Metals excluding iron. (Steel too, considering that the majority of steel consists of iron)
Edit: Ooh ooh! Bank vaults are comprised of steel-reinforced concrete. She wouldn't be able to do anything about the Steel bars reinforcing it, but one of concrete's main ingredients is aluminium which Mercury can dissolve. If it's hydraulic cement it'd take a bit(at worst it'd be about 1/4 dissolvable material, and at best about 50% dissolvabe material) but if it's non-hydraulic it's pretty much just Calcium and water with a little bit of carbon.