Failure of a Waterborne Primer Applied to Zinc Coated Steel

The durability of a high‐performance coating system based on a polyester top coat and a water‐based primer system has been studied by exposure of coated panels to salt spray and 100% RH. The humidity exposure is less deleterious than the salt spray with a higher concentration of an adhesion promoter further enhancing durability in each case. XPS analysis establishes that in both cases failure occurs as a result of a weak boundary layer of zinc oxide corrosion product. Further testing of these pre‐exposed panels using a wet adhesion test and a mechanical pull‐off test confirms the superior performance of the formulation with a higher level of adhesion promoter in both tests. The locus of failure is generally in the organic phase, although those specimens at the lower end of the performance spectrum show a significant amount of failure at or close to the coating/substrate interface. For samples pre‐exposed to the salt spray, the presence of chloride ions at the failed interface indicates that such a treatment may have a deleterious effect on subsequent performance. Copyright © 2012 John Wiley & Sons, Ltd.