Tin Plated Copper

Copper used in bus bars is sometimes plated with Tin.  This is because copper oxidises in the presence of air.  The oxides of copper fall off the surface, this exposes more copper and further oxidation takes place.

These oxides increase the contact resistances in electrical contacts causing localized heating.  This heating causes further oxidation.  Thus, the copper is steadily eroded.

Tin plating is used protect the copper from corrosion.   Tin does not react easily with either air or water.  Tin is a soft metal which can be easily polished to give a shiny finish.   Tin Plated contacts are also used in the contacts of  isolators and switches.