The evaluation of coating performance by the variations of phase angles in middle and high frequency domains of EIS

Electrochemical impedance spectra (EIS) of six coating systems in 3.5% NaCl solution were measured and the relations between coating resistances and phase angles at different frequencies were analyzed. The results indicated that in middle frequency range, the phase angle and the coating resistance show similar decreasing tendencies. For the coating systems studied, the variation of phase angles at 10 Hz with immersion time was very close to the variation of coating resistance, hence may qualitatively reflect the coating performance. For the studied coating systems, the phase angle at 10 Hz decreased continuously from the beginning, indicating the permeation of the coating system by the electrolytes. When the phase angle reached a relatively stable stage, for different coating systems which was below 40°–20°, meaning the coating has been permeated through and electrochemical reactions under the coatings occurred. In addition, the phase angle at 15 kHz may reflect the state of coating in later stage. These phase angle parameters may be used as quick measurements to evaluate coating performance.