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topic 61112

What maintenance is needed on trivalent chromate bath?


May 22, 2020

Q. Hi good day everybody.
I have a trivalent chromate conversion coating line, and I noticed some "dust" in the tank. The supplier told me that fine dust is the reaction of the conversion and doesn't cause any effect. But they gave it over for maintenance, stopped mixing the solution to decant the dust. After that they removed the solution from the tank and removed the dust manually.

Do any of you have the same dust or do the same maintenance? Is anyone trying to filter the solution?

I tried it but I feel the concentration of the solution dropped drastically. I used a micron filter, maybe a bigger porous filter would be better?

Aaron Garcia
- Mexico


May 27, 2020

A. Hi Aaron,
We filter ours through 20µm polywound.
When you say 'dust', I'm thinking... suspended sludge? It's very important to maintain pH of the tank within the very tight range that the manufacturer recommends. If this is TCP-HF, you gotta keep it right at 3.8. If it gets too high, the tank will go cloudy from the Chromium falling out of solution. If you're doing pH adjustment with a hydroxide (ammonium hydroxide is correct for this material), it should be very dilute, and added very slowly so as not to make pockets of elevated pH in the tank that haven't had time to mix in. And yes, the concentration can drop because some portion of the actives are now in the form of a suspended, filterable, solid.
Also this material is very sensitive to Iron contamination so having it tested for buildup periodically is useful.
If the 'dust' is sitting on top...
Is there any possibility of drag-in of grease from parts, a leaky crane, ...??

rachel_mackintosh
Rachel Mackintosh
Plating Solutions Control Specialist / Industrial Metals Waste Treatment - Brattleboro, Vermont


May 27, 2020

Q. Hi Rachel. Thanks for your constant support; you have responded to a couple of my doubts now and on past occasions.

Yes, I mean the suspended sludge.

I tried to filter the solution to remove the suspended sludge and the concentration of solution dropped radically.

If I understood correctly, probably the pH of my tanks is too high and unable to dissolve it?

If I have suspended sludge this means I have sludge in the bottom of the tank -- is this right ? Can I remove it with a good recirculation system and the 20µm filter? Or do I need to draw out the solution and remove it the sludge manually?

Aaron Garcia [returning]
- Mexico


May 28, 2020

Hi Aaron! Any time : )

Hey so your best bet is to try to drop the pH of the tank using acetic acid down to around 3.7 or whatever the manufacturer says is the 'lower limit' for pH and see if the remaining cloudiness clears, which it should, mixing of course but not scraping the bottom, and re-test the concentration for a TRUE look at where you're at. Also double check your meter calibration. The range is pretty narrow so if you're trying to really hit the limit, accuracy is key.

As a humorous (in retrospect!) aside/cautionary tale... We have a company legend about a previous GM here who didn't calibrate the pH meter and failed to identify that the probe was shot, kept adding hydroxide, and basically did a batch waste treatment of the TCP tank in situ... by the time anyone walked by and saw what she had done, the pH was up over 8 and the tank was unrecoverable. There goes several thousand dollars out in a truckload of portable waste totes!!!

Next time the tank is due for a change, you can really get in there with a shovel and manually remove any sludge you find, rinse and vacuum (drum head wet vac!) and clean out the tank before re-making it, then in the future, keep on top of your pH adjustment every day the tank is in use to prevent it recurring. As for any sludge that has managed to settle under your current agitation- the sludge is pretty dense and will sit there unless you increase your mixing/agitation.

Also, don't be alarmed if you have light aqua green to sage colored scale on the tank sides; it happens. I approach the scale as, "you just sit there and spectate and don't flake off into the tank, and I'll leave you be til the next dump date!"

Since the tank is run at about 90'F it does evaporate off pretty quick so unless you find that you've also got iron contamination, you can simply add more material to top it up until you decide to formally dump it, which I would do if salt spray results started coming back spotty and other tanks have been ruled out. I think ours is about 4 years old, crusty as all get out, but I havent seen a single pit from it in years with the exception of one month when we fouled the deox with someone's nasty sand castings right before running test panels. The TCP itself is surprisingly resilient!

Best of luck!

rachel_mackintosh
Rachel Mackintosh
Plating Solutions Control Specialist / Industrial Metals Waste Treatment - Brattleboro, Vermont


May 28, 2020

I did the oposite. I added too much sulfuric acid and dropped the pH alot D:

And honestly the sludge in the bottom or suspended sludge until now doesn't make me any problem, but I like to have everything clean and that stresses me, haha. Also I want to avoid future problems with that.

I am going to follow your advices and put the pH in the lower limit and I will be very cautious with that.

Again Rachel, thanks for your time and advice. Have a wonderful day. Hope the next doubt I have you will be there.

Aaron Garcia [returning]
- Mexico


May 28, 2020

A. IMVHO Acetic acid added carefully is superior to Sulfuric acid, and I say this for a couple reasons- I seem to recall someone telling me years ago to avoid any chance of sulfate contamination in conversion baths. Do I have any proof of this? No, and of course I didn't write down my source, but as a general rule I don't like stray inorganic ions floating around in there. Acetic doesn't seem to interfere at all.

Also it's harder to overshoot the target. I spend about 10 minutes or so making small adjustments, waiting, go bother the dye tanks for a bit, come back, recheck, and readjust until I get to the target. On the up side, once you hit the target, it's pretty stable as long as you don't drag in anything from the deox.

I have no idea what the lower limit on pH is before this tank is rendered 'dead' but my suspicion is that, like other proprietary mixes, it contains a chelator and if you go low enough break chelation, components will interact undesirably amongst themselves and with the dissolved aluminum, which may be allowed to build up to nearly a full 3 GRAMS per liter under normal conditions, and there may be no bringing it back. This would definitely be a question for your technical sales rep- it's beyond my firm knowledge of the tank.

rachel_mackintosh
Rachel Mackintosh
Plating Solutions Control Specialist / Industrial Metals Waste Treatment - Brattleboro, Vermont

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