GDOES analysis of niobium de-hydrogenation after electropolishing processes

Abstract

Niobium, as pure metal and alloying element, is used in a variety of applications, among them in nuclear industries. Niobium is incorporated into nuclear fission reactors due to its enormous strength and relatively low density. Surface finishing of niobium is often performed in electrochemical polishing processes in view of improving its smoothness, corrosion resistance and its surface cleanability. However, the presently used electropolishing process (EP) is intrinsically linked to the subsurface hydrogenation of niobium, which measurably degrades its properties. The annealing operation, which is used to remove hydrogen from electropolished niobium, is quite a costly and time-consuming process. The traditional electrolyte consisting of a mixture of 96% H2SO4/49% HF acids by volume in a 9: 1 ratio has been substituted for the new one, being a mixture of 70% methanesulfonic acid with 49% hydrofluoric acid by volume in a 3: 1 ratio. The additional imposition of a magnetic field during the electropolishing process-magnetoelectropolishing (MEP) further increases hydrogen removal, when compared to the hydrogen content achieved by the electropolishing process (EP) alone. The aim of the study is to reveal a methodic approach and showing decreasing hydrogenation of niobium samples after consecutive steps of electrochemical polishing. Glow-Discharge Optical Emission Spectroscopy (GDOES) measurements were used to measure the hydrogen content in the surface layer of as-received (AR) niobium and in the samples after EP and MEP processes, and prove its close-tozero content after MEP.