If you do not use dielectric fittings, in time (couple years / decades, who knows) the connections will start to fail. Usually at the thinnest point, the threads. You will start to see a rust build up around the nipple entering the cast iron threaded boss or the steel nipple coming form the radiator. Eventually it will leak worse and worse.

Connections between dissimilar metals without the use of dielectric fittings will produce galvanic corrosion (often called electrolysis). Metals have potential and each type of metal has a different potential (potential is measured in volts). When you make a connection between two or more dissimilar metals you create a potential difference. Now just introduce an Electrolyte (water) and you've created a very weak battery.
With this weak current flow traveling from one metal to the other it turns one into an Anode and the other into a cathode. Making the anode corrode at a faster rate than normal while the cathode will slow or completely stop corroding. To know which metal becomes the anode and which is the cathode you must know their potential. Zinc, cast iron (your radiator), steel, aluminum alloy are at the low end of the scale while copper, brass, bronze, stainless steel are more to the middle of the scale and platinum and titanium are at the high end. The lower metal on the scale, your radiator, becomes the anode and the higher metals on the scale, the copper tube / brass valves, becomes the cathode.

: I'm installing some cast iron radiators in a new addition we built, I'll be connecting the radiators to 3/4" copper tubing. How important would the use of dielectric nipples be in the connection? If not used, what can I expect?

• Re: Dielectric Nipples, how important? e-plumber 18:41:07 03/15/04(0)