Fretting corrosion is surface damage that appears where two clamped, press-fit or nominally stationary metal surfaces undergo small oscillatory micro-motion under load, so that mechanical wear and oxidation combine to produce shallow pits and a fine oxide debris that is reddish-brown on steel or black on aluminium. It is a slow, insidious mechanism because the joint looks tight and nothing visibly moves, yet slip amplitudes of only a few micrometres are enough to break the surface and drive damage.
Two conditions must be present at the same time: a normal clamping or contact load, and a repeated relative slip between the surfaces. The slip is tiny, typically in the range of about 3 to 50 micrometres, driven by vibration, cyclic bending, thermal cycling or fluctuating load. Each micro-slip shears the protective oxide film and exposes fresh metal. That metal immediately re-oxidises, and the hard oxide particles cannot escape the closed contact. They act as an abrasive, accelerating wear in a self-feeding loop of wear plus oxidation.
The trapped debris is the tell-tale sign. On ferrous surfaces it is red-brown iron oxide, often called cocoa or red mud; on aluminium and titanium it is a black powder. Underneath, the metal shows shallow pitting, a matt worn patch, and frequently small surface cracks that can act as fatigue initiation sites. This link to fatigue is why fretting matters beyond cosmetics: fretting fatigue can lower the endurance limit of a component substantially compared with clean, unfretted metal.
Fretting concentrates at joints that are meant to be fixed but still carry vibration or cyclic strain:
A special case is false brinelling, which affects bearings that are stationary but exposed to external vibration, for example machines in transport or standby units next to running equipment. The rolling elements micro-oscillate against the raceway without rotating, so lubricant is never replenished in the contact and fretting damage forms at each ball or roller position. The result is a pattern of marks spaced at the rolling-element pitch that mimic true brinell indentations from overload, hence the name. Unlike true brinelling there is no plastic denting; the metal is worn and oxidised. False brinelling shortens service life and, once running resumes, seeds noise and vibration. Understanding it alongside rated fatigue life helps set inspection intervals; see our note on L10 bearing life for how rated life and real service conditions diverge.
| Site | Dominant mechanism | Primary prevention |
|---|---|---|
| Bearing ring on shaft/housing | Micro-slip from cyclic load and loose fit | Correct interference fit; clean, lubricate mating surface |
| Idle bearing (false brinelling) | Vibration with no rotation; lubricant not replenished | Periodic rotation; anti-fretting grease; vibration isolation |
| Bolted / riveted joint | Slip from insufficient clamp preload | Raise and verify preload; add friction; gaskets or shims |
| Spline / keyway / hub | Torsional reversal and backlash | Tighter fit; solid lubricant coating; MoS2 or PTFE |
| Electrical contact | Thermal cycling; oxide raises resistance | Gold or tin plating; contact grease; higher contact force |
The most reliable cure is to eliminate the relative slip. Increasing clamping load or interference so the surfaces cannot move is the first lever: a properly preloaded bolted joint or a correct shaft fit removes the driver entirely. Where motion cannot be fully suppressed, the strategy shifts to managing the contact.
Because the resulting pits and cracks resemble other localised attack, confirm the diagnosis before acting. Trapped oxide debris and micro-slip point to fretting; isolated cavities under a stagnant electrolyte point instead to pitting corrosion, which needs a different remedy.
Fretting is hidden inside joints, so detection leans on indirect signs: red or black debris weeping from a bolted seam, rising vibration or bearing noise, growing electrical contact resistance, or fretting marks found at teardown. Vibration trending catches developing bearing problems early, and torque checks on critical fasteners catch preload loss before slip begins. Logging these inspections against each asset turns scattered observations into a trend. A CMMS such as Fabrico can schedule the fastener re-torque and lubrication routines and hold the inspection history, so recurring fretting on a specific joint becomes visible rather than a surprise at failure. Book a Fabrico demo to see how those routines are tracked.
On steel it appears as reddish-brown powdery debris, often called cocoa, around a matt worn patch; on aluminium and titanium the debris is black. Under the debris the surface shows shallow pits and sometimes fine cracks.
False brinelling is fretting at the rolling contacts of a stationary bearing exposed to vibration. It produces marks spaced at the rolling-element pitch that look like overload dents but are actually worn, oxidised patches with no plastic denting.
Lubrication reduces it by lowering friction and keeping oxide debris from packing, but it rarely stops fretting alone. The durable fix is to remove the micro-motion through correct preload or fit, then support that with lubricant or coatings.
The small cracks that fretting seeds can initiate fatigue failure, cutting the endurance limit of the part. A joint can look tight yet fail well below its expected load because fretting has weakened the surface.
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