copper oxidation phenomenon
If the optimal chemical parameters of the stator winding cooling water are not met, the oxygen in the water reacts with the copper. The optimal operating parameters for copper cooling systems are dissolved oxygen contents below 50 ppb or above 2 ppm. In these ranges, the transition of copper ions into solution occurs slowly and poses no hazards. On the other hand, in the 200-500 ppb range, corrosion processes proceed relatively quickly and amplify the risk of sediment deposition.
A similar problem of unwanted oxidation can occur during generator shutdown when improperly protected channels are exposed to atmospheric oxygen.