Leaching properties of chromate-containing epoxy films using radiotracers, PALS and SEM

Model chromate-containing epoxy primer (CEP) films were applied to aluminium alloy substrates using doctor blades. The effect of exposure to NaCl solution on the chromate-containing epoxy film was followed by monitoring the leaching behaviour of Cr. The continuous leaching of chromate inhibitor from the epoxy film is a self-healing characteristic that is designed to prevent corrosion of the aluminium alloy substrate. The CEP films were doped with 51Cr chromate inhibitor (t1/2 = 27.7 days), were exposed to NaCl solutions over time, and the 51Cr gamma emission was used to monitor its release from the film into the aqueous solution. Pulsed beam positron lifetime spectroscopy and scanning electron microscopy (SEM) were used to probe structural changes that might transport properties of chromate species in the epoxy matrix. Positron Annihilation Lifetime Spectroscopy (PALS) was used to determine changes to free volumes at the nano scale in the films and SEM was used to ascertain the chromate distribution and structural scales at the micron level and above. Both PALS and SEM studies were preformed as a function of exposure time. It was found that a SrCrO4 depletion zone was formed at the surface as a result of the complete dissolution of the inhibitor particles, suggesting that transport of the Sr2+ and CrO42− might be through a network of channels created by the voids remaining in the epoxy matrix.