I've been an A&P in GA since the early 80s and that looks like simple surface corrosion to me. Neglected surface corrosion to be sure, but once removed, zinced, and repainted it's fine. I can't, of course be 100% sure, just looking at photographs, but I worked near the ocean and have seen tons of fretting corrosion like this. As long as there's no real deep pitting or stress cracks it's repairable. And I doubt any A&P IA would risk their ticket signing off this A/C as airworthy if it wasn't.
BTW, 1st post here, Hi everyone!