Possibilities for Life Time Prediction of Organic Coatings with Electrochemical Impedance Measurements

The technical lifetime of a coating is defined in this paper as the period which leads, under well-defined circumstances, to a degree of deterioration of the coating system, resulting in a well-defined, not acceptable lowering of the protection against corrosion. Based on this definition, we focus on the degradation rate of a coating, and the relation between laboratory data and practical exposure. Relevant for the degradation rate of a coating are the water uptake rate of a coating as a function of the number of dry-wet cycles, the delamination rate at the interface for defect free coatings, the delamination area growth rate at defects, and growth rate of blisters. The paper shows that EIS enables technical life time prediction due to its high sensitivity, if the interpretation is undertaken very carefully. Capacitance values obtained for coatings under immersion are compared with CPE (YO,n) parameters. The reproducibility of these values after repeated immersion is discussed, while the influence of small defects on long term C values is shown. (Author)