I measured the voltage difference again between the bolt and the mast again more that a week after that I turned the bolt a few revs and there is still hardly any potential difference at all. Newer the less I worked out the bolt it toke a while and in principle the bolt is totally isolated from any other metal contact as there a "plastic" washers and plastic "bearings" where the bolt is, so the only reason I can think of for the voltage difference is salt water and salt crust.
I will treat the bolt with TefTec before mounting it back.
By the way what paint do you use for repainting the mast around the corroded are?= I need to paint as well
Paul on S/Y Kerpa SM#259