Conducting Polymers for Corrosion Protection: A Critical View

Abstract

Two possible mechanisms for the corrosion protection by conducting polymer coatings are discussed in this paper. The ennobling mechanism based on the assumption that the high potential of the polymer is able to keep the metal surface in the passive state and the possible capability to smear out the sites of oxygen reduction. Using a special model sample set up it could be shown that electrochemically deposited polypyrrole films can indeed protect small pin hole like defects from corrosion. In the case of immersion into chloride free solution the polypyrrole is able to maintain the defect in the passive state for a few hours while in chloride containing solution the passivitiy breaks down after few seconds. The size of the defect significantly determines the degree of possible protection. The polypyrrole showed ability to recharge itself after disconnection from the defect so it could provide a real protection under cyclic climatic conditions. In the presence of a larger defect, however, the positive effect of the conducting polymer turns into a negative one and fast delamination is observed. Experiments carried out in 18O2 containing atmosphere gave answer to the question concerning the oxygen reduction site. Sputter profiles measured using ToF-SIMS indicate a presence of 18O− and 18OH− ions created during the delamination just on the top of the polypyrrole layer. That excludes the possibility of the high conductive polymer like pure polypyrrole to smear out the sites of oxygen reduction.