Brazing Pickle

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Brazing Pickle

This topic has 21 replies, 11 voices, and was last updated 3 May 2010 at 19:59 by the artfull-codger.

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6 April 2010 at 15:06 #50454
Peter Gain
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@petergain89847
I have 500gm of citric acid in powder form, purchased from a wine brewing shop. What is the recommended mixture for use as a post brazing/silver soldering pickle?

Peter Gain.
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6 April 2010 at 15:06 #21690
Peter Gain
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@petergain89847
6 April 2010 at 17:55 #50456
mgj
Participant
@mgj
About so much. Its not critical.

Last time I used 2 of those small packets.

This time I used a bit more.

If its too weak it’ll take a bit longer, and if its quite strong it’ll take a bit less.

Its about that easy.
6 April 2010 at 19:27 #50461
Sub Mandrel
Participant
@submandrel
I have trouble finding citric acid, so I use W5 limescale remover, from either Lidl or Aldi. It is ready mixed,works wonders, is ‘irritant’ rather than ‘toxic’ and contains ‘corrosion inhibitors’ which suggests it’s less likely than most to create rust if splashed. Does smell a bit odd though.

Neil
6 April 2010 at 19:36 #50463
mgj
Participant
@mgj
Probably better than citric too. Possibly based on phosphoric acid.

I’ll try it.

Thankyou.
6 April 2010 at 22:02 #50470
Hugh Gilhespie
Participant
@hughgilhespie56163
Hi Peter,

The ‘standard’ recipe for citric acid pickling is a 3% solution at 90 – 95 C. This used to be enshrined in a CEGB standard called General Operating Memorandum 95 (or 75, I forget which now, not done it for a good few years).

For large scale work, e.g. cleaning power station boilers to remove magnetite scale, it is normal practice to add a another chemical called an inhibitor to reduce the rate of attack on the tube metal to below about 3g loss per square metre per hour. The inhibitors are commercial products and are not easily available. However, for home use, they are not really required as the contact time will be much less than in boiler cleaning, 6 – 8 hours being typical for a large boiler.

For post soldering pickling, I would recommend using about 4% citric acid (40 g of acid in one litre of water) at about 40 – 60 C. It is very slow at lower temperatures and becomes increasingly aggressive as the temperature rises.

Phosphoric acid is also relatively safe – from the perspective of the metal rather than the person doing the pickling. Phosphoric acid will slightly passivate steel and this can be an advantage – or a disadvantage as the final surface will be a silvery grey and the colour change can be quite marked. Something around a 5% phosphoric acidsolution at room temperature would work and the piece being cleaned can be safely left in for long periods, overnight if required, as the phosphate film formed on the steel surface stops the acid from continuing to disolve the metal.

One point – this only applies to steels and iron based alloys. Copper based alloys MAY be OK but be very careful with brasses as the acids may selectively dissolve the zinc – dezincification – and hence basically destroy the metal. Aluminium is generally OK but always try a test piece first – strange things can and do happen.

Hope this helps.

Regards

Hugh
6 April 2010 at 22:39 #50474
mgj
Participant
@mgj
Hugh – thats very useful. But you are talking of a process which is removing surface metal at a measurable hourly rate, and which if uninhibited, would work a lot faster.

I thought we were trying to remove a “scale” and leave the base material?

So, we are talking of a bit of lemon juice to remove something which is mostly water soluble anyway (particularly if you get enough heat into it and the flux always remains pasty )

Also I would doubt that many will heat their pickling tank (other than by dunking warm objects in it) and in most cases its going to be used at room temperature, which may vary between darned near freezing in winter and about 20c in summer. Even fewer would have a thermometer for such use.

Perhaps I haven’t been brave enough, but I like most others, just use a weak solution of citric at room temp, and pickle brass, bronze and steel as necessary, without any ill effects?

Perhaps the surprising thing is it works quite well?

At our level it is a very simple and uncritical business, and the wise might refrain from overcomplicating it?

Notwithstanding, I daresay that those with a sharp eye for detail could feel there is value in the effort?

