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topic 61104

Baffling Citric Acid passivation bath issue

May 11, 2020

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 [affil. link to spec at Techstreet] Citric 1 and AMS2700 [affil. link to spec at Techstreet] Method 2:
140-160 °F
60-100 g/L Citric Acid (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
Plating Solutions Control Specialist / Industrial Metals Waste Treatment - Brattleboro, Vermont

May 12, 2020

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...

William Hogan
amphenol aerospace - sidney, New York, USA

May 12, 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
Plating Solutions Control Specialist / Industrial Metals Waste Treatment - Brattleboro, Vermont

June 9, 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

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