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Zinc plating

Zinc prevents corrosion of iron, cast iron, and other iron-based materials sacrificially (electrochemically). Anticorrosive (antirust) performance is indicated by the time of rust formation (H) in salt spray testing (SST). Conventionally, chromate treatment using hexavalent chromium was performed to prevent white rust of zinc, but because hexavalent chromium has become unusable according to an RoHS Directive, trivalent chromium chemical conversion treatment is now mainstream.
Type of plating Features SST white rust SST red rust
Zn plating
(plating thickness 8μm)
Hexavalent unichrome 48 H 240 H
Hexavalent colored chromate 96 H 480 H
Hexavalent black chromate 96 H 480 H
Hexavalent green chromate 240 H 720 H
Trivalent unichromeGreen mark 96 H 120 H
Trivalent colored chromateGreen mark 120 H 240 H
Trivalent black chromateGreen mark 120 H 240 H
Adoption examples
Zinc plating
Zinc plating; trivalent chromate
- In Japanese, “chromate” implies “hexavalent chromium.” However, at present, when trivalent chromium chemical conversion treatment is mainstream, the word “trivalent chromate” is full of contradictions. As a matter of fact, it should be referred to as “trivalent chromium chemical conversion coating or treatment,” but here, general chromium chemical conversion treatment is correspondingly called chromate treatment as an industry term.

- For materials having concerns about hydrogen brittleness, such as high-tensile steel materials (tensile strength 50kg/mm2 or higher), acid zinc plating is used. In relation to aircraft, cadmium (Cd) plating and cadmium-titanium (Cd-Ti) alloy plating are used.

- Under a high-temperature aqueous environment exceeding 60°C, use of zinc plating should be avoided because the sacrificial anti-corrosive action of zinc deteriorates.