corrosion in slewing ring bearing

[Gerry45]

I have recently been shown raceway/ball surfaces inside a slewing ring bearing which had supporting a blade of a large wind turbine for around 6 months.

The raceway surfaces showed the usual zones of fretting where the balls had spent most of their time. In addition to this, around approx 1/4 of the circumference of the rings, there was evidence of water induced corrosion. This existed mainly about the ball/raceway contact areas but with the odd spot randomly positioned on the raceways between the normal contact points. In my experience, the appearance suggests that somehow water had entered the bearing, probably either at assembly, greasing or in service. The puzzle is that an analysis of the grease (moly disulphide type 'longterm 2 plus') indicated:-
i) no water present
ii) unusually high levels of phosphorus.

I need to ask further questions as to how the water content of the grease sample was measured, but as far as the high P content goes, can anyone guess as to where this has come from ? I am not clear at present as to the cage material, but if this is brass, could the phosphorus come from this by some strange chemical mechanism ? None of the constituents of brass were detected in the grease. Any comments would be welcome.

 

[TVP]

Could the phosphorus have been present as a zinc phosphate layer on one of the steel components? This is a typical pretreatment for steel parts, often in conjunction with some type of soap (stearate) or polymer (low molecular weight wax-type) as a lubricant for metal forming.

 

[Carburize]

If the oil lubricant in the grease had an EP additive package it is possible that this is the source of the phosphorus. Lubricant oils with EP additive packages have in excess of 150 ppm phosphorus.