Energetics of aluminium alloys anodization for porous oxide films elaboration

From recent published data relative to the anodization of 99.9% aluminium and 2024T3 alloy at a given value of the current density, it is established that the energy density W/S is proportional to the anodization time t(a). In order to appreciate the applicability of such an empirical relation, we conduct the 1050A alloy anodization at a given value of the anodization voltage V-a (industrial procedure) in three different electrolytes (borate, phosphoric acid and boric acid/sulphuric acid). The proportionality coefficient k between W/S and t(a) appears to increase with the anodization voltage V-a, while the charge flow density k/V-a is a linear function of the anodization voltage V-a. The H3BO3 + H2SO4 electrolyte experimental data indicate, nevertheless, a sharp change of the energy transfer from a critical anodization voltage. Finally, our data fit to the more detailed form: W/S = (a + bV(a).).V-a.t(a), where the constants a and b depend on the electrolyte solution and its temperature. Such a relation, very important from a chemical engineering point of view, may be directly correlated to the growth of the porous oxide layer during anodization. Considering that its growth rate follows the Elovich law, we indeed show for B.t(a) much less than 1 that the energy density relation is: W/S = alpha b.(c + d.V-a)C-a.t(a), where alpha, B, c and d are constants, i.e. an expression identical to the empirical relation.