Soldering with Stay Clean paste flux

I just soldered a locknut on my keg using Stay Clean paste flux. I didn't get it right first time, so I thought I would share my technique.

Stay Clean liquid flux is generally recommended instead of paste flux. The fluxes are chemically similar in that they both contain zinc chloride and ammonium chloride. The main difference is that the paste flux has a base of petrolatum instead of water. This makes it easy to apply but it also makes it easy to burn and the clean up is messy. My local Airgas only had the paste flux (Radnor brand). It was $3 for 4 oz.

My first attempt involved fluxing both nut and keg, and trying to apply solder (4% Ag) directly to the join. I applied heat to the inside of the nut using a propane torch with a strong flame. But the solder balled and soon I had over heated the joint. The flux turned black and I had to remove the nut, sand everything down, and start again.

Second time around I decided to pre-tin the nut. To avoid heating the nut directly, I placed it on a scrap sheet of aluminium and applied the flame to the sheet. It took a couple of goes but eventually I had a layer of solder on the surface of the nut. The crucial thing was to stop heating the moment the nut was hot enough to melt solder. The aluminium plate acted as a heat sink to keep the temperature high.

After sanding and cleaning I applied flux to the keg and put the locknut over the hole. I heated much more gently applying the flame only to the inside thread of the nut. As soon as I saw liquid solder emerge I removed the flame. I touched up with solder around the nut, heating only as much as necessary to keep the solder liquid.

Finally, I cleaned up the joint. There were solder blobs here and there and some unsightly balling. I let the metal cool slightly, then brushed a little more flux around the joint. I turned the torch to a high flame and feathered it around the joint, holding the torch at a low angle so that it melted the solder but didn't apply much heat to the surface of the keg. Then I flicked away excess solder with the flux brush to make a neat join. (Not with the brush that came inside the flux pot, a Harbor Freight cheapie.)

While flicking the solder away I managed to knock the nut sideways and had to nudge it back in place. It didn't matter because both surfaces were already tinned. But I did have to let it cool slightly and go round again with the flux brush and torch to neaten things up.

By the way I did not flatten the keg wall or radius the nut. Silver bearing solder has a bit of a window between the solidus and liquidus so if you keep the temperature low enough you can build the solder joint up to cover the poor fit.

Overall it was pretty tricky. In the past I have used phosphoric acid for flux and found it much easier to work with. It is much more tolerant of excess heat and easier to clean up because it is water soluble and the phosphate residues, unlike chlorides, are non-corrosive.

Quick photo with the phone while it was cooling down. I'm going to clean up with warm soapy water, then with alcohol, and lastly with sodium bicarbonate solution to neutralize any residual flux.

Awesome...thanks for the tips. Doing some practice runs of a similar project this weekend and this will be helpful.

Stay Clean Paste will NOT work on stainless. I verified this with harris directly. If it appears that the solder joint is good, it's likely superficial at best and really hasn't wet the stainless. I'd bang that thing with a hammer to check.

Oh well...back to the drawing board.

@Bobby_M I should shoot a video of me banging it with a cold chisel. If it falls off I'll show it to the Airgas rep who sold me the stuff. To be fair, I wouldn't have posted about this if I didn't think it had stuck. I have soldered stainless a few different ways and this looks like one of my better efforts.

I certainly wouldn't recommend this flux. The MSDS lists zinc chloride and ammonium chloride same as the liquid stuff. But the technical spec says it is "mostly" used for copper to copper/brass, which is not terribly encouraging.

Technical documents:
http://www.harrisproductsgroup.com/en/Technical-Documents.aspx

I ordered a General Hydroponics pH kit and will try the pH down (buffered phosphoric acid) as flux by way of comparison. I am hoping its miles better.

In the past I have also used neat muriatic acid as flux and if I was really desperate I would try that to tin the nut, but certainly not on the keg. It fumes and stains like crazy.

I just soldered a load of triclamp ferrules to copper pipe using the same flux. 3/4" copper pipe inside 1" ferrules is a tight friction fit, you can get the pipe in by freezing it. Pretinned the copper pipe, wiped it flat with a fluxed rag while still hot, cleaned it up and put it in the freezer. Had to pretin because the fit is too tight for solder to flow. Then fluxed and warmed the ferrule, put the pipe in, then heated the joint by applying the torch to the copper pipe rather than the stainless fitting. Then I went round the rims of the join applying a little extra solder just to make sure. 1 out of 8 failed when I washed the flux out so I redid it. I tested them just now by freezing the copper while heating the stainless and there were no problems.

P.S. banged the locknut, not tested to destruction but gave it a good solid tap with a hammer on each side and I am happy with the join. The 4% silver solder is a really nice colour match to the stainless when cleaned up. I'll try the 1" locknut for the element tonight & maybe put up some more pictures if I can avoid getting flux on the camera.

I'm glad it worked out for you but I didn't want people buying the paste flux based on only seeing this thread. I bought a huge tub of the stuff based on Harris' website info. After screwing around with it for a few tries, I decided it wasn't wetting at all like my previous small silver solder kit. I contacted Harris tech support and they told me the paste is not meant for stainless. Ultimately their website was wrong, they agreed and also sent me a 16oz bottle of the stay clean liquid which works a treat.

Fair enough Bobby, I am not querying your experience or your motives at all.

Here is the product I bought as advertised on the Airgas site. If it's the same product you tried perhaps their website needs updating too. "Superior flux for most metals, copper, brass, bronze, steel, stainless steel, galvanized, Monel."

http://www.airgas.com/browse/productDetail.aspx?Category=21&product=RAD64001786

In my hands it seems sufficiently active to solder stainless. I don't recommend it particularly because it is easy to overheat. It was tricky soldering the locknut. The flux worked absolutely fine to solder ferrules, really easy, and perhaps would for other tube-in-tube applications where the flux is shielded from heat.

I never asked if I was using my EPA licence to buy Stay Clean liquid flux, but this stuff is nasty, It will eat steel. It's not the same as other flux's. Don't breath it, touch it, and yes it works wonders. I know plumbers who don't clean fittings and just use it in a spray bottle.

Just soldered the 1" locknut much as before. This is how i tinned it. I sanded cleaned and fluxed the nut then heated indirectly on an Aluminium plate. When the flux started to sizzle I applied a little solder. With no capillary action the solder tended to ball so I flicked the solder around the surface with a flux brush until the nut was coated all over.

Probably an easier way to tin the nut would be to solder it to a flat piece of copper. The capillary action would wick the solder under the nut. Then just take the nut off while the still hot.

This time I had to hammer the wall of the keg flatter to get the larger lock nut to solder properly. The curvature of the keg made for large gaps otherwise.