THE DIPOLAR AND CONDUCTIVITY RELAXATIONS OF EPOXY COATING STUDIED BY EIS IN AQUEOUS SOLUTION

摘要

The water-uptake and the protective effectiveness of epoxy-amide paint layer were examined by EIS technique. The use of classical equivalent circuit method was found to be not appropriate for the organic coating having a high protective effectiveness, especially when no flaw, such as pores or crevasses connecting both sides of coating, is formed. EIS results were therefore examined on the basis of dipolar relaxation of polymers. Experimental impedance data were reproduced suitably by this approach. The relaxation process of coating film showed an asymmetric frequency distribution (Cole-Davidson type) and the relaxation time-constant was determined by parameter regression calculations. Firstly, water-uptake process for a short immersion time (up to 16 hours) were followed using the high frequency limit of the interface capacitance on the basis of Brasher-Kingsbury relationship. The absorption of water was explained with the combination of the normal (diffusion through Fick's law) and the abnormal (interaction between water molecules and polymer molecules) sorption kinetics. The increase of the film conductivity (or decrease of the film resistance) is essentially due to the sum of water penetrated in the coating layer, whereas the relaxation time-constant showed a marked induction period. Secondly, the water-sorption kinetics were followed up to 38 days. The extrapolation of sorption kinetics determined in a short period experiment explained fairly well the long term variation of the water absorption into the paint film. However, a sudden increase of the film resistance and at the dipolar relaxation time-constant were observed at about ten days immersion. These changes were interpreted by the formation of blisters on the paint layer. It is also remarked that the barrier properties of organic coating is not always correlated to the protective effectiveness of thick paint layer, if no flaw was formed.