Zinc-rich coatings have the unique ability to provide barrier and sacrificial protection to steel surfaces. These coatings have a large amount of metallic zinc dust combined with the binder. There are two main types of zinc-rich coatings which are distinguished by the type of binder used in the paint. 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 on a steel substrate, the binder holds the zinc particles in contact with each other and the steel surface. This contact between two dissimilar metals will form a galvanic cell when in the presence of an electrolyte. The zinc particles serve as the anode of the galvanic cell, while the steel substrate serves as the cathode. Galvanic activity causes the zinc to corrode preferentially while the steel is protected from corrosion. Organic zinc-rich coatings are generally considered to be easier to apply and topcoat than their inorganic counterparts; however, they do not provide the same long-term corrosion protection as inorganic zinc-rich coatings.
Organic Zinc-Rich Coatings
Organic zinc-rich coatings are often formulated using epoxy polyamide, vinyl, urethane, and chlorinated rubber binders. The choice of binder in the formulation determines the drying and curing of the organic zinc-rich coating. These coatings contain high concentrations of zinc dust pigment to achieve a dry film pigment volume of 75 percent or higher. When zinc particles are formulated into organic vehicles, the binder encapsulates the zinc particles more thoroughly than with inorganic vehicles. This encapsulation reduces the sacrificial capabilities of the applied coating. However, this characteristic of the binder also allows the coatings to more readily wet and seal the prepared surfaces. For this reason, organic zinc-rich coatings are more tolerant of incomplete surface preparation. Organic zinc-rich coatings are often used to touch up and repair inorganic zinc-rich coatings because the organic binder provides better adhesion and wetting characteristics than another coat of inorganic zinc-rich coating. Organic zinc-rich coatings provide less heat and abrasion resistance than inorganic zinc-rich coatings.
Inorganic Zinc-Rich Coatings
Inorganic zinc-rich coatings require an extremely clean surface for application. SSPC-SP 5/NACE No. 1 White Metal Blasting, SSPC-SP 10/NACE No. 2 Near-White Metal Blasting, or a clean, pickled surface is generally required. Inorganic zinc-rich coatings have good mechanical properties that resist scuffing, scratching, and impact. These coatings also have good heat resistance, up to 750°F. Inorganic zinc-rich coatings are unique in that they provide protection to the steel surface at voids, scratches, pinholes, and other small defects in the coating system by filling the defects with the zinc corrosion product. Some coating formulations are suitable for immersion in salt water. Immersion in salt water increases the dissolution rate of the zinc particles in the binder, and as such, the protective life of the coating in these environments will generally be less than that of fresh water immersion or atmospheric exposure. Top coating with inorganic zinc-rich coatings tends to be more difficult than with other coatings because of the porous nature of the inorganic zinc-rich 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 exhibit pinholes, voids, or bubbles.
Pre-construction primer is a coating applied to steel plates prior to the fabrication of a part. Manufacturers use this type of coating to prevent and control corrosion during ship construction. Typically, a pre-construction primer is a two-part epoxy composed of inorganic zinc.
On U.S. Navy surface ships, pre-construction primer can be over-coated during the final painting phase, but only in certain spaces and only if certain requirements are met. Two requirements are that primer must be formulated with zinc silicate and the specified coating system must be compatible with the pre-construction primer. Refer to NAVSEA Standard Item 009-32 for all pre-construction primer requirements.
“Coatings”, NAVSEA Basic Paint Inspector Training Student Guide. Version 13, Naval Sea Systems Command and Naval Surface Warfare Center, Carderock Division, Code 614, October 2016, sec 10, pp. 10-9 – 10-10.
Richard W. Drisko, James F. Jenkins, “Properties of Generic Coating Types” in Good Painting Practice, SSPC Painting Manual Volume 1, 4th ed. Pittsburgh: The Society of Protective Coatings, 2002, ch 1.2, pp. 35-37.