Electrochemical machining of tungsten carbide
Electrochemical machining (ECM) is characterized amongst other things, by extremely high current densities and a high dissolution rate of material. Due to the extreme current densities under ECM conditions, tungsten carbide forms adherent, supersaturated, viscous films of polytungstates close to the interface. This film is permanently dissolved by electrolyte flow and is reproduced at the electrode surface. The dissolution proceeds in an active state up to 30 A cm−2. An additional layer is formed at higher current densities which means that there is a passive state and the presence of high-field oxide films with thicknesses around 10 nm. The complex interaction between current, field strength, and oxide thickness yields a constant resistance to the oxide film. The formation of an oxide film is also indicated by the onset of oxygen evolution which consumes about 20% of anodic charge. The interaction of ionic currents (oxide formation and dissolution) and electronic currents (oxygen evolution) is small due to completely different conduction mechanisms.