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Pitting is characterized by craters on a surface, or for thinner parts, holes that may go through the part. Pits are a localized form of corrosion that can be difficult to detect because they can be quite small or covered with corrosive by-products. By the time the pits can be seen using visual inspection, the part or structure may be beyond repair. Pits can manifest in many different shapes, from broad and shallow to narrow and deep. In some instances, a pit may have a narrow throat near the surface but expand subsurface.

Pits can form as a result of damage to a material’s naturally occurring oxide layer or passive film caused by chemical factors, such as ion attack or low oxygen, which destabilizes the oxide layer. Pits can also form as a result of mechanical damage to the oxide layer. Localized damage to coatings that are more noble than the substrate material can result in particularly aggressive pitting. Further, the pit can form a stress concentrator that can lead to structural failure as a result of stress corrosion cracking. 


Pitting corrosion can be seen in hydraulic rods used in mining operations where the ore and gangue may be acidic or high in aggressive salts. Because of the repetitive action of the steel piston along with the chemically active conditions, pits can form on the piston resulting in failure. Oerlikon Metco uses the high velocity oxy-fuel (HVOF) process to apply a tungsten carbide coating in a corrosion resistant matrix. Not only does the coating help to prevent pitting but it also provides wear resistance. A sealer is used to fill the already low porosity of the coating which further helps with corrosion resistance.