Fast optimization of polymers to boost adhesion and corrosion prevention in hexafluoride conversion coatings for aluminium

Hexafluorides associated with polymers are a growing alternative to chromium-VI metal treatments. The polymers in these conversions enhance both the adhesion of the coating and the corrosion protection. The underlying mechanisms involve chemical bonding and surface interactions, the latter being readily accessible by measuring surface energy. We here investigate the link between surface energy, paint adhesion and corrosion resistance of painted aluminium panels. We evaluate zirconium hexafluorides conversion coatings with and without co-polymers made of acrylic acid and phosphonate or phosphate groups using an automated measurement of oil/solvent/metal contact angles. First, experimental results highlight that increased aluminium surface energies correlate with improved paint adhesion while limited corrosion seems related to low polar surface energies. Given this critical role of surface energy in these performances, the balance between adhesion and corrosion can be optimized by targeting specific values of contact angles of water and di-iodomethane. Second, the addition of polymer to hexafluoride treatments increases the polar surface energy and gives higher performances in corrosion and paint adhesion. Besides, a kinetic study reveals that the addition of polymer in zirconium conversions slows down the surface ageing over several days and allows a delayed paint application. Finally, as highlighted by the addition of phosphate or phosphonate functions in polyacrylic-acid polymers, the balance between adhesion and corrosion is improved by decreasing the polymer hydrophilicity to avoid corrosion while maintaining a sufficient functional density in its structure to develop covalent bonds with paint. This insight from contact angles and screening possibilities may contribute to speeding-up the development of new polymer chemistries in conversion technologies. Graphical abstract