Rust, corrosion, pear skin and discoloration
How to tell rust from corrosion, what a dull, dimpled "pear skin" surface says about dirt, and how to tell a stain from heat colour with a wipe.
Rust and corrosion
Rust is a film of oxide, hydroxide or carbonate on the metal from chemical action. Corrosion is oxidation or dissolution of the surface by chemical action with acids or alkalis, including electrochemical action.
Where rust and corrosion come from
- A stopped machine cools to the dew point and humidity in the housing turns into water drops. They get into the lubricant, and rust forms on the bearing.
- Bearings stored for a long time in a humid place rust at spacings equal to the rolling elements.
- Sulphur or chlorine compounds in lubricant additives can decompose at high temperature and cause corrosion.
- Water that enters the bearing causes corrosion.
Countermeasures: rust and corrosion
- Improve sealing.
- Inspect the lubricant regularly.
- Protect bearings from rust during storage.
Pear skin
Pear skin is a rolling surface with many small dents made by foreign matter caught between parts. The surface looks dim and rough, and in the worst case turns colour from heat. It is mainly caused by dirt or by a lack of lubricant, so check those two first.
Countermeasures: pear skin
- Wash the shaft and housing carefully.
- Improve sealing.
- Filter the oil.
- Review the lubricant and lubrication method.
Discoloration
Discoloration is colour on the bearing from stains or from heat. Staining comes from a deteriorated lubricant or from coloured matter stuck to the surface, and some of it can be removed by scrubbing or wiping with a solvent. A brown colour on a rolling or sliding surface comes from acidic powder formed by wear that sticks to the surface, usually evenly around the circumference.
How to tell what you have
| Test | Result | Meaning |
|---|---|---|
| Wipe with an organic solvent | Colour comes off | Stain |
| Look under a microscope | Small dents from sparks | Electric pitting |
| Rub with fine sandpaper | Unevenness remains | Rust or corrosion |
| Rub with fine sandpaper | Colour disappears completely | Temper colour from heat |
Countermeasures: discoloration from heat
- Improve heat removal from the bearing.
- Improve lubrication.
- Review the operating conditions.