On Mon, 16 Feb 2004 06:19:24 GMT, "Carl Ijames"
wrote:

,; ,; It took a while, but the sheet stainless eventually looked like a
,;lace
,; ,; curtain. Worse, my solution turned yellow: hexavalent chromium.
,; ,;
,; ,;That's interesting. Did you use one of the standard, weak alkaline
,; ,;solutions?
,;
,; If you throw the kind of voltages that are used in this process at an
,; anode something is going to give. You can't pass a current from an
,; electrode into a solution without a chemical reaction. At the anode it
,; is either an oxidation of something in solution or an oxidation of the
,; anode. You must have one or the other to pass a current. If the
,; current is so high that there is nothing available from the solution
,; to oxidize then the anode is going to go.
,;
,;I completely agree that you have to have oxidation at the anode, I just
,;find it hard to belive that you will produce Cr(VI) and not Cr(III). So
,;far as I remember my electrochemistry courses, electrolytic dissolution
,;pretty much always produces the lowest stable oxidation state. I've
,;done a lot of electropolishing of stainless steel, and those solutions
,;always turn green, consistent with Cr(III). Have you chemically
,;verified the presence of hexavalent chromium or did you just trust the
,;color, which could be due to dissolved rust or other metal ions at low
,;concentration (no, I didn't on my solutions)?

I agree that Cr+6 is not going to be produced. I never said that Cr+6
was produced. I doubt if it happens and if it was produced it would be
reduced to Cr+3 at the cathode. There is no way one will accumulate
chromates in this procedure.

For those who don't believe that the anode can be dissolved note that
this fellow has been doing electropolishing. For the uninitiated the
piece to be electropolished is the anode. They ramp the voltage up so
one gets anodic dissolution in addition to the normal gas evolution.
The theory is that the peaks dissolve faster than the valleys so one
ends up with a flat surface. In fact the theory is correct and with
the proper voltage and proper solutions you can make some pretty nice
stainless steel mirrors. In order to make those mirrors one dissolves
stainless steel from the surface. I rest my case.

One of those solutions was a mixture of acetic anhydride and
perchloric acid. That one is not for the faint of heart. Due to some
faulty decisions an entire city block was removed in Los Angles. That
gave some more bad publicity to the perchloric acid business. My
research director owned the largest plant for producing perchloric
acid in the USA at the time. He was not pleased.