Analysis of the passivation film reveals that it primarily contains trivalent chromium, followed by water, hexavalent chromium, and other components such as sulfate, zinc, and sodium. Therefore, the chromate film is mainly composed of trivalent chromium, hexavalent chromium, and water, with the general formula:
xCr₂O₃·yCrO₈·zH₂O
The chromate exists as a colloidal substance in the form of chromium oxyacid chromate (CrOa-Cr₂Oa-nH₂O or xCr₂Oa-yCrOa-zH₂O) on the surface of the galvanized layer. On the other hand, when chromic acid reacts with zinc, it becomes zinc chromate (ZnCrO₂) and attaches to the film. Since it is soluble in water, it exists as an aqueous solution along with chromic acid and can be dissolved away from the galvanized surface during water 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 difficult to dissolve in water, so it covers the galvanized layer like a transparent film, preventing the intrusion of oxygen and moisture from the air into the galvanized layer and its pores. Additionally, the presence of a small amount of hexavalent chromium can repair scratched passivation films, restoring torn films to their original integrity and preventing anodic dissolution of the galvanized layer.
