Oxide layers thermally formed on copper have been studied using double sweep cyclic voltammetry in strongly alkaline electrolytes. It was found that the addition of 1 M LiOH in an electrolyte (6 M KOH) allowed perfect resolution of cathodic waves due to the reduction of Cu2O and CuO. Assignment of the two reduction waves has been achieved with the help of spectrophotometric techniques including X-ray photoelectron spectroscopy and X-ray diffraction; the cathodic wave appearing between −1.3 and −1.5 V (vs. Ag/AgCl) was attributed to the reduction of Cu2O, while that appearing at a less negative potential (−1.0 to −1.1 V) was attributed to the reduction of CuO. The electrochemical measurement of samples prepared under several conditions has revealed that CuO is reduced at once to Cu prior to the reduction of Cu2O. It was also confirmed that the formation of the oxide films was accelerated by elevating temperature, heightening humidity, and by preimmersion in electrolyte solutions. Water vapor was essential for the formation of CuO at a lower temperature (80°C). © 2001 The Electrochemical Society. All rights reserved.

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Chronopotentiometry ͑CP͒ using 0.1 M KCl as the electrolyte has been most frequently used for selective determination of cuprous and cupric oxides ͑Cu 2 O and CuO͒ formed on copper surfaces. However, there are conflicting views regarding the order of reduction of the oxides. This study was carried out to settle this problem. Differently prepared samples of Cu-duplex oxide films were partially reduced by means of CP with 0.1 M KCl and then submitted to X-ray diffractometry and also linear sweep voltammetry using a strongly alkaline electrolyte ͑6 M KOH + 1 M LiOH͒ in which reduction peaks of Cu 2 O and CuO could be obtained with good separation. The results clearly demonstrated that CuO was first reduced in 0.1 M KCl, followed by the reduction of Cu 2 O. However, the reductions of both oxides were found to occur simultaneously, to a greater or lesser extent, in 0.1 M KCl. It was also revealed that CuO was reduced to metal Cu in one step and that a partially reduced Cu-duplex oxide sample had an expected Cu͉Cu 2 O͉Cu sandwich structure.Oxide films on copper are composed of cuprous oxide ͑Cu 2 O͒ and/or cupric oxide ͑CuO͒. 1 Selective determination of the two oxides with different properties is important for corrosion characterization. Surface analytical techniques, including X-ray photoelectron spectroscopy ͑XPS͒, 2-4 Auger electron spectroscopy ͑AES͒, 4 X-ray diffraction ͑XRD͒, 5 and infrared reflection absorption spectroscopy ͑IRAS͒, 6,7 have been used for this purpose. These spectrophotometric techniques are useful for qualitative analyses but not necessarily adequate for quantitative analyses.Electrochemical techniques, including chronopotentiometry ͑CP͒ and linear sweep voltammetry ͑LSV͒, provide reliable information on the "quantities" of copper oxides, which can be accurately obtained from the quantity of electricity required for the reduction of oxides. CP has been most frequently employed for simultaneous determination of Cu 2 O and CuO formed on copper surfaces. However, there are conflicting views regarding the order of reduction of oxides. Miler, 1 Pops and Hennessy, 8 and other authors 9-14 have claimed that the reduction of Cu 2 O occurs first, followed by the reduction of CuO. However, no convincing evidence seems to have been presented. Several authors have proposed that CuO is first reduced. 15-19 Su and Marek 18 proposed a two-step reaction mechanism in which the reduction of CuO to Cu 2 O occurs first and both the generated Cu 2 O and the originally existing Cu 2 O are then reduced together.In 2003, a subcommittee on the cathodic reduction of copperoxide films was constituted in the Japan Society of Corrosion Engineering ͑JSCE͒ ͑of which two of the authors, S. Nakayama and T. Notoya, are members͒, and round-robin experiments were performed to determine which mechanism is valid in the cathodic reduction of Cu-duplex air-formed thin oxide films. In the committee's reports, 20,21 it was concluded that in CP measurements with 0.1 M ͑=mol dm −3 ͒ KCl as the supporting electrolyte, CuO was first reduce...

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