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topic 6789

Chemical polishing of stainless steel tanks


(2000)

We are using stainless steel tanks to plate electroless nickel. The stainless steel has started to etch as seen by a continuous build up of iron in the bath. Is there a way to chemically polish the inside of these tanks without having them removed from the process line?

Jim Wyrough
Sanmina - Owego, New York


(2000)

Jim,

That's a rather weird question of yours (pardon me, I should have said 'strange' as the previous appellation sometimes upsets people).

If your stainless is starting to corrode, join the club!

Perhaps the only thing to do, and I can't swear that it will work, is to scrub it with nitric. That might bring back the sheen or some of it. After all, stainless is not supposed to be attacked by nitric.

But I have a question ... why in the heck did you use stainless steel when there are so many GOOD plastics and even some GOOD fabricators of them, too?

Cheers !

freeman newton portrait
Freeman Newton
White Rock, British Columbia, Canada

(It is our sad duty to
advise that Freeman passed away
April 21, 2012. R.I.P. old friend).



(2000)

Hello, Mr. Wyrough:

I assume you are using an anodic passivation system. You probably should contact the supplier of that system as there certainly could be a relationship between etching of the stainless and the operation of the anodic passivation system.

Freeman:

Electroless nickel can spontaneously "plate-out" onto plastic vessels. It is true that polypropylene tanks were very widely used for electroless nickel, but there has been movement in the direction of using stainless tanks with anodic passivation instead, because it purportedly reduces the likelihood of this spontaneous plate-out. If even a miniscule amount of nickel inadvertently plates out onto the walls or bottom of a polypropylene tank, that nickel acts as a catalyst for further deposition, and you have a chain reaction that spontaneously wastes the tanks contents.

Ted Mooney, finishing.com Teds signature
Ted Mooney, P.E. RET
finishing.com
Pine Beach, New Jersey


(2001)

An iron buildup in your EN is very probably more from the part rather than the tank. If the tank is pitted, it is nearly impossible to passivate and polishing it out will be nearly impossible. Brush electropolishing it would work, but it is not practical on large tanks. You can buy a new tank for what it would cost to set up to electropolish it via normal methods. If it is already pitted, I have some serious doubts as to how well it would work.

Several companies make thin poly liners for standard size tanks. They are not cheap, but are a lot cheaper than a new tank and eliminate nitric passivation and subsequent waste disposal, so are not a bad economic choice.

Last choice, consider putting in a fabricated 1/4" polypropylene liner using your tank for support. Remember to use a sloppy fit as poly expands roughly ten times more than most metals and 70 to 195 F is quite a large temp difference, especially since the SS tank will never get that high with a liner in it.

James Watts
- Navarre, Florida


(2001)

Have you tried Ammonium Citrate passivation at pH 3-3.1 10% W/v Citric acid solution adjusting the pH to 3-3.1 with ammonia water stronger using only deionized water for the solution. Heat the solution to 80 C and circulate in the tank for 6 hours then rinse well. Clean the tank first with hot solution of 5% W/V TSP [trisodium phosphate [linked by editor to product info at Amazon]] for 1 hour at 80 C rinse well.

Do you see any discolouration of the Stainless with this Ferrous/Ferric solution? You could also try a HF/HNO3 solution on a section of the tank to see if this most rigorous method of passivation will have the desired effect.

If this works I would then try the milder method mentioned above first.

John Mclaughlin
- RTP, North Carolina



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