Potential-dependent evolution and degradation of the protective oxide film on 70Cu-30Ni alloy in marine environment

This work systematically investigates the effect of applied potentials ranging from −100 to −900 mV vs. SCE on the composition, structure, semiconducting properties, and pitting resistance of pre-formed oxide films on 70Cu-30Ni alloy in marine environment. A multi-technique approach combining electrochemical measurements (polarization, EIS, Mott–Schottky), surface and cross-sectional microscopy (OM, SEM, HRTEM-EDS), and XPS compositional analysis was employed. Results reveal a non-monotonic “increase–decrease–increase” trend in pitting depth with decreasing potential, determined by the continuous evolution of the electrochemical driving force, film property and Cl − interaction under the regulation of applied potential. At weak anodic polarization (−100 mV), the film remains compact and highly protective. As cathodic polarization increases, sequential reduction of oxides/hydroxides of Cu/Ni/Fe occurs, accompanied by rising vacancy density, film porosity and pitting depth. At −500 mV vs. SCE, a transient cathodic protection window is identified, wherein annihilation of cation and oxygen vacancies temporarily reduces defect density, repels Cl⁻ penetration, and partially restores film resistance and layered structure. Below −700 mV, extensive reduction of all protective oxides/hydroxides and vigorous hydrogen evolution reaction (HER) induce catastrophic film disintegration, marked by exponential increases in charge carrier density, porous macro-defects and pitting depth. The findings provide a theoretical basis for optimizing cathodic protection strategies for 70Cu-30Ni alloys in marine environment. This study clarifies pitting corrosion mechanism of pre-filming 70Cu-30Ni alloy in marine environment and oxide film evolution under potential gradients, identifying applied potential as the core regulator of the synergistic effect of film compositional reduction, defect generation, Cl − attack and micro-galvanic coupling among alloying elements, which controls the pitting behaviors. Different polarized potentials induce distinct compositional/structural transformations of the film, thus regulating pitting initiation and propagation. Weak anodic polarization (−100 mV) yields an optimized defect-free film with slight pitting. The selective dissolution of Ni triggers pitting under mild cathodic polarization (−300 mV). Enhanced cathodic polarization (−400 mV) leads to massive vacancies and aggravated pitting via Cl⁻-metal chloro-complexes. −500 mV acts as a transient cathodic protection window, mitigating pitting through vacancy annihilation and Cl⁻ repulsion. Strong (−700 mV) and extreme (−900 mV, HER dominated) cathodic polarization trigger film structure collapse and complete peeling, respectively, with the latter leading to explosive pitting (max depth 16.33 μm) due to unrestricted Cl⁻ enrichment and H 2 bubble impingement. • Effects of cathodic potentials on 70Cu-30Ni alloy oxide films are investigated. • Corrosion resistance correlates strongly with oxide film compositional reduction. • CP region around −500 mV is identified to reduce film defects and Cl⁻ penetration. • Excessive polarization causes film collapse via total oxide reduction and HER. • SACP optimization for balanced pitting resistance by potential control is guided.