A couple of months ago I installed a Whirlpool Gas 40 gal. water heater and I'm getting some corrosion on both the hot and cold lines where the new brass compression fitting meet the old copper pipe. I checked with my multimeter, and both lines are firmly grounded. As you can see from the pictures there is much more corrosion on the hot line. I used the dielectric unions that came with the install kit I bought and tried my best to do everything right. What am I missing?