The corrosion and passivity of aluminum exposed to dilute sodium sulfate solutions

The composition and thickness of the oxide/hydroxide film that forms on pure aluminum surfaces that are polarized in 0.05 M Na₂SO₄ in acidic, near-neutral, and alkaline solutions have been characterized using XPS. The results of this effort have been plotted on surface behavior diagrams to follow the evolution of the surface film during polarization. Although a thin layer of gibbsite Al(OH)₃] is often indicated to be the film that forms and protects aluminum from corroding in near-neutral pH solutions after 10–20 h of exposure, the composition of the film that initially forms on Al after a short period of time (1 h) was actually closer to boehmite (AlOOH), the oxyhydroxide phase. This film generally grows thicker during either anodic or cathodic polarization in both the pH 7 and pH 10 solutions. For the solution at pH 2, the air-formed film dissolves to a very thin oxyhydroxide film that is replaced by an oxide film, Al₂O₃, during anodic polarization. The growth of the oxyhydroxide film at each pH during cathodic polarization is attributed to the local buildup of hydroxyl ions at the surface of the working electrode.