In-situ evidence for the existence of surface films in electrochemical machining of copper in nitrate electrolytes

This contribution analyzes the electrochemical etching of copper in a sodium nitrate electrolyte using in-situ Raman spectroscopy. Experiments were conducted in a specially designed process chamber at voltages below, at, and above the limiting current plateau. Below the plateau, a Raman peak at 1050 cm–1 is detected, which is attributed to the symmetrical nitrate ion. However, the line shape changes at and above the plateau. Detailed line shape analysis indicates the presence of a copper nitrate surface film by ex-situ comparison with copper nitrate solutions. In addition, the conductivity and pH of copper nitrate solutions were determined. A 5 M copper nitrate solution exhibits a low conductivity of 55 mS cm–1 and a pH close to 0. A significant influence, especially of the low pH, on the surface structure and therefore on the etching mechanism is assumed.