The results in chloride-containing solutions revealed that, by microcrystallization, the resistance of aluminum to pitting corrosion was improved. The semiconducting property of the oxide film on the microcrystalline Al (mc-Al) was different from that of the bulk Al in chloride containing solution. The former had p-type semiconducting property. However, the latter had n-type property. The improved pitting corrosion resistance for the mc-Al was ascribed to the p-type semicomducting film with the smaller isoelectric point (IEP).
The variation in the critical pitting potential with the chloride ion concentration has been studied. A linear relationship was found for the bulk and mc-Al. The larger slope for the mc-Al indicated its higher susceptive degree to the change of the chloride ion concentrations. The critical chloride ion concentration leading to pitting corrosion for the mc-Al was about two orders of magnitude higher than that for the bulk counterpart, which was a result of their different pitting corrosion mechanisms. The differences in the electronic properties, structure and surface states of the oxide films on the bulk and mc-Al determined their different chloride corrosion mechanisms.
The effect of microcrystallization on the stability of the oxide film was investigated. The results indicated that the passive film on the mc-Al in chloride-containing solution was more stable than that in chloride-free solution. The addition of chloride ions had two competitive effects: one made the solution more corrosive; the other changed the film from an n-type semiconductor to a more stable p-type semiconductor. The latter is predominant, which is different from the case of the bulk Al. An experimental procedure was designed to investigate the effect of Cl- on the stability of the passive film on the bulk Al. That procedure was also applied to the microcrystalline Al. However, the result was unusual. This result was explained in terms of a positive p-n junction structure for the passive film.
The results of the grain size effect on the electrochemical properties for the mc-Al revealed that the resistance to pitting corrosion was improved with decreasing the grain size.
The results in NaF solution indicated that the microcrystallization led to the passive film on Al less stable. When F- ion concentration is high enough（0.03mol/L） to make Al have an active-passive behavior, the passive film is an n-type semiconductor on the mc-Al and a p-type semiconductor on the bulk Al.
The electrochemical behavior for the mc-Al in solution containing F- and Cl- ions was investigated. It was found that, when [Cl-]>[ F-], the resistance of mc-Al to pitting corrosion was improved with increasing F- ion concentration. Cl- and F- ion competed to affect the semiconductor type of passive film on mc-Al. When [Cl-]<[ F-], the effect of F- ion is predominant and the passive film is an n-type semiconductor. When [Cl-]>[ F-], the effect of Cl- ion is predominant and the passive film is a p-type semiconductor.
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