The degradation of sputtered columnar ZnO layers under DC polarization was studied by using electrochemical impedance spectroscopy and electron microscopy. It was found that the structure of the as‐deposited ZnO film was dense at the nanoscale. An equivalent circuit model including de Levie impedance accounted for the localized propagation of microscale cracks towards the copper substrate. This generates a capacitance (C ZnO) that represents the crack surface area in contact with the electrolyte. C ZnO is small enough not to be obscured by the double layer capacitance at the top of the layers and increases with increasingly negative potential and time. These results were compared to nanoporous ZnO layers that behave differently and exhibit a large C ZnO. The combination of in situ EIS analysis with the ex situ structural information provided by electron microscopy proved to be an efficient methodology to characterize very different microstructures of conductive coatings.