Some comments on the use of the EIS phase angle to evaluate organic coating degradation

The impedance of a disc electrode protected by an organic coating, with a thickness profile along its radius, was considered by EIS. The local and global impedances, the ohmic resistance corrected phase angle, and raw phase angle were calculated. The thickness profile leads to a well-defined minimum in the calculated phase angle curves, which can be observed in the high frequency domain. This effect is enhanced with higher permittivity, thinner coatings and low conductivity immersion baths. From these results, it appears that the graphical treatment of experimental phase angle curves to evaluate the coating degradation may lead to erroneous conclusions.