Steel substrates were coated with a water-borne epoxy polymer layer and investigated by in situ scanning Kelvin probe (SKP) experiments, X-ray photoelectron spectroscopy, electrochemical impedance spectroscopy, and peel tests. After ion transport processes along the polymer/substrate interface were initialized at an electrolyte covered coating defect, nonspecific potential profiles were detected with the SKP. An identification of the front position of the electrolyte was not possible in air of high relative humidity, because wet de-adhesion and hydrolysis processes determined the local interface potential. Although the established mechanistic model for cathodic delamination solely predicts a cation transport in humid air, anions of the defect electrolyte were also verifiable at the epoxy/steel interface. It is discussed which forces rather than ion diffusion were additionally effective and contributed to the overall driving forces for the ion transport in this case. The oxidation state of the resulting substrate structure was investigated to further analyze the epoxy/steel interface degradation. These results are compared to the initial surface condition of the uncoated steel surface. They are also compared to the surface condition after oxygen reduction induced electrolyte spreading proceeded along the uncoated steel substrate. (C) 2010 The Electrochemical Society. [DOI: 10.1149/1.3525239] All rights reserved.