Bimodal effects of tensile stress on the passive film stability and localized corrosion behavior of 7075 aluminum alloy

The localized corrosion behavior and underlying mechanisms of 7075-T6 aluminum (Al) alloy in 3.5 wt.% NaCl solution were investigated under distinct tensile stress regimes to resolve the non-monotonic stress-corrosion relationship. Utilizing in-situ scanning electrochemical microscopy, electrochemical impedance spectroscopy, and Mott-Schottky analysis coupled with Electron Backscatter Diffraction characterization, this study reveals a critical bimodal effect of tensile loading on 7075-T6 Al alloy. In the elastic regime (200 MPa), the alloy exhibits stress-enhanced corrosion resistance, characterized by a densified passive film with reduced acceptor defect density and suppressed localized cathodic activity. Microstructural characterization confirms that this improvement stems from the relaxation of pre-existing residual strains and corresponding reduction in geometrically necessary dislocation (GND) density. Conversely, transitioning to the yield (490 MPa) and plastic (540 MPa) regimes causes a sharp deterioration in electrochemical stability. Plastic deformation induces significant dislocation multiplication and strain localization, which maintain a high defect density in the passive film and drive accelerated anodic dissolution. These findings demonstrate that tensile stress acts as a regime-dependent switch: moderate elastic loading stabilizes the electrode interface through microstructural "healing", whereas plastic deformation promotes localized failure via a slip-dissolution mechanism.