Release of cerium organo corrosion inhibitors from primers - role of primer free volume

It has been recently shown that rare earth metal organo compounds, in this case cerium-organo compounds, when incorporated into epoxy primer coating can offer significant corrosion protection for AA2024 T3 alloy [1]. Two cerium compounds at fixed loading were evaluated in order to investigate the synergy between cerium and the organic-anion complexes. The compounds are dissolved in an epoxy-amine primer in liquid form and on curing the primer mixture, the cerium compound reprecipitated in nanoparticulate form dispersed throughout the coating. Primers containing 20 wt% of either cerium dibutylphosphate (Ce(dbp) 3) or cerium salicylate were able to reduce the extent of corrosion both in mild (0.001M NaCl) and aggressive (filiform corrosion test, saturated aqueous NaCl) corrosion environments on AA2024 T3 by leaching out from the coating into surrounding moisture without being depleted over a 30 hour period. Analysis using position annihilation lifetime spectroscopy (PALS) showed that Ce(dbp) 3 did not simply function as an inert filler in the primer system but altered the properties of the primer. This interaction was reflected in the free volume, water uptake properties, and the adhesion characteristics of the primer. These results indicate that design of the inhibitor coating system relies on an understanding of the delivery mechanism and release kinetics. Such an understanding will enable the replacement of hexavalent chromium (Cr 6+) compound inhibitors with environmentally benign alternatives.