Soft-solder Flux: Shelf-Life?

[Nigel Graham 2Participant @nigelgraham2]

Twice recently I have had unexpected difficulty soft-soldering small brass and copper components.

I was using plumber’s paste flux, and flux-cored solder, had cleaned the metal, etc. Plenty of heat, from a gas blow-lamp. Small parts so not a cooling problem; similar metals.

The electrical-grade solder seemed reluctant to melt, and just did not want to wet the metal properly, making hard a task I should have found simple and very satisfactory.

Is it possible the flux, either the paste in the tin (which is old but I don’t know how old) or inside the solder, has gone off with age?

Or that the solder itself has become too oxidised to work? (It is a bit grey.)

 

[Phil SParticipant @phils66830]

My experience with the white paste plumbers flux is that it does go off with little visible change after five to ten years. Might depend on how often you open the tub. The cored electrical tin lead solder I have from decades ago is still fine. The flux in surface mount solder paste has a very short life.

[John PurdyParticipant @johnpurdy78347]

I have some “Kester Soldering Paste” in a small square blue tin and the remains of a 7 lb. spool of Ersin 60/40 1/16″ solder that were bought, probably circa 1959. I use both regularly and have had no problems.  One thing I have found is that wire solder that is dark gray on the outside from oxidization doesn’t seem to flow as well as one that is still shiny.

John

[MikeKParticipant @mikek40713]

My plumbers flux is just fine after at least ten years.  It’s grey-ish and comes in a round tin by Oatey.  But I’ve only ever used it with lead-free plumbers solder.

[Michael GilliganParticipant @michaelgilligan61133]

No way of knowing for sure … but I suspect Nigel’s difficulty is due to use of Rosin-cored solder, which is not sufficiently aggressive to fully clean the oxidation off the wire.

Comparative tests using as-is and cleaned samples of the same solder would be interesting.

MichaelG.

[Speedy Builder5Participant @speedybuilder5]

John Purdy -1959 solder – “Lead Free”  I guess not.

I do find that the lead free stuff does not wet so easily as the old stuff (Bit like cadmium free silver solder). The plumbers flux you talk about is normaly quite corrosive which will even clean dirty copper surfaces and I guess that it could “Go off”.

Fluxes like the FLUXITE product seem to have eternal life and many of us have pots that were made before “shelf life” was invented.

However, there are some bronze alloys like Aluminum Bronze that are difficult to solder without a special flux.  If using solder with an oxidised surface, then the oxide has to be dispensed with by the flux perhaps lessening the effectiveness of making the actual joint.

[JasonBModerator @jasonb]

is there any chance that the “plenty of heat” on small items is actually too much and exhausting teh flux before it has a chance to work.

I prefer paste either old brown Fluxite or the white modern plumbers type as you can put that on before heating and as it bubbles and burn off indicated when the temp is about right. Heating a part and then applying flux cored solder gives the surface a chance to oxidise before any flux gets near.

depending on the job it is someimes better to tin each surfac efirst then sweat them together.

[Bo’sunParticipant @bosun58570]

A couple of mentions here about oxidised solder.  I find it helps if the solder to be used is cleaned with a little 0000 grade wire wool so it’s shiny again.

[bernard towersParticipant @bernardtowers37738]

Im using some time exp USAF flux out of date by 15 years and still ok and yes cleaning the solder works for me too.

[Nigel Graham 2]

On 4 June 2024 at 21:47 Nigel Graham 2 Said:

Twice recently I have had unexpected difficulty soft-soldering small brass and copper components.

I was using plumber’s paste flux, and flux-cored solder.

The electrical-grade solder seemed reluctant to melt, and just did not want to wet the metal properly, making hard a task I should have found simple and very satisfactory.

Many different plumbers fluxes and they are all incompatible with electrical fluxes.   Never mix different fluxes!

Is it possible the flux, either the paste in the tin (which is old but I don’t know how old) or inside the solder, has gone off with age?

