The role of the local nanometric thinning in trivalent chromium conversion coating for the electrochemical stability of buried metal-oxide-model epoxy interface formed on a rough (arithmetic average roughness parameter Ra=5 µm) electrodeposited substrate is presented. For different morphologies and sizes of thinned zones, the buried interface stability was tested in AC-DC-AC procedure. Local oxide thinning at zones of <0.25 mm2 was successfully detected by local electrochemical impedance mapping (LEIM). LEIM was able to measure in situ the interface degradation kinetics and distinguish the defects for which the interface underwent severe degradation from others in both terms of defect size and underpaint reactivity. The evolution of LEIM maps was in good agreement with the optical observations from 3D image reconstruction performed ex situ, thus validating quantitative LEIM as a tool for the local degradation quantification of buried interfaces. Based on the evolution of cathodic charge, LEIM and ex situ optical microscopy, it was demonstrated that the zone with local thinning of conversion coating on the rough substrates needs to reach some critical size to affect significantly the underpaint reactivity. Otherwise, the surface inhomogeneity related to the initial roughness hinders the effect of the local oxide thinning.