heater manifold leaking

[Deuce2]

Just noticed my Hayward H250 was leaking around where the manifold connects to it. After disassembly, I noticed that the aluminum wall where the 0-ring sits against is pitted out right where the o-ring sits. First question, why would it pit out here? (I always have my PH between 7.4 and 7.6). Second question, anyone have any experience fixing these? I'm thinking about using JB weld epoxy to fill in the pits, then put it back together.

37,000 gallon, IG Vinyl.

Thanks.