Pitting corrosion
| Pitting corrosion | |
| Type | Corrosion mechanism |
|---|---|
| Affects | Copper and other metallic pipe |
| Produces | Pinhole perforation rather than general thinning |
| Tube standard | BS EN 1057 |
Pitting corrosion is a localised form of corrosion that attacks a very small area of a metal surface while the surrounding material remains largely unaffected. In water pipework it is the mechanism behind most pinhole leaks in copper tube, which is specified for water and gas in sanitary and heating applications by BS EN 1057.[1] Because the attack is concentrated rather than general, a pipe can perforate while its wall thickness elsewhere is essentially unchanged, which is why an otherwise sound system can develop a leak without warning.
Mechanism[edit]
Pitting begins where the protective oxide film on the metal surface is broken down locally. The small anodic area that results corrodes rapidly while the much larger surrounding cathodic area does not, so material loss is concentrated at a point. Once a pit has formed, conditions inside it differ from the bulk water and tend to sustain the attack.
Contributory factors in domestic systems include water chemistry, flux residue left inside the tube after soldering, debris and particulate resting on the pipe wall, high local flow velocity, and, in heating systems, oxygen ingress and the resulting corrosion products.
Consequences for leak detection[edit]
The characteristic result is a pinhole: a very small opening passing a small but continuous volume. That produces a slow, steady loss rather than a burst, which has direct consequences for how it is found. Acoustic signal from a pinhole is weak. Damp may take weeks to appear, and appears where the water collects rather than where it left the pipe. A sealed heating system loses pressure gradually and is topped up repeatedly before anyone treats it as a leak.
Where one pinhole has been found in a system, others on the same run are more likely than the population rate would suggest, because the conditions that produced the first are usually present along the length.