Fluxes for silver soldering.

I've been using Tenacity 5 with SF55 to join both steel/steel and brass/brass joints. It seems to work, and I like it because when it it melts and flows I know that the parts are close to temperature.

However, planning an emergency dash to my local ME supplier tomorrow for supplies of SF55 I looked at their catalogue and noticed they they list Easy-Flo flux as suitable for SF55, Tenacity 4A without any qualification, and Tenacity 5 for joining stainless steel.

Question is - apart from the slightly higher cost of T5, is there a downside to using the stuff as 'one flux fits all' with SF55?

Robin.

Yes, Robin. I quote from Johnson Matthey's own blurb:

"The flux residues of this product are virtually insoluble in water. Immersion in a warm (>40 ̊C) 10% sodium hydroxide solution for 30 minutes followed by brushing in a stream of water is recommended. The residues are hard and will also respond well to mechanical removal methods such as grit blasting."

It's not called "Tenacity" for nothing.

The Upside is that T5 or HT5 will stay active longer so better for larger components that may take a while to get upto heat where standard flux will get exhausted.

I use Tenacity 5 for everything and, although JM's advice is sound, I find that the residues will often come away just by soaking in hot water and a wire brush or scraper.

If the heating has been prolonged then they can be more difficult to remove.

I always mix Ten 5 with a drop of washing up liquid as per an old Rolls-Royce Aero procedure, maybe that helps to soften used flux. I also use meths instead of water to mix the flux because when the water boils it often removes the flux right at the point when you need it the most.

Brian

I've always thought that Easy Flo flux was slightly more active than Tenacity, so, within it's temperature range, was more reliable. JM also say it is more fluid and sticks well to hot solder rods, hence perhaps a bit easier to use.

However, whatever works for you.

Used Borax as a flux some years ago (in an emergency). Seemed to work OK at the time, but I can't vouch for the integrity of the joint. Maybe I was just lucky.

Edited By Circlip on 09/10/2020 10:18:13

Devils Advocate here. Might we be paying "over the odds" for something with a fancy sounding name? Far from an expert, but just wondering.

Horses and courses again.

Borax (if I remember right) only works at red heat, using it in conjunction with low temp' silver solders doesn't work well. Good for gold soldering. OK on soldering Sterling silver, but bog standard yellow silver solder doesn't get hot enough to enter the range where borax is active.

EasyFlow Flux is suitable for low temperature (dull red heat) solders, easy to clean but exhausts quickly, and is intolerant of dirt. Tenacity solves some of these problems but is a devil to clean up. Tenacity will wet stainless steel (and some carbides), EasyFlow won't.

More complication comes when working with stuff for hall marking, and where the sequence of assembly means subsequent soldering operations, possibly at (slightly|) reducing temperatures. Easyflow won't survive being heated twice, Tenacity will.

For different grades of Tenacity, I'll defer to the maker's blurb.

Thanks. From what has been said it seems the only real downside to using T5 for everything is the difficulty of cleaning up afterwards. I have been using a caustic soda solution, which seems to do the job. So I shall stick with it.

Like Brian I use alcohol rather than water for mixing the flux, then let it flame off before applying serious heat.

Robin.

Edited By Robin Graham on 09/10/2020 23:12:03

Does anyone use killed spirits (zinc chloride) as a flux?

Killed Spirits is OK for soft solder jobs but not Silver soldering, Bakers Fluid is similar and often used particularly on sheet metal work.

I would like to try this - Is standard pink Methylated spirits OK for the job?

Regards

Dom

Not quite sure about pink meths, mine is blue/purple. This is fine as long as you don't keep reactivating dried flux as this can cause excessive contamination. I prefer to use a non-coloured material such as Isopropanol. This can be used to re-activate flux that has dried out in a container.

Brian

It's fine. The amount of dye in meths is tiny and makes no difference to the flux.

The advantage of using alcohol rather than water to mix the paste is it boils or burns off almost the instant the flame is applied, leaving the flux to do it's job. Water takes a tad longer because it has to be boiled off before the flux activates.

My brazing experience is very limited, but my learner gut feel suggests alcoholic paste is good for smaller jobs, say up to 100g of metal, but water is just as effective above that. Getting enough heat quickly on to the job with a suitably big torch and/or insulation is probably more important than the paste fluid. Slowly cooking a joint with a small torch deactivates the flux before the silver solder is hot enough to run. The results are miserable.

Dave

Thanks Dave - will give that a try next time. It was the smaller pieces I had in mind - my last soldering job was a tiny part and it was quite an amusing exercise trying to keep the bit of solder from wandering off when the water boiled. It was almost like it as deliberately trying to annoy me!

Dom

It's not the time it takes for the water to boil off it is the act of boiling itself that makes the flux bubble and spread to where you don't want it, this does not tend to happen when the liquid is burnt off.