Definition - What does Pitting Corrosion mean?
Pitting is a type of corrosion that occurs in materials that have protective films. It is an attack with localized holes on the metal's surface. The attack can penetrate the metal very rapidly, while some parts of the metal surface remain free from corrosion.
Pitting is vigorous when the solution on the metal surface contains chloride, hypochlorite or bromide ions. Other harmful solutions are those that contain fluorides and iodides, while sulfides and water are known to enhance the pitting process.
Corrosionpedia explains Pitting Corrosion
When metal is exposed, its available electrons are given up, and thus tiny pits begin to form on the metal surface. This then grows to become a rapid attack that results in massive damage of the metal. The oxidizing cation of iron, copper and mercury, among others, enables the formation of pitting even when there is no supply of oxygen in the metal surface. Stainless steel, chromium, passive iron, cobalt, aluminum, copper and associated alloys are all prone to pitting corrosion.
A tubercular morphology can be seen where pits develop. Pitting is not always local in nature, as even when intrinsic defects in the solution-metal interface, the potential nuclei remains intact. Their development and stabilization show a random nature, and galvanic coupling established in the zones of discontinuity where metal dissolution occurs, lead to the formation of small anodes.
Pitting corrosion can be controlled by:
- Use of a more resistant material
- Ensuring that the fluids in contact with the material are either washed away or are injected at a high velocity
- Reducing the medium's aggressiveness
- Use of cathodic protection
- Avoiding stagnant zones
- Use of appropriate materials for service conditions
- Proper use of inhibitors or control of fluid chemistry
- Use of a coating that will prevent pitting on metal surfaces
- The ability to maintain the protective film of the same material