CORROSION OF SYNTHETIC METALS

Intrinsically conducting polymers as polypyrrole or polythiophene are new materials which have found widespread interest in the last ten years. Acceptor type doping leads to a solid, which is well comparable to graphite salts. For any kind of practical application of these polymers, the problem of adequate long term stability under environmental conditions must be solved. There are only a few measurements of conductivity vs. time in the air to derive conclusions in this direction, e.g. for polypyrrole. For the first time we present results of a systematic research into the understanding of corrosion phenomena, which occur in aqueous electrolytes at different pH. Polypyrrole is selected as a well established candidate for a synthetic metal. Layers at a nominal thickness of 10-mu-m on platinum, doped with BF4- -anions, were investigated in aqueous buffers in the pH-range of 1-13. Corrosion was determined via the measurement of the residual redox capacities. The material corroded after a rapid electrochemical mechanism and a relatively slow chemical mechanism. Both processes involve the radical cationic centers (polarons) in the solid. The time constants varied from 10(2) to > 10(8)s. Our findings allow for a better understanding of corrosion and selfdischarge of polypyrrole. Other conducting polymers were found to corrode after the same mechanism.