Zinc plating is a soft, ductile, decorative, marginally solderable, corrosion-resistant finish. Unlike most other commonly plated metals, zinc protects the substrate by sacrificing itself and thus corrodes before the base metal. For corrosion protection, chromates are applied over the zinc. Chromates are chemical conversion coatings. The substrate metal participates in the coating reaction and becomes a component of the coating. The collaboration has a profound effect on the properties of the coating. Among the metals commonly chromated are zinc, zinc die casting, steel, aluminum, and sometimes copper and silver. Chromate films are typically very thin, on the order of .0000001" and contribute no measurable thickness to the overall coating.
The most commonly used specification for zinc plating in ASTM B 633, which replaced QQ-Z-325 in 1982, although a great many drawings still call for QQ-Z-325. One of the main advantages of the ASTM specification over QQ-Z-325 is that ASTM B 633 distinguishes between colored and clear chromate conversion coatings. It calls out four types of post finishes for zinc plating, Type I: as plated; Type II: colored chromate (default yellow, but olive drab or black may be specified); Type III: colorless chromate (these are often designated as clear or bright, blue bright, or blue); and Type IV: phosphate coating. Thicknesses are spelled out by an SC designation: SC1 0.001in., SC2, 0.0005 in., SC3, 0.0003 in., SC4, 0.0002 in. and SC5, 0.0001 in.
Types of Zinc Finishes - (Their resistance to white corrosion)
Clear - (8 to 12 hours) It has a slightly iridescent blue appearance.
Black- (48 hours)
Yellow - (in excess of 96 hours) Excellent paint base.
Olive Drab- (150 hours to white corrosion) dark green finish
Seals are offered for after chromates to increase corrosion resistance.