Zinc dust (metallic zinc)
The sacrificial metal in zinc-rich primers: zinc corrodes so the steel underneath does not.
Zinc dust is finely divided metallic zinc, typically spherical particles a few micrometers across, made by condensing zinc vapor. Lamellar (flake) zinc is also used in some systems.
It is the active pigment in organic (epoxy, urethane) and inorganic (ethyl silicate) zinc-rich primers for structural steel, bridges, tanks and offshore structures, usually topcoated with epoxy and polyurethane.
Zinc is more electrochemically active than iron. With enough zinc in the dry film for particles to touch each other and the blast-cleaned steel, zinc corrodes preferentially, protecting the steel galvanically. Zinc corrosion products then seal pores, shifting the primer toward barrier protection over time.
Fine zinc powder can be combustible and may react with acids, bases and water to release flammable hydrogen. Avoid dust clouds and sparks, store dry, and protect waterways because zinc is toxic to aquatic life.
Galvanic protection requires electrical contact between zinc particles and bare metal. Mill scale, rust or contamination break that contact.
Specifications commonly set a minimum metallic zinc content in the dry film, and research shows galvanic protection needs high loadings. Check the product data and governing specification.
