A Fundamental Study On The Corrosion Electrochemistry Of The Electrically Conductive Polymer

During the recent years, there has been a fast growth of research in the field of the material performance and electrically conductive mechanism of both intrinsic and complex electrically conductive polymers (ECP) owing to their interesting electrical properties and their potential applications in various fields, while the attempts on the corrosion failure of the ECP are less. Although some studies on corrosion of ECP had made, such as short-cut of silver-filled adhesives due to migration, contact resistance shift in the interface of dissimilar metals, conductivity drop of copper-base complex polymer and changes of resistance of polyaniline (PANI) on metals and so on, there is lack of systematic study on corrosion electrochemistry of ECP, especially, it is quite lack of study on corrosion electrochemistry of intrinsic ECP. In this work, fundamental aspects on corrosion electrochemistry of ECP are studied. The thesis contains six chapters including a reference review on corrosion of ECP, synthesis and character of polyaniline (PANI), the electrochemical behavior of the separate PANI film electrode, the galvanic coupling of the PANI film electrode with different metals, the applied study of the galvanic anodic protection effect of the PANI on ferrous metal, and the corrosion failure behaviors and mechanism of the complex ECP. The main results and conclusions are summarized as follows:(1) The soluble and electrically conductive PANI is polymerized by electrochemical method and it is characterized as half- oxidized emeraldine state salt by some spectroscopic techques, such as FTIR, SEM and XPS. The separate PANI film electrode with electro-active and repetitive performance is prepared.(2) Based on the separate PANI film electrode, the effect of pH value, counter ion, temperature and dissolved oxygen concentration of the solution on the open circuit potential (OCP) of the PANI film electrode are examined in detail. The results show that the OCP of the film is more obviously determined by the pH of the solution rather than by these particular counter ions. The lower pH of the solution results in a higher protonation level and the more positive OCP of PANI film electrode, while in the near neutral solution, the dedoping results inthe more negative OCR Nevertheless the temperature and concentration of dissolved oxygen have almost no effect on OCR(3) The results of polarization and XPS present that the cathodic polarization process of the PANI film is divided into two stages: under the polarization potential above OVvs.SCE, the PANI with bipolarons structure first obtains one electron and the counter ion removes from the PANI simultaneously, then the PANI becomes to polarons structure. Owing to the half-oxidized structure does not change above OVvs.SCE The polarization and the redoping process can complete by self- doping without other oxidant, thus the polarization current in the solution with or without dissolve oxygen has no difference. When the PANI with polaron structure is polarized subsequently lower than OVvs.SCE, the polaron structure would be kept and then the half-oxidized state would be reduced to leucoemeraldine salt, which has the tendency to recover to half-oxidized state by other oxidant such as dissolve oxygen or electrochemical oxidation process, thus polarization current in the solution with dissolved oxygen would be larger than that in the solution without dissolved oxygen apparently.(4) Owing to the more positive open-circuit potential and the large charge density of the separate PANI film electrode, it can provide the galvanic anodic protection to the passive ferrous metals when it couples with these metals in the acid solution. And changed with the different passivity parameter of the metals, the protection current, i.e the area of PANI required, would be different. In the 0.5M Na2SO4(pH=l), it requires more than 25 time area of PANI to promote 20A carbon steel into passivation state when the PANI couples with 20A due to its more large critical passive current, while only an equal area of PANI can promote 2Crl3 stainless steel passivation. The dissolved oxygen in the solution has no effect on the cathodic polarization of the PANI film electrode above OVvs.SCE, which means galvanic anodic protection on the ferrous metal does not require oxygen, not follows the "catalyzing oxygen reduction" mechanism.(5) If the area of PANI film electrode would not be enough to promote 20A into passivation when the PANI couples with the 20A, the PANI is reduced to insulating leucoemeraldine salt by the 20A and the couple should be disconneted by the insulatingleucoemeraldine salt that is similar as an intelligent disconnections switch. Because the re-oxidization of leucoemeraldine salt consumes the dissolved oxygen nearby the 20A electrode, decreasing of the concentration of the dissolved oxygen near the 20A surface, the corrosion rate of the 20A with a coating of PANI would be lowered. These characters of PANI improve the safety of the galvanic anodic protection.(6) When the PANI coupled with non-passivation copper in the 0.5M Na2SC>4(pH=l), the mixed potential is about OVvs.SCE between the OCPs of the PANI and copper. The PANI keeps conductive polaron state and copper is in active anodic dissolved state. The larger cathodic current supplied by the PANI accelerated the corrosion of copper. And the corrosion rate increases with the increase of the area of the PANI electrode.(7) The PANI powder was synthesized by chemical polymerized method. An more high efficient powder electrode is prepared by pressing the mixture of PANI powder and the adhesive with a certain ratio onto a stainless steel or titanium mesh, which shows excellent conductivity, mechanical strength and anodic protection efficiency.(8) The silver-plated copper powder by the electroless plating without cynade shows excellent conductivity, high conductive stability and high migration resistance. The cyclic voltammetric experiments of the powder microelectrode show the high migration resistance of the silver-plated copper powder may be produced by the galvanic effect that copper anode restrains the dissolution of silver cathode during the electrolyzing, and decreases the rate of silver dentritic deposition.It is the first time using the separate PANI film electrode to study the galvanic coupling of the electrically conductive polymer (ECP) with the metals. In this work, a lot of original data and results are obtained about the interaction between the ECP and the metals. The main creative achievements are shown as follows:(1) The experiments prove that the PANI can promote the ferrous metal such as 20A and 2Crl3 passivation in the acid solution and provide the galvanic anodic protection for the metal. It is found for the first time that this anodic protection effect can take place in the solution without dissolved oxygen. Based on the experimental results, it is a mechanism named as"self- reduction" is proposed.(2) A pressed PANI powder electrode was prepared, which offers the high efficient galvanic anodic protection to the 1Q13 stainless steel in the 2M H2SO4, with an efficiency of 100%. This is the latest investion in corrosion protection.(3) The silver-plated copper powder by the electroless plating without cynade shows excellent conductivity, high conductive stability and high migration resistance. The mechanism of migration resistance is studied by the powder microelectrode. It is provide a new approach to solve the stability of the conductivity of the complex ECP.