After analyzing the passivation film, it was found that trivalent chromium is the most abundant component, followed by water, and then hexavalent chromium. Other components include sulfate, zinc, and sodium. Therefore, it can be concluded that the chromate film is mainly composed of trivalent chromium, hexavalent chromium, and water, with the general formula:
xCr₂O₃·yCrO₈·zH₂O
The chromate exists on the surface of the galvanized layer in the form of colloidal substances of chromium chromate (CrOa-Cr₂Oa-nH₂O or xCr₂Oa-yCrOa-zH₂O). On the other hand, after reacting with zinc, chromic acid becomes zinc chromate (ZnCrO₂) and attaches to the film. Since it is soluble in water, it coexists with chromic acid in aqueous solution and can be dissolved away from the galvanized layer surface after rinsing. Its presence is detrimental to the film. Therefore, it is evident that the chromium component in the passivation film provides the protective effect.
The principle of corrosion protection by the passivation film mainly lies in its high content of trivalent chromium. The passivation film is insoluble in water, which acts like a transparent coating covering the galvanized layer, preventing the intrusion of oxygen and moisture from the air into the galvanized layer and its pores. Meanwhile, the presence of a small amount of hexavalent chromium helps repair scratched passivation films, restoring torn films to their original integrity and preventing anodic dissolution of the galvanized layer.
