Stress-Driven Iron Diffusion in Passive Films on Laser-Desensitized 316L Stainless Steel

Austenitic 316L stainless steel is protected in corrosive environments by a chromium-rich passive film that forms rapidly on its surface. When this film is compact and enriched in Cr₂O₃, it limits charge transfer and helps prevent aggressive species from reaching the underlying metal. In chloride-containing environments, however, even local disturbance of the film can initiate pitting, which make the stability and repair of this surface layer critical under mechanical load. This protection can be weakened by sensitization and in 316L stainless steel, prolonged exposure to humid and elevated-temperature conditions may promote Cr₂₃C₆ carbide precipitation along grain boundaries, producing chromium-depleted regions that are less able to sustain corrosion resistance. Carbide precipitates can be dissolved by pulsed laser treatment without melting the surface, thereby reversing sensitization while preserving the treated surface. The more important question is how this restored surface performs afterward, because the renewed passive film can lose stability when tensile stress and chloride exposure act together.

Previous studies have linked tensile loading with poorer passivity, increased defect density, and altered electrochemical response. What remained less clear was the atomic-scale origin of this deterioration, especially how stress changes the elemental distribution within the passive film. For laser-desensitized 316L stainless steel, the problem therefore extends beyond removing chromium-depleted zones at grain boundaries. The restored surface must also maintain a chemically protective oxide film under mechanical loading. A film may even become thicker without becoming more protective if tensile stress changes its composition, compactness, and ability to resist chloride attack.

In a recently published research paper in Corrosion Science, Dr. Xu-dong Li, Dr. Zi-wen Zhao, Dr. Muhammad Arslan Hafeez, Professor Cheng Zhang, and Professor Lin Liu from Huazhong University of Science and Technology established a multiscale mechanism linking tensile stress to the deterioration of corrosion resistance in laser-desensitized 316L stainless steel. Their multiscale approach connected electrochemical degradation with passive-film chemistry, nanoscale elemental redistribution, and Fe/Cr diffusion.

The researchers prepared pulsed-laser-desensitized 316L specimens and applied increasing elastic tensile stress using a three-point bending arrangement. Electrochemical testing in 3.5 wt% NaCl solution showed a steady decline in corrosion resistance as tensile stress increased. Impedance spectroscopy identified a sharp reduction in total polarization resistance, and circuit fitting showed decreases in both passive-film resistance and charge-transfer resistance under stress. Polarization measurements confirmed this deterioration and that the unstressed and moderately stressed specimens retained a passivation-pitting transition, but the highest-stress condition no longer showed a stable passivation region.

The authors performed Potentiostatic measurements and observed frequent metastable pitting events under high tensile stress, with current transients indicating repeated local rupture of the passive film and slower repassivation. Surface examination after polarization showed that pits became larger and more numerous as stress increased. They were also no longer concentrated mainly near grain boundaries, but appeared increasingly within grain interiors.

The team conducted as well passive-film analysis to explain why the apparent barrier became less effective. Current-decay behaviour showed that tensile stress reduced the efficiency of film densification during growth. Capacitance measurements indicated that the stressed surface became more favourable to chloride adsorption. Mott-Schottky analysis showed n-type semiconducting behaviour in all conditions, but donor defect density increased progressively with tensile stress. XPS then identified the accompanying chemical change: the protective Cr₂O₃ fraction decreased, while FeO and Fe₃O₄ became more prominent. The stressed film therefore contained more lower-valence iron oxides and less protective chromium oxide.

The research team used aberration-corrected TEM and elemental mapping to show that tensile stress did not physically thin the passive film. Under the highest tensile loading, the film near grain boundaries became about 19% thicker, yet its chemical composition shifted in a less protective direction. This finding indicates that mechanical stress degrades the passive film mainly through compositional transformation rather than simple structural thinning. The unstressed passive film was relatively chromium-rich. Under tensile stress, Fe concentration increased toward the outer surface, chromium content was lower across the film thickness, and the Cr/Fe ratio declined substantially. Chlorine content also increased, consistent with the electrochemical evidence for stronger chloride adsorption.

The team’s DFT and AIMD simulations provided the atomic basis for this compositional transformation. Tensile stress made Fe segregation toward the surface more thermodynamically favourable and increased the mobility of both Fe and Cr. Fe nevertheless diffused about 1.5 times faster than Cr under the simulated tensile condition. This preferential Fe migration explains how tensile stress drives the formation of a Fe-enriched, Cr-depleted passive film despite the restored surface condition produced by laser desensitization. It also moves the explanation beyond a general increase in passive-film defects by showing how tensile stress selectively alters Fe and Cr migration during film formation.

The findings of Huazhong University of Science and Technology researchers are important for laser-restored 316L stainless-steel components that operate in chloride-containing environments under sustained elastic loading. For engineering practice, this means that a visually intact surface may still have reduced resistance to localized corrosion. Components restored by pulsed laser treatment may recover protection against sensitization-related attack, but their corrosion performance should be assessed under the loading condition expected in service. Electrochemical testing under pre-applied tensile stress could provide a more realistic basis for qualifying treated material than stress-free corrosion tests alone. Indeed, effective process design should pair Cr₂₃C₆ precipitate removal with control of residual or service-induced tensile stress near exposed surfaces.

Where feasible, reducing high tensile stresses, limiting stress concentrations, or introducing favourable compressive surface states may help preserve the Cr-rich barrier needed for corrosion resistance. Inspection after laser restoration should evaluate localized corrosion across the exposed surface, including grain interiors as well as grain-boundary regions, when the component is expected to experience tensile loading. Although laser desensitization chemically restores the alloy surface, tensile stress can still alter Fe and Cr transport during film formation, leaving the surface layer less protective.