Polypyrrole electrochemistry: Environmentally friendly corrosion protection of steel: (im)possibilities

Chromate compounds have been widely used to improve the corrosion protection of galvanised steel and aluminium objects in the past decades. The hexavalent chromium in chromate enhances the adherence of coatings to galvanised steel and aluminium. Additionally, if the passive layers on these materials should become damaged, hexavalent chromium supports the repair of these layers to restore passivity. The carcinogenic nature of hexavalent chromium has been well-known for many years, but it is still used because no viable alternatives are available. The mechanism of corrosion protection by hexavalent chromium is heavily researched, as is the development of less toxic alternative corrosion protective treatments. In the near future, the use of chromate will be restricted or even completely banned (chapter 1). As the improvements in corrosion resistance provided by chromate are still necessary, alternative treatments are called for. Solutions are being sought in, among others, the following areas: molybdate and cerate treatments, trivalent chromium compounds, silane coatings and conducting polymers. This thesis exclusively deals with conducting polymer coatings on steel substrates. In chapter 2 some background information on conducting polymers is provided, including the serendipitous discovery of the first of the conducting polymers (polyacetylene) in the 1970s. Chapter 3 provides a theoretical treatment on semiconductor electrochemistry, which is a relevant topic as conducting polymers may possess semiconductive properties. The main difference between semiconducting and metal electrodes is the presence of a space charge layer in the former, which may dominate the electrochemical behaviour of the electrode under certain circumstances. The potential range in which semiconductive properties dominate this behaviour ismarked by the flatbandpotential. Chapter 4 deals with the production of conducting polypyrrole layers on steel substrates by electrochemical deposition. The method used is wellknown from literature, yet the steps involved in the reaction mechanism have not all been explained. By stressing that the desorption of ferrous oxalate must occur prior to the deposition of polypyrrole, this thesis contributes to the clarification of the reaction mechanism of polypyrrole electrodeposition. Chapter 4 also describes experiments to determine the polypyrrole layer thickness as a function of deposition time. Similar experiments have been performed to determine the influence of large (polymeric) anions on the obtained layer thickness. The chapter concludes with an analysis of semiconducting properties of polypyrrole layers on steel substrates. From this analysis, a flatband potential of approx. -400 mVNHE is determined. As this potential marks the upper boundary of the range in which the space charge capacity influences the electrochemical behaviour of the electrode, it can be assumed that such influence is not observed in the potential range relevant to corrosion protection (-400 to +400 mVNHE). The electrochemistry of polypyrrole layers on steel is described in chapter 5. An overview of several possible mechanisms for corrosion protection of steel by conducting polymers is given. From this overview, it is observed that the mechanism most accepted in literature (anodic protection) requires some refinement. The Schottky barrier model proposed by Jain et al. is scarcely mentioned in literature, but section 5.2 shows that a combination of this model with the anodic protection model yields an interesting perspective. The remainder of chapter 5 deals with experimental results obtained by open circuit potential monitoring and electrochemical impedance spectroscopy. Both single and dual layer configurations produced according to the description in chapter 4 were used to examine the influence of the polymeric anions in the dual layer configuration. It is shown that the presence of large anions in the dual layer systems effectively blocks the exchange of ions with the electrolyte solutions. In immersion experiments the time to failure of single layer systems is exceeded significantly by comparable dual layer samples, especially in chloriderich solutions. It is shown that in dual layer samples, the low frequency transport of charges is effectively blocked by the presence of large (polymeric) anions in the layer. In addition to this results, the impedance experiments reported in chapter 5 show that the development of a corrosion process underneath the coating can be detected before the coating physically fails. In other words, the developing corrosion process can be monitored well before it becomes exposed to the electrolyte solution. To date, this has not been reported in literature. Chapter 6 deals with the commercial development of a corrosion protective coating system based on conducting polymers. The poor processability of conducting polymers is circumvented by the used of core/shell latex systems. In such systems, on attempts to combine the processability of the soft core material with the functional properties (e.g. conduction) of the shell material. After several iterations to optimise the system, coatings were produced with satisfactory coating qualities. The experiments described in chapter 6 show that the corrosion protective properties of these coatings were mediocre at best, although many of them could protect the substrates until a defect in the coating occurred. The final chapter of this thesis summarises the conclusions drawn in the preceding chapters. In addition, the general discussion in this chapter links together some of the preceding chapters