How to fix this corroded zipper?

This is more general repair than home DIY, hopefully right place for it.

I tried getting something out of a snowboard bag and found it was sealed by corroded zippers. I'm not sure why this happened as I've stored it like I have for years: off the floor in a dry-ish garage. Here are photos:

The zippers barely budge, and I applied a decent amount of force to the zipper pulling the zipper and bag away from each other almost parallel to the zipper track. I'm considering using plyers and pulling much harder, and I wondered about first cleaning the corrosion with vinegar or using a lubricant. I figured I'd get some better informed opinions first.

How can I get this zipper back in working order? Less important, I wonder how this happened in the first place and how to prevent it from happening again.