Electropolishing pure vanadium

457

Metallography

Electropolishing Pure Vanadium J. PELLEG Nuclear Research Center - Negev, Israel

It is often necessary to obtain highly finished and free-of-cold-work surfaces on vanadium materials. However, prolonged electropolishing (which is necessary to minimize effects of cold work) has a tendency to roughen the surface and produce undulations. In this note, an electropolishing technique is reported that satisfies both requirements; it provides a high-quality surface and, as indicated by Laue x-ray diffraction data, one that is free of cold work. In addition, the technique is not affected by temperature fluctuation and other such variables and precautions that must be taken with solutions based on perchloric acid. 1,2 Also, the excessive vibratory polishing times required when NaOH base solution 3 are used are eliminated. Vanadium polycrystalline specimens (15 to 20 grains/cm 2) with a maximum impurity content of 450 ppm, as determined by spectrographic analysis, were used. Before electropolishing the specimens were mechanically ground, although such grinding is not necessarily required. The electropolishing solution was composed of 10% hydrofluoric acid, 37~o sulfuric acid, 52% lactic acid, and 1 °/o glycerine. The proper conditions for electropolishing were determined by running a potential-versus-current density curve (Fig. 1). The curve was based on data obtained from experiments carried out at room temperature in a cell containing 500 ml of the electrolyte. A graphite cathode was placed about 1.2 cm from the specimen, which was connected to a rotative rod. Electropolishing occurs in region B C , with the optimum polish obtained at an applied voltage of 3.2 volts and 26 mA/cm 2. Microscopic investigations did not reveal any preferential attack along the grain boundaries or matrix, even after electropolishing for 1 to 1½ hours. Care must be taken, however, to maintain a clean surface before immersion to make sure the current is not impeded. In the region A B the solution can also be used as an etchant. In the lowvoltage region, for example, grain boundaries are delineated within 2 to 3 minutes. Examples of the surfaces obtained are shown in Fig. 2, 3, and 4. SHORT COMMUNICATION Metallography 3, (1970) 457--460 Copyright © 1970 by American Elsevier Publishing Company, Inc.

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APPLIED VOLTAGE, V FIG.

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Current density-versus-potential curve.

Care m u s t be taken to avoid the CD region, or grain b o u n d a r y delineation a c c o m p a n i e d by excessive pitting, surface roughening, and oxidation will occur.

Thanks are due to Mr. Y. Elenhorn and Mr. A. Levi for their contributions to the experimental phase of this work.

Electropolishing Pure Vanadium

459

Fie,. 2. As ground through a series of grinding papers and finished on 2/0 paper. Magnification 96 x .

FIG. 3.

As polished; 3.2 volts, 26 m A / c m 2, time 1 3/'4 hours. Magnification 96 × .

J. Pelleg

460

FIc. 4.

Etched at 1 volt and 9 mA/cm 2 for 5 to 10 minutes. Magnification 96 ×.

1. D. Godel and S. Steeb, Prakt. Metallog., 5 (1968) 625. 2. Book of A S T M Standards, Part 31, Metallography, American Society for Testing and Materials, Philadelphia, 1964, p. 26. 3. R. B61sing, E. Braun, U. Heubner, and M. Rfihle, Prakt. Metallog., 8 (1969) 475.

Accepted July 23, 1970