What Does Micro Pitting Mean?
Micro pitting is a form of fatigue failure that occurs on a material's surface, typically seen in rolling gears and bearings.
It is similar to pitting corrosion—the only difference is the size of the pits. The pits formed through micro pitting measure 10-20 µm in depth and often exhibit a gray or frosted appearance. This form of corrosion damage is commonly caused by inadequate lubrication.
Micro pitting is also known as micro spilling, frosting and staining.
Corrosionpedia Explains Micro Pitting
Micro pitting is a type of local corrosion that produces damage in the form of pits or spots. This type of corrosive attack usually occurs in locations with weakened passive areas, such as in imperfect or damaged passive layers and through slag inclusions. When the attack begins, a certain material can become perforated within a short period.
Alloys that are most prone to micro pitting are the ones in which corrosion resistance results in a passivation film, such as:
- Aluminum alloys
- Nickel alloys
- Stainless steel
Metals that are prone to uniform types of corrosion do not undergo pitting. Hence, carbon steel undergoes corrosion under seawater, while metals like stainless steel undergo pitting.
Compared to fresh water and brackish water, natural seawater is highly corrosive due to its chloride concentration. This is due to the fact that biological activity can take place. Thus, biocide and chlorination are usually used to control the proliferation of microorganisms in pipe systems, especially in seawater.
In order to determine resistance to pitting, certain tests should be employed. Such tests evaluate the grade of the steel and its resistance to corrosive attacks such as pitting. One example is the critical pitting temperature (CPT).