|Figure 1 – Schematic polarization diagram displaying transitions from active corrosion to passive behaviour and to the transpassive state|
One of the strategies employed in the protection of metals is the anodization process. By applying a potential in the passive state (between Epp and transpassivation), and choosing the right media and applied current, it is possible to grow very thick oxide layers to protect the metal surface from oxidation. This constitutes one of the most used techniques in the protection of aluminium alloys. There are several models that try to explain the formation and structure these oxide films; however, much is still uncertain. There are two basic theories: the crystalline oxide model and hydrated polymeric oxide model.
In the crystalline oxide model, as the name implies, passivation depends on the formation of an oxide/hydroxide layer. The exact structure of the oxides is very uncertain and seems to vary from crystalline all the way to completely amorphous. The oxides may contain oxygen and/or hydrogen under several different forms (H+, OH- or H2O), and the number of layers may change according to specific systems as well as the stoichiometry of the oxide.
In polymeric model, on the other hand, water molecules have an important role in passivation as they connect chains of polymeric oxide, whose structure varies from partially crystalline to amorphous.