A 2-STEP ELECTROPOLYMERIZATION OF PYRROLE ON ZN IN AQUEOUS-MEDIA

Substrate-adherent polypyrrole films are obtained by electrochemical o xidation of pyrrole on a Zn electrode in an aqueous sodium oxalate med ium provided that the zinc surface has been treated previously in a su lfide medium with the view to reduce its reactivity. The best pretreat ment was achieved when the electrode was polarized anodically at 0.35 mA cm(-2) for 3 min in 0.2 M Na2S solution. Various electrochemical te chniques (cyclic voltammetry, chronopotentiometry, impedancemetry) acc ompanied by XPS studies were used to characterize the surface modifica tions of the zinc electrode induced by this pretreatment. It was found that the formation of surface film occurs in two steps. At low anodic potentials, a porous layer is formed and at high anodic potentials th e surface film is modified and acquires good protective properties. Th e surface layer consists of a mixture of ZnS (ca. 65%) and ZnOxHy, (ca . 35%) with 1 less than or equal to x less than or equal to 2, and 0 l ess than or equal to y less than or equal to 1. Best electropolymeriza tion conditions for obtaining homogeneous and adherent polypyrrole fil m involved an aqueous solution of 0.9 M pyrrole + 0.1 M Na2C2O4 With g alvanostatic polarization and after the substrate metal had been treat ed as indicated previously. (C) 1998 Elsevier Science Ltd. All rights reserved.