1. University Chemical Laboratory, Keio University, 4‐1‐1 Hiyoshi, Yokohama 223, Japan
2. Faculty of Science and Technology, Department of Chemistry, Keio University, 3‐14‐1 Hiyoshi, Yokohama 223, Japan
3. Institute for Materials Research, Tohoku University, 2‐1‐1 Katahira, Sendai 980‐8577, Japan
4. Corrosion and Protection Centre, UMIST, PO Box 88, Manchester M60 1QD, UK