TCP Trivalent Chromate Bath Maintenance & filtration

A. Hi Kelsey, we'll check if Rachel is available for an update on this. She and Aaron say on this thread that they filter their TCP trivalent bath, but in thread 526/67 from 2 years later Rachel recommends against filtering Iridite 14-2. It would be nice to get clarification whether she had a change of heart or she treated TCP differently than 14-2.

In my own industry travels I don't recall ever seeing chromate conversion coating baths filtered, but nearly all of those travels were pre-TCP.

Luck & Regards,


Ted Mooney, P.E. RET
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finishing.com - Pine Beach, New Jersey
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A. I think it may depend on the brand name. I don't know too much about Iridite TCP or others that you may be using, but Chemeon TCP would not be filtered except for a short time after adds if you use the powdered chemical product to assist with full dissolving. The bath is intended to run still without agitation, so you have to turn off pumps (and optional heaters) in order to run parts. Filtration during the non-working periods wouldn't hurt anything, but it is not necessary- for Chemeon at least.

A. My apologies, as I read the post now I see that you were asking about Iridite 14-2 and not about a TCP as was in the chain heading.

For 14-2, it is a good idea to filter and heat up the bath when you are dissolving new chemical, but beyond that, you would normally turn the heat and agitation off. Mild agitation and/or filtration is not harmful to the coating, it is just not necessary. Some people like to run 14-2 at a temp like 80 °F (just over ambient) so that they can have more consistency in part process time. In that case, you want to have some movement for temperature uniformity. A small pump with or without a filter would be fine.

Ed. note: Sorry about the confusion; as Jon says, Iridite 14-2 is not a TCP coating; it is an older hexavalent chromium coating.

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, ...??

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pH 2.0

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A. Hi Aaron! Any time : )

Hey so your best bet is to try to drop the pH of the tank using acetic acid ⇦ on eBay or Amazon [affil link] 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 haven't 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!

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 sulphate 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.

A. Hi Aaron,
Trivalent chromate bath maintenance:

1) check ph 2.0 is ideal if the ph high adjust using nitric acid, if ph low add sodium hydroxide liquid
2) check zinc contamination - if it is too high ,decant bath and adjust chemistry
3) Fish the tank everyday, take the parts out.
4) Check detail analysis - add iron inhibitor and conditioner
in trivalent chromate bath if it is necessary.