Enhanced paint adhesion to metals using in situ phosphatizing coatings

Adhesion of an organic coating is related to the density of weak secondary bonds, in general, and strong primary chemical bonds, in particular, produced between the coating and the substrate as well as their distributions, and also on the morphology and quality of coating. A 'green technology' for metal finishing, based on in situ phosphatizing coatings (ISPCs), is described to enhance the presence of primary bonds and applied to cold-rolled steel, 2024 T3 aluminum alloy, and Ti-6Al-4V titanium alloy. When an ISPC is applied to an untreated metal substrate, the in situ phosphatizing reagent (ISPR) reacts chemically and/or physically with the metal surface to produce a metal phosphate layer and simultaneously forms covalent phosphorus–oxygen–carbon linkages with the polymer coating. In an ISPC, the ISPR is designed both to phosphate the metal surface in situ and, at the same time, to catalyze the curing of the polymer coating or paint. Paint systems cured by ISPR give a dense and corrosion-resistant and a thermally stable and defect-free coating. The primary bonds and the less permeable paint coatings produced in ISPCs are shown to have enhanced adhesion and inhibit substrate corrosion. Moreover, an ISPC is a chromate-free single-step process in which the phosphate formation and polymer curing can occur independently, but simultaneously.