Parameteroptimierung für das Elektropolieren von Molybdän in Schwefelsäure

Research output: ThesisMaster's Thesis


Plansee SE, based in Reutte, is one of the world's leading manufacturers of refractory metals. Among other things, tubes made of pure molybdenum are also produced. Due to customer requirements, the surface quality of the inner side should be particularly high. Since the desired quality cannot be achieved by a mechanical polishing process, the surface treatment must be carried out by electropolishing. The aim of the present work is to optimize the parameters for the electropolishing system molybdenum-sulphuric acid by using statistical design of experiments. In preliminary tests the sample geometry was determined for further investigations. Electropolishing of samples of 3, 5 and 7 cm length with inner diameters of 2 and 3 mm was carried out. The duration of the test was controlled by charge (4 minutes of electropolishing with a current density of 0.5 A/cm²). The temperature and the distance of sample movement were constant for all preliminary tests and amounted to 52 °C and 15 mm respectively. All samples could be sufficiently electropolished. As good results were already achieved under the standard conditions given, the sample length was increased to 10 cm with an inner diameter of 2 mm for all further tests. Galvanostatic and potentiostatic test procedures were compared as part of the work. Charge-based experiments were carried out at different current densities and potentials, while keeping the temperature and the sample movement constant. It was found that an increase in current density is accompanied by an increase in the proportion of electropolished surface and a decrease in the Ra value. Above a certain current density, a further increase in current density neither lead to a significant improvement of the Ra value, nor to an increase in the proportion of electropolished surface. Similar results were obtained for the potentiostatic tests. For parameter optimization, experiments with pulsed current flow were carried out by varying the anodic current density, the ratio of the cathodic current density to the anodic current density, the anodic time, the ratio of the anodic pulse half-wave to the cathodic pulse half-wave, the temperature and the distance covered during sample movement. In addition, the influences of the cathode material and bath ageing were examined. The results show that the current density is the decisive influencing factor and that the effects partially override other parameters. Thus, no electropolishing is achievable at low current densities and always at high current densities. Depending on the ratio of anodic to cathodic pulse duration, the ratio of cathodic to anodic current density and the anodic pulse duration, the conduction of pulsed current causes a deterioration of the electropolishing results. Furthermore, progressive bath ageing and high temperatures have a negative effect on electropolishing, whereas sample movement has no big influence. The choice of cathode material is of great importance for the feasibility of the pulsed current electropolishing process but shows no significant influence on the Ra value and the percentage of electropolished surface. Furthermore, a current density-potential curve proves, that molybdenum is always in a state of active metal dissolution under electropolishing conditions and that there is no evidence for passivation.


Translated title of the contributionParameter optimization for electropolishing of molybdenum in sulphuric acid
Original languageGerman
Awarding Institution
  • Mayr-Schmölzer, Bernhard, Supervisor (external), External person
  • Mori, Gregor Karl, Supervisor (internal)
Award date25 Jun 2021
Publication statusPublished - 2021