Chromium
Posted on 11-03-2010Chromium is a hard, steel-gray, shiny, metal that breaks easily. The pure metal chromium can be electrochemical machined, but more often chromium will be machined as alloying element in steels or other alloys. Using chromium in steel increases the hardness and the corrosion resistance of the steel.
Chromium dissolves in the trans-passive state during electrochemical machining; the protecting oxide film dissolves at sufficient high potential, leading to the dissolution of the metal. Chromium will dissolve with high current efficiency independent of potential or electrolyte type, copying accuracy is hard to control.
Under electrochemical machining conditions chromium can dissolve as trivalent chromium and as hexavalent chromium. The hexavalent chromium will appear in the electrolyte as chromate ion (CrO42-) or dichromate ion (Cr2O72-) depending on the pH, and causes a health risk because of its carcinogenic character. For that reason the chromate has to be reduced to the much less hazardous trivalent chromium, for example by adding iron(II)sulphate. (see: chromate reduction)