Inorganic Zinc Coatings Review

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Inorganic Zinc Coatings Review

Industrial Coatings Application and Design

Zinc 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 coatings, which differ in type of binder. Inorganic zinc 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 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 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 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 zinccoatings 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.

Top coating with Inorganic zinc 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.

Inorganic Zinc coating when applied to blasted steel provides excellent corrosion protection, weathering protection, and very good resistance to salt water and solvents. Inorganic zinc coating provides excellent protection when used in chemical plants, refineries, and coastal or offshore installations. This coating can provide even better corrosion protection when a topcoat is applied. Inorganic zinc coating is compatible with epoxies, phenolics, acrylics, silicones, and many other coatings.

Inorganic Zinc Coating General Specifications:

Use Temperature 800°F max
Chemical Resistance (ASTM D543) good
Salt Spray Resistance (ASTM B117) 4300 hrs @ 2 mils DFT
Water Absorption (ASTM D570) < .01%
Color Flat Gray
Thickness .001" - .003"
Adhesion (ASTM D4541) 1500 psi
Slip Coefficient (ASTM A325) .668
Hardness (ASTM D3363) H pencil