Chromate-free corrosion inhibition of aluminum alloys: Vanadates and anionic exchange clay pigments

In this study, aqueous vanadates and vanadate pigments were studied for possible use as chromate replacements to inhibit corrosion of Al 2024-T3. Vanadate inhibition on Al 2024-T3 was characterized as a function of pH and concentration using anodic and cathodic polarization experiments and scanning electron microscopy (SEM). The results showed a strong correlation between inhibition and the availability of tetrahedrally coordinated vanadate species in test solutions. In particular, solutions containing predominately tetrahedrally coordinated vanadates were observed to act as modest anodic inhibitors and to reduce cathodic kinetics through the suppression of oxygen reduction kinetics. Further, the effect of these tetrahedral vanadates on individual intermetallic particles commonly found in Al 2024-T3 was characterized using a microcapillary electrode. Tetrahedral vanadates were generally found to increase breakdown potentials and decrease cathodic kinetics on all tested materials. Open circuit potential (OCP) was observed to shift in the active direction as a result of decreased cathodic kinetics, just below the observed breakdown potential of Al2CuMg; a phase that plays a critical role in corrosion susceptibility of Al 2024-T3. OCP measurements, SEM images, and potentiostatic hold experiments were used to show suppressed Al2CuMg dissolution and damage accumulation in vanadate solutions.;Furthermore, synthetic hydrotalcite anion exchange clay pigments were synthesized with vanadates and other possible inhibitor anions. Hydrotalcites allow the use of inhibitors that are too soluble for direct use in organic coatings without leading to coating blistering. A number of hydrotalcite pigments were synthesized and compared to a SrCrO4 standard using electrochemical impedance spectroscopy and salt spray exposure of scribed organically coated panels. Typically, vanadate pigmented PVB coatings were observed to have total impedance within an order of magnitude of SrCrO4 (2 x 10 7 ohms cm2). Further, all vanadate coatings were observed to provide some scribe protection during 750 hours of salt spray exposure, however, these coatings also had a tendency to blister. Release from vanadate hydrotalcites was characterized using neutron activation analysis. Interestingly, vanadate hydrotalcite pigments that released relatively small total concentrations of vanadium resulted in the best performance. Low concentrations of vanadium may promote the formation of tetrahedrally coordinated species which were shown to act as inhibitors earlier in this study.