Zinc phosphate
The leading inhibitive anti-corrosion pigment, which replaced lead and chromate pigments in many steel primers.
Zinc phosphate, Zn₃(PO₄)₂, is a white, slightly soluble pigment, usually supplied as a hydrate. Modified grades combine it with other ions (such as aluminum or molybdate) to boost activity.
It is used in epoxy, alkyd, acrylic and polyurethane primers for steel to provide inhibitive corrosion protection, especially where lead and chromate pigments are no longer acceptable.
When moisture reaches the primer, small amounts of phosphate dissolve and help form a passivating layer on the steel, slowing corrosion at defects. Its effectiveness depends on formulation, binder permeability and pigment volume concentration, and it is generally slower-acting than the chromates it replaced.
Zinc phosphate is classified as hazardous to aquatic life; prevent release to drains and waterways. Control dust when handling dry pigment.
Zinc phosphate inhibits corrosion chemically by passivating steel. Zinc dust protects sacrificially by corroding in place of the steel and needs very high loading.
It is much less toxic but generally less potent, so formulations are optimized and often use modified zinc phosphates.
