Critical view of the results of the 2nd international round-robin test on EIS measurements of organic coatings

Although electrochemical impedance spectroscopy (EIS) is a well-established and powerful technique to inves-tigate protective properties of organic coatings, there is no comparative study to show how reliable and reproducible are the results of this technique between different laboratories. One of the tools to fill this gap is international round robin tests (RRT). Following up the conclusions of 1st international RRT, presented in 11th workshop on AETOC Valencia (Spain), this paper will present the results of 2nd international RRT. 30 labora-tories have participated this RRT using various equipment (24 different electrochemical systems from 8 manu-facturers). To reduce the complexity of the testing in the 2nd RRT only one well defined E-coating material on a single kind of substrate (steel) was used as sample for all participating labs. Results were analyzed in terms of estimated capacitance (RC model, at single frequencies at 1 kHz and 10 kHz, respectively) and magnitude of impedance at low frequencies (100 mHz and 10 mHz). The provided data has been evaluated in a balanced approach using equal statistical weighting for each laboratory. The general conclusion drawn from the tests performed with the coated panel is that the results in terms of repeatability and reproducibility are scattered (high variance). To understand this issue, an additional RRT was organized with 11 participating laboratories (which showed the best results in the E-coating test), using a dummy cell as a test subject. Scattered EIS Bode plots for E-coting as well as for the dummy cell were observed and will be discussed. Although the careful collection and evaluation of the EIS raw data is crucial, it became apparent that this is not always being done. The observed sources of error and measurement artifacts will be presented and discussed. Recommendations for best lab and best evaluation practices will be presented.