Dielectric union
A fitting that joins two different metals without causing corrosion.
When two dissimilar metals — like the copper pipe and steel tank of a water heater — touch in the presence of water, they corrode each other through galvanic action. A dielectric union is a special fitting with an insulating barrier that separates the metals electrically, slowing that corrosion. They're commonly used at water-heater connections; a corroded, leaking union there is a frequent, fixable culprit.