Short definition
Corrosive water is water that chemically attacks metal pipes from the inside — typically because of low pH (below 7), low alkalinity, or both. In Washington, the classic source is Cedar/Tolt surface water (Seattle, Bellevue, Tukwila) and acidic private wells on the Olympic Peninsula. The visible signs are blue-green stains, metallic taste, and over years, pinhole leaks.
What it is
Water aggressiveness is determined by chemistry, not appearance. Three properties matter:
- pH — acid below 7, neutral at 7, alkaline above 7. Below pH 6.5 the water is increasingly aggressive on metal.
- Alkalinity — capacity to buffer pH changes. Low alkalinity means small acid additions (CO2, organic acids) shift pH downward. Soft surface water typically has low alkalinity.
- Langelier Saturation Index (LSI) — calculated value combining pH, alkalinity, hardness, and temperature. Negative LSI = corrosive (will dissolve pipe). Positive LSI = scaling (will deposit on pipes).
Cedar/Tolt water — Seattle’s primary source — has historically had a negative LSI, meaning it dissolves rather than scales. SPU’s corrosion-control treatment (added around 2003) shifts the chemistry toward neutral, but the legacy effect on pre-2003 pipes persists.
Symptoms in homes with corrosive water:
- Blue-green stains on porcelain at every fixture.
- Copper aftertaste, especially first-draw.
- Pinhole leaks in copper supply lines (eventually).
- Lead leaching (in pre-1986 solder homes).
- Premature pipe failure.
Why it matters to a homeowner
Corrosive water doesn’t damage anything overnight — it works on copper, lead, and zinc over years to decades. The economic question is whether to mitigate (raise pH at the service entry, install RO at the kitchen sink) or to repipe (PEX is unaffected by acidic water). The answer usually depends on home age, copper condition, and pinhole-leak history.
For homes with kids in pre-1986 solder + corrosive water, the lead angle is the priority — first-draw lab tests cost $30-$100 and answer the most important question definitively.
When a plumber recommends a calcite filter or a “pH buffer” or asks about pinhole leak history, they’re describing the same thing. Corrosive water is the input; pinhole leaks and stains are the output.
Common failure modes
- Pinhole leaks — the dominant failure.
- Lead leaching from pre-1986 solder.
- Blue-green staining — system-wide cosmetic warning.
- Premature copper failure at high-velocity fittings (erosion-corrosion overlap).
Common variants
- Corrosive water (homeowner term) = soft acidic water (trade term) = aggressive water (utility term) = plumbo-solvent water (older trade term). Same concept, different audiences.
- Corrosive water vs. hard water. Opposite chemistry; opposite problems (corrosion vs. scale).
Washington note
Risk distribution by region:
- High risk: pre-1990 Seattle Cedar/Tolt-fed homes; private wells in Mason / Jefferson / Olympic Peninsula counties; Tukwila and parts of Bellevue.
- Moderate risk: Tacoma surface-water sources (varies by neighborhood and season).
- Low risk: Spokane Aquifer, Eastside Bellevue/Sammamish private wells in Cascade Water Alliance area — hard water buffers corrosion. Different problem (scale).
For private wells, water testing is the only reliable answer. Annual coliform testing is recommended for WA private wells; pH and metals (Cu, Pb, Fe, Zn) are typically separate add-ons. Olympic Peninsula wells often run pH 5.5-6.5 and need a pH neutralizer / calcite filter at the service entry just to make basic copper supply viable.