Electropolishing of Passive Materials in HF-Free Low Viscosity Aqueous Electrolytes

Abstract

A pulse reverse electrochemical surface finishing process for electropolishing passive materials is described. Unlike conventional electrochemical surface finishing processes, the pulse reverse process does not require low conductivity/high viscosity electrolytes and does not require the addition of chemical species to remove the passive film associated with electropolishing of passive and strongly passive materials. This paper focuses on pulse/pulse reverse electropolishing of niobium. © 2013 The Electrochemical Society. [DOI: 10.1149/2.044309jes] All rights reserved. On some scale, all surfaces are rough. Electropolishing is the process whereby the asperities are preferentially removed by an electrolytic reaction, which may be generally represented as: For large asperities, generally defined as features larger than ∼1 μm, 1 low conductivity electrolytes are used to affect the primary current distribution such that the voltage gradient between the asperities and the recesses of the surface is magnified, and the asperities are preferentially removed. High resistance electrolytes are generally reported for electrochemical removal of large features, such as deburring applications. 2,3 For small asperities, generally defined as features smaller than ∼1 μm, 1 high viscosity electrolytes are used to affect the tertiary current distribution such that under mass transport control the limiting currents are higher at the asperities than in the recesses and the asperities are preferentially removed. Jacquet 4 was one of the first to report that the optimum region for electropolishing is the mass transport or current limited plateau in the polarization curve. Furthermore, during anodic metal dissolution (Eq. 1) some metal surfaces can form a passive oxide film, generally described as: For these strongly passivating metals (e.g. stainless steels, titanium, nickel, niobium, and their alloys), continued electropolishing under direct current (DC) electric fields in a simple electrolyte can lead to a roughened surface similar to pitting corrosion. To remove the passive film, aggressive chemicals are added to the electrolyte to remove the passive film. For example, in the case of niobium, hydrofluoric acid is added to the electrolyte to depassivate the surface. 5 In addition to the electrolyte handling and safety issues associated with concentrated hydrofluoric acid, conventional DC electropolishing of niobium presents process control issues, and reject rates are often 40 to 50%. In 7 Generally speaking, for uniform polishing of a surface, for hydrodynamic boundary layers conforming to the roughness features (i.e. a macroprofile) the anodic on-time should be relatively small. For hydrodynamic boundary layers much larger than the roughness features (i.e. a microprofile) the anodic on-time should be relatively large. Furthermore, for oxide forming or passive materials, anodic only pulses lead to a rougher surface due to the non-uniform breakthrough of the passive film