Please refer to the attached article on "Measurement of Oxide Barrier-Film Thickness of Al-Alloy by Electrochemical Impedance Spectroscopy at the Nanometre-Scale".
It would be great to make your argument a bit scientific regarding to the questionable issues of the equations. Naturally, I am interested & eager to hear from you about the suggested model of the attached article.
Indeed, the charges(3+)in the entire Eqs.4&5 should be removed from Al.
Eq.5(without the charge(3+) is a suggested model as indicated in the attached article, without any experimental data, for the oxidation of Al in air. The model of the oxide barrier-film formation of Al in air then the subsequent formation of a porous oxide layer of Al during anodization was what the attached article attempted to shed light on the formation of the porous structure of Al2O3.
You can tell the suggested over all model is quite complex one.