Well, you have four elements, you have powers that draw energy from them, you want every power to have a unique way of damaging targets, I can sort of see the thought process. And I can see how corrosive chemicals can be tied to the least-energetic element, earth. But, I mean, oxidation requires air, too, doesn't it? And water is sometimes acidic. And acid burns like fire. You could make an argument for any of the four elements to be tied to acid damage. Thunder is vibration. It damages you by bludgeoning you with pressurized shockwaves. It feels like the most "solid" energy type, if that makes sense?
Acid I'd put in the same category as poison. Maybe make some creatures' venom acidic to make up for the huge amount of things that resist poison damage.
EDIT: Also, rock music is loud. Sonic damage!