With non-noble materials, these spot
contacts are clean-metal junctions formed by
the mechanical breakthrough of thin oxide
films on surface. If a slight relative motion
occurs between the contact surfaces, the
original spot contacts are broken and new
ones are formed. The old spots quickly
oxidise. If the motion is then reversed,
additional spots are exposed to oxidation.
With repeated cyclic motions of limited
amplitude so that the motion is ineffective in
cleaning away the wear debris and
accumulating oxides, a highly localized,
thick, insulating layer is formed in the contact
interface. This sequence of events leads to
rapid and dramatic increases in corrosion
resistance, proceeding to virtual open
circuits.