Transpassivation, traditionally regarded as a damaging process in corrosion science, has been re-examined through atomic-scale analysis, revealing a more nuanced role in metal surface stability. At potentials beyond the passive range, such as 1.1 V/SCE, anodic current densities rise sharply, a phenomenon long interpreted as rapid dissolution of a once-protective passive film. In Fe-Cr alloys and stainless steels, this was linked to oxidation of trivalent chromium to soluble hexavalent species, transforming a Cr-rich film into a Fe-rich transpassive layer. However, the mechanism of chromium dissolution remains contested, with some studies, including operando synchrotron work by Langberg et al., showing preferential Fe dissolution at lower transpassive potentials and rapid Cr dissolution only at higher ones.
In the present study, transpassivation at 1.1 V/SCE was found to enrich the film in Cr³⁺ without detectable hexavalent chromium, suggesting direct metal matrix dissolution across the oxide film. This dissolution proceeds anisotropically, with atoms escaping along the <110> direction, exposing residual {111} planes at the metal/film (Me/F) interface. The process roughens the interface, creating undulations and concaves of varying depth, initiated at misfit dislocations where local strain induces potential drops. As the passive film crystallizes under transpassivation, more dislocations form, propagating dissolution and generating a fractured interface dominated by close-packed {111} planes.
The inhomogeneous composition of the passive film also plays a role; nanoscale Cr-enriched regions resist dissolution, shaping the concave depth distribution. These structural changes have implications for chloride attack resistance. Previous work identified chloride transport, interfacial undulations, and lattice tensions as key in pitting initiation. The low-surface-energy {111} planes weaken chloride interaction, while dislocations disperse chloride, and metal vacancies act as sinks, reducing local accumulation. Direct measurements beneath the transpassive film confirmed diminished chloride concentration, correlating with improved pitting resistance in FeCr15Ni15 austenitic alloy.
Extending the approach to commercial 304 stainless steel revealed grain boundaries (GBs) and inclusions as critical factors. In small-area samples with minimal inclusions, transpassivation markedly improved resistance to reductive dissolution in sulfuric acid, increasing activation time by up to two orders of magnitude. Polarization tests showed reduced passive current density and a noble shift in pitting potential to approximately 520 mV in acidic chloride media. SEM and AFM imaging demonstrated preferential dissolution along GBs, forming gully-shaped concaves, and within grains, generating shallow cones. High-resolution HAADF-STEM confirmed that these features were enclosed by {111} planes or dense {111} zigzag facets, restructuring the interface into a low-energy configuration that stabilizes the passive film.
In larger-area samples with coarse sulfide-oxide inclusions, corrosion resistance did not improve, as inclusions acted as local electrochemical cells, undermining interface engineering. Testing a 304 stainless steel with smaller inclusions restored the beneficial effects, highlighting the importance of inclusion control in steel manufacturing for both mechanical and corrosion performance.
The findings demonstrate that transpassivation, when applied under controlled conditions, is not inherently destructive. Instead, it can serve as a deliberate interface engineering strategy, creating inactive, close-packed crystallographic planes that enhance passive film stability. By coupling film crystallization, misfit dislocation formation, and vacancy generation, the process mitigates chloride-induced pitting and prolongs surface integrity in aggressive environments. This atomic-scale reconfiguration offers a pathway for designing corrosion-resistant alloys, with potential applications in sectors where material longevity under harsh conditions is paramount.