Pitting Corrosion
Pitting corrosion most often takes place at points where the passive layer might be weakened, for example by slag inclusions, a damaged surface or imperfections in the passive layer.
Pittings may take the shape of small holes, 1-2 mm in depth and diameter to begin with. At times they are almost hidden below the surface of the steel with an "entrance hole" of only a few tenths of a mm. with a cavity of 1-2 mm below. They are therefore sometimes very difficult to detect, but might cause severe damage if left unattended.
Pitting may develop in a generally corrosive surrounding, especially when the surface is disturbed or when some extra chemical aggressive agent is present such as:
- Chlorides (seawater, "salt") Contents above 200 ppm are generally dangerous when together with some other corrosive agent such as phosphoric acid. Fluorides have a similar effect.
- Particles of iron or other materials (fremdkorper) on the surface
- Craters or pores in weld deposits
- Surface defects such as micro cracks and rough surfaces
- Lack of oxygen renders formation of chromium oxide difficult
- High temperatures above 400oC corrosion rates increase rapidly
Pitting Corrosion - Brief Facts
- Pitting most often takes place at points where the passive layer might be weakened, for example by slag inclusions, a damaged surface or imperfections in the passive layer
- Pitting corrosion takes place in neutral or acidic solutions containing primarily chlorides (Cl-), for example in neutral seawater
- Factors influencing Pitting: Cl- content, pH value, temperature, presence of oxidizing agent
- Starting Pitting (see pic) can be detected by white dots on the surface and need to be properly cleaned before further deterioration of the surface into actual pits
- Pitting corrosion can cause severe damage if left unattended
