Chapter 6 - Environmental and Solubility Issues Related to Novel Corrosion Control

Abstract

This chapter describes the thickness and chemistry of the layers of a conventional three-layer paint system: the pretreatment, primer, and topcoat layers. The corrosion characteristics of a few industrially important metals and the novel silane-based pretreatments and primers of the new and future systems are also described in the chapter. Industrially important metals are often protected by a Cr-containing pretreatment, a Cr-containing primer, and a topcoat. The chromate pretreatment layer, which is also called the chromate conversion coating (CCC), varies in thickness depending on the chemistry of the process and the application method used. The CCC layer is, however, usually not thicker than a few microns, which in coating weight is somewhere between 5 and 25 mg/m2. The hexavalent chromate in the CCC layer is known for its low solubility and the self-healing effect, which means that it only leaches out on demand when the base metal has been scratched. The primer paint layer is, however, much thicker than the CCC layer and in many applications, the primer coating is still loaded with chromate, typically strontium chromate, because its solubility is optimal in primer coatings. The topcoat is usually ∼50 μm thick in many paint systems, and it provides a decorative and durable exterior surface to the metal. Silane-based pretreatments that are based on trialkoxysilanes have been shown to be competitive replacements for CCCs. They provide good corrosion protection to the metal substrate by improving adhesion between the primer and the substrate and by imparting hydrophobicity to the metal surface.