Passivity breakdown of high-strength 7068 aluminum alloy in borate buffer solutions containing chlorides

Passivity breakdown mechanism of high-strength 7068 alloy is studied. Breakdown potential varied linearly with log a_Cl^‒, pH, and square root of scan rate in potentiodynamic polarization tests. Mott-Schottky analysis showed that the dominant defect is cation vacancy. Passive layer characteristics like cation vacancy density and defect annihilation rate are determined. Critical cation vacancy density for pitting obtained from point defect model and the theoretical values are somewhat compatible. Chloride concentration of 0.01 M is too dilute to cause severe localized corrosion, whereas beyond 0.5 M, saturation is achieved in terms of breakdown potential and the cation vacancy density.