Influence of Temperature on Diffusion Behavior and Infiltration Layer Structure of Sherardizing on Structural Steel

• • At 380 °C for 6 h, the layer reaches ~36.1 μm with 84.1% δ phase and no observable cracks; this specific window delivers the best combination of corrosion resistance (icorr 1.16×10⁻⁶–9.78×10⁻⁷ A/cm²) and process efficiency, avoiding the through-thickness cracking that occurs beyond 6 h and the interfacial cracking caused by excessive Γ phase at 340 °C. • • Temperature dominates growth kinetics: increasing from 340 °C to 400 °C at 6 h raises layer thickness from 10.80 μm to 43.90 μm (a ~3× increase), while extending holding time from 2 h to 10 h at 380 °C grows the layer from 12.10 μm to 81.60 μm; the measured diffusion coefficient at 380 °C is 2.341×10⁻¹³ m²/s, providing a quantitative basis for furnace time–temperature scheduling. • • Phase constitution is temperature-sensitive: all layers contain δ and Γ phases, but higher temperatures increase the Zn/Fe ratio and δ-phase fraction, which correlates with denser layers and improved corrosion resistance; conversely, low-temperature processing (340 °C) produces excessive interfacial Γ phase that initiates cracking at the substrate–layer interface, a critical failure mode for load-bearing structural components. • • The three-stage growth mechanism—active Zn atom generation from ZnCl2 decomposition, bidirectional Zn/Fe interdiffusion along substrate defects, and continuous inward advancement of the intermetallic front—identifies NH4Cl activator chemistry and substrate defect density as levers for industrial scale-up; holding times beyond 6 h are counterproductive because through-thickness cracks degrade uniformity and corrosion protection.