Novel, one-step, chromate-free coatings containing anticorrosion pigments for metals—An overview and mechanistic study

The concept of superprimers, i.e., primers for metals with the conversion coating built in, has proven to be feasible. Such primers do not require a chromate (or other) conversion treatment of the metal and can be applied on any bare metal. The VOC content in the primer is kept low by using water-dispersed organic resins. Because of the high concentration of organofunctional silanes contained in the superprimer, its adhesion, both to the substrate and to overcoats, is excellent. The corrosion resistance of the primer can further be improved by adding corrosion-inhibiting pigments such as micronized zinc phosphate into the formulation. The corrosion protection performance of these coatings has been evaluated using performance tests and electrochemical methods. The primers have also been characterized with a variety of surface analytical methods. It will be shown that 2000 h of salt spray resistance has been obtained with the primers both on AA2024-T3 and HDG steel. This study will mainly present an overview of the chromate-free primers studied in our laboratory and will attempt to explain the mechanism by which one of the coatings, the epoxy–acrylate-based primer deposited onto aluminum, protects the metal from corrosion.