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De-plate copper bucket. Unknown grey metal coating.

I have a vintage copper bucket, about 8 litre capacity, plated on the inside with some unknown grey metal. My guess is tin or zinc, but I am not sure and don't know how to find out. The coat is thin enough that the copper shows through in places. I want to remove it completely.

Following advice given to penny polishers on this site, I tried NaOH with sodium carbonate ⇦this on eBay or Amazon] with some results, but not good enough. My problem is keeping the solution hot enough. The bucket has a small leak and the only vessel I have that is large enough to contain it is plastic.

More digging on this site, someone suggested dilute hydrochloric or sulfuric acid to a similar question (also about pennies, letter 40372). Will this work at room temperature? How about below 10C, as in the garage? How long will it take and how much acid do I need? Will it not damage the copper? I used to etch PCBs with HCl/H2O2 solution in my younger years, so I am a bit weary of using HCl around copper...

Peter Pichler
Hobbyist - Oxford, UK
March 1, 2011

While waiting for my question to appear, I tried some kitchen science. Patched the leak with duct tape and poured two bottles of toilet cleaner into the bucket. Left at room temperature for about an hour until my better half banished me from the kitchen due to the smell, then left it overnight in the garage at about 5C. It must have had some effect as more copper is definitely showing through. I feel encouraged to go off and buy a litre of HCl just for the lark. In the worst case, I can clean my drains with it.

Peter Pichler
- Oxford, UK
March 3, 2011

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Peter,

You said that you have used Hydrochloric acid in the past and are wary of it. Quite rightly so, but please, please, please do not add it to toilet cleaner in your bucket, you are likely to kill yourself! Stick with the toilet cleaner, it may be slow, but it is obviously doing the job. If you are impatient you can also try abrading the surface of the plating as well.

Brian Terry
Aerospace - Yeovil, Somerset, UK
March 8, 2011

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Thanks Brian, I am not afraid of HCl as such, I have used strong chemicals before. It's only that I wasn't sure of using it on copper as that is what I used to etch copper with. Admittedly, it was with hydrogen peroxide as oxidizer.

For the record, the toilet cleaner was HCl based (6.5%, if I remember correctly). I know better than mix acids with bleach :-) And I flushed it out before inspecting the results. I had to: it was so thick I could not see anything.

My second attempt was with some patio cleaner, also HCl based. The label didn't mention what concentration, but it said corrosive rather than irritant so I guess at least 25%. Three days on and big patches of pure copper are showing up, so I will stick with that. It is worse than watching paint dry, but time is not really much of an issue.

This experiment and research surrounding it have taught me a lot about chemistry and metallurgy. I wasn't sure what the metal was, for example. It was dark gray, almost black to start with, dull light-grey after my experiments. Based on my observations, I am now convinced it is tin.

Peter Pichler
- Oxford, United Kingdom
March 8, 2011