The corrosion of overlay bearings Available to Purchase

AN OVERLAY bearing is a bearing with a precision electroplated overlay of lead‐tin or lead‐indium alloy, 20 to 40µ thick. This lead‐alloy overlay is generally applied to a copper‐lead or lead‐bronze substrate, which is itself bonded to a steel backing. Bearings of this construction are probably the most extensively used type of engine bearings; many millions are manufactured annually. The main functions of the overlay are to provide a seizure‐resistant surface, to increase the tolerance of the bearing for dirt and wear‐debris, and to protect the lead in the underlying copper‐lead or lead‐bronze alloy from corrosion by oxidized oil. When mineral oils are exposed to air oxidation for prolonged periods at elevated temperatures, weak organic acids are formed which can dissolve pure lead. The lead in copper‐lead or lead‐bronze is present as a separate, unalloyed phase, very susceptible to corrosion by weak organic acids. However, it has been known for some time that if lead is alloyed with indium or tin it is not corroded in this way. Most bearing manufacturers incorporate at least 4%w indium or 8%w tin in overlays to make them resistant to corrosion, but precise information on the amounts of alloying element required is not available.