Before you removed the heat exchanger, where was the leak presenting at? I had a leak in my Hayward H400 that was showing at the endplate of the heat exchanger by the water header. Cleaning the O-ring gasketing surfaces and replacing the water header O-ring solved the problem. It might have been easier to pinpoint the place where it was leaking before actually removing the heat exchanger. When you talk about corrosion, are you talking about the blue/green coating/coloration of the heat exchanger fins/tubing? That is the end stage of oxidation of metals containing copper and is actually preventative of further oxidation. As long as there is a normal clear space between the oxidized fins, you will only lose a small amount of heat transfer due to the oxidized surface.