Pitting corrosion occurs when a passive film or another protective surface film breaks down locally. When this happens, an anode forms where the film has broken,
while the unbroken film, or protective layer, acts as a cathode. This process accelerates localized attack and pits will develop at the anodic spots. The electrolyte
inside the growing pit may become very aggressive, with a low pH, which will further accelerate the rate of corrosion.
Pitting corrosion is significant due to the fact that the pitting action is auto-catalytic, meaning the corrosion rate accelerates and can rapidly perforate through steel
plates leading to serious failures.
Key Properties of Pitting Corrosion
- Is caused by localized anodic area surrounded by strongly cathodic regions.
- The bottom of the pit becomes anodic compared to the top of the pit.
- Corrosion rate increases as electrolyte becomes acidic.
- Pitting corrosion is auto-catalytic.
- Rate is reduced in flowing applications.
- Stainless steels are particularly susceptible to pitting corrosion.
Where does it occur?
- Stainless steel tanks
- Piping
- Pumps
- Valves
- Heat exchangers
How to Control Pitting Corrosion?
Pitting corrosion may be controlled in a variety of ways, including:
- Materials selection
- Coatings
- Avoid stagnant areas in design
- Removal of chlorides
Source: Chesterton ARC Manuals
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