This isn't an Apple issue.
The corrosion came from the outside and not the inside, so it isn't an internal issue.
The damage does not resemble a defect in the material, but rather a chemical reaction.
There are several things that could have got on your phone, in normal usage. When I am working in my lab, I keep my phone in a specific area, where it isn't in contact with anything harmful. When I work on my car, I keep the phone a safe distance away.
I work in a science lab with a lot of corrosive chemicals, and I have never seen this type of damage. This doesn't just happen. I don't know if it was caused by the case, something the phone was exposed to, or a mixture of both.
You are most likely out of luck here. Unless you can prove it was the cases fault, which would be difficult.
Good luck.