CORROSION INHIBITION OF AL-CU-MG ALLOY 2024-T3 BY VANADATES: UNDERSTANDING THE ACTION OF COMPLEX AQUEOUS SPECIATION ON A COMPLEX MICROSTRUCTURE

Vanadate compounds dissolved in aqueous solutions can be powerful inhibitors of localized corrosion on 2024-T3 (Al-4.4Cu-2.5Mg-0.6Mn). These compounds act to stifle the oxygen reduction reaction, which in turn stops the corrosion cell process. Solution phase speciation of vanadate is complex. The types and proportions of species present depend on solution pH and vanadate concentration. Results from solution phase ⁵¹V nuclear magnetic resonance work show excellent corrosion inhibition in the presence of tetrahedrally coordinated forms of vanadate and poor corrosion inhibition when octahedrally coordinated forms predominate. These findings establish the conditions where useful levels of corrosion inhibition by vanadates can be expected. Microfocal Raman spectroscopy shows that vanadates act specifically at Cu-rich intermetallic particles found in 2024-T3. Results suggests that a complex bi-layered polyvanadate polymer forms on Cu-rich intermetallic particles presumably interfering with the process of oxygen reduction. Results from microelectrochemical cell experiments carried out on a range of intermetallic compounds that typify the constituent particles in the alloy have been used to quantify the extent to which the ORR is inhibited on a phase-by-phase basis. From a microstructural-electrochemical viewpoint, these results show that vanadate inhibits corrosion by decreasing the corrosion potential of the alloy below the breakdown potential of any phase present in the microstructure.