Or that the solder itself has become too oxidised to work? (It is a bit grey.)

 

Yes,  plumbers flux, can go off, and yes dirty solder gives flux more work to do and increases the chance of failure.    Shouldn’t be a surprise that a flux that’s lost some of it’s power is more likely to fail than one in tip-top condition.     Electrical flux can also ‘go off’.  How long a flux takes to ‘go off’ depends on the particular chemicals in the flux and how it is stored.   Individual reports of long life are pretty much hearsay, not worth much unless they identify the specific product, the condition of the packaging and the nature of the temperature and humidity cycles it was stored in.

From a chemist’s point of view designing a flux is complex.   He has to find a chemical, or mix of chemicals, that melt, clean, and seal out air when heated, and then dissipate without leaving too much nasty residue behind.  Any residue should be easy to clean, ideally water soluble, and non-toxic.

Cost matters.  Cheap is OK for plumbing because it can tolerate a lot of dirt, electrics can’t.  Soldering electronics with plumbers flux is a disaster, because the acid residue attacks every delicate metal surface within range – several feet!   Soldering electronics requires a much higher standard of cleanliness because the clean resin based fluxes used are weak compared with the cheap rough, tough chemicals used by plumbers.  Even though it’s reassuringly expensive electrical solder high-risk for plumbing jobs.  In both cases the flux is wrong for the job.

The chemist also has to consider the metals being soldered.    Fluxes such as Zinc Chloride, Sodium Phosphate and Ammonium Chloride work well on Copper, but are a poor choice for Brass, for which Sodium Borate is more suitable.  Iron, Nickel, Steel, Bronze, Aluminium and Zinc are another story.   Doesn’t help that another chemist alters the ratio of Tin and Lead in soft-solder to suit particular types of work, and the flux has to be formulated to deal with that too.

Old-time fluxes such as Sodium Carbonate, Potassium Carbonate, Calcium Oxide and Charcoal aren’t ideal for various reasons.  Many others are off the list because they are costly, toxic, have poor shelf-life or some other disadvantage.  Killed Spirits / Bakers Fluid was the first ‘easy to use flux’;  compared with earlier methods it’s semi-fool-proof!  Killed Spirits is an easily made acid solution of Zinc Chloride, long-lived, but it tends to leave muck inside the joint.  Being very corrosive makes the spilled liquid a pesky nuisance, so the chemist developed pastes, which are also easier for the workman to apply.   The chemical used to hold the paste together this might degrade over time, making the flux action patchy.

In the workshop, customers like to keep it simple by buying a brand-name flux that works on almost everything, possibly not aware that the contents must be a compromise.  Fortunately the chemist can get quite close to ideal by blending different flux chemicals so that – within limits – one or other will do the job when heat is applied.  Some of the chemicals are more stable than others, but note that all fluxes melt at below soldering temperatures, and can’t be totally stable over time.  They may also absorb water, leading to poor performance or separation of the mix.   Discerning customers study the catalogue, looking for a flux well suited to the job.  Using a suitable new flux for a Lead Free soldering job is a better bet than relying on a rusty tin of government surplus flux picked up cheap in 1952 and race-tuned for lead solder only!

By all means try ancient flux on a low-risk job as Nigel has done, hurrah if it works, because that saves a few bob.  Don’t rely on it though!  Unwise I think for a beginner to attempt to solder a complex boiler using unknown materials because the cost of failure is  unpleasant!

I think Nigel’s main problem is mixing plumbers flux, which might be ‘off’,  with electrical solder, which might also be ‘off’.   So, either approach this as an electrical soldering job without plumber’s flux, or – probably more practical – use plumber’s flux with plumber’s solder.  And clean all the metal before starting.

