BigWallop wrote:

A relay will arc a little as the contacts make and break. It is why the
relay may have stuck in the first place, because the arc can cause a carbon
deposit to build on the face of the studs.

No. What normally happens is that localised corrosion sets in, and the
contacts don't 'make' properly. Its not carbon..its usually silver or
copper oxide.

The spark jumps the gap, and welds the things together. Usually leaving
a little 'pip' of metal.

Now knocking it may un-weld it, but the pip is there..a nice high
resistance point that will weld again.

Filing the silver flat gets you back to square one tho.