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Post cleaning flux residue - silver brazing
petertha:
Thus far I've been using Harris brand Stay-Silv white paste flux for silver brazing. Still novice level but I'm getting satisfactory joints. My issue is the cleanup stage. It takes be significantly longer to get the part looking nice again. Most of the heat affected area becomes bright with scuff pads but the flux residue is kind of a glassy material that surrounds the joint area. It is much more stubborn & seems like it can only be removed by scraping or chipping away with magnifiers on.
My next project is a little gear pump where the steel shaft & brass gear will be brazed together. The flux or solder won't flow into the gear area but the thought of cleaning the overall discoloration & expectant flux residue around the axle joint & gear I'm not looking forward to. When I see examples here like boiler work & similar, they look so clean by comparison. Is there a bath or soak process I should be aware of? Does it loosen flux & heat stain or completely remove it? I see these (link) 'salts' but its not clear to me if this for PRE brazing cleanliness or a POST cleanup, or maybe both depending on concentration. I tried some citric acid powder I had left over from use cleaning minerals like kettles & such. It didn't do much good. Or maybe this is more related to the Harris flux & i should be looking to something else?
https://www.harrisproductsgroup.com/en/Products/hpg-na-staysilvwhite
FLUX REMOVAL: Post braze flux residue should be removed to avoid potential corrosion. A hot water wash and brushing with a rag or non-woven abrasive pad, (e.g. Scotch-Brite™), is often sufficient. Additional measures include mechanical cleaning with a wire brush, steam jet, or abrasive blasting media such as grit, soda, or dry ice.
Production post-braze cleaning can also be accomplished with proprietary chemical formulas such as Bernite® 45 available from Harris Products Group. This approach is suitable where parts lend themselves to a water bath/tank immersion.
https://cupalloys.co.uk/product/cleaning-salts-100g/
petertha:
Hmmm... I have been adding (very small) quantities of water to make a thick slurry as my flux is now a bit crumbly. Not hard but drier. I thought adding water that was OK. I also thought I was being gentle with the torch. the flux bubbles & shortly thereafter the silver solder (usually already sitting in position on the joint) begins to flow. I have also done the dip into water within a short time span after soldering to try & thermally shock the flux. No real improvement. This link hints at stubborn stains as being charred, but they do say it is glassy.
https://www.harrisproductsgroup.com/en/Resources/Knowledge-Center/Tech-Tips/Tech-Tips-OLD/Five-misconceptions-about-flux
Because the active temperature for the flux is the same as the active temperature for the alloy, you can actually tell when your torch temperature is ideal for brazing based on how the flux is behaving. When it begins to go transparent, you know you are up to temperature. If it doesn’t go clear, you know you need more heat.
Misconception #3: Cleaning the flux off is a pain.
Cleaning off the flux residue is an important step in the brazing process. When flux is used properly, it can be removed with a rag or brush and some water. If you don’t clean the parts, flux can become corrosive over time. It may also affect the performance of the part over time.
However, if you overheat during the brazing process, flux can char or burn on the part. To remove this, you may need a wire brush or even grinding or other mechanical means. Many brazing applications in a manufacturing environment will benefit from a consistent flame height and temperature. This will prevent overheating of the part and overheating of the flux.
Misconception #4: I can get my flux to last longer by diluting it with water.
Fluxes lose their effectiveness when diluted with water. Diluting flux can result in a significant impact to the quality of your braze, including:
· Burning or charring of the metals. If there aren’t enough chemicals in the applied flux, it won’t be able to absorb any more oxides. Those oxides then burn and char, affecting the metal. To remove this burning and charring, you may need additional steps to clean the flux – including grinding or other mechanical means. This is often more costly than simply using the flux at its intended strength.
· Preventing the alloy from flowing into the joint or space. Without the presence of flux, because it is seriously diluted or unevenly mixed with water, the consumable alloy won’t ball-up as it should and flow into the joint. The integrity of the joint is compromised by the lack of flux. Even if you get it to braze, you’ll likely have to rework or re-braze the joint.
crueby:
Yeah, I have read the easy clean off of the flux too, never had it that easy! Maybe on other metals that they test with! I always soak brass or bronze parts in Sparex 2, which comes dry and you mix up a gallon. Store it in a plastic screw top cannister. I put the parts on a copper wire to hang in the solution and get out easily. Then rinse and wire brush. Sometimes needs a second bath. I leave parts for half to a full hour. Works on steel too, but dont use same solution as the brass, or it will transfer color. Another option is to soak in kitchen type white vinegar, 5%. Takes twice as long but is safe to handle and dispose of, and is very cheap.
There are other acids some people use, very quick but can be very nasty to handle or breath around. The sparex is a mild acid so always rinse well, hanging on the wire keeps it away from your hands till its rinsed. Careful not to splash back at you! Either solution will soften and remove the glassy flux and any soot.
Thats how I do it, anyway. Sure there are other methods.
Kim:
I'm with Chris here. I have NEVER found flux to easily wipe off. I don't believe that statement at all.
I use a Citric Acid solution to soak my parts in after silver soldering. I used to use Sparex (like Chris suggested) but when I wanted to make a large enough tub of solution to fit my whole boiler in, Sparex started to look pretty expensive. And I know that Citric Acid is very gentle. After all, you eat it, right? (Guess you could say the same for vinegar, which I've also used.) I got a 5 lb bag of citric acid off Amazon for around $20 and used about half that to make a large vat of pickle solution. I know you're supposed to not mix brass, steel, and copper in the same tub, but I do. Yeah, it makes steel come out a little pink. But I just buff that off. It's never bothered me much.
And yes, I see that same thing where the flux turns to a glass-like substance around the joints. But leaving it in the pickle bath for 30-60 min gets rid of it. And after a little buffing, I'm all set. If I were more careful about not mixing metals in my pickle bath, I probably wouldn't have to do the buffing step. But it works for me. If I take it out of the pickle bath too soon, I will have to scrape some of the flux away. That's kinda how I know I need to put it back in for longer.
I do think the citric acid is a little slower than the Sparex. Maybe if I made my citric acid solution stronger, it would work faster. But I put as much of the citric acid powder in the solution as would dissolve. Once I ended up with stuff parcipitating to the bottom of the container, I decided to stop adding powder. I keep it in my garage, which isn't heated. It doesn't get too cold usually, but it gets down to 50F consistently in the winter. I'd have to find a way to heat the solution to get a stronger solution, I think.
So make sure to soak your part in a pickle solution for some time after you solder your parts.
Some (many?) people soak their parts before soldering too, as a method of cleaning. I generally buff the parts before soldering - use a mechanical method to get any oil, dirt, and oxidation off the parts. Seems to work fine for me. Or rubbing them down with Acetone. I do that too. But I don't tend to pickle the parts before soldering. Just my method.
Kim
Jasonb:
Citric acid is reasonably safe and will remove the residue but can be slow particularly if usinng teh higher temp fluxes.
I use a stronger acid that not only removes any flux but also any black scale that may form on steel parts. Sold here as brick cleaner it is a dilute hydrochloric acid. Use outside and rinse well.
Couple of before and after shots. Any slight pink/copper colour is due to the acid getting a bit old but that does come of easily. All where HT flux and dipped for about 30mins
Brass & bronze
Steel
For things like fabricated castings then a sandblast after the acid also works well but keep it off any shafts or finished holes.
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