By the by, my other hobby provides the gear for electronic soldering and I’ve attempted to apply it to mechanical engineering with low   success.  60/40 and high Tin solders both struggle.   Not sure why.  I guess my irons don’t put enough heat into slightly larger joints to melt the solder before the rosin flux burns off.  The biggest is a 140W Weller Solder Gun.  Conversely my blowlamp flames are too high temperature and burn the rosin off too quickly. The characteristics of electrical solder and irons that make them good for electronics, don’t good seem suitable for general purpose work.

Dave

 

[SillyOldDuffer]

On SillyOldDuffer Said:

Many different plumbers fluxes and they are all incompatible with electrical fluxes.   Never mix different fluxes!

I don’t seem to get that problem, often I will coat the surface with plumbers flux and then use cored electrical solder on that as the smaller diameter is more useful.

[Bo’sunParticipant @bosun58570]

Recently did the same as Jason above, soldering small 20swg mild steel sheets and it went OK.  Joint didn’t need to be gas or liquid tight, and seems to be fine.

[peak4Participant @peak4]

If the electrical grade solder was reluctant to melt, as opposing to forming molten droplets which don’t flow over the substrate, is it a modern lead free solder?

Bill

[Dave HalfordParticipant @davehalford22513]

Use a soldering iron on small components, if you don’t have one a random chunk of copper held in mole grips and heated in your blowlamp will do. Tin both pieces with the makeshift iron then warm them. Small parts can be heated and married together with an old fag lighter.

My old rosin core lead/tin solder is 30 years old and still works, you don’t need any other flux.

it’s quite legal to still use this for non plumbing work, lead tin will not go over modern lead free.

[john fletcher 1Participant @johnfletcher1]

I’ve never had any fluxes GOING OFF or heard of going off and have several type. I have the remains of a tin of Fluxite which must be around 60 plus years old which I used some of last week all OK. Often it just to much heat or mounting the object on some thing which is leaching the heat away, cooling. The only flux which sort of going off is flux for Silver Soldering, but its easy to jog it back to life with a drop of water. After reading your post a second time I think to much heat, I suggest you have another try. John

[Nigel Graham 2Participant @nigelgraham2]

Thank you all for a lot of useful advice!

Errr….

Ummmm….

Ever had One Of Those Moments…?

I looked carefully at what I’d been using, to prepare for this reply.

Flux:  good old Fluxite, unknown age, about 1/3 used of a tin labelled ” 450g / 15.87g” , (if ’twere jam it’d say ” ~ / 1lb” ).

Solder: multicore, two reel. One has long since lost its labels, the other retains them, but they are very grubby.

A short test with a piece of brass sheet, a touch of Fluxite and large electric soldering-iron proved the anonymous one is lead-tin solder.

The other? Well, now…. That was the one I’d been using.

My sister likes to write witticisms on Christmas present labels, and insists we ” Read the Label!”  She should have been with me yesterday when I was trying to join those two small, simple pieces of freshly machined and degreased brass.

Under the grime it reads, “R.S. Multicore Solder for Aluminium” .

 

 

[Michael GilliganParticipant @michaelgilligan61133]

Well-found, Nigel

… useful stuff, when you actually need it.

MichaelG.

[Nigel Graham 2Participant @nigelgraham2]

Thank-you!

The wrong solder worked well enough for the two small pieces of newly-machined brass, but would be wrong for anything critical. The assembly, an injector water-valve’s control-rod guide, will not carry any significant load. Failure would be a nuisance but the control would still work due to other supports.

The union-nipple may be more problematical, being in the injector’s water supply, where a poor joint is a potential air leak under suction.

 

However, this all does show how easy it is to go diving after the complicated answer because the easy real one seems too easy, or is not at all obvious.

As here, where the solder looked like any tin-lead solder and its reel labels are all grimy.

…

Incidentally, I was thinking how to stop the control-rod from jumping up off the plug-cock handle it engages. Preparing to make a collar with grub-screw I realised an ordinary wiring-grommet or slice of polythene tube fitting the rod tightly, just below the lower guide, is fine: it will be invisible on the finished work!

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