Nanni 3.75he; Corrosion in aluminum heat exchanger

[fourlyons]

I recently bought a older, used Nanni 3.75he, it is a 21hp Kubota block.

The heat exchanger was leaking antifreeze slowly from below the exit cap for the tube stack. I discovered an epoxy repair, that has failed, to an area of corrosion in the aluminum just inside the bore where the exit end of the tube stack is capped. The corrosion is inside the freshwater chamber, again in the aluminum, so it shouldn't be due to saltwater. The tube stack is a copper alloy, but it is, in theory, isolated from the aluminum by the o ring and paint. Plus, there is no corrosion at the other end of the tube stack where it sits in an aluminum bore.

I have ground out all of the corrosion, cleaned it, epoxied, and trued the bore. It no longer leaks.

My question is, why did it corrode, and how do I keep it from corroding in the future?
Is it possible it's as simple as the anti corrosion additives in the antifreeze went away because it wasn't changed often enough?

Should I add a zinc to the freshwater chamber, I didn't think antifreeze is an electrolyte.

Thanks