An EIS study of ion diffusion through organic coatings

A very important mechanism of organic coating deterioration is the cathodic delamination. In this study a double electrochemical cell has been used in order to keep separated the solution in contact with an artificial defect and the solution in contact with the rest of the intact organic coating (polyester coated phosphatised steel). In this way it is possible to polarise the sample in the cathodic region and to control the environment (ions and concentration, oxygen, water activity) in the defect and around the defect. Moreover it is possible to measure electrochemical parameters in the two different areas of the coating. The cathodic delamination was measured, in different solutions, by measuring the distance of delamination from the artificial defect after different times of exposure under cathodic polarisation. The coating properties have been evaluated by analysing electrochemical impedance data obtained in the two parts of the cell. The behaviour of different cations and anions has been compared and a different diffusion mechanism was found for the cations couple Li and Na and the ions couple K and Cs. The second couple, because of the smaller dimension of the hydrated shell can diffuse through the coating, while Na and Li diffuse mainly through the defect. The anions diffusion plays an important role in the case of positive ions diffusing through the coating, partially balancing the positive charge and increasing the cations diffusion rate. For these reasons the delamination rate in KCl solution is higher that the delamination rate in KBr.