Edited By mgj on 06/04/2010 22:40:11
7 April 2010 at 09:20 #50478
Gordon W
Participant
@gordonw
I’ve mentioned this before in a different thread, but hope it may be of interest. I wasstuck at home with no transport etc. and needed to clean a small silver soldered boiler, so I used what I could find, washing powder as used in w/ machines boiled up in pan. Worked very well. Next did a stainless silver soldered cylinder, just in hot water in sink and scrubbed, worked very well, also got browny points for cleaning sink. This stuff is caustic so careful with skin etc. and rinse well. Both these items are laying about in w/shop and show no sign of corrosion etc. after 2 months.
7 April 2010 at 09:39 #50479
KWIL
Participant
@kwil
Drain cleaner from the hardware shop is a acid based cleaner, suitably “watered down”, makes a good cleaner for copper at room temperature, check frequently, wear rubber gloves with gauntlets and old clothes. Wash off copper goods with copious water to removes all traces. As it is a drain cleaner, it also cleans drains!
7 April 2010 at 09:55 #50480
Hugh Gilhespie
Participant
@hughgilhespie56163
Thanks for the comments. I ought to point out that although I had many years experience in chemical cleaning of industrial plant, I have never silver soldered or made a brazed joint in my life – I have all that good fun stuff to look forward to – so I don’t have any real experience of removing fluxes and general cleaning up after hot work.

My points are pretty general. As you say, all we are trying to do is to remove a ‘scale’ and leave the base material unharmed. Unfortunately, all the common acids used for metal cleaning WILL attack the base metals, it’s just in their nature to do it! Slight qualifier here – copper, some grades of stainless steel and some fairly exotic high nickel content alloys such as R55 probably won’t be attacked but normal carbon steels, aluminium and zinc containing alloys will be chewed at by the acids. In fact even pure water will attack steel like an acid (i.e dissolve the metal and liberate hydrogen) if the temperature is high enough.

So, don’t underestimate that bit of lemon juice, it can, does and will happily dissolve steel. Just takes a long time to do it so the effects aren’t obvious. For what it’s worth the order of ‘aggressiveness’ on carbon steel is something like:

Hydrochloric, Sulphuric

Phosphoric acid

Citric acid

Hydrofluoric acid – NB Do not try this one at home!!

Then there are various descalers that work in alkaline conditions that are based on chelating agents such as EDTA. Very safe but need to be used very hot and are not easy to get hold of anyway. Caustic soda (Sodium hydroxide) is fairly good for general cleaning but nasty to handle.

In fact I agree very much with your points about keeping it simple. Stick with a weak solution of citric acid, use it cold if you want and just fish the bits out occasionally to see if they are clean. If you are in a hurry, warm the solution and if at all possible, stir it continuously, cleaning will be much, much faster.
7 April 2010 at 12:09 #50482
Ian S C
Participant
@iansc
I found an old aquarium pump some years ago and just recently tried it when pickling some parts in citric acid, I think it did speed up the prosess a little, have to try a few more times to see if there is a real effect, I supose a bit of warmth would help. Ian S C
7 April 2010 at 18:04 #50484
mgj
Participant
@mgj
Hugh – thanks for your understanding.

I do agree – in the same way as there is no such thing as a rigid beam, almost everything is eaten by something – the important question is how much in how long.

Its just that sometimes one gets into the business of counsels of perfection which often take the process out of the realms of the model engineer – either in terms of practicality, or risk, or time available.

The best being often the enemy of the good?

Talking of the good. Seen my first swallow of the year – just from the desk.!
7 April 2010 at 23:40 #50502
ChrisH
Participant
@chrish
From Hughs comment above:

“Hydrofluoric acid – NB Do not try this one at home!!” TOO RIGHT!

Hydrofluoric acid, if I remember correctly is a really nasty acid with the power to do great and very lasting damage to humans – avoid at all cost, just have absolutely NOTHING to do with it, don’t even think about using it, if you splash yourselves with it you will have the devil’s own job to wash it off, if you can at all.

Happy pickling!

Chris
8 April 2010 at 11:40 #50513
Doddy
Participant
@doddy
Hugh

oh thanks – just remember we’ve two “Outages” this year so guess the fun is double for the Acid wash !

