When retrieving some tinned tomatoes, my mum found the can bulging, evidently due to some gas produced, some of which bubbled to the top as it was opened. The inside of the tin wasn't coated as it should be and the metal had corroded.

I decided a little detective work was in order and after a bit of searching discovered this was due to hydrogen swell, a result of electrolytic action. A satisfactory explanation for most, but being an A level chemistry student with some knowledge of electrolysis I'd love to know exactly what compounds and ions are involved.