Effect of Copper Enrichment on the Electrochemical Potential of Binary Al-Cu Alloys

Using Al-Cu alloys, containing between 0.1 and 26 atom % Cu, deposited by magnetron sputtering and etching in sodium hydroxide solution, enrichments of copper have been developed selectively in the alloys. Rutherford backscattering spectroscopy and medium energy ion scattering quantified the enrichments and their locations just beneath the alumina-based oxides remaining from the etching. In some cases, the enrichment was sufficient for oxidation of copper to take place; in other cases, it was not, so that only aluminum was oxidized, with copper being confined to the alloy, enriching in an alloy layer about 2 nm thick. The potentials of the etched alloys in 0.1 M ammonium pentaborate solution at 293 K increased as the copper content of the enriched alloy layer increased. The enrichment of copper in the alloy beneath an alumina-based film free of copper species, i.e., without requirement for oxidation of copper and incorporation of copper species into the oxide, was sufficient to generate increases in potential of similar magnitude to those found for specimens in which oxidation of copper had taken place. The potential increased by ∼410 mV with an increase in the level of enrichment to Cu atoms