The heterogeneous structure of Al alloys renders them susceptible to localized corrosion due to the different electrochemical properties existing in the Al-rich solid solution matrix and secondary phase particles. The galvanic interactions between these two phases can result in pit formation either through dissolution of the particles or corrosion of the matrix adjacent to the particles. This detrimentally localized corrosion behavior is closely related to the corrosion properties of the particles and the Al-rich matrix. The comprehensive characterization of this behavior under various and varying conditions is critical to understanding the mechanism of pit formation, selecting appropriate inhibitors, and developing protection strategies. The corrosion properties (corrosion potential, pitting potential and corrosion rate) of both secondary phase particles and Al-solid solutions in Al alloys are summarized in this review, aiming to provide a database for corrosion research applicable to the localized corrosion of Al alloys.

Corrosion inhibition of AZ31 Mg alloy with aqueous vanadate was studied and has been attributed to the pH dependence of vanadate speciation. Immersion in tetrahedral coordinated vanadate species, present in neutral and alkaline solution, was shown to decrease corrosion current density and increase the breakdown potential, both of which were enhanced with longer immersion times. Exposure to octahedral coordinated vanadate, predominant in acidic solution, only slightly decreased corrosion current density. An acidic solution was adjusted to alkaline conditions and samples were immersed in the adjusted alkaline solution. Inhibition of these samples was weaker than that of samples immersed in initially alkaline solutions. Anodic inhibition was observed on samples treated in solutions containing tetrahedral species. SEM images showed that vanadate formed a film across secondary particles and the Mg matrix, and provided qualitative evidence that inhibition efficiency increased as the pH increased. XPS results indicated that film formation was associated with the reductive adsorption of vanadium oxoions. Exposure at pH 5.0 produced a film predominated by V⁴⁺. Exposure at pH values of 7.7 and higher, however, produced a film containing predominantly V³⁺. Raman analysis confirmed the formation of a vanadate film on the Mg surface after exposure at all pH values.

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.