Studies on Aging and Regeneration of Electropolishing Bath Composed of phosphoric Acid and Chromic Acid

The electropolishing bath prepared from phosphoric and chromic acid is regarded to be very important because it gives high lustre and anticorrosive property to the polished iron, steel, stainless steel, aluminium and copper. However, the short life of this bath, owing to the reduction of CrO3, is considered to be its only defect. In the electropolishing baths for iron. for example, the aging appears as soon as about 30% of CrO3 is reduced. Therefore, it is prerequisite to prolong the life of bath. So the authors made experiments for that purpose by the following two methods: (1) Regeneration of CrO3 by means of the electrolytic oxidation of Cr3+, and (2) Prevention of the cathodic reduction by using a ceramic diaphragm to separate the cathode. The result obtained were as follows: In case of concentrated phosphoric acid bath, the electrolytic oxidation of Cr-Alum used as trivalent chromium salt was observed, which resulted in the low current efficiency and poor yield. While, in the oxidation of diluted acid bath, such as below 40 Vol.% of the acid, both values of current efficiency and yield were relatively high, although inferior to those examind with sulphric acid bath. So that the generation of CrO3 is not desirable for prolonging the life of concentrated phosphoric acid bath for clectropolishing of iron, stainless steel, or aluminium. But it is possible to regenerate the CrO3 for the relatively dilute phosphoric acid bath for electropolishing of copper. On the other hand, when the diaphragm was used, little chromic acid in the bath was reduced even after 4 hours electropolishing, and good results were obtained by further using of the bath. But, without diaphragm, chromic acid was readily reduced, and therefore good results were no more obtained after 4 hours' polishing. Considering these results, we can say about electropolishing baths that it is more preferable to prevent the reduction of CrO3 by using diaphragm than to regenerate CrO3 by means of electrolytic oxidation.