None; seemy comment above. The nickel in contact with the puck excludes air from the interface; the rubber gets carbonised (emitting some water and, I guess, sulphuric acid in the process which helps keep the air out) and the carbon layer gradually gets thicker until it forms a fairly impervious shield.
In the same way, large timbers such as are used in old house beams and ships may become quite self-extinguishing in a fire once a thick carbon layer has formed. Ship timbers were often saturated in salt from inside as a preservative, and this increased their fire resistance still further.
