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Trivalent chromium plating



(-----) 2006

My question is: if we start with trivalent chromium for our chrome plating operation, how do we prevent the trivalent species from oxidizing to Cr+6? Thank you

Jeff Moraitis
- Van Buren Twp., Michigan
^


2006

To my knowledge there are no generic trivalent chromium plating processes, only proprietary ones. My understanding is that one type uses semi-permeable membranes around the anode compartment that do not let the hexavalent ions reach the work. Another brand uses anodes of a special material that supposedly prevents the formation of hexavalent ions.

I can't be of much help except to say that I think you need to learn from the supplier, depending on which type of system you are using, what ptotocol to use to deal with hexavalent chromium.

Ted Mooney, finishing.com
Ted Mooney, P.E.
Striving to live Aloha
finishing.com - Pine Beach, New Jersey
^


2006

The simplest way of stopping Cr(III) being oxidised to Cr(VI) is to use a divided cell, so the chromium ions don't get the chance to see the anode.

trevor crichton
Trevor Crichton
R&D practical scientist
Chesham, Bucks, UK
^


2006

I have a question for Trevor. What do you mean by a divided cell? Is the anode behind a wall but still sharing the same solution. Can CR VI in a CR III bath cause faint black Staining?

Kelly Pichel
- Aguanga, California, USA
^


2006

A divided cell is one where two electrolytes are kept apart by a membrane or sintered barrier. The electric current can pass across the barrier, but large metal ions cannot. I have successfully used a porous clay flowerpot as a cell divide; one electrode (anode) was inside the pot with an electrolyte (10% sulphuric caid) and the other electrode (cathode) was outside the pot in another container. The workpiece was in the outer container and an electric current was passed between the two electrodes. The voltage drop was higher than one would normally expect when plating, but the process worked and I didn't get Cr(III) oxidising to Cr(VI).
As far as the other question is concerned, trivalent chromium is very susceptible to contamination of any type, so Cr (VI) could well cause these problems. However, faults with trivalent chromium are also dependent on the bath makeup, so I would check with your supply house when trying to trace faults.

trevor crichton
Trevor Crichton
R&D practical scientist
Chesham, Bucks, UK
^


2006

I can only answer on one of the commercially available TriChrome processes. Hexavalent chromium is a contaminate in trivalent chromium plating and will cause significant problems. Unless your solution is out of balance you should not be forming hexavalent chromium at the anodes.

George Shahin
George Shahin
Atotech - Rock Hill, South Carolina
^

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