plating, anodizing, & finishing Q&As since 1989
Chromic Anodize - Deterioration of bath
Q. We are still using a chromic anodize bath according to MIL-A-8625 / MIL-PRF-8625
[affil link or DLA] and BAC-5019 specs. We have trouble with our hexavalent chrome reducing to trivalent chrome. The specs give ranges for free chromic acid and total hexavalent chromate. Every six months or so we end up having to remove and discard about half of the tank (150 to 200 gal). We then replenish the tank with water and chrome flakes (CrO3). Analysis of the tank afterwards shows that a good portion of the chrome is in the oxidized form (hexavalent), but over the course of a few months reduction occurs until the tank can't be kept in spec and the process is repeated.
We have pots that we can run in the tank to oxidize the trivalent chrome, but we haven't noticed much of a difference when we run them.
Is there a better way to maintain the tank? Having to change out half the tank twice a year, plus paying to have the waste shipped off and disposed of is expensive. I don't imagine the solution is as simple as adding an oxidizing agent (such as nitric acid)?
I've tried suggesting that we change to a different type of anodize, but management and/or customers still want us to use chromic.
Thanks for your help. I have found many answers to questions on this site.
A. If memory serves me correctly, chromic acid anodizing calls for using cathodes at about 10% of the anode area. If you are using a 1:1 or worse ratio You will convert hex to tri faster than the pots can get rid of it.
What are you using for cathodes? What alloy are you anodizing?
A. Sorry Chad but that is the way it goes with chromic, the bath needs dumping once every so often.
There are people how sell an ion exchange unit to recover the hex chrome from the trivalent.
Just check how the numbers stack up on the costs and the purity of the recovered chrome
surface treatment shop - Stroud, Glos, England
July 19, 2011
Does anyone have any idea of how much trivalent chromium would be present in a typical chromic anodizing bath and what is a harmful level?
Going forward, we will be checking for trivalent chrome. We have found a titration procedure to test it, but no levels or standards as to what is good or bad.
July 22, 2011
A. If you are running a proper anode to cathode ratio, a gross excess of tri is not probable.
I do not remember seeing any limit, but have seen several references to it being desirable to keep it to a minimum.
If you think that it is a problem, dummy the tank with a 1% cathode at the highest voltage that you can get.
A probably larger problem is tramp metals, especially if you do a lot of 2xxx or 7xxx alloy parts.
A porous pot will work great on both problems, with store bought formal ion exchange working well, but at a higher cost.
Search a lot before you buy.
July 22, 2011
A. Dear Jim, I just spent 3 hours searching ASTM, Boeing specs, Lockheed specs, Ken Graham's Electroplating Engineering, AES Slide Show. Sheasby/Pinner, Arthur Brace, an old AEC Proceedings published out of the non-existing Alcoa New Kensenton laboratories. In text the following statement kept repeating: "Trivalent chromium, normally present to the extent of a few milligrams per liter may cause darkening". One source dared to give a hard number, Slide 24 in the AESF Light Metals Finishing does say, "Keep Below 5 mg/L". That is not good enough for me but it is all I can find. Let us know if you do find a more reliable number.
Meanwhile, although the difficult titration analysis discourages measurement, it is very easy to electrolytically dummy it out with two electrodes running overnight.
Robert H Probert
Robert H Probert Technical Services
Garner, North Carolina
July 26, 2011
A. Two numbers I see crop up very frequently are related to total chrome, rather than trivalent chrome.
The total chrome I see often quoted are either 100 g/l or 107 g/l (not a lot of difference between the two, I know). The total chrome covers the hex chrome available for anodising plus complexed hex chrome and the traces of tri chrome.
Thank you for all of your replies. You have helped me greatly!Jim Black
July 29, 2011
Q. I have never understood the term "free Chromic acid" as used in the analysis of Chromic acid anodising baths.
When making up the bath, Chromium trioxide is used and water,nothing else. we make ours up at 50g/l.As the bath ages there will be some reduction, especially if the cathode/anode ratio is not right, producing chrome3+.I always assumed there was quite a lot of Chrome 3+ produced, thus accounting for the gradual increase in total Chrome. However, on measuring this by titration, and UV, only about 2g/l was found in a six month old bath.
If we are looking at "free Chrome" how can there be another form presumably "bound" Chrome, and why so much, given the low amount of trivalent Chrome?
August 22, 2011
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