Development of electrically conductive polymer coatings for coil coated steel sheets

This project had the aim to investigate the potential of intrinsically conducting polymers (ICPs) for corrosion protection. On the one side, over the last 20 years there have been numerous publications claiming improved corrosion performance by application of conducting polymer containing or based coatings on steel, galvanised steel and other metals, on the other side there have been nearly as many stating the contrary. In this project it was tried to study on simple model systems free of additives, or at least free of unknown additives. Since the commercial suppliers of ICP based anti-corrosion coatings were not willing to cooperate by the way to supply such clearly defined coatings, it was tried to prepare such coatings within this project. Since most works on ICPs for corrosion protection are on redox polymers such as polyaniline or polypyrrole, these polymers were also at the focus of this project. Pure polypyrrole coatings were prepared for investigation of the fundamental principles of the possible protection mechanisms. Composite coatings containing particles of polyaniline were prepared in order to study the potential of such easier to apply and technically more relevant coatings. Coatings containing polypyrrole coated particles were also prepared. The corrosion properties of commercial coatings based on polyethylendioxythiophene were also investigated. Although a steady progress in coating improvement of the polyaniline/polyester coatings was made, the finally obtained coatings were still far from being good enough to be suitable for technical tests. The adhesion was very poor for the polyaniline/polyester coatings containing conductive polyaniline, but could be improved by adding extra crosslinking agent to the coating system. Several corrosion tests were performed, but the polyaniline/polyester coatings behaved worse than the chromated reference samples. It is however unclear in how far the poor performance of the coatings containing ICPs is due to the inherent properties of the ICPs or a decrease in adhesion and the barrier properties caused by the addition of the ICP. Also, heat-treatment experiments and conductivity measurements together with DSC experiments showed that a large amount of polyaniline decomposes and loses conductivity already during the curing process, especially if an additional topcoat has to be cured. Infrared spectroscopy was performed on pure polyaniline and on several of the polyaniline/polyester coatings in order to identify and determine characteristic peaks of the doped and undoped polyaniline. It proved to be a very suitable method for monitoring the degree of induced degradation of the ICPs. The studies of the corrosion behaviour of commercial coatings based on polyethylendioxythiophene showed that the corrosion behaviour can be improved in comparison to a corresponding specimen without the conductive polymer coating. The prepared coatings containing polypyrrole coated particles showed very good adhesion with steel, but no positive effect was seen on the corrosion protection. Most of the mechanisms proposed in the literature about how ICPs might improve corrosion performance of coatings were shown to be effective only under very special conditions. Delamination experiments in a heavy oxygen containing atmosphere was performed in order to localise where the oxygen reduction takes place, at the metal-polymer interface or inside the bulk of the polymer or at the polymer surface. The proposed smearing out of the site of oxygen reduction was not found by ToF-SIMS. Instead it was found that on conductive coatings, the oxygen reduction is just shifted from the interface substrate/polymer to the surface of the coating. The reduction of the polypyrrole is a concurrent reaction to the reduction of the oxygen and was studied by delamination experiments in nitrogen atmosphere. The objectives of the experiment were to find out which reactions determine the delamination speed and how reduction of the polymer proceeds.