Surface coating of UNS G15130 steel by thermo-reactive diffusion using recycled titanium machining chips: process feasibility and coating characterization

Abstract Titanium machining chips lose up to 95% of their commercial value upon generation and are typically discarded. Addressing this critical sustainability gap, this study introduces a novel approach to surface engineering by demonstrating the feasibility of valorizing recycled Ti F-67 Grade 4 machining chips as the sole diffusion source in a thermo-reactive diffusion (TRD) process. This low-cost treatment utilized a NaCl-KCl eutectic molten salt bath under ambient atmosphere to coat low-carbon UNS G15130 steel (0.14 wt% C) without any prior carburizing step, effectively extending TRD applicability beyond conventional carbon thresholds (typically ≥ 0.3 wt% C). Treatments were conducted at 1000 °C in ambient atmosphere (1–5 h, 10 wt% Ti). The 4 h condition yielded the optimal duplex TiC/Ti(α) coating architecture with a total thickness of 5.7 ± 0.4 μm, representing a 5.2-fold increase compared to the 1 h treatment; a subsequent decrease to 4.3 ± 0.4 μm at 5 h marked the onset of oxidative degradation outpacing further coating growth. Potentiodynamic polarization in 3.5 wt% NaCl revealed significant corrosion resistance enhancements, characterized by anodic dissolution suppression, stable TiO₂ passive film formation, and corrosion potential (Ecorr) shifts of + 227 mV (1 h) and + 302 mV (4 h) relative to the uncoated substrate. These quantitative findings establish the technical viability of exploiting the intrinsic reactivity of titanium machining waste, offering a sustainable, low-capital manufacturing route for surface engineering of low-carbon steels.