Protection of infrastructure from corrosion is essential in preventing costly failures, with risk to both human life and the structure itself. This is frequently achieved through the use of complex, multi-layered coating systems. Current organic coating systems designed for harsh environments are typically comprised of a number of different types of coating layers, each providing a different set of properties. A basic system usually consists of three layers, which may include a primer coat, an intermediate coat and a final top coat. Primer coats are typified by epoxy-based formulations containing a relatively high loading of a more anodic metal such as zinc (zinc rich), which provides a sacrificial protection to the metal substrate. Intermediate coats are usually formulated around a solvent/epoxy base with a pigment blend containing a relatively large fraction of micaceous iron oxide. The intermediate coat or tie coat serves to promote adhesion between the primer coat and the top coat layers. The intermediate coat may also provide a barrier-type protection against corrosive species such as water, ions or oxygen, slowing their diffusion to and from the metal surface, although it is recognized that there are some limitations to these barrier properties due to the permeability of organic coatings to such corrosive species. Finally, the top coat of the system is normally composed of a polyurethane or polysiloxane. These materials usually offer UV resistance in addition to any aesthetic finish.
A key component of these systems in high-risk environments where there is significant risk of corrosion is the zinc-rich primer. These primers have been widely used since the 1930s for the protection of steel structures. Unlike regular organic paints, they provide galvanic protection at edges or the places where the coating is defective. In the FHWA-sponsored “PACE” study,1 the coatings with zinc-rich primers performed best compared to other generic types of coatings evaluated under similar conditions. Zinc-rich coatings are described in a number of industry and military specifications. SSPC Paint 202 is an industry specification that categorizes zinc primers according to vehicle types. Type II coatings are the most common, involving organic vehicles such as epoxies and moisture cure urethanes.