A Galvanic Corrosion Approach to Investigating Chromate Effects on Aluminum Alloy 2024-T3

Abstract

The effects of chromate treatment on the corrosion of an aluminum/copper aircraft alloy were examined with a galvanic corrosion apparatus composed of two electrodes and a zero resistance ammeter. Combinations of pure Al, pure Cu, and AA2024-T3 electrodes were immersed in 0.1 M NaCl solution, which was saturated with air, O2, or argon. Selection of electrode material or differential aeration resulted in partial segregation of corrosion reactions, with one electrode becoming a net cathode and the other a net anode. For the case of an Al/Cu galvanic cell, chromate significantly reduced the observed cathodic current on Cu but had little effect when added to the Al cell. For an AA2024/AA2024 couple, chromate decreased the observed current when present in either the net anode or net cathode compartments. The results indicate that CrVI in solution, or CrVI pretreatment, inhibits O2 reduction on Cu sites. Inhibition is preceded by a spike in the reduction current, implying that CrIII is formed at approximately a monolayer level on Cu. The results for CrVI effects on the AA2024/AA2024 galvanic couple are consistent with observed polarization curves. Considering the results in their entirety, CrVI acts as a strong irreversible cathodic inhibitor on Cu and AA2024, but the results do not exclude the action of CrVI as an anodic inhibitor on AA2024-T3.