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Zinc plating is a soft, decorative, corrosion-resistant finish. Zinc protects the substrate by sacrificing itself and thus corrodes before the base metal. This means that zinc will protect even if the zinc coating sustains minor damage, such as scratches or small punctures because of the galvanic protection of the zinc; however, the zinc coating can be attacked or dissolved by ordinary liquids such as soft drinks and vinegar. The ultimate corrosion resistance of zinc is dependant on the thickness of the coating. To increase the corrosion-resistance of the zinc, a conversion coating is usually added. The primary use of chromate finishes on zinc is to retard or prevent the formation of white corrosion products on zinc surfaces.
Specification Comments
ASTM B 633
Type I As plated, no chromate
Type II With supplementary chromate treatment
Type III With supplementary colorless chromate treatment
Type IV With phosphate conversion treatment. Used to provide a paint base.
SC 4 (very severe) .0010"
SC 3 (severe) .00050"
SC 2 (moderate) .00030"
SC 1 (mild) .00020"
Corrosion resistance requirements:
  Type II         96 hours
  Type III        12 hours