A Dielectric Nipple is used to help prevent corrosion of the steel tank of the water heater caused by galvanic corrosion when dissimilar metals such as copper are used for pipes connecting to the water heater. Galvanic corrosion can occur when two different metals contact each other in an electrolyte solution. Metals are ranked on a nobility scale and the further apart the two metals are in nobility the more likely they are to corrode. Copper and steel are far enough apart that damaging corrosion can occur in many cases. Water acts as the electrolyte solution and depending on water conditions there may be either almost no corrosion or, major corrosion that occurs when copper and steel are connected.
Many water heaters are supplied with Dielectric Nipples and in many cases the supplied Dielectric Nipples are also heat traps which can limit heat loss from a water heater. The Dielectric Nipples are essentially a short piece of pipe, which is threaded at each end, and constructed with plastic lining the inside surface of the nipple which prevents the water or, electrolyte from contacting the metal. This failure of the water to contact the metal stops the galvanic corrosion from occurring. Care must be taken with the female adapter that threads onto the Dielectric Nipple to ensure that the water is not able to touch both the nipple and adapter where they connect to avoid corrosion problems.