Electropolishing of an Fe-Ni-Co Alloy in Acetic Acid-Perchloric Acid Mixture

Abstract

Among Fe-Ni-Co alloys, Kovar (53% Fe-29% Ni-17% Co) designated as ASTM F-15[1], is a well-known glass-sealing alloy. It also is classified as low–expansion alloy [2]. Low expansion is required to avoid internal stresses for applications that involve strong ceramic to metal joining. One of the most significant problems in vacuum systems is the outgassing which is the liberation of a gas that was dissolved, trapped, frozen or absorbed in a material. To decrease the outgassing rate from the chamber walls, the material should have smooth and passivated surface finish [3]. Electropolishing, one of the finishing methods applied to decrease surface roughness, is appropriate for the systems involving parts of simple and complex geometries. However, there is very limited information on the electropolishing of Kovar alloy in the literature. Acetic acid (glacial)–perchloric acid (60%) mixture, stated as a cleaning solution before soldering Kovar [4], was used as electropolishing electrolyte in this work. A systematic study was conducted in 5 to 15% perchloric acid containing electrolytes at 25 to 35°C to find the optimum polishing conditions. A two–electrode setup employing Kovar anode and stainless steel cathode was used to perform voltammetric studies to determine low and high levels of current densities. According to linear sweep voltammetry results, shown in Figure 1, 200 to 600 mA/cm2current density range was selected to optimize electropolishing. Experimental route was designed using three parameters (current density, concentration and temperature) and three levels. The duration was kept constant as 5 minutes. Effect of electropolishing duration was determined by utilizing the condition that revealed the lowest roughness in 5 minutes. It was found that, the surface roughness of Kovar samples (having dimensions of 10×25×2 mm), was decreased from 0.21µm to 0.06 µmRa in a 15% perchloric acid containing solution. ADDIN EN.REFLIST 1. ASTMInternational, Standard Specification for Iron-Nickel-Cobalt Sealing Alloy. 2009.2. Special-Purpose Nickel Alloys, in ASM Specialty Handbook: Nickel, Cobalt, and Their Alloys. 2000, ASM International. p. 92-101.3. Tajiri, K., Pit-free electropolishing of aluminum and its application for process chamber. Journal of Vacuum Science & Technology A: Vacuum, Surfaces, and Films, 1998. 16(3): p. 1196.4. Vianco, P.T., F.M. Hosking, and J.A. Rejent, Solderability Testing of Kovar with 60Sn-40Pb Solder and Organic Fluxes, in 19th International AWS Brazing and Soldering Conference. 1988: New Orleans, LA. Figure 1