Battery acid may drain onto/around that location. Road salt, sediment stream in from underneath headlamp and settle in that area.
Pay attention to the ground wire sitting underneath the battery box, exposed to that battery run-off. It rusts, you can't fix that; can only add other(s) some place else to augment it. Had to do it on the prior car when it started to burn-up alternators.
Hopefully the frame/threads are okay.