Significance of waveform design to achieve bipolar electrochemical jet machining of passivating material via regulation of electrode reaction kinetics

Electrochemical machining (ECM) using the bipolar pulse technique shows significant potential for the effective machining of prone-to-passivation materials. However, despite its success in machining specific materials (e.g. tungsten carbide), the underlying mechanism remains unclear. This study presents an in-depth analysis of the interplay between cathodic polarisation (CP) and anodic oxidation in bipolar pulse electrochemical jet machining (bipolar-EJM). An observational study of the structural changes in the anodic oxide film indicated that the oxide broke down by electrochemical reduction and mechanical damage when subjected to CP. The electrochemical kinetics at the electrode interface, particularly the proton transport and discharge reactions, were discussed to elucidate the mechanism. This study first discloses that the bipolar pulse waveform plays a crucial role in overcoming the oxide film and achieving machining in bipolar-EJM. Variations in waveform shape result in distinct interactions of the cathodic pulse with the anodically formed surface oxide. A principal approach of bipolar-EJM was thus developed to effectively remove or eliminate passive oxide films by controlling the bipolar pulse. The material removal mechanism of the developed method is described and the corresponding kinetic model of the electrode reactions is presented. For verification, the bipolar-EJM of the strongly passivating metal niobium (Nb) is first demonstrated in a pH-neutral NaNO aqueous solution, demonstrating the effectiveness of the proposed method. The new insights into the bipolar pulse process established here will is believed to provide valuable guidance to advance the bipolar electrochemical process, including ECM and electrochemical polishing.