Testing of Protective Organic Coatings on Metals: Comparison of Salt Spray and Electrochemical Impedance Spectroscopy

Abstract Both electrochemical impedance spectroscopy (EIS) and the salt spray chamber are very popular techniques for testing the protective properties of organic coatings. It is common that the results obtained by the two methods often show significant differences and it is difficult to compare the performances. This paper discusses some experimental results obtained on polyester-coated galvanized steel, analyzing the data dispersion of 36 identical samples tested both with EIS and salt spray. The morphology of degradation is very different because blisters develop in the salt spray chamber, but not in immersion. In order to quantify the degradation in the salt spray test, we propose a blister index (BI); the data dispersion of BI and the data dispersion of the parameters obtained by EIS are compared and discussed. A mechanism is proposed: only the double-layer capacitance can be compared with the development of blisters in the salt spray chamber while the barrier properties of the coating measured by EIS follow a completely different behavior.