So much easier at home than on a “real boiler”

Dave D

Assistant Shift Charge Engineer
8 April 2010 at 14:08 #50514
Ian S C
Participant
@iansc
There was a warning in ME about Hydrofluoric acid formed by over heating some type of O ring, and I think it mentioned the antidote that is required to neutralise the effect of the acid. Sorry a wee bit off course. Ian S C
8 April 2010 at 14:29 #50515
mgj
Participant
@mgj
Viton and gloves.
8 April 2010 at 16:20 #50518
Hugh Gilhespie
Participant
@hughgilhespie56163
David,

Two outages – lots of fun!! Don’t forget to ask your boiler cleaning contractor for a a kilo or two of citric acid, always comes in handy and they will have tons of the stuff – literally – so a small sample ‘for checking’ never hurts.

Regards

Hugh
8 April 2010 at 16:28 #50519
Hugh Gilhespie
Participant
@hughgilhespie56163
ChrisH, Ian and mgj,

Off topic slightly but you are right about the dangers of hydrofluoric acid. It is very toxic and above about 15% concentration it is absorbed through the skin – without much burning – and then it wreaks havoc with bones. The pure acid is actually a good solvent for proteins and as we are built of proteins, people and HF don’t really mix well.

The basic antidote is soluble calcium in a form that can get to the acid – it then forms calcium fluoride which is inert – a lot of Derbyshire is built on calcium fluoride or fluorspar rock . When we used it industrially we all carried tubes of calcium gluconate gel for immediate application to splashes plus we always had medical personnel on site with calcium gluconate injections.

Just thought I’d cheer you all up while your waiting for your brazed bits to clean up in the acid.
8 April 2010 at 18:54 #50524
Fowlers Fury
Participant
@fowlersfury
Don’t forget basic chemistry if using sulphuric acid in an attempt to remove limescale.

Whilst hard water is a very complex subject, our primary problem is calcium carbonate created by the boiling water. We need to convert this to a soluble calcium salt and using sulphuric acid will convert it to insoluble calcium sulphate. Furthermore, this will form on the surface of the scale and prevent the acid attacking the underlying carbonate.

If you want to use a strong mineral acid, then hydrochloric is the choice because it will convert the calcium carbonate to soluble calcium chloride:-

CaCO3 + 2HCl → CaCl2 + CO2 + H2O.

I don’t like citric acid for removing scale or pickling oxidised copper because the calcium citrate formed is only partially soluble in water and if left to dry, the particles of calcium citrate (and copper citrate) are irritants to the respiratory tract, eyes and skin.

One of the best articles on hard water and model engineering is in EIM vol 21, No.2 p 34 et seq August 1999 (well worth reading IMHO)
8 April 2010 at 20:59 #50526
mgj
Participant
@mgj
Yes I met the pleasures of hydrofluoric acid as the exhaust from fluorine lasers.

Fortunately the idea was dropped as not very practical – your own personal cloud of the stuff!

Still – fluorine lases well if you hit it hard enough!

I remember walking up near the fluorspar mine at Eyam. i think most of its production went into fluxes for steel smelting.
8 April 2010 at 21:51 #50536
Sub Mandrel
Participant
@submandrel
I think easyflow flux has flourides in it as well as borates.

The chemistry of flourine is not nasty, its downright terrifying. When they made the first atom bombs in WWII, they used uranium hexaflouride to get the radioactive material into a gaseous form they could enrich. The big challenge was to make unlubricated airtight seals, and the UF6 attacked all and everything. I think the solution they came up with was PTFE – which is usually given as a product of the space race, not the nuclear arms race!

Please correct me if I’m wrong!

Neil
3 May 2010 at 19:59 #51395
the artfull-codger
Participant
@theartfull-codger
I agree with Hugh regarding hydrofluoric acid penetrating the skin & through to the bone, we used to use gallons of the stuff in the glass trade for embossing / decorating glass as it dissolves glass but makes beutifull designs on glass, for myself I’m a little bit old fashioned & I still use dilute sulphuric acid on the boilers I / we make, usual precautions taken of course.
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