Fretting corrosion is combined mechanical and chemical damage where two metal surfaces rub together and corrode at the same time. The rubbing strips away the passive film that protects stainless steels, nickel alloys and copper alloys, exposing fresh metal that corrodes. Repeated over many cycles, this removes significant material and can turn light polishing into deep gouging.
What is fretting?
Fretting is the mechanical damage caused when two metal surfaces rub against each other. It happens where surfaces are not designed to move relative to one another, under vibration, or at the extremities of rotating components where there is a relative speed difference. The damage ranges from light burnishing or polishing to heavier gouging and grooving under higher loads.
What causes fretting corrosion?
Fretting corrosion is fretting and corrosion acting together, each worsening the other. The passive film that protects stainless steels, nickel alloys and copper alloys is rubbed away by the fretting action, exposing fresh, unprotected metal that then corrodes. Repeated over many cycles, this steadily removes significant amounts of metal.
Which materials are affected?
Any alloy that relies on a passive film is vulnerable, including stainless steels, nickel alloys and copper alloys. For a related surface-corrosion topic, see preventing galvanic corrosion.
How can fretting corrosion be mitigated?
Fretting corrosion cannot be reversed, so prevention is the better route:
- Remove the source of vibration where possible, or damp it if it cannot be removed.
- Coat surfaces designed to move against each other to improve hardness or lubricity.
- Pair metals carefully, combining a relatively hard, strong metal with a softer, less strong one, as with bearings and bushes.
- Add seals, gaskets or bearings between the surfaces of rotating components to separate them.
For remediation once damage has already occurred, see how to remove fretting corrosion.