Chemistry – long forgotten

[Kiwi BlokeParticipant @kiwibloke62605]

I had some vinegar-salt solution that had been used successfully to clean some lightly-rusted steel. Having used vinegar to clean brass, in the past, I dropped some brass into this used solution. The brass cleaned up reasonably well, but areas of copper-colour spoiled the finish. I suppose this was due to ‘de-zincification’: am I correct? What is a good way to chemically clean brass without this risk?

Then I thought I’d clean some slightly rusted steel shimstock, in the same, contaminated solution. It instantly became copper-plated. Oh, like steel in copper sulphate solution, I thought to myself. Then I suppose my aged brain shut down, because I tried to remove the Cu with fresh acid. Silly! It just instantly re-plated – as any thinking person would expect (sigh). Light abrasion got rid of it. But the question remains: how would you chemically remove Cu deposits from steel/iron? Are there Cu-specific chelators, for example? I suppose I could have increased the volume of acid wash. Any better ideas?

[Nigel Graham 2Participant @nigelgraham2]

Unless you left the steel in the Cu-salt solution for a very long time, the copper-plating is likely to be so thin and fragile it would yield simply to metal-polish and elbow grease.

[FulmenParticipant @fulmen]

Ammonia or even better a mixture of ammonia and ammonium carbonate (hartshorn salt) will dissolve copper.

[Fulmen]

On Fulmen Said:

Ammonia or even better a mixture of ammonia and ammonium carbonate (hartshorn salt) will dissolve copper.

Thanks! Doh! I knew that (well, half of it). ! now I remember that ammonia is the standard way of removing Cu-fouling from firearm barrels! I wonder whether there’s anything left in my head – I seem to have forgotten so much!

Going on from the above – one is told not to use ammonia to clean Cu-fouling from stainless steel barrels. Why?

[FulmenParticipant @fulmen]

I haven’t heard of that, maybe because I’ve never had a stainless barrel. It might cause corrosion if left for an extended time, people often overestimate the chemical resistance of “stainless” barrels. I also found something about the combination of ammonia and chlorinated solvents, but I didn’t go down that rabbit hole.

[Simon CarpenterParticipant @simoncarpenter97524]

<p style=”text-align: left;”>I use citric acid for cleaning brass, it’s not as cheap as vinegar but not as strong smelling. I’ve also used sodium bisulphate, which is a bit harsher, probably better for copper and steel. With any acid ” pinking” is a risk, so don’t leave it in the bath too long, and don’t put brass and steel in together or put brass in acid that’s previously been used for steel as apparently the presence of iron salts in the acid promotes dezincification: this may well have been the problem with your brass pieces.</p>
Interesting to read about the effects of ammonia on copper: a lot  of pressure lantern and stove enthusiasts say you shouldn’t use Brasso on brass tanks because it contains ammonia and  can cause weakening of the metal.

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