A dielectric union is a term used to describe two-part fittings of dissimilar metals which are electrically isolated from each other to prevent galvanic corrosion. The term is most commonly used to describe a family of plumbing pipe fittings. Galvanic corrosion occurs where two different metals are exposed to an acidic solution. This combination effectively forms a battery and causes a flow of electrical current between the two metals. To prevent the corrosion caused by this current flow, the dielectric union uses an insulating insert between the two metals to prevent electrolysis from occurring.
When two different metals are placed in contact with each other in an acidic solution, electrolysis occurs which causes an ion flow between the two metals. This flow of electric current causes molecules from one of the metals to be deposited on the other, thereby resulting in what is known as galvanic corrosion. The metal which donates molecules will slowly be eroded away while the one that receives the material will build up a layer of corrosion byproduct. Most domestic water sources are mildly pH biased; galvanic corrosion is likely to occur wherever dissimilar metals are used in plumbing fittings. One way of preventing this destructive process is to insulate the two parts of the fitting from each other.
The dielectric union achieves this insulation by a plastic insert located between the two parts of the fitting. Bi-metal fittings typically consist of brass and steel halves for connecting similarly differing pipe types. A dielectric union has an additional plastic or non-conductive fiber sleeve which surrounds the brass part of the fitting and a washer to form a flange barrier. The sleeve and washer effectively isolate the two materials from one another and prevent the galvanic battery effect from occurring. One point to keep in mind when considering this phenomenon is that the voltages involved in galvanic electrolysis are very small and pose no danger.
Dielectric union fittings meant for use with copper pipes generally require the brass part of the union to be soldered onto the copper tubing. The plastic sleeve should be removed from the fitting to prevent melting before this is done and only replaced once the brass has cooled down. The washer should also be moved as far down the copper tubing as possible while soldering for the same reason. When the fitting is assembled, care should be taken to avoid tearing or nicking the plastic sleeve. A small area of contact between the two metals will lead to far more aggressive, localized corrosion than full area contact.