Study on the deterioration process of bipolar coating using electrochemical impedance spectroscopy

A bipolar coating, which is composed of inner layer epoxy with nano SiO2 modified by cetyltrimethylammonium bromide (CTAB) (containing positive fixed charge) and outer layer epoxy with nano SiO2 modified by sodium dodecylbenzenesulfonate (SDBS) (containing negative charge), was prepared in this paper. Its deterioration process after exposure to 5% KCl solution was also studied by EIS measurement and SEM observation. The results indicate that the impedance module of the bipolar coating is about 1E+9 ohm after a longer time immersion period. The bipolar coating has a better anti-corrosion capacity than that of epoxy coating. The cation-selective outer layer in bipolar coating inhibits the aggressive anion, such as Cl− ion, passing through the outer coating. Similarly, the anion-selective inner layer inhibits the metal cation passing through the inner coating. Thus the bipolar coating can protect the metal substrate from corrosion effectively. The p–n junction of bipolar coating, which has great charge storage ability, is the key factor in the anti-corrosion capacity of bipolar coating.