Pickling brass after silver soldering.

Jason,

Thanks for that, that finish looks OK for my purpose, just to get some brick cleaner now! Not sure I want to spend half my day stood in B&Q car park!

Paul.

Steve

Most likely hydrofluoric acid. As far as I know ( which is probably not much ) it was only ever used for passivating stainless steel, mostly after welding. It is, or was, also used for etching glass.

It was very effective but is very nasty stuff. I still have some, probably enough to see me through my lifetime, but I have also had training and have used a lot of particularly nasty chemicals so I am fairly comfortable, but careful, in using it.

I don't know the concentration when used as a pickle, but did go to university with a geological laboratory specialist who died after spilling concentrated acid on himself when a vessel broke. That knowledge reminds me to be extra careful in using it.

Pero

Hi, this Brick Cleaner works well for removing the mill scale from steel, so brick cleaners won't work very well though. I don't know if it will clean off flux residue though. I Pero's post, he mentions hydrofluoric acid, it is definitely nasty stuff and you should not use it without realising its dangers, if you get it on your skin, you are not likely to notice it straight away, but it will burn through and attack your bone and that's something you do not want to happen. The vapour from it can also lead to serious lung problems.

Regards Nick.

I am no Chemist, just A level many many moons ago.

I have been looking at the Sodium Bisulphate (Sodium Hydrogen Sulphate) Hydrogen Peroxide mix and what effect it has on brass and scale.

After much searching I found this paper

It looks at the dezincification of brass using various concentrations and temperature. However they do acidify the solution with sulphuric acid.

On page 62, Table 2 shows the effect of temperature on the dezincification factor of brass treated for 6 h in acidified sodium sulfate solution (1 M Na2SO4 and 0.1 M H2SO4) in the presence of 0.5% H2O2.

It shows that if the solution is between 60C and 80C the rate coper and zinc are dissolved are equal. Perhaps this could be the answer to not getting the pink skin.

I have not found any information on the relative rates of reaction between zinc/copper oxides, zinc/copper and sodium bisulphate. So I cant say if the oxides or brass will react faster.

Adrian

Hi Adrian, there maybe some truth in what you are saying. When I silver soldered this Ali-Bronze block to a Brass plate at work back in the early 80's, I dunked them (I made two of them) in the companies 70% sulphuric acid storage tank for a little while and when they came out, they were all nice and shiny brass and bronze, the silver solder was nice a clean also and no hint of any pink, however the acid was at best only 18 C. Of course I dunked them into a bucket of clean water before I handled them and had my PPE on.

Regards Nick.

Edited By Nicholas Farr on 25/04/2020 10:30:59

Cracking find Adrian!

But I read it as a study into enhanced de-zincification, that is it shows adding Oxygen in the form of Hydrogen Peroxide, increases the rate and penetration of de-zincification. In other words, adding Hydrogen Peroxide makes an acid pickle even more unsuitable for use on Brass.

I guessed Hydrogen Peroxide was a bleach, but from this paper it seems adding Hydrogen Peroxide to an acid pickle makes it stronger, so well worth trying Peroxide on Steel or Copper. But definitely not on anything with Zinc in it, like Brass.

Nicolas mentions success with 70% Sulphuric Acid. Yes, because Zinc, and other metals, don't react with Concentrated Sulphuric Acid. Water is needed to release the ions necessary to the reaction. Counter-intuitively, although concentrated Sulphuric Acid can be stored in a steel drum, dilute Sulphuric Acid attacks it.

What's needed in a pickle is a mixture that removes oxides and other crud whilst leaving the underlying metal well alone. A tricky balance.

Dave

There is a patent here for a chemical polish for brass. Interesting that it uses the aforementioned Hydrogen Peroxide.

Stay well.

Rod

It may help to know that Hydrogen Peroxide is an oxidising agent. When used as a bleach it is the active oxygen that attacks the stain and the colour. It may well be that this extra oxygen changes the balance so that the more reactive (and more easily attacked) zinc is not attacked in preference to the copper.

A factor which might be helpful to some is that if you leave the pink surface it will darken progressively and take on a patina which can be pleasing. But no good if your main object is to dazzle by the super shiny appearance of polished brass.

It seems, too that most acid solutions will help to remove borax-based fluxes, and will also affect the sufaces of brass, bronze, copper, etc in rather different ways. Experiments with acetic, oxalic, formic, etc on a sample of the actual brass you are using could be helpful.

And in any acid dip, avoid the use of steel tools or welding wire to dip the parts. The iron will amplify the copper-plate effect - just the opposite of what is sought here.

Cheers, Tim

Edited By Tim Stevens on 25/04/2020 17:49:43

Dave

In the paper I too thought it was saying adding Hydrogen Peroxide made dezincification worse. Then I spotted the table about temperature. It shows that Zinc and Copper loss are the same between 60C and 80C. So if you heat the solution up to 60C-80C it will etch real fast, but the rate of Copper increases faster than Zinc with temperature.

I have asked my son ( A Chemist) so sort out what recipe I would need to try it. From what I can tell so far the Sulphuric Acid and Hydrogen Peroxide strengths are well bellow the legal limit.

Incidentally it may be possible use electrolysis instead of Hydrogen Peroxide as it is possible to produce it using electrolysis. But that would take quite a bit more testing.

Adrian

In the world of silversmithing, the traditional pickling method for silver, gilding metal, brass and copper in order to remove oxides and the borax flux was dilute Sulphuric acid.This was heated in a lead bath to speed the process. Then 'safety pickle' a Sodium Hydrogen Sulphate powder (available HS Walsh, Kernowcraft etc ) was introduced and used widely in schools colleges. The user added water. For small items it can be used in a slow cooker! I now put water in the cooker and keep the pickle in a glass jar in the water as I've got through a few car boot purchased slow cookers over the years. Needless to say that good ventilation and eye protection are essential. Retrieve items with plastic tongs ie do not use fingers or steel tweezers

I'm not sure of the exact composition of 'Sparex 'which is similar to 'Safety Pickle' but when I worked in the US, the metalsmiths were keen on mixing in some Hydrogen Peroxide at times, to avoid the coppery surface which appears on Nickel silver and brass when pickled. It was very effective.