抄録
銅が腐食すると表面に生成する銅酸化物 (Cu_2O 及び CuO) の分離定量には,これまで中性∿弱アルカリ性の電解液を用いるクロノポテンショメトリー法がよく用いられてきたが,測定時間が長いことや,測定データを解釈する際のあいまいさなどの問題がある.本研究では,Cu_2O 又は CuO の一方のみが存在する標準試料と両酸化物が共存する実サンプルを準備し,2回の掃引を行う二重掃引サイクリックボルタンメトリー (DSCV) 法による測定を行った.その結果,Li^+ イオンを含む強アルカリ性電解液 (6M KOH+1M LiOH) を用いると,Cu_2O→Cu及びCuO→Cu の還元による各ピークを完全に分離して得ることができた.各ピーク面積から算出した銅酸化物の量は,不活性ガス融解法による酸素分析から求めた値と 3% の実験誤差範囲内でよく一致した.また,DSCV 法から評価した酸化被膜の厚さも,走査型イオン顕微鏡によって観察された膜厚とよく一致した.
The quantitative characterization of oxide films formed on copper is an important subject in corrosion research. So far, chronopotentiometry has been most frequently applied to the selective determination of cuprous oxide (Cu_2O) and cupric oxide (CuO) in copper oxide films. However, it is time-consuming, and has a serious problem that there are two different doctrines about which oxide is reduced first. In this study, we prepared two standard samples of Cu_2O film (Dumet wires) and CuO film (by treating a copper sheet with a commercial oxidizing agent) and also a real sample in which the two oxides coexist on a copper sheet. We then performed quantitative analyses of the oxide films using a recently proposed voltammetric technique, i.e., double-sweep cyclic voltammetry (DSCV). It was found that the use of a strongly alkaline solution (i.e., 6 M KOH+1 M LiOH) allowed a perfect separation between two cathodic peaks due to the reductions of Cu_2O and CuO. One of the peaks, appearing at a less negative potential, was due to the reduction of CuO to Cu, whereas the other peak, appearing at a more negative potential, was due to the reduction of Cu_2O to Cu. These peak assignments have been justified by analyses of standard samples by X-ray diffraction, inert-gas fusion analysis for oxygen, and scanning ion microscopy (SIM). Also, the thickness of the oxide layer of each standard sample, calculated from the peak area, was in fair agreement with that estimated by either gas analysis or the SIM image.
