Micro-tomography (CT) can be successfully employed to characterize ex situ the structural changes occurring in graphite felt electrodes during vanadium redox flow battery (VRFB) operation. Coupled high resolution X-ray and electron microscopy in conjunction with XPS are used to elucidate the microstructural and chemical changes to the high voltage RFB carbon electrode. The results reveal the onset of corrosion of the carbon felt structure relatively early in the VRFB life-cycle, extended operation is expected to result in extensive microstructural evolution effects. Display omitted •High resolution X-ray CT is used to characterize the microstructure of RFB felts.•3D microstructural evolution effects are presented revealing significant degradation.•Corrosion of the RFB felt structure starts relatively early in the VRFB life-cycle.•RFB felt corrosion is clarified by SEM, XPS and electrochemical characterization.