Electropolishing Valve Metals with a Sulfuric Acid-Methanol Electrolyte at Low Temperature

This study reports the electropolishing of Ti and Nb metals using a fluoride-free electrolyte of sulfuric acid and methanol at low temperature (−70 °C) without prior treatment. A fluoride-free electrolyte provides a less hazardous and more environmentally friendly option for electropolishing procedure. Experimental studies are presented on electropolishing with sulfuric acid electrolyte, which provides high quality macro- and micro-smoothing of the metal surfaces. Optimal conditions yielded leveling and brightening of the surface of Ti and Nb metals beyond that by the currently utilized electropolishing procedures with fluoride-containing electrolytes. The root mean square roughness (R_q) from atomic force microscopy analysis was 1.64 and 0.49 nm for Ti and Nb, respectively. Lower temperature experiments led to noticeable kinetic effects, indicated by a dramatic drop in current densities and the expansion of the steady-state current density plateau in anodic polarization curves. In addition, the voltage range of the current plateau expanded with increasing acid concentration. Surface characterization of Ti and Nb metals after polishing provided evidence of salt film formation. In addition, these metals were used as substrates in the formation of nanostructured metal oxides. The overall quality of the electropolishing led to a dramatic improvement in the uniformity of the nanostructures.