Pitting corrosion forms when localized corrosion creates small holes in a metal surface. According to provided references, pitting corrosion specifically occurs when the cathode area is large relative to the anode area.
The Pitting Corrosion Process:
Pitting corrosion is a localized form of corrosion that results in the creation of small holes in the metal. This can happen due to several factors:
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Damage to the Protective Layer: Often, metals are protected by a surface layer or film. If this layer is damaged or cracked, it exposes the underlying metal.
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Formation of Cathode and Anode:
- The damaged coating typically becomes the cathode, which is the area where reduction occurs.
- The exposed metal becomes the anode, which is the area where oxidation (corrosion) occurs.
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Area Disparity: Pitting corrosion is more likely when the cathode (damaged coating) is large, and the anode (exposed metal) is small.
Illustration:
| Component | Role | Description |
|---|---|---|
| Damaged Coating | Cathode | Large area where reduction occurs due to damage or cracks. |
| Exposed Metal | Anode | Small area where oxidation (corrosion) is concentrated. |
Example:
Imagine a stainless steel surface with a scratch. The scratched area (exposed metal) becomes the anode, while the surrounding undamaged stainless steel (protective film) acts as the cathode. Because the undamaged area is much larger, the corrosion is focused on the small anodic area, leading to pit formation.
Factors Contributing to Pitting:
- Presence of Chlorides: Chloride ions are especially effective at penetrating passive films and initiating pitting.
- Stagnant Environments: Lack of oxygen in certain areas can promote anodic reactions.
- Material Inhomogeneities: Variations in the metal's composition can create anodic and cathodic sites.
