How does a corrosion problem looks like? I'm curious. Care to take a pic?

but judging it's external, I'm pretty sure unless it's manufacturer defect, it most likely gonna be tagged as "wear and tear"...

For warranty stuff, suggest to contact Apple support online first. They concur and ask to bring to service centre, do so. Going direct to QCD, machines, mac city SC first despite what some says, most I hear warranty claim is a "No due to X reasons"