Pitting Corrosion is the localized corrosion of a metal surface confined to a point or small area, that takes the form of cavities. Pitting corrosion is one of the most damaging forms of corrosion. Pitting factor is the ratio of the depth of the deepest pit resulting from corrosion divided by the average penetration as calculated from weight loss.
Non
uniform corrosion over the entire metal surface, but occurring only in small
pits is called pitting. It is caused by lack of uniformity in metal.
The attack of pitting corrosion is in the form of highly localized holes that can penetrate inwards extremely rapidly, while the rest of the surface remains intact. A component can be perforated in a few days with no appreciable loss in weight on the structure as a whole.
For a defect-free "perfect" material, pitting corrosion is caused by the ENVIRONMENT (chemistry) that may contain aggressive chemical species such as chloride. Chloride is particularly damaging to the passive film (oxide) so pitting can initiate at oxide breaks.
The environment may also set up a differential aeration cell (a water droplet on the surface of a steel, for example) and pitting can initiate at the anodic site (Centre of the water droplet).
For a homogeneous environment, pitting IS caused by the MATERIAL that may contain inclusions (MnS is the major culprit for the initiation of pitting in steels) or defects. In most cases, both the environment and the material contribute to pit initiation.
The passive layer on stainless steel can be attacked by certain chemical species. The chloride ion Cl- is the most common of these and is found in everyday materials such as salt and bleach. Pitting corrosion is avoided by making sure that stainless steel does not come into prolonged contact with harmful chemicals or by choosing a grade of steel which is more resistant to attack.
How to prevent pitting corrosion?
The stainless steels are particularly sensitive to pitting corrosion, but other metals, such as passive iron, chromium, cobalt, aluminium, copper and their alloys are also prone to this form of damage.
Pitting corrosion can be prevented through:
- Proper selection of materials with known resistance to the service environment
- Control pH, chloride concentration and temperature
- Cathodic protection and/or Anodic Protection
- Use higher alloys (ASTM G48) for increased resistance to pitting corrosion