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Platinum-Clad Anodes for Chrome Plate
I've been trying to get some additional "life" out of my very expensive platinum-clad anodes. We used to retire these after about 35 uses, I raise this to 50 cycles with no problem, now I'm basically using them until some sort of plating problem manifests itself. These anodes are generally treated very gingerly, never abraded or polished, cleaned in dilute HCl and wiped dry with a soft cotton cloth after each use, visually inspected and straightened, if needed, after every use. My question is: what should happen as the platinum gets thin? Will the plating efficiency change? Will there be a visible change in the chrome deposit? Will the physical properties of the deposit (hardness, etc.) be affected? Or will it just take more voltage to maintain plating current?
George Brackett IIISr Manufacturing Engineer - Maine
April 10, 2009
George,
As long as the entire anode (front, back, interior of the mesh) still has some kind of Pt thickness, it will not affect anode distribution, nor the plating. If you think about electroplated electrical contacts with a final finish of Au, Ag, or Pt, current is measured and flows on the surface of the final plated finish. This explains the low contact resistance of these plated noble metals. The same goes for Pt clad anodes. It would be a good idea if you could obtain a standard of Pt over Ti and measure your Pt thickness as part of your inspection process. A quick spot check would be to apply a propane torch flame to the anode in one spot for 15 seconds. If the anode turns black and does not wipe off clean, it's time to replace them. I prefer measuring the surface though. Good Luck!
Fellow Plater - Syracuse, NY, USA
April 20, 2009
First of two simultaneous responses --
For any platers thinking about using clad anodes for the first time, they are wonderful for spots needing precision conforming anodes but they cause a fairly rapid rise in the trivalent chrome which if not oxidized in some manner to hex chrome, will slow/stop the plating.
James Watts- Navarre, Florida
April 21, 2009
Second of two simultaneous responses --
Thanks for the response, Mark. Your torch idea may work, I'll try it. We're not using mesh, our anodes are long copper rods, clad with niobium, then platinum. We recently had an anode at our post-plate inspection that seemed to have a lot of platinum missing (maybe 10% or more), I tried it one more time, the part came out perfectly. I wondered if, as we lost platinum, if the "effective" anode area would be reduced, raising the anode current density, and what effect this would have on the cathode side...
George Brackett III- Maine
April 22, 2009
George,
As far as the loss of Pt it would all depend on the anode to cathode ratio. In other words, if you are running 30% over the minimum requirements on the anodes, a 10 to 15% loss of Pt coverage would not hurt you. I like to record my voltage vs a given amount of amperage applied. Platinum has a lower resistance than niobium or tantalum, so as the Pt on the anode decreases, voltage will rise accordingly. Good Luck!
Fellow Plater - Syracuse, NY, USA
April 22, 2009
First of two simultaneous responses --
Hello George,
normally a thickness of 2.5 microns of Pt would be enough for
1'000'000 Ah. To avoid build up of trivalent chrome just hang some led anodes in it, or run over an external tank and oxidize it.
Regards,
Dominik
- Mexico City, Mexico
April 23, 2009
Second of two simultaneous responses --
I could speculate that, as the Pt gets thinner, and eventually exposes underlying niobium, the latter will be anodized and that portion of the surface will become non conductive. That will increase the current density on the remaining Pt coated surface.
And, I could also be full of beans. :) Feel free to correct me if you know better, anyone.
It's interesting, Mr. Watts, what you point out about the build up of Cr3+; I gather that Pt lacks the reoxidation capability of conventional Pb/Sb anodes. Could some kind of DSA be used, like one clad with Ru, Os, or Ir oxide? or would the chromic eat them up?
Dave Wichern
Consultant - The Bronx, New York
April 24, 2009
If I were using Pt clad anodes, I would certainly use a small porous pot to control the trivalent as well as remove tramp metal ions.
I THINK that it has to do with the oxygen overvoltage potential and that it does not release as much O2 at the anode.
- Navarre, Florida
April 24, 2009
If I were running any kind of hexchrome bath I'd have a porous pot purifier in there.
If I may pose a question: why use Pt clad auxiliary anodes instead of casting up some made of antimonial lead? Where lies the advantage?
Dave Wichern
Consultant - The Bronx, New York
April 29, 2009
We're on an environmental kick, we've dismantled our lead-tin plating line. Platinum clad is a lot easier to use than lead-tin, for us, at least.
George Brackett III- Maine
April 30, 2009
May 7, 2009
Advantage of Pt => No sludge development, like if you use led-Anodes
Dis-advantage of Pt ==> Slow oxidation of trivalent to hexavalent chrome
If you'd like to kick out lead completely from your working tank, that might you should use a semipermeable ceramic pot with an own circuit to oxidize trivalent to hexavalent by using a small piece externally of led.
regards,
- Mexico City, Mexico
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