Tankless coil corrosion on cold water supply

John McCary

I have an Ultimate Boiler (hot water boiler) that has had a constant corrosion problem where the cold water supply enters the coil for the domestic hot water. There is a small intermitent seepage of water from what appears to be the boiler jacket that is corroding the fitting welds (flaking and scaling) and the piping that is connected. Di-electric unions have been put on the cold and hot water lines about 3' from the coil and they have rusted quite badly in about 3 years time. There is a voltage of 2 mili-volts to ground with about 1 mili-amp of current on either hot or cold water lines on either side of the unions.
Any ideas as to the cause of the corrosion or ways to mitigate it?