Journeyman Plumber ExamWater Supply SystemsEasy
A plumber is installing a water distribution system in an older building where renovation is taking place. The existing piping is galvanized steel, which needs to be connected to new copper piping. What is the most important fitting to use at the connection point between galvanized steel and copper piping to prevent accelerated corrosion?
- AA threaded steel nipple
- BA dielectric union
- CA PVC adapter
- DA standard brass coupling
Show answer & explanationAnswer & explanation
Correct answer: B. A dielectric union
A dielectric union is specifically designed to separate dissimilar metals, such as galvanized steel and copper, in a plumbing system. This prevents galvanic corrosion, which occurs when two different metals are in electrical contact in the presence of an electrolyte (water), leading to accelerated deterioration of the less noble metal (galvanized steel in this case).
Why the other options are wrong
- A. A threaded steel nipple would be the same material as the galvanized pipe and would not prevent corrosion with copper; it would likely be part of the problem.
- C. A PVC adapter could separate the metals but is typically not rated for the pressure or temperature of a potable water system and may not be code-compliant for this application.
- D. A standard brass coupling would directly connect the two dissimilar metals, accelerating galvanic corrosion.
Galvanic Corrosion Prevention
Galvanic corrosion occurs when dissimilar metals are in electrical contact in an electrolyte (water), causing the less noble metal to corrode. Dielectric unions prevent this by electrically isolating the metals.
- Common with copper and galvanized steel connections.
- Less noble metal (galvanized steel) corrodes faster.
- Dielectric unions use non-conductive materials to break the electrical path.
Memory trick: Dielectric Divides Dissimilar Duos, Detouring Degradation.