Short definition
Corrosion is the chemical or electrochemical breakdown of metal pipes and fittings. In Washington plumbing, it shows up as pinhole leaks in copper, rust-through in galvanized, lead leaching from old solder, blue-green stains, and tuberculation that strangles flow. Different mechanisms drive different failures — and Washington’s water chemistry exercises most of them somewhere in the state.
What it is
Pipes corrode in several distinct ways:
- Uniform corrosion. General thinning of the pipe wall over time.
- Pitting corrosion. Localized attack at pinpoint locations on the pipe interior — the cause of pinhole leaks in copper.
- Galvanic corrosion. The less-noble metal at a dissimilar-metal joint sacrifices itself to protect the more-noble metal. Common at copper-to-galvanized transitions in pre-1970 PNW homes.
- Electrolysis. Stray-current or galvanic-cell-driven decay at metal joints.
- Erosion-corrosion. High-velocity water (>5 ft/s) at undersized fittings strips the protective layer faster than it can re-form.
- Microbiologically induced corrosion (MIC). Biofilm bacteria interact with metal surfaces under specific water chemistries.
- Stress corrosion. Mechanical stress plus aggressive chemistry produces accelerated cracking.
- Crevice corrosion. Oxygen-deprived gaps (under deposits, in fittings) corrode faster than open surfaces.
Symptoms vary by mechanism:
- Leaks (pinhole, joint).
- Stains (blue-green for copper, brown for iron/galvanized).
- Reduced flow (tuberculation in galvanized).
- Metallic taste.
- Premature pipe failure.
The diagnostic order: identify the pipe material first (copper, galvanized, cast iron, lead service, brass), then narrow down the attack mode based on the symptom pattern and water chemistry.
Why it matters to a homeowner
Corrosion is the slow-motion plumbing failure mode. Pinholes, joint failures, and reduced flow develop over years to decades, then arrive at a single moment that costs thousands of dollars. The economically catastrophic conversation is the one nobody had with the homeowner before purchase about which pipe materials they’d inherited and what water chemistry was working on those pipes.
Insurance carriers complicate the picture: most homeowners policies cover sudden water damage from a corrosion-caused leak, but exclude long-term seepage or “wear and tear” — and corrosion is wear and tear. A pinhole leak that’s been weeping for two months may be partially or fully denied. Catching corrosion early is also catching insurance coverage.
When a plumber recommends a repipe, partial replacement, or expensive water-treatment installation, they’re describing decisions tied to specific corrosion mechanisms. The vocabulary of corrosion is the homeowner-protection vocabulary.
WA-specific risk profiles
Washington’s water chemistry isn’t uniform — different regions exercise different corrosion mechanisms:
- Cedar/Tolt soft acidic water (pre-2003 era), Seattle and Bellevue. Copper pitting corrosion, pinhole leaks, lead leaching from pre-1986 solder.
- Spokane / Eastside Cascade Water Alliance hard water. Tuberculation in galvanized; scale plus metal interaction in valves and water heaters.
- Olympic Peninsula well water. Iron, manganese, sometimes acidic — calcite filter common.
- Pre-1960 galvanized supply (city-wide). Rust-through, zinc release. Most pre-1960 Seattle/Tacoma stock affected.
- Pre-1986 lead solder (Seattle, Tacoma, Everett). First-draw lead release; intersects with soft acidic water for highest exposure.
Common failure modes (umbrella)
- Pinhole leak (copper).
- Galvanized rust-through.
- Cast-iron channel scaling and bottom-channel rot.
- Lead leaching at first-draw.
- Tuberculation (flow loss).
- Galvanic-joint failure at copper-galvanized transitions.
- Brass dezincification in chloraminated water systems.
Common variants and what corrosion is not
- Corrosion (chemical attack on metal) vs. pipe scale (mineral deposit; can occur on plastic too). Different chemistries.
- Corrosion vs. erosion. Chemistry vs. mechanical wear; can co-occur.
- Internal corrosion (water-side) vs. external corrosion (soil or atmosphere side). Different agents, different mitigations.
- Aboveground vs. buried. Different agents.
Washington note
If your home was built before 1990 in Washington, corrosion is a maintenance category, not a one-off emergency. The combinations vary by region:
- Pre-1990 Seattle / Bellevue copper: plan for pinhole leaks; budget for partial or whole-house PEX repipe.
- Pre-1960 galvanized anywhere in WA: plan for tuberculation, pinhole leaks at threaded fittings, and a repipe decision.
- Pre-1986 home with kids: first-draw lead test is cheap and answers the most important question.
- Spokane and Eastside: softener installation extends fixture and water-heater life dramatically.