Surface engineering by non-equilibrium plasma remelting processing for supreme stress corrosion cracking protection

The degradation of stainless steels (SS) in corrosive environments severely limits their long-term service reliability. While numerous bulk and surface processing techniques have been employed to prevent stress corrosion cracking (SCC) in SS, their success in mitigating this form of corrosion has been limited. To address this situation, we need to resolve challenges that persist in retaining passivation-promoting elements (e.g., Cr), neutralizing corrosive ions via surface interactions, and preserving microstructure integrity under coupled mechanical-corrosive stimuli. Here, inspired by nitriding-induced surface protection, we propose a novel non-equilibrium plasma transferred arc (PTA) remelting surface strategy to retain passivation-beneficial elements and introduce metastable phases to dynamically neutralize the local corrosion environment. This strategy, together with in-situ processing window optimization, overcomes the limitation from conventional nitride precipitation of consuming Cr and produces a heterogeneous 1 μm-scale novel metastable ε-Fe3N surface layer and a ~10–20 μm grain-refined Cr-retained sub-layer just below, which even eliminates corrosion-adverse δ-ferrite. Further analysis reveals that the ε-Fe3N phase reactively manipulates oxygen vacancies and corrosive ions, effectively inhibiting chlorine adsorption and pit initiation. Meanwhile, the retained Cr in the grain-refined layer enhances p-type passivation after ε-Fe3N dynamically reacts to moderate corrosion environment. With this, PTA-remelting achieves superior SCC resistance with retained high elongation-to-fracture up to ~75%. Coherent with the corrosion mechanism, this SCC resistance is attributed to the unique multi-layer configuration to neutralize crack initiation.