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Tool Talk Discussion Forum

Phosphoric acid as solder flux ?

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Charles in Aus.

11-11-2023 21:20:14




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Decades ago a friend was soldering stainless steel and used a clear , water like rust converter as a flux I was surprised at how well it worked. Now I find I'd like to solder a shower waste grate , a small job that needs two dabs of flux . He's long gone now and I don't think the place he went to has a phone service .

Would I be right in thinking the rust converter was a non latex based phosphoric acid ?
Any help welcome .

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Charles in Aus.

11-23-2023 01:08:12




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 Re: Phosphoric acid as solder flux ? in reply to Charles in Aus., 11-11-2023 21:20:14  
A follow up .
The advice to use a higher silver content solder was excellent . Flux was easy enough, in the end , a sterene wax paste made for the refrigerant solder I used . Wiping the area with Hydrochloric acid slaked with zinc chips was all the preparation needed .
Result was strong and acceptable .
I had thought stainless steel would be very difficult to solder , it isn't.



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Charles in Aus.

11-14-2023 00:10:59




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 Re: Phosphoric acid as solder flux ? in reply to Charles in Aus., 11-11-2023 21:20:14  
Thank you very much Dave , genuinely helpful advice .



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Dave G9N

11-13-2023 23:41:48




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 Re: Phosphoric acid as solder flux ? in reply to Charles in Aus., 11-11-2023 21:20:14  
Quoting Removed, click Modern View to seeYou probably should look for an inorganic acid flux such as Kester 817. This is a good example, but not the only one. I have no connection, but I am familiar with the company from aerospace applications Link
The SDS that I found for a phosphoric acid flux for stainless steel says 75% phosphoric, which is about as strong a phosphoric acid solution as you will find. Milkstone cleaner has 56%. It may be worth a shot, but you will have to boil off the water before it gets up to temperature. Could be messy. If you have to go out and buy a gallon for $20 you might consider $5-7 for a small bottle of stainless steel flux on ebay.
75% H3PO4 flux SDS:

Link

The other thing to mention is that the silver bearing tin solder is supposed to stick better. This is from reliable sources, Indium Corp for one. Lowes carries it, so it has to be pretty common.
Dilute muriatic acid is also supposed to work. I have not found how dilute, but I would try 5% on a piece of scrap. HCL strips chrome plating, so it should work as flux. I don't know what to expect other than boiling, spattering and a foul odor.
Citric acid is a weak acid and may not be strong enough to work on stainless. You really need an inorganic acid flux Link

Passivation>Link is the removal of iron and other contaminants from a stainless steel surface. It generally leaves a denser oxide layer at least when done with the old school nitric acid solutions. The chromium oxide that is so good for corrosion is not solderable. I know nitric acid passivation forms a heavier (10 nm) oxide than exposure to air.

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Charles in Aus.

11-12-2023 17:26:06




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 Re: Phosphoric acid as solder flux ? in reply to Charles in Aus., 11-11-2023 21:20:14  
Thanks Hemmjo and Showcrop , at least now I know how to proceed. It also seems that If I find any Milkstones in our shower I'll know what to do with them as well !



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DoubleO7

11-12-2023 07:20:55




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 Re: Phosphoric acid as solder flux ? in reply to Charles in Aus., 11-11-2023 21:20:14  
The presence of milkstone is means it can't and be sanitized.



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showcrop

11-12-2023 07:07:04




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 Re: Phosphoric acid as solder flux ? in reply to Charles in Aus., 11-11-2023 21:20:14  
Quoting Removed, click Modern View to see

Virtually all sanitation codes require cleaning to precede sanitizing



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showcrop

11-12-2023 05:30:02




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 Re: Phosphoric acid as solder flux ? in reply to Charles in Aus., 11-11-2023 21:20:14  
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Yes, geo. removing milkstone but not sanitizing.



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Geo-TH,In

11-12-2023 05:35:12




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 Re: Phosphoric acid as solder flux ? in reply to showcrop, 11-12-2023 05:30:02  
Why did the inspector want the milestone removed?



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Geo-TH,In

11-12-2023 05:15:12




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 Re: Phosphoric acid as solder flux ? in reply to Charles in Aus., 11-11-2023 21:20:14  
If I recall Phosphoric acid was used to sanitize SS milk cans. The milk inspector used a UV light to see if all bacteria was gone.



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Geo-TH,In

11-12-2023 06:38:36




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 Re: Phosphoric acid as solder flux ? in reply to Geo-TH,In, 11-12-2023 05:15:12  
If I recall Phosphoric acid was used to sanitize
SS milk cans.

A hard deposit of milk residues that accumulates on imperfectly cleansed dairy utensils, stainless steel, and serves as a substrate for bacteria and contributes off-flavors to milk.

I think we are splitting hairs if we say Phosphoric acid isn't used to sanitize, remove milk stone from SS.



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Hemmjo

11-12-2023 04:42:42




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 Re: Phosphoric acid as solder flux ? in reply to Charles in Aus., 11-11-2023 21:20:14  
Search for Phosphoric acid solder flux. You will find it mentioned often as an ingredient in various fluxes. Seldom more that 10% , so you get 90% something else.

Jewelers are mentioned often in the search results as they work with many types of metals. I would that just as important as the flux is the composition of the solder you use.



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showcrop

11-12-2023 04:33:52




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 Re: Phosphoric acid as solder flux ? in reply to Charles in Aus., 11-11-2023 21:20:14  
Basically you are cleaning contaminants from the stainless steel. You can look at a process called "passivation". They use nitric, citric, or phosphoric for that. The manufacturer that I work for has a phosphoric based product that breweries use for passivating new stainless steel equipment, so I am pretty sure that it would work. You certainly don't want a product with latex in it. You can get phosphoric acid packed as rust converter at any auto parts store, or you can get it as delimer at a foodservice store or as milkstone remover at any dairy supply.

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