Pitting
Pitting turns up in 3 fields: tribology, corrosion, gears. Here is what it means to the people in each one.
Pitting in tribology
Rolling contact fatigue that leaves visible craters on gear teeth or bearing races.
Fatigue in which cracks grow under repeated contact stress and break out small pieces of metal, leaving craters usually half a millimetre across or larger. Initial pitting on new gears can stop once the high spots wear in; progressive pitting keeps spreading into spalling, noise and eventually tooth fracture. Oil analysis catches it as a rising PQ and fatigue particles on the ferrogram, and vibration picks up the gear mesh sidebands.
"Progressive pitting across the pitch line on two teeth, trend it monthly on vibration until the shutdown."
Pitting in corrosion
Localized corrosion that drills small, deep cavities into metal while the surface around them stays nearly intact.
Localized attack in which small anodic spots corrode rapidly while the surrounding surface stays passive or protected. Inside the pit, chloride concentrates and acidity rises, so the pit feeds itself and grows deeper rather than wider. Stainless steels, aluminum and poorly protected coated steel are the usual victims. A pit can perforate a pipe or tank wall with only a tiny loss of total metal, which is why average weight-loss rates understate the risk.
"UT shows nominal wall everywhere, but the cut-out has pitting under the deposits nearly through to the OD."
Pitting in gears
Surface fatigue of gear teeth that breaks out small craters, typically on the flank just below the pitch line.
Contact fatigue caused by repeated Hertzian stress, in which cracks start at or just below the surface and release small pieces of metal, leaving craters. It usually appears first in the dedendum near the pitch line, where rolling combines with negative sliding and a single tooth pair carries the whole load. Initial pitting may stop once high spots wear in, while progressive pitting spreads and leads to spalling, noise and tooth fracture.
"Pitting is just a band of little pits below the pitch line, it's not spreading yet, re-inspect next outage."