A dielectric bond or fittings of two dissimilar metal parts can be used to prevent electrolysis from occurring.
A dielectric bond is a term used to describe two-part dissimilar metal fittings that 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 when two dissimilar metals are exposed to an acid solution. This combination effectively forms a battery and causes an electrical current to flow between the two metals. To prevent corrosion caused by this current flow, the dielectric bond uses an insulating insert between the two metals to prevent electrolysis from occurring.
When two dissimilar metals are brought into contact with each other in an acid solution, electrolysis occurs, causing a flow of ions between the two metals. This flow of electrical current causes the molecules of one of the metals to deposit on the other, which is known as galvanic corrosion. The metal donating molecules will corrode slowly, while the metal receiving material will form a layer of corrosion by-product. Most domestic water sources are slightly biased by pH; Galvanic corrosion can occur whenever dissimilar metals are used in plumbing connections. One way to avoid this destructive process is to isolate the two parts of the connection from each other.
The dielectric union achieves this isolation through a plastic insert located between the two parts of the connection. Bimetallic fittings generally consist of brass and steel halves for connecting similarly different types of pipe. A dielectric union has an additional sleeve of non-conductive fiber or plastic that surrounds the brass portion of the connection and a washer to form a flange barrier. The sleeve and the washer effectively isolate the two materials from each other 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 do not pose any danger.
Dielectric union fittings intended for use with copper tubing generally require the brass portion of the union to be soldered to the copper tubing. The plastic sleeve should be removed from the fitting to prevent melting before doing so and should only be replaced when the brass has cooled. The washer should also be moved as far down the copper tube as possible during brazing for the same reason. When mounting the accessory, care must be taken to avoid tearing or damaging the plastic sleeve. A small area of contact between the two metals will lead to much more aggressive localized corrosion than a full area of contact.
