Anion-selective, cation-selective and bipolar
phenolic coatings were prepared by mixing phenolic varnish with ion exchangers.
MoO42- anion exchanger resin and ammonium molybdophosphate were utilized as anion and cation exchangers respectively. The bipolar coating consisted of an anion-selective inner layer on the metal side and a cation-selective outer layer on the solution side. The polarization curves and Nyquist plots of copper electrodes covered with various ion-selective phenolic
coatings and nonionic-selective phenolic coating as contrast in 3% NaCl solution were measured by poltentio-dynamic method and EIS technique. The results indicated that, compared with the nonionic- selective coating, the anion-selective coating accelerated the propagation of metal
corrosion, cation-selective coating decelerated corrosion propagation, and bipolar coating was the most corrosion-resistant among the four types of phenolic coatings. With the bipolar coating, the anodic current was greatly suppressed, while no current blockage occurred in the cathodic direction. Thus there arose rectification of ion movement retarding the ionic current in the anodic direction. The rectification phenomenon blocked the flow of anodic ion transport and led to metal passivation. This showed that the bipolar coating possessed some function of passivation membrane and had important effect on the process of anodic dissolution.