Either a bright or dull finish is acceptable. Bright zinc plating closely resembles bright chromium. However, bright zinc does not have the permanence of surface appearance. Zinc coated steel will not rust even when exposed by scratches because of the galvanic protection of the zinc. On weathering, zinc turns to a drab gray color. Zinc should be deposited directly on the base metal (Nickel is permissible undercoat if base metal is a corrosion resisting steel).
The primary use of chromate finishes on zinc is to retard or prevent formation of white corrosion products on zinc surfaces. The primary purpose of phosphate coating on zinc is to provide a paint base.
Type I Without supplementary treatment.
Type II With supplementary chromate treatment.
Type III With supplementary colorless chromate treatment.
Type IV With phosphate conversion treatment.
Zinc coatings prevent oxidation of the protected metal by forming a barrier and by acting as a sacrificial anode if this barrier is damaged. Zinc oxide is a fine white dust that (in contrast to iron oxide ) does not cause a breakdown of the substrate's surface integrity as it is formed. Indeed the zinc oxide, if undisturbed, can act as a barrier to further oxidation, in a way similar to the protection afforded to aluminum and stainless steels by their oxide layers. The majority of hardware parts are zinc plated, rather than cadmium plated .