Prevention of Hexavalent Chromium Formation by Iron-ion-added Electrolyte in Electrochemical Machining of Chromium-containing Steel (2nd report)

Abstract

Electrochemical machining (ECM) of Cr-containing materials such as stainless steel produces hexavalent chromium, which is often considered a problem in ECM. In this study, using the relationship of the standard electrode potential, we investigated whether the formation of hexavalent chromium itself could be suppressed during ECM and developed the method of preventing the formation of hexavalent chromium using divalent iron ions in the electrolyte. If the electrolyte contains much easily oxidized ion, the oxidation of trivalent chromium is prevented by the oxidation of the ion. To supply divalent iron ions to the electrolyte, an iron-ion-supplying device composed of iron chips was fabricated. In our first report, it was shown that the formation of hexavalent chromium can be prevented by this method, but it was also found that it is difficult to maintain the prevention effect. In this report, a method to continuously prevent the formation of hexavalent chromium is described. It was found that the required amount of Fe2+ ions could be supplied to the electrolyte by temporarily acidifying the electrolyte. And it was shown to be possible to carry out ECM with a neutral electrolyte while preventing a decrease in Fe2+ ion concentration by neutralizing the acidic electrolyte before supplying it to the machining tank.