Research on the Formation Mechanism and Processing Effect of Black Film in Bipolar Pulse Electrochemical Etching of Micro-Rod

Tungsten micro-rods are core components of micro-probes and micro-tool electrodes. Electrochemical etching, featuring no cutting force and hardness-independent machining, enables efficient fabrication of tungsten micro-rods. Nevertheless, the formation and removal mechanisms of surface black films during ECE are not fully understood. This work investigates bipolar pulse electrochemical etching of tungsten micro-rods, examining how pulse amplitude and temporal parameters affect etching and clarifying the evolution mechanism of the surface black film. Results show that positive pulse amplitude has a valid processing range. Excessively large positive pulses trigger black film formation, suppressing anodic dissolution and lowering machining efficiency. Negative pulse amplitude has a clear process window, and weak negative pulses cannot remove black films, whereas overly strong pulses cause violent reactions and uncontrolled micro-rod morphology. Furthermore, the positive pulse duty cycle strongly governs black film evolution. A high duty cycle shortens the negative pulse duration, weakens film removal, and impedes etching. Negative pulses boost interfacial mass transfer via cathodic hydrogen evolution and neutralize residual charges in the electric double layer. The black film consists of low-valence tungsten compounds from cathodic reduction. Bubble nucleation, growth, and detachment effectively strip this film and renew the electrode interface.