The correlation between the pitting corrosion and the activating process of aluminium and its alloys has been investigated in Cl− containing borax buffer solutions with pH values of 5.5∼8.5. Effects of the critical activating potential, pH, concentration of Cl− and anodic polarization behavior on the pitting corrosion of aluminium are studied by means of potentiostatic electrolysis and the potentiodynamic method. The experimental results obtained are as follows.(1) A critical activating potential exists in anodic polarization curves of aluminium. Near this potential, aluminium is rapidly activated and the anodic current density is increased.(2) The critical activating potential is affected by the concentration of Cl− and the surface condition of aluminium and shifts toward the basic potential side in proportion to the concentration of Cl−. In case the aluminium oxide film is stable, it shifts toward the noble potential side. But additional alloy elements and cations de not affect the activating potential.(3) Aluminium is affected after an induction period when it is held under prolonged potentiostatic electrolysis at the potential more basic than critical activating potential. The activating induction peroid depends on the potential and concentration of Cl−. The interval of the period is reduced with increasing potential and also with increasing concentration of Cl−.(4) Aluminium is activated even at the potential lower than activating potential and pitting corrosion occurs proportionally with activating. Therefore, it is uncertain whether the critical activating potential of the anodic polarization curves is a pitting potential.
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Izaki et al. (1967) studied this question.