Zinc-Rich Coatings

Zinc-rich coatings and primers have the unique ability to provide galvanic protection to the steel surfaces to which they are applied. These coatings have a large amount of metallic zinc dust combined with the binder. There are two main types of zinc-rich coatings, which differ in type of binder. Inorganic zinc-rich coatings generally have a zinc silicate binder while the organic variety uses an organic resin such as an epoxy, butyl, or urethane. After proper application of a zinc-rich coating to a steel substrate the binder holds the zinc particles in contact with each other and the steel surface. This contact between two dissimilar metals, when in the presence of an electrolyte, will form a galvanic cell. The zinc particles become the anode in the galvanic cell and the steel substrate serves as the cathode. Galvanic action causes the zinc to be preferentially corroded while the steel is protected from attack. Zinc-rich coatings are unique in that they provide protection to the steel surface even at voids, scratches, pinholes and other small defects in the coating system.

Inorganic Zinc-Rich Coatings
Inorganic zinc-rich coatings require an extremely clean surface for application. SSPC-SP 10, SSPC-SP 5, or application to a clean, pickled surface is generally required. Inorganic zinc-rich coatings have good mechanical properties and resist scuffing, scratching, and impact. These coatings also have good heat resistance, up to 750F. Some formulations of these coatings are suitable for immersion in salt water while other formulations are not intended for such use. Immersion in salt water increases the dissolution rate of the zinc particles in the binder and as such, the protective life of coatings in these environments will generally be less than that of fresh water immersion or atmospheric exposure. Topcoating with inorganic zinc-rich coatings tends to be more difficult than with other coatings because of the porous nature of the inorganic zinc primer. When applied the binder partially wets and binds the zinc particles together and to the substrate but it does not completely cover and seal the surface. As a result, topcoats applied over this porous surface can cause pinholes, voids, or bubbles within the topcoat.