Silver soldering question

[Chris GillParticipant @chrisgill22114]

I encountered a bit of a puzzle when silver soldering recently and I wondered if anyone could advise.

I was trying to solder brass valve chests onto steel cylinders (EN1 or EN3, not Pb) for a model I was tinkering with. I cleaned everything, applied flux and heated inside the cylinder primarily.

I picked the finished item in dilute citric acid for a short time. All the cylinders had a black coating on the inside and I assumed it was just a little oxidation but wet-and-dry paper wouldn't touch it.

Then a little bit flaked off and I discovered it was a thick, glass-like glaze. After a few experiments I found I could turn the glaze off using carbide tools but HSS didn't seem happy.

Any idea what the glaze might have been?

In terms of the model, there were several other "lessons learned" so it runs off a small motor rather than air or steam!

[Chris GillParticipant @chrisgill22114]

Soldering steel to brass

[David George 1Participant @davidgeorge1]

Hi the glass like coating is flux left over. I use an acid descaler and use brick cleaner.

David

[JasonBModerator @jasonb]

Also possible it was an oxide like you get on "black bar" if the heating was prolonged and is quite hard to remove manually. This will flake off in brick cleaning acid.

[rachel tibbrianiParticipant @racheltibbriani25638]

Posted by Chris Gill on 16/02/2023 21:39:56:

I encountered a bit of a puzzle when silver soldering recently and I wondered if anyone could advise.

I was trying to solder brass valve chests onto steel cylinders (EN1 or EN3, not Pb) for a model I was tinkering with. I cleaned everything, applied flux and heated inside the cylinder primarily.

I picked the finished item in dilute citric acid for a short time. All the cylinders had a black coating on the inside and I assumed it was just a little oxidation but wet-and-dry paper wouldn't touch it.

Then a little bit flaked off and I discovered it was a thick, glass-like glaze. After a few experiments I found I could turn the glaze off using carbide tools but HSS didn't seem happy.

Any idea what the glaze might have been?

In terms of the model, there were several other "lessons learned" so it runs off a small motor rather than air or steam!

if the flux is not thoroughly cleaned off, it can react with the citric acid and form a glass-like glaze on the surface of the metal. This residue can be difficult to remove, especially if it has been allowed to sit for a long period of time.

To prevent this issue in the future, it's important to make sure that all flux is thoroughly cleaned off after the soldering process is complete. You can try using a wire brush or sandpaper to remove the residue, or you may need to use a more aggressive cleaning agent to dissolve the glaze.

It's also possible that the glaze could be a byproduct of the citric acid reacting with the brass or steel. However, this seems less likely given that you were able to remove the glaze with carbide tools.

[Simon Johnson 2Participant @simonjohnson2]

I suspect your problem was "dilute citric acid for a short time".

Next time, try a stronger mix for 20 mins or even better a commercial pickling product such as 'Safety Pickle' -its about a tenner/kilo from jewellery tool suppliers and should be used warm

[noel shelleyParticipant @noelshelley55608]

If your near me I can give you some pickle – free. N Norfolk. Noel.

[Chris GillParticipant @chrisgill22114]

Thanks all

I never thought of a reaction between citric acid and flux but I have some brick acid which I can try next time.

Noel – many thanks for the offer but I'm in Leeds.

Simon – I had heard of sodium bisulphate pickles but I just happened to have citric acid in stock. I couldn't find it at quite that price although I did find a supplier offering 25kg sacks. I think I'll start with something smaller

[shaun meakin 1Participant @shaunmeakin1]

Hi Chris,

it sounds like you have overheated the flux. A flux is like a sponge absorbing oxide as well as creating a barrier to further oxides forming. After a while, like a sponge, it can become 'saturated'. Plus if you get above the working temperature of the flux it will 'bake' on leaving a glassy hard residue difficult to remove. This is especially true if the joint takes a long time to reach brazing temperature. In future please consider a flux with a longer working life, for example our HT5.

Hope that is of interest.

Shaun.

[Chris GillParticipant @chrisgill22114]

Shaun – that sounds like good advice. I've added HT5 to my shopping list. If you're at the Harrogate show I'll call in

Chris

[bernard towersParticipant @bernardtowers37738]

Really surprised that you failed with your soldering as EF flux should have coped with that and I remove that flux in boiling water. If you use HT5 flux CUP Alloys recommend a 10% caustic soda solution

[Tim StevensParticipant @timstevens64731]

The silver-solder fluxes I am familiar with are all based on boric acid. Some of the patented recipes include salts which enable the glass-like residue to dissolve quickly in very hot water. Otherwise the borate glass will dissolve slowly in dilute suphuric acid (used on sterling silver, but not so clever for steel).

Even if you don't get it all off, it will (in my experience) weather away and rub off – looking like whitish mould in the process.

Another way to remove glassy deposits which have melted to a metal surface (including real enamels) is to heat the object to a dark red, and sprinkle saltpeter* on the glassy surface. This results in a more liquid glaze which includes nitrates and so (when cooled) dissolves in water.
* Potassium Nitrate – use a bit more than the same volume of it as the deposit to be removed.
[Or if you are making your own fireworks, ask for Nitre – it's the same stuff but an even older name. If you use the scientific name the next question will be 'What do you want it for, sonny?']

Cheers, Tim

[JasonBModerator @jasonb]

Without knowing the size of the steel cylinders and what Chris has in the way of heating equipment it is impossible to say if standard "easyflo" type flux would have worked as it may have taken a long time to get upto temp or got overheated in which case as Shaun says the flux would have been exhausted but HT5 will stay active for a lot longer.

Chris it would be interesting to know if you covered the whole inside of the cylinder with flux, or just where the joints were and whether the hard layer was all over the fluxed surface or on bare metal too.

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