The Electrochemical Corrosion Mechanism Of Conductive Polypyrrole

Polypyrrole (PPy) has been studied and used widely owing to its advantageous physical and chemical properties. With regard to its applications, the stability of PPy is a key issue because PPy may undergo irreversible degradation and loss of performance in service environment, which is also considered as the corrosion of PPy according to the European Federation of Corrosion Working Party on Corrosion of Polymer Materials. Researches on the corrosion of PPy just more consider the chemical actions between PPy and media, including the changes of conductivity, electroactivity, structure and chemical components. PPy has a large surface conjugated molecular structure and conducts through the activation of conjugated π electrons. After being doped, it can reach the conductive level of metals. Moreover, PPy has similar charge transfer ability and electrochemical catalytic as metals, so it can act as good interface for electrochemical redox reactions. In this case, corrosion cells can be formed on PPy surfaces in electrolyte media, just like their formation on metals. If so, apart from the chemical actions between PPy and media it may also suffer electrochemical corrosion. From the perspective of corrosion electrochemistry, in this dissertation, the mechanism of electrochemical corrosion of PPy is proposed and investigated.Corrosion behaviors of PPy are first studied. Severe corrosion of PPy mainly occurs in alkaline aqueous solutions. The nucleophilic attacks by OH" on the PPy chains destroy conjugated structures and result in corrosion. In1M NaOH+0.1M NaCl solutions, environment factors such as high concentrations of supporting electrolyte (NaCl), low temperature and absence of dissolved oxygen inhibit PPy corrosion. At positive potentials simultaneously, corrosion of PPy is promoted, while corrosion is inhibited at negative potentials (vs. OCP). PPy doped with large counterions show much better corrosion resistance than those doped with small counterions. The corrosive galvanic cell with anodic oxidation of PPy and cathodic reduction of dissolve oxygen can form on PPy and plays an important role in PPy corrosion. According to these corrosion behaviors, we propose the mechanism of electrochemical corrosion of PPy.In neutral0.1M NaCl media to avoid the nucleophilic attack of OH", with continuous anodic polarisation, the chloride-modified reactions and atomic oxygen, HO· and ClO-generated from the oxidation of H2O and Cl" will accelerate PPy corrosion. With continuous cathodic potentials, at a small polarisation, PPy suffers attacks of OH-generated form the reduction of dissolved oxygen and is corroded slightly. At a strong cathodic polarisation, PPy chains are neutral, counterions are all expelled from the film, and conjugated structures remain intact. However, the lack of counterions in PPy makes the cation insertion/ejection process disappear and deep cathodic polarisation makes the oxidation peaks corresponding to the anion insertion process move to more positive/higher potentials.Some galvanic couples are designed with stand-alone PPy films to simulate possibly formed corrosion cells on PPy. The results indicate that the PPy redox state difference cell just occurs in the initial corrosion period and cannot be sustained. The H+ion and electrolyte concentration difference cells can be sustained but do not influence PPy corrosion. The direct chemical erosion of oxygen on PPy is not obvious. The oxygen concentration difference cell can also be sustained and severely accelerates PPy corrosion through the electrochemical reactions.The redox states, doping ratios, densities, thicknesses and porosities of a PPy film cannot be completely uniform at different locations. A nonuniform distribution of molecules and ions from the solutions can be assumed in the film. Apparently, the concentrations of various molecules and ions near the film outer surface are the highest, while those in the film, especially in the more compact PPy areas, are lower. So the PPy redox state, the oxygen, H+ion and electrolyte concentration difference cells can occur at PPy film. If the dissolved oxygen is removed, i.e. eliminating the electrochemical processes, the PPy corrosion rate in0.1M NaOH alkaline media significantly reduced by an order of magnitude, which means PPy corrosion is greatly inhibited and reflects the key role of PPy’s electrochemical corrosion mechanism.The metal substrates have a great impact on the electrochemical behaviors of PPy films, owing to the galvanic interactions between PPy and metals. In the electrolyte, because of its relatively positive potential PPy mainly acts as a cathode and will be reduced when it coupled with many kinds of metals, simultaneously, the metals will be oxidized as an anode. Different effects of metal substrates on PPy result from different metals, mass of PPy (film thickness or area) and medium conditions. More stable metal substrate has less influence on PPy.Moreover, PPy film also affects the corrosion behaviors of metal substrates. Generally PPy film can inhibit the corrosion of metal substrates in a certain extent. The PPy coating is able to mitigate the corrosion of NiTi substrate in chloride solutions, shifting corrosion and pitting potentials positively and increasing the impedance of corrosion process. This is attributed to the effective barrier of the polymer layer and the stabilization of the passive oxide film through interaction between PPy and underlying NiTi alloy. The corrosion protection is significantly related to the anions exchange behavior of PPy, i.e. the ability of inhibiting the migration of corrosive Cl-to the substrate. PPy coating with poor anions exchange ability performs excellent corrosion protection.