Short definition
Galvanic corrosion happens when two dissimilar metals are joined in the presence of water (an electrolyte). The less-noble metal becomes the anode and dissolves; the more-noble metal stays protected. The classic Washington pattern: copper-to-galvanized transitions in pre-1970 Seattle and Tacoma homes, where the galvanized side fails first within a few years if no dielectric union is installed.
What it is
When two different metals contact each other through water, they form a small battery (a galvanic cell). The relative position of metals on the electromotive series decides which one corrodes:
- More noble (cathode, protected): copper, brass.
- Less noble (anode, sacrificed): zinc (galvanizing layer), iron, steel, magnesium (intentional anode rods).
The greater the gap between the metals on the series, the faster the galvanic cell drives corrosion. Copper paired with galvanized steel is one of the more aggressive common pairings — the zinc strips off first, then the steel rusts.
Where galvanic corrosion shows up in residential plumbing:
- Copper-to-galvanized transitions (the dominant WA case). Installing new copper into older galvanized creates a galvanic cell at every threaded transition.
- Brass valves in galvanized systems. The galvanized side around the brass corrodes faster.
- Steel pipe straps on copper pipe. Surface galvanic at the contact point — use copper or plastic straps.
- Water heater anode rods — intentional galvanic protection: a magnesium or aluminum-zinc rod sacrifices itself to protect the steel tank. Replace every 3-5 years (soft Cedar/Tolt) or sooner (hard Eastside).
Mitigation:
- Dielectric union between dissimilar metals — an insulating coupling that breaks the electrical circuit while maintaining a water seal.
- Replace the older metal (usually the failing galvanized side).
- Compatible-metal selection at design time.
- Sacrificial anode in tanks and large vessels.
Why it matters to a homeowner
In pre-1970 PNW homes that have had partial copper repipes — kitchens, bathrooms, water-heater swaps — there are usually copper-to-galvanized transitions somewhere in the system. Without a dielectric union at each one, those transitions are corroding right now. The galvanized side weeps, pinholes, or fails at the threaded fitting first.
When a plumber says “I need to install dielectric unions on the water heater connections” or “this transition is a galvanic problem,” they’re describing exactly this. Dielectric unions add a few dollars per joint at install; retrofitting them later means cutting drywall.
For water heaters, galvanic chemistry is harnessed for benefit: the sacrificial anode rod is meant to corrode in place of the tank. Replacing the rod every 3-5 years is the cheapest way to extend tank life. Many Seattle plumbers report getting an extra 5-8 years out of water heaters with regular anode service.
Common failure modes
- Galvanized side of copper-galvanized transition fails first. Zinc strips, then steel rusts.
- Brass valve in galvanized system. Galvanized corrodes faster around the brass body.
- Anode rod consumed faster in hard water (Spokane, Eastside) than in soft Cedar/Tolt water.
- Steel pipe straps on copper pipe. Surface galvanic at the contact point.
- Pre-1960 lead service line to copper in-house — galvanic plus lead leaching combined.
Common variants
- Galvanic corrosion (dissimilar-metal cell) vs. electrolysis (broader: any electrochemical corrosion, including stray current).
- Galvanic vs. pitting. Pitting can occur in single-metal systems; galvanic always two metals.
- Sacrificial anode = the same mechanism harnessed for benefit.
Washington note
In Seattle, Tacoma, and Everett, pre-1970 homes that had copper added during a kitchen or bath remodel — without full repipe — typically have copper-to-galvanized transitions somewhere. The pattern repeats often enough that pre-purchase inspectors specifically look for missing dielectric unions on water-heater fittings.
For water heater anode-rod cadence in WA:
- Cedar/Tolt soft water (Seattle, Bellevue): 3-5 years; some homeowners get 8-10 years with attention.
- Spokane / Eastside hard water: 2-3 years; high-conductivity water consumes anodes faster.
- Olympic Peninsula private wells (acidic, sometimes high-iron): 1-2 years; check annually.