Most ball bearings are made of a type of steel known as high carbon chromium steel, often called chrome steel. This is used for reasons of cost and durability. Bearings are also made from other materials such as stainless steel, ceramics and plastic.
Chrome steel will corrode if exposed to moisture so, where corrosion resistance is needed, bearings made from 440 grade stainless steel are often used. This is a very hard, magnetic steel with good corrosion resistance but not suitable for use with salt water and many chemicals. 316 grade stainless steel is used for more corrosive applications but it is much softer so can only be used where loads and speeds are low.
Ceramics (zirconia and silicon nitride) are good for very corrosive or extreme temperature use while various types of plastic are suitable for good to very good corrosion resistance although only suitable for low load and low speed. 316 stainless steel bearings, ceramic bearings and plastic bearings are non-magnetic.
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