This is very odd. Corrosion? In the time frame of what, a few hours? To be honest, it sounds like they accept that there was no user negligence and that it is a manufacturing failure - otherwise, once it's fixed, why wouldn't you continue warranty? What else might cause the problem?
If it happened here, in Australia, I'd be pressing for warranty repair and full warranty to continue as normal. What if you have some other fault with the unit, completely unrelated? How can they deny warranty on that?
You can't cancel a warranty unless you can show user neglect, and if that's the case, why would you offer to repair it for free? The offer smacks of "we know it's a warranty failure, but we don't want to admit it, so we'll try to make it sound like it's your fault but we won't flat out say that because then you might take legal action and we'd lose so we'll try to sweet talk it with an 'offer' to make you think we're being nice".
If it's not a warranty failure, deny the claim. *IF* a good customer then presses you, then you might consider offering some sort of compromise as a business decision, but doing this sort of thing up front is just poor.