Microelectrochemical characterization of the effect of rare earth inhibitors on the localized corrosion of AA2024-T3

The inhibition effect of three rare earth metals (REMs), Ce³⁺, La³⁺ and Pr³⁺, on the electrochemical properties of synthesized single-phase intermetallic compounds (IMCs) representing phases commonly found in AA2024-T3 were studied and compared with chromate inhibition. REM additions had little effect on corrosion potential, pitting potential and i_corr in short-term tests. This is in contrast to effects observed with chromate inhibitor additions where the pitting potential was seen to increase dramatically. All three REMs decreased the oxygen reduction reaction (ORR) kinetics on each synthesized phase, but none as much as chromate. The results in these studies are consistent with the idea that inhibition by REMs results from hydroxide precipitation at IMCs due to a local pH increase associated with the ORR. As hydroxide precipitation is a comparatively slow process, REMs cannot inhibit the initial dealloying of S phase, leading to the formation of dealloyed layers under hydroxide layers. Once the hydroxide precipitate forms, the subsequent corrosion of S phase is strongly inhibited. The dependence of inhibition on the formation of hydroxide deposits demonstrates that REMs are "slow" inhibitors, which must be considered in their applications.