CORROSION OF CONDUCTING POLYMERS IN AQUEOUS-MEDIA

The corrosion of conducting polymers (polypyrrole and polythiophene, e ach with two derivatives) was investigated in aqueous buffers (pH 1-13 ). The corrosion rate was determined experimentally by potentiodynamic discharge of the residual redox capacity of the conducting polymer af ter exposure to the corrosion medium for some time. The corrosion reac tions were found to proceed in two steps, both following a pseudo-firs t-order mechanism. The initial rapid process is due to an electrochemi cal mechanism. The cathodic dedoping is balanced by an anodic overoxid ation reaction, even at relatively negative potentials. A rather slow second process is caused by chemical attack of nucleophiles dissolved in the solid at the remaining radical cationic centers. The pseudo-fir st-order rate constants cover a range of 3 x 10(-5) -2 x 10(-2) s-1, f rom polypyrrole to polythiophene, for the fast process. Anodic overoxi dation is rate determining. Corrosion increases with increasing pH, wh ich is attributed to a negative shift of the overoxidation potentials. The rate constants could be evaluated from the exponential decay of t he corrosion potential with time. In contrast with polypyrrole, the po lymer backbone conjugation for polythiophene is not interrupted becaus e -SBAR- --> -SO2-, and recharge is possible to some extent.