![]() 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. Zinc coatings prevent oxidation of the protected metal by forming a barrier and by acting as a sacrificial anode if this barrier is damaged. Type IV With phosphate conversion treatment. Type III With supplementary colorless chromate treatment. Type II With supplementary chromate treatment. The primary purpose of phosphate coating on zinc is to provide a paint base. The primary use of chromate finishes on zinc is to retard or prevent formation of white corrosion products on zinc surfaces. Zinc should be deposited directly on the base metal (Nickel is permissible undercoat if base metal is a corrosion resisting steel). On weathering, zinc turns to a drab gray color. Zinc coated steel will not rust even when exposed by scratches because of the galvanic protection of the zinc. However, bright zinc does not have the permanence of surface appearance. ![]() Bright zinc plating closely resembles bright chromium. Either a bright or dull finish is acceptable.
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