Abstract
The effect of paint adhesion on the corrosion propagation and the type of corrosion products formed in painted hot-dip galvanized steel panels has been studied. The HDG panels were phosphated and EC-coated and exposed in the GM scab corrosion test. The adhesion between the paint and the phosphated steel surface was varied by curing the primer at different temperatures. In another series of experiments, the interfacial bonding was modified from hydrogen (acid-base) bonding to covalent bonding by using a series of functional silanes replacing the standard chromate post rinse. In order to estimate the strength of the paint-metal interface, a new technique was developed which is presented and discussed here. It consists of swelling the paint in N-Methyl Pyrrolidone (NMP) at 60°C. The time required to lift the paint is a measure of the adhesive strength of the interface in the system. Differences in cure state, pretreatments and types of paints can be detected with this technique better than by previously used paint adhesion methods. The major results of this study are that the rate of underfilm corrosion is somewhat dependent on the quality of the phosphate-paint interface and further, that the corrosion products formed also differ in composition and morphology if this adhesion is varied. The results are discussed in terms of a model which is based on diffusion through the paint film from the top downwards and also along the paint-metal interface.
