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Baffling citric acid passivation bath issue
Q. Hello all!
I've been dealing with a baffling issue in my Hot Citric passivation bath, set up to process according to ASTM A967 Citric 1 and AMS2700 Method 2:
140-160 °F
60-100 g/L citric acid [on
eBay or
Amazon] (from anhydrous granular)
Optional 10 g/L Sodium nitrate (rarely used to begin with, and has been removed from the 'official' makeup as we have ZERO problems burning or passing water immersion without it)
Deionized water makeup
Mechanical mixing (stainless bladed lab stirrer)
It shares an enclosure with a number of Nitric and Nitric/Dichromate tanks, both heated and unheated.
Iron upon makeup is <.1ppm and upon disposal, reaches no more than 30ppm at worst condition. We don't have parts burning, or failing testing.
There's no possible Nitric dragover. The Citric bath is the first actual passivate bath on the line, only preceded by a soap tank and 3 city rinses.
I tried to have the tank tested for Nitrate, on a worst-condition bath that did not get the NaNO3 treatment to begin with, and the results were that the lab could not resolve the nitrate peak on the GIANT tail of a massive Chloride peak (!).
So here's what's going on:
Over the course of only a few weeks, the bath will test at an ever-increasing apparent free acid concentration (titration, not pH). This started when we bought an automated line with an enclosure. Prior to that, the tanks were just out in the open with simple negative pressure lip-vent exhaust ports. These are present in the new line too, but I'm having to dump my bath after a very short time because of the rising acid that ends up going out of spec, even were it to be all Citric, which obviously it ISN'T because of the known Chloride contamination and the Nitric-laced atmosphere. So it's not like I can just decant it back to a passes-free-acid-assuming-it's-all-citric-wink-wink level, and pretend everything is dandy.
This is frustrating. Good thing Citric is cheap and can be pH adjusted right down the drain but this is still driving me crazy.
Has anyone had this problem? What is going on!? Is this really the tank sucking up atmospheric contaminants? It's 10 g/L Citric Clean tank sister behaves fine (aside from bioslime but that's neither here nor there). And our town water, which we use for running rinses, is not heavily chlorinated. There's no off taste in the drinking water, and anodized parts going through rinses from the same town water source NEVER show pitting or spotting, and the Chloride drag-in to the ano tanks has never built up to more than 27ppm even going on average nearly 2 years between only about 40% decants!
I guess I'm hoping that an experienced Citric passivator will tell me that this is just what happens when you put your tanks in a stuffy box, and that it's not me losing my marbles, just so I can stop losing sleep over it. And also so I have something from 'A Real Professional' (....!) to show the line technician, who is skeptical that I'm not feeding him a baloney sandwich that after 8 years of relative simplicity, he suddenly has to remake the tank every 3-6 weeks, forgetting that I'm the one stuck with the disposal project and am definitely not doing this for fun!
Thanks in advance for any words of wisdom (or even validation, haha, assuming I'm not way off base!) you can pass my way!
Rachel Mackintosh
- Greenfield, Vermont
A. Howdy Rachel,
My initial reaction would be to question the exhaust system. Does anything containing HCl come upstream in the ventilation? Are chlorides increasing in your rinse tanks before/after, or is it purely your citric bath?
There is also the extreme possibility that rainwater could get into the tank...
amphenol aerospace - sidney, New York, USA
May 11, 2020
Q. Hi William,
Thanks for stopping by. There's no HCl anywhere in the building. It's a machine shop and this means the primary indoor air pollution is cutting oil mists, which, despite our pretty extensive air handling system, do deposit on surfaces and everything ends up with a sticky film that keeps the janitors busy. I would have expected this to appear as a problem- likely a worse one- when the tank lived out in the open.
There's no possibility of rainwater inclusion into the 'robot line'. It has a roof on it, and the building as a whole is in good shape in general, with the exception of a recurring roof leak in the ladies room (of course !)
Okay how about this: In the presence of an organic acid at high temperature, will Chlorine leach out of the PVC fittings in the tank and its filter loop?
Rachel Mackintosh
- Greenfield, Vermont
May 12, 2020
A. Burning PVC will release chlorine, but if it is just getting hot it should not leach any out. We run a burn off stripping company and cannot burn any pvc coatings because of the chlorine and hydrochloric acid fumes that are emitted.
