Tyinksy.....Irn Bru would work much better than Coke
Seriously though Geoff I use Phosphoric acid various mixure strengths on all my brass plaques and copper items and even cleaning old electrical connectors wires before re soldering.
If you look at any decent alloy wheel cleaner or rust/oxydisation cleaner you will see it is a high content usually of phosphoric acid.
We have a brand name in UK called Jenolite and you can just brush on and wash off. We also have another product called Cillit Bang but this is a very weak solution for the home use but does work and the dirty old two pence piece comes out the tank sparkly. I have also in the past used 'Silver Dip' another brand name here in UK.
Look for a decent brand spray bottle of alloy wheel cleaner in the autoshops and see it contains a solution of phosphoric in its contents. All others mentioned though will work too