The chemical properties of zinc are lively, and it is easy to oxidize and darken in the atmosphere, and finally produce white rust corrosion. Zinc plating is treated with chromate to cover a layer of chemical conversion film on the zinc, so that the active metal is in a passive state, which is called chromate passivation. A film thickness of 0.5um can increase the corrosion resistance of zinc by 6~8 times, and endow zinc with a beautiful appearance and anti-pollution ability.

The passivation film can be divided into white passivation, light blue passivation, iridescent passivation, golden yellow, black passivation and army green passivation from the appearance. The order of corrosion resistance of these passivation films is army green>black>rainbow color>golden yellow>light blue>white. Therefore, all galvanized mechanical parts used for corrosion resistance must be iridescent passivated.

According to the concentration, the passivation solution can be divided into three types: high concentration, medium concentration and low concentration. Because less than 5% of chromic acid is consumed in passivation, 95% is lost by parts, causing serious environmental pollution. The use of low-concentration passivation solution can reduce production costs, reduce pollution, passivation film quality and high-concentration chromic acid passivation, so low-chromium passivation is the main method.

The three elements that make up the chromic acid passivation solution: 1. The main salt and its concentration; 2. The activator and its concentration; 3. A certain hydrogen ion concentration (PH value).