Generator rotor shaft pock marked

[eeprom]

I've got a generator, a 6MW GE unit from the early 60s. During a recent turbine maintenance we noticed that the shaft under the #3 bearing there were significant pock marks due to electrolysis. This is a 3 bearing machine; the #3 is at the collector end of the generator rotor. This is the only bearing which has markings.

The bearing insulation tested good. The clearances of the bearings were tightened during the last turbine maintenance. I have been told that a grounding brush was in place, but I have not seen it.

Any ideas if this is related to the change in clearances? And if a grounding strap is being used, how could there be ground current to cause this?

thanks,
EE

 

[eeprom]

Additional information....
There are no unusual marking on the bearkings. There is one scuff mark where the shaft must have scratched the babbett.

The change in clearance was done to the oil seals only, not the bearing itself. The only embedded device in the bearing is the RTD.