Jason Meekhof- Norton Shores, Michigan
June 9, 2020
A. I'd look at what you're putting into it.
If you can let it sit over the weekend without anyone running it, then you can figure out if it's the parts you're running or the system.
If you're using tap water to store parts before washing, or if your rinse tank evaporates faster than the water drains... you could be adding tap water to makeup for evaporation, resulting in an increased concentration of salts, even though the town water is fine to start.
I'd also look at your sanitization procedure, you mentioned biofilm, if your sanitization people aren't rinsing the system down properly, you could have a dead leg holding chlorine in a filter housing or in a fill line resulting in an increase. That would also question if the tanks weren't getting dumped to sanitize, but I'll let you work that one out.
- Newmanstown, Pennsylvania
September 28, 2020
October 6, 2020
Hi Josh! Thank you so much for your response, and your points are well taken.
It appears to rise even without parts going through, but running jobs speeds up the increase. When it sits idle, the tank heater is turned off, as is the mixer, so I'm also taking into consideration the increased reaction rate and exposure to atmospheric contaminants provided by that combination of factors. We only get the occasional job for this tank and fire it up only as needed. I am certain to tag it TEST BEFORE USE the minute the job is finished, because even though I have not seen any 24hour water immersion failures come out of it, I don't trust it!
Parts are never stored in tap water. They come from our sister company next door, are vapor-degreased (n-propyl bromide), and then ultrasonic cleaned in Brulin815. Next, they are rinsed in tap water prior to entering the Citric tank, but there is a dwell time over the rinse tank to allow draining. The rinses are cascading, and never reach a total TDS of greater than 50 ppm as NaCl equivalency, and I drink the tap water (not out of the rinse tanks of course) and it's really good. Not that this is a scientific test of Cl content, but just something to note.
On that entire line, we only ever get bioslime in the 1%Citric ambient temperature tank nearby. The rinse tanks in the Passivation system never show algae or that black mildew that appears in summer in a couple of our Anodize rinse tanks.
These are very small tanks of less than 100L with minimal plumbing and not a lot of volume for contaminants to hide. It's just baffling! I appreciate your troubleshooting pointers!
Rachel Mackintosh
- Greenfield, Vermont
October 24, 2020
So I wanted to post an update, and a thanks to the kind folks who have chimed in.
I did a little test! The tank is used quite infrequently, so I dumped it, cleaned it out, and refilled it with plain DI water.
And then I tagged it offline and let it sit for a week. Well wouldn't you know it ... just for ha-has, I pulled a sample off the "empty ready to remake" tank and toddled off to the lab with it, thinking I'd just run a quick free acid titration and see if I found anything weird.
Y'all. It tested AS IF IT WAS WITHIN SHOP PARAMETERS TO RUN. Yikes. Nothing has been added, the tech is as baffled as I am due to the plumbing not allowing for leaks except to Containment, and my next project is to send a 1:10, 1:100, and straight DI makeup "blank" out to my lab to see if they can resolve actual peaks on the HPLC, rather than just the interference whale (giant early rounded peak coming off the column soon after eluent solution is applied) they reported last time as "excessive chloride". As I recall from my previous life in a university lab that actually owned a few of those things that were my toil to run (and fix when the grad students broke them), I seem to remember that on certain columns, Bromine elutes early, barely off the chloride peak, and I'm starting to wonder if residue from our vapor degreaser has fouled the soap tank and dragout has dripped into the citric, 3 tanks down, on an 18 tank line ... everything passes over it. If it turns out to be fouled with HBr, I'm going to facepalm hard enough to leave a handprint.
I will update with results! For now, that tank is indefinitely going into "make from scratch to run a job, then dump" status.
Ciao!
Rachel Mackintosh
- Greenfield, Vermont
Update!
The lab was able to dilute my specimen down enough to resolve and quantify the peaks.
It's Nitrate... at 2.3% (!)
So anyway there's the culprit. Still doesn't explain why the ambient 1% Citric tank is unbothered, but I guess I can stop fretting about Chlorine and Bromine and focus on how to go about changing the ventilation in the enclosure to draw out fumes better. Or just keep making the tank fresh when the rare job comes in for it.
Rachel Mackintosh
- Greenfield, Vermont
November 17, 2020
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