Abstract
The anodic polarization behavior and time dependence of anodic current of 99.99% aluminum in 0.5N NaCl solution (pH 4, 6, 8.5, 10 and 12) were examined potentiostatially. The surface structure and the composition of corrosion product films formed on the surface were also observed by a scanning electron microscope, X-ray microanalyzer and energy dispersion type X-ray spectrograph, and the pitting mechanism of pure aluminum was discussed. Pitting corrosion of pure aluminum in 0.5N NaCl at pH 8.5 seemed to be caused by Cl- which enters through cracks at the interface between the surface film and a rhombic corrosion product formed at less noble potentials than the pitting potential. This rhombic substance was observed only at pH 8.5, and was not formed at pH 4, 6, and 10 in 0.5N NaCl solutions. In 0.5N NaCl solution at pH 12, the pitting corrosion was affected by chemical dissolution, thus the pitting potential became rather noble. The limiting current observed in the anodic polarization curves in the solutions of pH 8.5, 10 and 12 seems to be controled by the diffusion of OH-. The potential at which pits are induced for pure aluminum in 0.5N NaCl was about -0.743V at pH 4, -0.757V at pH 6, -0.760V at pH 8.5, and -0.710V at pH 10. The main components of the rhombic corrosion product were ascertained as aluminum, iron, and silicon.
