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Stainless passivation dip tank and chemical?

NewGunPlumber

Aluminum
Joined
Dec 18, 2019
Hi All,

We are doing a fair amount of small stainless fabricated items. Anything from welded tube assemblies 2"X2" to sheet metal assemblies 2'x2'.

Majority of this is TIG welded with some MIG and has varying degrees of weld discolouration.

Currently using a TIG brush and it works but very labour intensive for batches of complex parts so looking at using some sort of dip tank process. The place I worked at 20yrs ago had one but I never asked what the chemical was and guess products and processes have improved over that time as well.

If any of you guys have experience with something like this I'd be interested to hear your process and chemicals your using for submerged passivation?
 
bigger the area the more amps you need to be able to clean it for the machine that runs it, then it gets stupid expensive that way. doesn't take much to get into the 100K territory for a power supply.
if it was me, id send it to a local shop that does it, or just stick to a portable machine that can be 10K with the brush and consumeables.
the dip is nothing special, just an acid.
 
commercial places use a phosphoric acid based product. I use a company that has a tank that is 4' x 4' x 8', with a 1000 amp 100volt power supply. They have to pay to have the acid hauled away as toxic waste when they clean the tank.
I have used citric acid for small parts- its much less nasty than phosphoric, and no biggie to get rid of. I use products from these guys.
I dont do any tank work, I use the commercial guys for that, but I use the citric for cleaning welds with a small power supply and a wand.
But if you call them up, they can tell you which one is good for a soak tank.
With the big power supply, the commercial shop still may leave something in the tank for 20 or 30 minutes. My work is usually forged stainless, so its black when I give it to them- it takes more like an hour at 1000 amps.
Small battery charger, or maybe dc welder would probably work. I think my portable unit is 40 volts.
https://citrisurf.com/
 
The tank we had was just a dip with no power source. It was a simple dip and then pressure wash and worked well but have no idea what chemical it was.
 
The tank we had was just a dip with no power source. It was a simple dip and then pressure wash and worked well but have no idea what chemical it was.
Acid. The kind that burns thru your skin til it hits bones. You can get phosphoric acid at most paint stores, even a version at home depot called cocrete etch. Its relativey safe, dont get it on yer skin. use a five gallon bucket or a plastic storage bin. Takes a lot longer without the power supply, but overnight often works.
 
Passivation can be done with nitric or citric acid. You are talking about weld scale removal or electropolishing, different animals.

Be especially careful with some of the products as they will contain hydroflouric acid.

The best power sources will vary the waveform in a very controlled manor to polish faster.
 
You're basically looking to do a pickling bath, which is some NASTY stuff. Without the aids of electricity or physical agitation, you're relying on the harshness of the chemicals to do the work. And believe me when I say you need some harsh chemicals to pickle off stainless oxides without any help from electrical current or scrubbing.

I've used a gel product from Bradford Derustit called Wondergel, and it works reasonably well, but still not as nice as an electro passivation machine like a Walter Surfox. The SDS sheet lists the active chemicals as 15-40% Calcium Nitrate, 10-30% Nitric Acid, 10-20% Ammonium Bifluoride and 1-4% Hydrofluoric Acid. Some serious stuff in that gel.

Before we got our Surfox, we used a home grown contraption that consisted of an old stick welder, some high temp ceramic wool on a copper wand and as high a concentration of phosphoric acid as the chemical supply house would sell us. And even then, we needed to use electricity to make it do much of anything. The raw acid would work, but it was not a fast process.

I worked for a few years at a large brewery, and we dealt with some dangerous acids and caustics, and none of them scared me quite as much as the pickling gel. I can't imagine having a dip tank of this stuff in my shop. The corrosion to exposed steel and tools would be enough to turn me off, nevermind the large amount of safety issues. This stuff is not good for your skin and bones. You'd need to implement a pretty involved program of how to deal with this stuff properly, assuming you like your organs and your employee's wellbeing. Can it be done safely? Sure, but it'll likely be more headache than just using a passivation machine.

Nobody likes running the passivation machine. It's messy, wet and boring. But it is so much safer and easier than just having a large tank of hot acid just waiting to become an issue.

YMMV, this advice was free and worth what you paid for it.
 
Citric acid is much nicer to work with than the harsher acids. A passive citric tank would be very easy. Weld discoloration might be tricky to remove passively though, so I'd probably send a sample to the acid company and have them run it.
 








 
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