Surface Oxides
WARNING: This page is about a different car, the 2004 Volvo S40, 2003 Volvo S40, 2002 Volvo S40, 2001 Volvo S40, and 2000 Volvo S40. However, it is still accessible from the selected car via links, so may be relevant.
All metal surfaces oxidize in contact with air. This makes it important to fit a cable terminal on a stripped cable for crimping as soon as possible after stripping the cable.
Most cable terminals are tin plated. Tin oxidizes rapidly and the surface oxide coating makes a hard layer on the soft tin which cracks easily when a pin or socket is attached to create a contact.
The surface oxide around contact points protect build a gas tight joint which prevents further oxidization and provides a good electrical contact.