Corrosion protection of steels by coatings containing electrically conductive polymers

The anti-corrosion performance of conducting polymers coated on the mild steel and stainless steel samples which exposed to artificial brine and dilute hydrochloric acid was investigated. Mechanistic information, and quantitative corrosion data were obtained by several common DC and AC monitoring techniques such as Tafel Extrapolation, Potentiodynamic Polarization, Galvanic Coupling and Electrochemical Impedance Spectroscopy(EIS). These characterization studies, in conjunction with surface analysis by SEM and XPS, indicate that the corrosion protection, even for purposely exposed steels, occurs by the formation of passivating iron oxides layer at interface between conducting polymer and steel and on direct-exposed areas. The possibility for anodic protection of steels and other metals was proposed over 10 years ago by several researchers. This work attempts to rationalize the observed corrosion processes in order to elucidate protection mechanisms from the view of a corrosion engineer. However, the mechanisms by which steel can be protected against corrosion are quite varied, and sometimes, not well understood. A systematic correlation between the inhibition performance of electrically conducting polymers and its electronic, molecular, and electrochemical properties was also established. The main function of most organic coatings for corrosion protection of metals is to provide pinhole-free barrier on the surface. When these barriers break down through pin-holes or scratches in the coatings, a secondary method of corrosion protection is desired. It is this secondary method of protection that conductive polymers appear most suited. In this work, experiments were conducted to evaluate the electrochemical kinetic behavior of the oxidation of metal surfaces for the purpose of understanding the interfacial stability, oxidation response and porous film diffusion phenomena of oxidation films on metal surface. Monitoring of electrochemical parameters alone provides enough information in terms of redox details; other in situ techniques were employed to further investigate the nature of why conductive polymers provide this corrosion protection in certain damaging environments. Several reaction models and revised corrosion principles are presented to discuss the protective properties of intrinsically conducting polymers from electrochemical macroscopic theories and microscopic molecular details.