Adhesion of epoxy primer to hydrotalcite conversion coated AA2024

Hydrotalcite-based (HT) conversion coatings are being developed as an environmentally benign alternative to chromate conversion coatings (CCC). Accelerated exposure tests were conducted on epoxy primed, HT-modified AA2024 to gauge service performance. HT-based conversion coatings did not perform as well as the CCC when used with an epoxy primer. The current HT chemistries are optimized for stand-alone corrosion protection, however additional research into the primer/HT interactions is necessary before they can be implemented within a coating scheme. The relative contribution of mechanical and physico-chemical interactions in controlling adhesion has been investigated in this study.; Practical adhesion tests were used to assess the dry and wet bond strength of epoxy primer on HT coatings using the pull-off tensile strength (POTS) as the figure of merit. The practical adhesion of HT coated samples generally fell between that observed for the CCC and bare AA2024. Laboratory testing was done to assess the physical and chemical properties of HT coatings.; Contact angle measurements were performed using powders representative of different HT chemistries to evaluate the dispersive and acid-base character of the surface. The wet POTS correlated with the electrodynamic (dipole + dispersive) parameter of the surface tension. The HT surfaces were found to be predominantly basic. Given the basicity of epoxy, these results indicate that increasing the acidic character of HT coatings may increase the adhesion performance. This was supported by electrokinetic measurements in which the dry POTS was found to increase with decreasing conversion coating iso-electric point. The correlations with the dry and wet state adhesion are interpreted as indicating that dry state adhesion is optimized by minimizing unfavorable polar interactions between the basic epoxy and HT interfaces. Wet state adhesion, where polar interactions are disrupted, is dictated by non-polar bonding. FTIR spectroscopy suggested that covalent between HT coatings and epoxy primers may occur, but could not definitively indicate so.; Present results suggest that the limited chemical interactions, as governed by substrate wetting and acid-base interactions between the epoxy and HT, have minimized the possible mechanical interactions between the resin and the conversion coating.