For several years an investigation of the physical properties of thorium and methods of preparing higher purity thorium metal has been conducted 5 at the Ames Laboratory of the U. S. Atomic Energy Commission. In the metallographic examination of high purity thorium, an electropolishing procedure which did not obliterate the inclusions was desired . The mechanical polishing of thorium to reveal small inclusions is difficult and slow, and becomes more difficult as the hardness of the metal decreases with higher purity. In addition, a number of tensile specimens were to be examined to determine the grain size, and a convenient method of revealing the grain structure was needed. A previous study of the electropolishing of thorium had shown that a solution of hydrochloric and hydroiodic acids in formic acid had some promise as an electropolishing solution. A more extensive study of electropolishing solutions which used formic acid as the solvent was undertaken to see if a satisfactory solution and procedure could be evolved. The results are incomplete and the solutions used are not recommended for general use but some very interesting observations have been made on thorium metal.
Peterson, David T., "Some metallographic obsrvations on thorium metal" (1960). Ames Laboratory Technical Reports. 24.