Combined influence of Ce(III) and iodide ions for corrosion protection of AA 2024-T3 in acidic to neutral chloride-rich environments: Electrochemical and surface characterization studies

Abstract

It has been reported that trivalent cerium salts are effective inhibitors for corrosion of AA2024-T3 in neutral to alkaline corrosive environments, but poor in acidic environments. In this study, work was done for extending corrosion resistance provided by Ce(NO3)3 to acidic pHs as low as 2.5 through the addition of iodide ions. To this end, potentiodynamic polarization was used to ascertain the optimum Ce(III) concentration for the inhibition of AA2024-T3 corrosion in 3.5%NaCl solution. This optimum concentration is found to be in a range between 0.1 and 0.3 mmol/L Ce(III). This optimum concentration also gives a maximum width of the passive region under the pitting potential (Epit). Based on this optimization, further work was performed as a function of pH without and in the presence of 0.01 mol/L KI at five pH values (2.5, 4, 5, 7, and 8). Without iodide the best inhibition provided by the Ce(III) cation is at pH = 7 but active corrosion is observed at pH = 2.5 with an estimated inhibitor efficiency (−37.6%). The addition of iodide ions (I–) reverses the poor corrosion resistance whereby the inhibitor efficiency becomes 47.7%. Moreover, at pH = 4 the efficiency is also increased from 54.0% to 81.0% with iodide addition providing a significant improvement over Ce(III) cations alone under acidic conditions. There is a synergistic effect between I– anion and Ce(III) cation, which is determined to be 2.4 and 2.1 at pH = 2.5 and 4.0, respectively. At pH=5 and 7 there is virtually no change in inhibition but at pH = 8 the inhibition falls away due to a considerable amount of cerium precipitating out of solution as cerium carbonate. The protective film formed on sites in the aluminium alloy surface at different pH values was examined by both scanning electron microscopy with energy dispersive X-ray analysis and X-ray photoelectron spectroscopy to determine the role of these ions as a function of pH in surface inhibition. It is proposed that at pH = 2.5 protection is provided by a bilayer adsorption of iodide with “capping” cerium ions, but at pH = 4 protection occurs through precipitate formation on active cathodes thereby shutting down the oxygen reduction reaction.