Electropolishing Valve Metals with Sulfuric Acid-Methanol Electrolyte

To develop uniform oxide nano-structures on the surface of valve metals, via anodization, it is desirable to start with a polished surface. Electropolishing is a common method to produce highly polished surfaces. However, common procedures utilize toxic, fluoride containing electrolytes. This study reports on a novel method for electropolishing titanium and niobium, in a sulfuric acid/methanol electrolyte, at low temperature (-70 oC). Electropolishing at low temperature has a significant effect on reaction kinetics. Experiments show an expansion of the steady-state current density plateau of anodic polarization curves. Additionally, increasing the sulfuric acid concentration led to broadening of the current density plateau. Optimization of conditions produced a root mean squared roughness of 1.64 nm and 0.49 nm for titanium and niobium, respectively. An improvement over results obtained with fluorine-containing electrolytes. We believe it is possible to apply this method to other valve metals, like zirconium and tantalum. Preliminary experiments with zirconium have shown a brightening and smoothing of the surface. However, there is further work required to optimize results with this metal. Additionally, we show that polished valve metal surfaces produce more uniform nano-structures, formed via anodization