What happens if you connect copper directly to galvanized pipe?

The two metals form a battery in the water, and the steel side corrodes at an accelerated rate, building rust at the joint that restricts flow and eventually leaks. The joint can survive years, but it is failing the whole time.

Is brass okay between copper and steel?

Yes. Brass sits between the two on the galvanic scale, and a brass fitting or a short brass nipple is the traditional, plumber-preferred buffer, often more reliable in service than a dielectric union.

Where are dielectric unions required?

Codes commonly require dielectric protection wherever dissimilar metals join in a water system, with the water heater's steel tank connections to copper house piping as the everyday case.

Why did my dielectric union clog with rust?

Dielectric unions concentrate the transition in one place, and mineral-rich water can still deposit corrosion products right at the union's plastic sleeve. It beats direct contact, but inspecting old unions at any service is fair practice.

Dielectric Unions and Galvanic Corrosion in Plumbing

Page last reviewed: August 2026 · Reviewed by Harold R. Bennett, retired home inspector

Connect copper pipe directly to galvanized steel and you have built a battery: two dissimilar metals in an electrolyte, quietly trading electrons at the steel’s expense. Dielectric union galvanic corrosion questions come up in exactly two household places, old-pipe transitions and water heater connections, and both have standard answers. This reference explains the electrochemistry in plain terms and the fittings that break the circuit.

The battery you did not mean to build

Metals differ in how eagerly they give up electrons, ranked on the galvanic series: magnesium most eager, then zinc, steel, brass, copper among the common plumbing metals. Join two different metals in contact, immerse them in water (a serviceable electrolyte, more so as minerals rise), and current flows: the more active metal becomes the anode and corrodes preferentially, protecting the nobler one. This is precisely the mechanism a water heater’s anode rod exploits on purpose, magnesium sacrificing itself for steel. Uninvited, the same chemistry runs at any copper-to-steel joint, with the steel playing the anode and losing, faster in hot water, faster in mineral-rich water, and always at the joint itself, where the rust buildup narrows flow before it finally weeps.

The distance rule of galvanic attack is merciful: the corrosion concentrates within inches of the contact point. Which means the fix is local too: break the electrical continuity, or buffer it, right at the joint.

The two everyday battlegrounds

Old-pipe transitions. Every partial repipe of a galvanized system creates copper-to-steel joints, and every one of them needs the treatment below. The classic symptom of the untreated joint is a rust bloom and eventual weep exactly at the transition fitting, years ahead of the surrounding pipe’s schedule.

Water heater connections. The tank’s fittings are steel; the house piping is commonly copper. Manufacturers and codes anticipate the mismatch, which is why heater installations use dielectric nipples (steel nipples lined with plastic, often factory-installed in the tank ports), dielectric unions, or brass/copper flex connectors, and why a heater hooked up with bare copper directly to its steel nipples grows the telltale green-and-rust crowns at the connections. Corroded heater connections deserve attention on their own merits: they are a leak path above an appliance, and they read as a warning sign in the aging-heater assessment.

The fittings that break the circuit

The dielectric union is the purpose-built answer: a union whose two halves are separated by a plastic sleeve and gasket, so copper threads to one side, steel to the other, and no metal touches metal. It works, with two honest caveats from field experience: the rubber-and-plastic internals make it the joint most likely to weep with age, and in hard water the union’s throat can still accumulate corrosion deposits, so unions earn a glance during any nearby service.

The brass buffer is the traditional alternative many plumbers prefer: a brass fitting, valve, or a short brass nipple (six inches is the folk standard) between the copper and steel. Brass sits mid-series, so neither joint spans a large galvanic gap, and the corrosion driver shrinks to background levels. Full-brass ball valves at heater connections do double duty as service valves and buffers.

The flexible connector solves it by composition: braided stainless or corrugated copper heater connectors with brass nuts interpose enough mixed, gasketed metal that direct galvanic contact never occurs, which is one reason flex connectors dominate modern heater installs alongside their convenience.

One boundary note: dielectric isolation must respect the house’s electrical grounding and bonding, since metal water piping is often part of the bonding system; isolating sections is a known interaction, handled with bonding jumpers where required, and a fair thing to mention to the electrician rather than discover later.

The bottom line

Copper touching steel is a slow battery that eats the steel at the joint: buffer it with brass, isolate it with a dielectric union or lined nipples, or span it with a flex connector, and do it everywhere the two metals meet, especially at the water heater. The chemistry never takes a day off, but it is entirely local, and a few dollars of the right fitting retires it for the life of the pipe.