brass corrosion

We recently bought a house in a small town with a municipal water system. They treat with chlorine (but claim not to use chloramine). I'm concerned about corrosion, particularly in the lead-free brass that's becoming so common; I know there's DNZ (dezincification-resisitant) brass, but not...

House is 4-1/2 years old, and kitchen island was getting intermittent hot water, so I planned to try cleaning the sweated "brass" check valve downline from the recirc pump. When I removed the insulation sleeve, however, I found massive electrolysis corrosion on the outside of the valve body...