Rusty Biscuit Tins

Reader David asks:

Hi Dr Chemical,
Hope you can help? I am a collector of old biscuit tins. I have recently received one that is in poor condition and is showing signs of rusting all over. Is there anything that i can use to at least clean tin up, without damaging print on tin?

Yes, there sure is – Citric Acid.

Rust is ferric oxide, and like many metal oxides is very chemically stable. To break the oxide bond you either blast it apart with hot sulphuric acid, or you treat it with a chemical that forms an even stronger bond with the iron than the oxide.

Citric acid fits the bill perfectly. it’s a weak acid, so it won’t damage the paint or metalwork, but it forms a very strong complex with the iron and will therefore remove the rust very effectively,

Incidentally, this is why the traditional breakfast involves an iron fortified cereal with orange juice.  The citric acid in the orange juice binds with the iron in the breakfast cereal and makes it more bioavailable (that is, more readily absorbed into the body).

So I would use the citric acid with a wet sponge – dip the wet sponge into the citric acid power and use it as a type of powder cleaner as you rub the tin.

So where do you get citric acid from?  Unfortunately, it doesn’t appear to be readily available from hardware stores.  So I’d look in yellow pages under chemical suppliers.  It’s not a restricted or dangerous chemical, so there is no reason you can’t buy it over the counter.