Short definition
Pitting corrosion is localized electrochemical attack on copper pipe walls — pinpoint-sized holes that grow deeper while the rest of the pipe stays sound. Type-1 (cold-water, soft acidic) is the PNW signature: pre-1990 Cedar/Tolt-fed Seattle homes show it as pinhole leaks, often clustered on the same horizontal run.
What it is
Where uniform corrosion thins a pipe wall evenly, pitting concentrates attack at pinpoint locations on the inside surface. Most of the pipe stays intact; one tiny anodic spot dissolves through the wall over months or years until water finds the outside. The hole — a “pit” — is often narrower than a pencil lead but goes all the way through.
Three types matter in residential plumbing:
- Type-1 pitting. Cold-water attack, low pH, low alkalinity, soft water. The classic Cedar/Tolt PNW pattern; produces small, deep pits.
- Type-2 pitting. Hot-water attack, harder water, higher pH. Less common locally.
- Type-3 pitting. Cold-water, high pH, with microbial component. Rarer.
The Type-1 risk profile in WA: pre-1990 thin-wall copper installed in Cedar/Tolt-fed Seattle and Bellevue. Pre-2003 Cedar water (pH 6.5-7.0) was firmly in Type-1 territory; after SPU’s corrosion-control upgrade brought pH to 8.0-8.2, new Type-1 pitting slowed but pre-existing damage continues to manifest as leaks years later.
The diagnostic pattern:
- Pre-leak indicator: chronic blue-green staining at every fixture.
- First leak: pinhole on a hot-side horizontal run, often at the 2-o’clock or 10-o’clock position (turbulent flow concentrates attack).
- Cluster pattern: more pinholes on the same run within months to years.
Why it matters to a homeowner
Pitting is the engineering term behind the “pinhole leaks cluster” homeowner reality. The single most important fact: a pipe that has produced one pinhole has been corroding everywhere along its length. Repairs cluster; one this month, two more within a year is the standard PNW pattern in pre-1990 Cedar/Tolt-fed homes.
When a plumber says “Type-1 pitting” or “soft-water pinhole” or recommends repipe after the first leak, they’re reading the same field pattern. The decision is patch and watch (acceptable for one-off pinholes in modern copper) vs. repipe (the realistic choice for chronic pitting on pre-1990 Seattle copper).
Common failure modes
- Pinhole leaks — the structural manifestation.
- Multiple pinholes on same run — Type-1 cluster pattern.
- Hot-side fails first. Higher temperature accelerates the chemistry.
- Pinhole at 2 or 10 o’clock on horizontal copper. Turbulence concentrates attack.
- Whole-house pitting in pre-1990 Cedar/Tolt-fed homes — repipe decision.
Common variants
- Pitting corrosion (localized) vs. uniform corrosion (general thinning).
- Pitting vs. erosion-corrosion (velocity-driven).
- Pitting vs. galvanic corrosion (single-metal vs. dissimilar-metal).
- Type-1 (cold soft) vs. Type-2 (hot harder) vs. Type-3 (cold soft + MIC).
Washington note
Cedar and Tolt source water — Seattle’s primary supply since the 1890s — runs soft and historically slightly acidic. Hardness around 22 ppm (1.3 GPG, very soft); historic pH 6.5-7.0. Copper installed before SPU’s corrosion-control upgrade (around 2003) saw the most aggressive water and shows the highest Type-1 pitting rates today. Bellevue, Tukwila, and several smaller utilities pull from the same regional supplies and share the pattern.
For pre-1990 Cedar/Tolt-fed home owners:
- Visible blue-green staining is pre-pinhole evidence — the chemistry that produces stains also produces pits.
- A single pinhole leak is a leading indicator, not an isolated incident.
- Whole-house PEX repipe (Seattle: $4,000-$15,000 depending on access) is the durable answer; a pH neutralizer / calcite filter slows attack on remaining copper but doesn’t reverse it.