A galvanic corrosion approach to investigating chromate effects on aluminum alloy 2024-T3

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, O-2, 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 Cr-VI in solution, or Cr-VI pretreatment, inhibits O-2 reduction on Cu sites. Inhibition is preceded by a spike in the reduction current, implying that Cr-III is formed at approximately a monolayer level on Cu. The results for Cr-VI effects on the AA2024/AA2024 galvanic couple are consistent with observed polarization curves. Considering the results in their entirety, Cr-VI acts as a strong irreversible cathodic inhibitor on Cu and AA2024, but the results do not exclude the action of Cr-VI as an anodic inhibitor on AA2024-T3. (C) 2002 The Electrochemical Society.