Hi Lisa .
The tin can is made of low carbon
steel which is essentially the element iron. On top of that is a coating of tin metal. The tin normally
acts as a barrier which protects the steel from corrosion. Unfortunately, if
the tin barrier is broken or incomplete, the exposed iron will corrode in
preference to the tin. Zinc would be a better protector of the iron than the
tin. The zinc would corrode in preference to the iron, but the zinc would go
into the food contained within and probably make the consumer bilious.
What you have observed is called
galvanic corrosion. When you put two dissimilar metals together (i.e. touching)
you get an electric potential across them. Add water and the more reactive metal will
corrode.
In this case, at the anode ;
Fe --> Fe2+ + 2e-
(The Fe2+ will go on to form rust, more importantly the above reaction supplies 2 electrons)
Hydrogen will evolve at the cathode (this should answer your question);
2H+ + 2e- --> H2
(Because tomatoes are acidic there are plenty of hydrogen ions to latch on to electrons from the anodic reaction.)
In other words you have electrolysis happening.
As you have observed, iron corrodes very well in an acidic
environment.
