by E.J. Taylor,* H. McCrabb, H. Garich, T. Hall and M. Inman, Faraday Technology, Inc., Clayton, OH 45315
In contrast to plating or electrodeposition of coatings onto a substrate, an important surface finishing technology involves the removal of material from a substrate. Some specific examples of material removal surface finishing technologies include electropolishing, electrochemical deburring and electrochemical through-mask etching. This paper presents a brief introduction to the electrochemical principals involved in conventional electrolytic electropolishing, which generally utilize high viscosity and high resistivity concentrated acid electrolytes. The bulk of the paper will focus on a recent approach to electropolishing/electroetching, based on pulse/pulse reverse electrolysis in low concentration/low viscosity aqueous electrolytes. A number of pulse/pulse reverse electropolishing and electroetching development activities, ranging from automotive planetary gears to fluid control valves to medical stents to superconducting radio-frequency cavities and materials including carbon steel, stainless steel, nickel/titanium-based alloys, and niobium will be described.
Electropolishing and electrochemical deburring are industrially important edge and surface finishing technologies. Applications include (1) deburring of carbon steel automotive planetary gears, (2) polishing of stainless steel fluid control valves, (3) polishing of nickel-titanium medical implant stents and (4) polishing of niobium superconducting radio-frequency cavities for high energy physics applications. An emerging derivative technology with numerous industrial applications is electrochemical through-mask etching. Conventional surface finishing technologies utilize high viscosity and/or low conductivity electrolytes, such as concentrated acids (e.g., sulfuric, phosphoric) and non-aqueous solutions (ethylene glycol, methanol-sulfuric acid).1 For strongly passive materials (e.g., titanium and titanium alloys and niobium), hydrofluoric acid is added to the electrolyte to depassivate the surface.2,3,4 Alternatively, some have suggested electropolishing strongly passive materials in acid-alcohol electrolytes with low water content5 or organic solutions containing fluoride salts.6 Due to the nature of conventional edge and surface finishing electrolytes, process control and robustness are difficult7 and reject rates are often as high as 40 to 50%.8,9
Background and technical approach
Conventional electropolishing. As depicted in Fig. 1(a), on some scale, all surfaces are rough. Electropolishing is the process whereby the asperities are preferentially removed by an electrolytic reaction [Fig. 1(b)], generally represented as:
M0 à M+n + ne- (1)