Fretting corrosion is a wear failure at the bearing mounting interface — repeated micro-motion between the bearing ring and the shaft or housing generates reddish-brown iron oxide powder (Fe₂O₃) that progressively loosens the fit and damages the shaft surface.
Fretting corrosion occurs when two surfaces clamped together under pressure experience small-amplitude oscillation (typically 1–100 µm amplitude). See how to diagnose and prevent this and related failure modes at bearing damage and failure modes, and choose spherical roller bearings with the correct interference fit for heavy rotating loads. Under high contact pressure, the natural passive oxide layer is continuously broken and reformed — fresh metal exposed to oxygen and moisture produces fine Fe₂O₃ powder. This powder becomes trapped between surfaces and acts as an abrasive, accelerating the damage cycle. The result: reddish-brown pits on the shaft surface, reduced shaft diameter, and a loosened inner ring fit.
Causes and Triggering Conditions
Fretting corrosion appears in two main situations: (1) Inadequate interference fit — a bearing mounted on a shaft with insufficient press fit allows micro-motion under cyclic loading. (2) Stationary equipment exposed to external vibration from nearby running machinery or during transportation — similar to false brinelling but occurring at the mounting interface rather than the raceway.
Distinguishing fretting corrosion from similar failure modes per ISO 15243: fretting corrosion produces reddish-brown Fe₂O₃ powder on the outer face of the inner ring bore and the shaft surface — not on the raceway. False brinelling produces reddish-brown indentations with oxide powder on the raceway itself. True brinelling produces bright, shiny indentations on the raceway with no rust.
The location of the oxide powder is the diagnostic key: outside face of inner ring and shaft surface = fretting corrosion. Raceway surface = false brinelling. No oxide, shiny indentation = true brinelling.
Practical Example: Bearing 6308 C3 on a Vibrating Pump
At a water pumping station in Can Tho, a 6308 C3 bearing (d=40, D=90, B=23 mm, C=32.5 kN, C₀=19 kN) mounted on a pump shaft developed fretting corrosion after 8,000 hours. A technician found reddish-brown powder around the inner ring bore during scheduled maintenance. Shaft diameter measured 39.97 mm — 30 µm below the nominal 40 mm — insufficient interference fit remained. Shaft restoration required chrome plating and regrinding to nominal diameter.
| Parameter | Design value | Measured after failure | Unit |
|---|---|---|---|
| Shaft diameter (d) | 40 k6 (40.002–40.018) | 39.97 | mm |
| Fit tolerance | k6 (interference) | clearance | — |
| Shaft surface finish Ra | 0.8 | 2.5–4 | µm |
| Fe₂O₃ powder present | None | Yes (reddish-brown) | — |
Fit Selection and Prevention
The fit tolerance between inner ring bore and shaft is the primary lever for preventing fretting corrosion. ISO fits for rotating inner ring applications:
| Load condition | Shaft tolerance | Interference range (d=40) | Typical application |
|---|---|---|---|
| Light radial | j5 | 0–10 µm | Light conveyor rollers |
| Normal radial | k6 | 2–18 µm | General machinery, pumps |
| Heavy radial | m5 | 9–25 µm | High-vibration, impact loads |
| Very heavy | n6 | 17–33 µm | Rolling mill, crusher bearings |
For a 6308 C3 on a pump shaft (d=40), k6 tolerance provides 2–18 µm interference — correct for normal load. Using j5 or h6 instead creates clearance fit risk and significantly increases fretting probability.
Shaft surface finish also matters: Ra ≤ 0.8 µm is specified for bearing seats. Surfaces above Ra 1.6 µm reduce effective contact area and lower the true interface pressure.