Dissimilar Metals, Dielectric Unions, and Galvanic Corrosion Control

Why Dissimilar Metals Cause Problems in Plumbing

When two different metals are connected in the presence of an electrolyte — any water that carries dissolved minerals or salts — a small electrical current flows between them. This is galvanic corrosion, and it eats away one of the metals silently inside a wall or underground, often for years before a leak appears. Understanding this process is not optional for a plumber: codes require dielectric isolation at certain joints, and failing to install it correctly can destroy a system in a fraction of its expected service life.

The Galvanic Series

The galvanic series ranks metals by their electrochemical potential in water (seawater is the standard reference, but the ranking holds in potable water as well). Metals near the active (anodic) end of the series corrode preferentially; metals near the noble (cathodic) end are protected at the expense of the active metal.

For everyday plumbing, the approximate ranking from most active to most noble is:

1. Zinc (galvanizing coating)

2. Carbon steel / black iron

3. Cast iron

4. Stainless steel (active grades)

5. Brass

6. Copper

7. Stainless steel (passive grades)

When two metals from this list are connected, the one higher on the list (more active) becomes the anode and corrodes. The one lower on the list (more noble) becomes the cathode and is protected. The greater the separation between the two metals on the series, the faster the corrosion reaction.

Key plumbing pairings to know:

• Copper connected to galvanized steel: zinc coating and then the steel itself are consumed; failure appears at or just past the joint on the steel side.
• Copper connected to black iron: the iron is anodic and corrodes; expect pinhole leaks near the transition fitting.
• Brass connected to galvanized steel: zinc corrodes slowly; brass is close enough to copper on the series that the attack is moderate but still present.

What Is a Dielectric Union?

A dielectric union is a threaded union that contains an insulating plastic or rubber sleeve and washer between the two metal halves. This sleeve physically separates the two metals, breaking the electrical circuit so galvanic current cannot flow. When current cannot flow, corrosion stops.

Dielectric unions are the most common solution at exposed or accessible copper-to-galvanized and copper-to-black-iron transitions — for example, where copper water supply lines connect to a steel water heater. They consist of:

• A copper-solder or copper-threaded half
• A steel or galvanized-threaded half
• A plastic insulating collar and washer between the two halves

Dielectric nipples serve the same purpose in a compact form: a short steel nipple with a factory-applied plastic liner and coating that prevents metal-to-metal contact with the mating copper fitting.

Plastic-lined or plastic-insert fittings (such as push-fit fittings with non-metallic bodies, or transition fittings with plastic seats) can also provide isolation, but verify they are listed for that service.

Code Requirements

Both the International Plumbing Code (IPC) and the Uniform Plumbing Code (UPC) require that joints between ferrous (iron- or steel-based) and non-ferrous (copper, brass) metals be made with an approved dielectric fitting where galvanic incompatibility exists. Always consult the current adopted edition of the code in your jurisdiction, because specific section numbers differ between editions. The general rule is consistent: dissimilar metals that are far apart on the galvanic series must be isolated.

Note that copper-to-CPVC (chlorinated polyvinyl chloride) is a dissimilar-material transition but not a galvanic one, because CPVC is plastic and carries no electrical potential. Code still requires appropriate transition fittings (such as CPVC-to-copper adapter fittings that are listed for the application), but the reason is compatibility of thermal expansion and joining method, not galvanic corrosion.

Recognizing Galvanic Failure

When you examine a corroded sample or a failed joint:

• Identify which metal is pitted, thinned, or perforated — that is the anode.
• The cathode metal is usually undamaged or may have a crusty deposit on its surface.
• Corrosion is typically worst at the joint itself and diminishes as you move away from it along the pipe.
• In copper-to-galvanized systems, you may see a white or gray powdery crust (zinc oxides) on the galvanized pipe right at the joint, followed by rust-colored staining where bare steel has been exposed.

Safety Points

Galvanic corrosion is a hidden hazard. Pipes that look intact from the outside may be paper-thin inside. Before cutting into any older system:

• Shut off and depressurize the system; apply lockout/tagout where required by your worksite procedures.
• Wear safety glasses and gloves; corroded pipe can collapse or release pressurized water unexpectedly when disturbed.
• Inspect both sides of any dissimilar-metal joint before reusing the existing pipe. Do not rely on visual inspection of the exterior alone — probe with a pick tool if the condition is uncertain.
• Dispose of corroded pipe samples as metal scrap per your shop's waste procedures; some older galvanized coatings may contain lead or cadmium in trace amounts.

Specifying the Right Fitting

Use this decision process:

1. Identify both metals.

2. Determine their positions on the galvanic series.

3. If they are dissimilar ferrous-to-non-ferrous (e.g., copper to galvanized), a dielectric fitting is required.

4. Choose the fitting type based on accessibility and system type: dielectric union for accessible locations where future servicing is expected; dielectric nipple where space is limited; verify pressure and temperature ratings match the system (water heater connections typically require fittings rated for the water heater's maximum output temperature).

5. Confirm the fitting bears the appropriate listing mark (NSF, UL, or similar) for potable water service if it is on the supply side.