The Corrosion Mechanism Of Conducting Polypyrrole

As one of the most widely researched and applied conductive polymer materials currently, conductive polypyrrole has many advantages such as good conductivity, specific electrochemical and optical properties, good biological compatibility and easy to polymerized, et al. PPy and their composite materials have been aroused widespread attention and applied as electric catalytic materials, sensors, electrode materials, catalysts, electric ion exchange materials etc. With regard to these applications, the stability of PPy, especially the electrochemical stability, is a key issue, which determines the commercial application of these materials and is the main currently encountered challenge. Most studies on the corrosion mechanism of PPy just focus on the chemical action between PPy and environment. It is generally believed that the nucleophilic attacks from H2 O, OH-, CH3 OH, and NH3 etc. destroy the conjugated structure of the PPy chains and result in the chemical corrosion of PPy. However, our previous work has proved that electrochemical processes can occur in the corrosion of PPy and influence its corrosion extent. In this thesis, based on the electrochemical corrosion theories, we studied the corrosion mechanism of PPy films under continuous anodic polarization and under natural immersion in electrolyte solutions. In addition, the effects of chemical action and electrochemical action on the corrosion of PPy were compared.It is well known that polypyrrole doped with different size of anions behaves different morphology and ion exchange behavior, so counterions may have a nonnegligible effect on the corrosion process of polypyrrole. In this thesis, we use electrochemical methods including cyclic voltammetry(CV) and electrochemical impedance spectroscopy(EIS), and spectroscopic analyses to research the influence of doped anions(SO42-, p TS-, DBS- and PSS-) on the corrosion behavior of PPy films in different polarisation states(different polarization time and polarization potentials). The results indicate that the PPy films with different doped anions show similar corrosion behaviors, i.e. the corrosion extent(rcorr) of these four PPy films increases with an increase in anodic polarisation potential(Ea) and polarisation time(t). At a small Ea(? 0.5 V vs. SCE), the PPy films with large counterions(PPy/PSS and PPy/DBS) show much better corrosion resistance than those doped with small counterions(PPy/SO4). When Ea is more than 0.5 V(vs. SCE), there is an abrupt increase in the rcorr value for the four PPy films(> 90%), suggesting that they nearly lose their electroactivity completely. The corrosion of PPy/SO4 film in deaerated electrolyte without Cl- is much slighter than that in the solution with Cl-. However, this effect of Cl- on the corrosion of PPy films doped with large and medium counterions(PSS- and p TS-) is not obvious, which may be due to the complex impact of these doping ions on the main chain and structure of PPy films.The corrosion of PPy/SO4 films in aqueous solutions, including 0.1 M Na Cl, 0.1 M Na2SO4, 0.1 M H2SO4 and high pure water(HPW, conductivity = 0.55 ?S cm-1), and gas environments, including humid air, dry air, humid O2, dry O2 and dry N2, were studied. The results indicate that pure chemical effects of H2 O and O2 on the corrosion of PPy are not apparent, but the combination of them can cause electrochemical corrosion on PPy films, which can significantly aggravate their corrosion degree. Comparing with the corrosion of PPy/SO4 in 0.1 M H2SO4, the effect of the intermediate products from the electrochemical reduction of O2 on the corrosion of PPy was verified, and the electrochemical corrosion of PPy in the acid solution was proved.The influence of film thickness(0.3-2.0 ?m) on the corrosion of PPy/SO4 in humid air environment was studied. The rcorr values of the PPy/SO4 films with different thickness increase with an increase in the corrosion time and gradually reach stable after 48 h. In general, the thicker films(?1 ?m) show better electrochemical activity over time. Comparing the corrosion behavior in aqueous solutions and gas environments, it is seen that the corrosion of a PPy film starts from its surface, gradually develops to the inner film and eventually may stop in a certain depth.