How to fix bad contact on busbar?
A picture postcard example of "wrong breaker type"

Note the alien breaker in position 27, a GE, an obvious misfit. If that one is still working, it's only because it hasn't been put under heavy load as this one was. 

People who think "if it snaps in, it's good" fail to notice that the snap-in pressure is all wrong compared to proper breakers.  That's because even though breakers often roughly fit, each manufacturer's busbar is a different shape. The correct breaker gets surface contact.  The wrong breaker gets edge or corner contact - works for a few lights but when you load it, it starts arcing.  Since this is on the supply side of the breaker, nothing stops this.  Anyway it's a series arc, current won't be more than the load's draw. 

Siemens makes correct breakers for ITE panels. 

The contact isn't corroded.  It is destroyed.  The arcing removed metal from the contact. The metal is gone, and cannot be restored.   That slot is done for and you will not be fitting breakers there anymore. 

Your best bet is to plug the hole with something.  Your choices are a panel hole cover UL listed for that panel, though those can be hokey and hard to find; or a spare Siemens/ITE breaker, labeled "Unused", under $5.