Effect of the breakdown time of a passive film on the electrochemical machining of rotating cylindrical electrode in NaNO3 solution

Abstract

In electrochemical machining (ECM), a passive film is generally present on the anode surface at low current density but is broken down at high current density in a passivating electrolyte. This film can impede unwanted metal dissolution in a low-current-density region and thereby improve the accuracy of ECM. However, it may sometimes affect metal dissolution at high current density. In this paper, the breakdown time of the passive film is determined for different current densities by varying the applied anodic potential pulses. It is found that the breakdown time has a significant magnitude of a few seconds, which can be long enough to affect metal dissolution in ECM.To illustrate the effect of the breakdown time of a passive film, ECM tests with cylindrical electrodes at different rotational speeds are conducted. The results indicate a correlation between the breakdown time and the rotational speed. At a high rotational speed, the surface of the anode workpiece is always in a passive state due to the existence of the breakdown time of a passive film, and hardly any material is removed. When a slower rotational speed is used, the steel passivity can be overcame, and a high material removal rate is obtained. A numerical simulation of the current density distribution on the cylindrical anode surface is performed, and the relationship between total metal dissolution time and breakdown time is found. It is seen that the breakdown time of the passive film can have a strong effect on the metal removal rate in ECM.