1. Corrosion and Protection Centre, University of Manchester Institute of Science and Technology, Manchester M60 1QD, England
2. Institute for Materials Research, Tohoku University, 2‐1‐1 Katahira, Aoba‐Ku, Sendai 980‐77, Japan
3. University Chemical Laboratory, Keio University, 4‐1‐1 Hiyoshi, Yokohama 223, Japan