Electropolishing of Kovar Alloy for Vacuum Applications

Kovar (53% Fe – 29%Ni - 17% Co) is one of the most commonly used materials in high vacuum applications which quite often require electrical isolation(s). Kovar has thermal expansion coefficient close to that of alumina over the range from brazing to cryogenic temperatures. Therefore, joining problems due to difference in thermal expansion coefficients can be minimized when alumina and Kovar alloy are used together. 

For the applications that require long vacuum life, Kovar parts should have smooth surfaces to minimize outgas from the surface due to inadequate cleaning. The surface modifications obtained by electropolishing, i.e., preferentially dissolving macroscopic and microscopic surface roughness without grain attack in the depressions, and surface metal oxide layer dissolution [1,2], are indispensable in high vacuum applications. Electropolishing has also long been proven to be a powerful tool for complex parts that are difficult to polish mechanically. Although it is widely used in the vacuum industry, there is very limited information about the Kovar electropolishing in the literature. There is no systematic study known by the authors that aims to reveal the optimum electropolishing conditions for the Kovar alloy.

A two-electrode linear sweep voltammetry was conducted in a solution of acetic acid, chromic acid and water mixture, where a copper plate was used as the counter and reference electrode.  A plot of current density versus applied potential given in Figure 1 was obtained at 9°C. In the light of almost constant current density, namely limiting current plateau region observed in this plot, some experiments were conducted at 30 V for durations of 30, 60, 90 and 120 seconds. Rectangular Kovar samples of 15x15 mm dimensions having about 0.50 μmRa initial surface roughness were electropolished to final surface roughness of about 0.25 μmRa. The surface roughness profiles and their microstructures before and after electropolishing process (EP) are given in Figures 2 and 3, respectively.

Figure Captions:

Figure 1: Current density vs. potential graph in acetic acid, chromic acid and water mixture.

Figure 2: Surface profiles of samples, (a) before and (b) after EP. The horizontal and the vertical scales are embedded to left hand side.

Figure 3: Microstructures of samples, (a) before and (b) after EP. 

References

[1] Sova, V. (1986). U. S. Patent No. 4,563,257.  

[2] Popov, K. I., Djokic, S. S., & Grgur, B. N. (2002). Fundamental Aspects of Electrometallurgy. New York: Kulluwer Academic Publishers. pp.226-227.