The study of copper oxidation in hot de-ionized water was initiated in response to observed cooling efficiency losses that potentially affect turbo-generator stator winding conductors. These losses are explained by the accumulation of copper oxides in hollow strands. Corrosion mechanisms involved are very slow and their direct approach requires performing long duration tests in an instrumented corrosion loop. Results show that exposure of pickled copper in aerated water (dissolved O2 content: 2–3 mg 1−1) enables an external CuO protective layer to form in about 200 h. If the dissolved oxygen content is low (about 20 μg 1−1) many of the oxide nuclei formed on copper surface are swept along by the water flow and accumulate on filters and downstream hydraulic circuit discontinuities. Once an oxidizing water chemistry has been specified, the satisfactory operation of generators then requires a cautious gas treatment of cooling water so as to prevent an accidental change in reducing conditions.