EVALUATION OF COATING DEGRADATION WITH ELECTROCHEMICAL IMPEDANCE SPECTROSCOPY AND ELECTROCHEMICAL NOISE-ANALYSIS

The performance of two coating systems on cold-rolled steel has been evaluated with electrochemical impedance spectroscopy and electrochemical noise analysis during exposure to 0.5N NaCl for five months. The decrease of the pore resistance R(po) and the polarization resistance R(p) and the increase of the double layer capacitance C(dl) show qualitatively that the disbonded area A(d) increased with time for the alkyd system (CR-2). No deviation from capacitive behavior was observed for the epoxy polyamide coating (CR-9) which showed excellent performance. Potential and current noise data have been collected simultaneously. This approach allows evaluation of the frequency dependence of the spectral noise response and determination of the spectral noise resistance R(sn)o. From the statistical analysis of the noise data the noise resistance R(n) has been obtained. Good agreement between these two parameters has been observed. R(n) and R(sn)o show the same time dependence as R(po) and R(p). However, their numerical values are much lower. For CR-2 a parallel shift of the potential power spectral density (PSD) curves to lower values and of the current PSD curves to higher values with a constant slope of -20 dB has been observed for CR-2. No obvious time dependence was indicated for CR-9.