pipe corrosion

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Pitting corrosion is a localized form of corrosion by which cavities or "holes" are produced in the material.  This occurs when the material of the metal pipe is slowly worn away over time, but the wear is not uniform across the pipe. Instead, small holes or pits are created. Most cases of pitting are believed to be caused by local cathodic sites in an otherwise normal surface. Apart from the localized loss of thickness, corrosion pits can also be harmful by acting as stress risers.

This type of corrosion can be extremely difficult to detect because the damage is not often known until the pipe starts leaking. The pits in the pipe may be narrow and deep, elliptical-shaped, wide and shallow, horizontal or vertical. It’s also difficult for piping manufacturers to predict and design against, due to the plethora of factors that can contribute to metal pipe pitting. Regular camera inspection may be able to detect pitting before it compromises the integrity of the pipe.

Pitting is caused by:

1. Localized chemical or mechanical damage to the protective oxide film; water chemistry factors which can cause breakdown of a passive film are acidity, low dissolved oxygen concentrations (which tend to render a protective oxide film less stable) and high concentrations of chloride (as in seawater)
2. Localized damage to, or poor application of, a protective coating
3. The presence of non-uniformities in the metal structure of the component, e.g. nonmetallic inclusions.

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