Research On Electrosynthesis, Application And Corrosion Protection Mechanism For Polypyrrole

Due to the special structure and the excellent physical chemical properties, conducting polymers have attracted great interest in many applied fields such as metal protection, energy, optoelectronic devices, molecular devices, electromagnetic shielding, stealth technology etc.. The anti-corrosion efficiency and mechanism of conducting polymers have not yet been fully understood, and thus are still open for further investigation. Polypyrroly (PPy) is one of the most promising conducting polymers for corrosion protection application. In this work, the electrosynthesis processes of PPy were firstly studied, then the protection mechanism of PPy was investigated by methods such as scanning electronic microscopy, infrared spectrum, cyclic voltammetry, potentiodynamic polarisation curve, open circuit potential-time curve and electrochemical impedance spectroscopy (EIS) etc.. The application researches for PPy in the field of corrosion protection and electrochemical super-capacitor were also carried out after the study of some fundamental problems related to its electrochemical properties were done.1. Electrosynthesis of PPy and its propertiesCyclic voltammetry and potentiostatic method were adopted to investigate the synthesis process of PPy. Results show that the pre-formed PPy films have a catalytic effect on oxidative polymerization of pyrrole in sodium dodecylbenzenesulfonate (SDBS) solution, thus lowering the polymerization potential. Therefore, a two-step process was proposed: the first step is the formation of a very thin PPy film at a higher potential, and the second step is the PPy film formation on the freshly produced thin film at a lower potential. PPy films prepared by this two-step method were found to be uniform and smooth in appearance, more flexible and adherent than those prepared using the conventional potentiostatic technique. Mostly, anodic metal substrate dissolution and over-oxidation of PPy can be avoided.The effects of applied potentials on the electrochemical, structural and protective properties of PPy films have been explored. Films electrodeposited at lower potentials have more homogeneous morphology, less overoxidation, better ahersion to metal surface and higher electroactivities. Applying lower potential for the synthesis improves the anti-corrosive efficiency of PPy film in 3.5% NaCl solution.2. Corrosion performance and mechanism research for PPy coated stainless steel The comparison of electrochemical properties of stainless steel/PPy in 3.5%NaCl solution for different thicknesses was performed. Interesting results were obtained that the thicker the PPy film was, the smaller the semicircle of EIS at high frequencies was, and thus the semicircle should correspond to the charge transfer impedance and the impedance diagram response is mainly ascribed to the polymer film rather than to the underlying stainless steel substrate. The anodic polarization current of stainless steel/PPy increased as film thickness increasing, and rose rapidly at about 0.50 V (vs SCE) indicating that the corrosion of stainless steel in 3.5%NaCl solution is inhibited. The polarization current mostly comes from redox current of PPy before the potential reaches 0.50 V.EIS of stainless steel/PPy and Pt/PPy in 3.5%NaCl solution were measured for the sake of comparision. Results show that the semicircle for stainless steel/PPy related to charge transfer impedance was found to be similar to that for Pt/PPy at high frequencies. The charge transfer impedance measured reflects the redox reaction of PPy, and the corrosion resistance of stainless steel substrate coated with PPy is very large indicating that the stainless steel is under protection of PPy film. The electrochemical behavior of stainless steel/PPy in 3.5%NaCl solution was investigated for different immersion time. The anodic polarization current is mainly ascribed to the dissolution of stainless steel at pinhole defects for initial immersion, and to PPy oxidation after 1 h immersion.The comparison of electrochemical properties for stainless steel/PPy and Pt/PPy in 1 mol L-1 SDBS solution was carried out. Results show that electrochemical reaction between PPy and stainless steel will take place when electrolyte reaches the surface of stainless steel causing passivation of stainless steel and dedoping of PPy. The dedoped PPy can be re-oxidized in doped form.An equivalent circuit of EIS for stainless steel/PPy in 3.5%NaCl solution was proposed, with which the charge transfer resistances for stainless steel corrosion and redox reaction of PPy were separated. Then the corrosion behavior of stainless steel/PPy was analyzed through the evolution of corrosion resistance and the redox reaction resistance with time in 3.5% NaCl solution for PPy coated stainless steel. Results show that PPy can inhibit corrosion of stainless steel by passivation and release of dodecylbenzenesulfonate anion.3. Electrochemical properties of free-standing PPy filmPotentiodynamic polarization curves of free-standing PPy film electrode in 3.5% NaCl with no-deaeration and deaeration were measured. Results show that the redox reaction of PPy is under electrochemical control even in large overpotential.Free-standing PPy films were firstly polarized at -0.8 V for 10 min to reach full reduce state, then potentiostatic anodic polarizations were performed. It is found that film resistance evolution as a function of time shows exponential decay. The relation equation between film resistance and time was analyzed in theory, the more higher the potential, the shorter the half-life.The influences of pH value on properties for free-standing PPy film in 1 mol L-1 NaCl were investigated. Results show that line slope of potential-pH curve is -0.029 V/pH for pH < 11, and is -0.083 V/pH for pH > 11. In acidic solution, proton-acid doping occurs and film conductivity increases. While in alkaline solution, PPy films are de-protonated and confronted over-oxidization thereby becoming less conductivity. The higher the value of pH is, the more negative the initial potential and peak potential are.4. Application of conducting polymerPPy-Pani (polyaniline) composite were prepaired through layer by layer deposition or copolymerization in order to improve the anti-corrosive efficiency and capacitance properties of conducting polymer, and subsequent material test or characterization were carried out. Results show that pitting potential of stainless steel in 3.5% NaCl solution can be raised by covering with bilayer PPy films or copolymerizated PPy films. The composite polymers with PPy as primer have better protection effect than that with Pani as primer. Copolymerizated PPy films synthesized with higher pyrrole concentration show better anti-corrosive performance. The capacitor properties quite depend on the deposit order: the composite polymers with PPy as primer have better capacity than that with Pani as primer. The specific capacitance of stainless steel/PPy/Pani and stainless steel/PPy/Pani/PPy electrodes is far higher (up to 206.11 F g-1 and 216.13 F g-1) than that of pure conducting polymers or other composite polymers.