Electrolysis (electrolytic corrosion)

Short definition

Electrolysis in plumbing is electrochemical corrosion driven by electric current at metal pipe joints — most often the copper-to-galvanized transition in pre-1970 PNW homes. The current can be galvanic (from the dissimilar metals themselves) or stray (from improperly grounded electrical service). Dielectric unions and proper bonding limit the damage.

What it is

Plumbers and electricians use “electrolysis” loosely for two related phenomena:

1. Galvanic-cell corrosion. Two dissimilar metals joined in an electrolyte (water) form a galvanic cell. The less-noble metal becomes the anode and dissolves; the more-noble metal becomes the cathode and is protected. This is the most common residential mechanism — see galvanic corrosion for the dedicated entry.
2. Stray-current corrosion. External DC current (improperly grounded electrical service, nearby buried-pipe systems, transit DC return paths) drives metal dissolution at points where the current leaves the pipe and enters soil or water. More common in commercial and utility settings; rare in residential plumbing but possible.

Symptoms:

• Rust at a joint between dissimilar metals — copper-to-galvanized is the classic.
• Pinhole leak at a copper-galvanized transition without dielectric union.
• Hydrogen gas accumulation in upper radiators of legacy hydronic heating systems.
• Anode rod consumed faster than expected in a water heater.

Diagnosis:

1. Identify the joint type — is it dissimilar metals (galvanic) or single metal (suspect stray current)?
2. Inspect for proper dielectric union (an insulating coupling between dissimilar metals).
3. Verify electrical bonding is correct — both required by code and a possible source of stray current if mis-installed.
4. Inspect anode rod in the water heater — premature consumption is a galvanic-activity indicator.

Mitigation:

• Install dielectric unions at every dissimilar-metal joint.
• Verify electrical service grounding is correct per NEC and WA amendments.
• Replace consumed sacrificial anode rods on schedule (3-5 years in soft Cedar/Tolt water; sooner in hard Eastside water).

Why it matters to a homeowner

For most WA homeowners, “electrolysis” actually means galvanic corrosion at a copper-to-galvanized transition somewhere in their house — typically where a partial repipe added new copper to old galvanized supply. The galvanized side fails first. A pinhole or weep at a transition fitting is the warning sign that the dielectric coupling is missing or has failed.

Stray-current corrosion is rarer and usually involves an electrical issue. If a pre-purchase inspection or a plumber finds severe corrosion at multiple locations on the same metal, it’s worth having an electrician verify grounding and bonding before assuming chemistry is the only cause.

Common failure modes

• Pinhole at copper-galvanized fitting in pre-1970 Seattle/Tacoma homes.
• Failed dielectric union with deteriorated insulator.
• Anode rod consumed in under a year in some hard-water + high-conductivity scenarios.
• Hydrogen gas accumulation in upper radiators of legacy hydronic systems.
• Stray-current corrosion on buried metal pipe — mostly commercial/utility scale.

Common variants

• Electrolysis (broader chemistry term: any electric-current-driven corrosion) vs. galvanic corrosion (specific: dissimilar metals without external current). Heavy overlap in trade usage.
• Electrolysis vs. erosion-corrosion (velocity-driven) vs. pitting (localized chemistry). Different mechanisms.

Washington note

Pre-1970 Seattle, Tacoma, and Everett homes often have copper-to-galvanized transitions installed during partial repipes — adding new copper kitchens or bathrooms to homes that still had galvanized everywhere else. These transitions are the most common WA location for galvanic/electrolytic failure. The signature: a weep or pinhole on the galvanized side of a fitting, often within a year or two of the transition being installed without a dielectric union.

For new work in WA, code-compliant grounding and bonding requirements (under WA-amended NEC) include the metallic water service and may require a bonding clamp where the service enters the building. Both improper bonding and properly-installed bonding can be implicated in stray-current concerns; if you’re chasing unexplained corrosion, an electrician’s verification before a plumber’s repair is sometimes worth the visit.