HUANG Qizhong; YANG Qzaoqin and WU Lijun(Powder Metallurgy Research institute, Central Sonth University of Technology, Changsha 410083, China Materials Research and Test Center, Hnnan University, Changsha 410082, ChinaLab. of Atomic Imaging of Solids, Institute of Metal Research, Chinese Academy of Sciences, Shenyang110015,ChinaState Key Laboratory for Corrosion and Protection, Institute of Corrosion and Protection of Metals,Chinese Academy of Sciences, Shenyang 110015, China)Manuscript received 4 August 1995; in revised form 6 April 1996