusion-based boriding) was chosen due to its fast and green nature. CRTD-Bor of AISI 304L substrates was carried out in a borax-based molten electrolyte at temperatures ranging from 950 to 1050 °C for periods of 15 to 60 min at a constant current density of 200 mA/cm². The x-ray diffraction analyses revealed the mixed iron boride phases including Fe₂B, FeB. Moreover, cross-sectional scanning electron microscopy examinations confirmed the growth of these phases. Additionally, a phase homogenization (PH) step was adapted into CRTD-Bor to eliminate brittle FeB layer. It was founded that after 70 min of treatment at 1000 °C (15 min of CRTD-Bor + 55 min of PH) it is possible to grow ≈ 40-µm-thick Fe₂B layer exhibiting 1700 ± 100 HV on the surface with excellent adhesion to the substrate (HF1). Besides, kinetic calculations showed the activation energy (Q) of boride layer growth as 181.45 kJ/mol.