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48 OCTOBER 2017 MATERIALS PERFORMANCE NACE INTERNATIONAL: VOL. 56, NO. 10 MATERIALS SELECTION & DESIGN P Hollow copper strands in power gen- erators can suffer plugging and flow restrictions caused by copper oxide deposition in the stator cooling sys- tem. Response surface methodology was used to optimize the individual effects and interaction for the oper- ating temperature, pH, and dissolved oxygen (DO) to decrease the Cu cor- rosion rate. Scanning electron mi- croscopy was employed to investi- gate the morphology of oxides coupled with a proprietary software to define species formed on the Cu surface. The optimum condition was temperature 40 °C, pH 9.0, and DO 0.1 mg/L. At this condition, the corro- sion rate could be lower than 0.005 mm/y. Plugging of hollow copper strands in w a t e r - c o o l e d g e n e ra t o r s i s a n i s s u e occasionally encountered in large nuclear and thermal power plants, which results in load limitations or unplanned shutdowns. Plugging mechanisms of Cu strands in demineralized water, which is the most commonly employed coolant in stator cool- ing water (SCW) systems, were investigated through various methods. Moliere, et al. 1 demonstrated that cuprous oxide (Cu 2 O) nuclei were formed with traces of dissolved oxygen (DO). These nuclei can be swept by the water and produce colloidal suspen- sions, which deposit on the downstream hollow strands and filters. A black passive film was formed at DO content higher than 50 µ g/L, which was caused by the growth and oxidation of an adhering Cu 2 O nuclei layer. Then, as Svoboda 2 demonstrated, the oxides moved and redeposited, resulting in plugging. The mechanism of plugging can be summarized as four steps: oxidation of Cu, release of the oxidized Cu, transport of the free Cu, and subsequent redeposition of the oxides. Park 3 investigated the electrochem- ical corrosion potential (ECP) of the Cu, which shifted from 180 mV vs. saturated hydrogen electrode (SHE) to 260 mV vs. SHE when the water chemistry changed from low to high oxygen content. Cu oxides transformed from one type to another when the potential was more positive than 240 mV vs. SHE. It is believed that local redeposition of corrosion products was responsible for the increase in hydraulic resistance. An investigation made by EPRI further confirmed the conclusions demonstrated by Park on the measurements of ECP by in situ experiments in the Graig and Tarong power plant. This program revealed that the particle sizes were in the range of 4 to 11 µ m, with the great majority in the range of 4 to 5 µ m, which was detected by a 0.45- µ m filter membrane and laser particle counter. 4 Nevertheless, it is difficult to con- trol DO content in weakly sealed genera- tors, especially in China, even though this method is crucial for avoiding corrosion. Alkaline treatment, which directly or indi- Optimizing Conditions to Control Copper Corrosion in Stator Cooling Water z ha N g r uiyua N a N d c ao Shu N 'a N , Wuhan University, Wuhan, Hubei, P.R. China z ha N g Xuefe N g, Central Southern China Electric Power Design Institute Co., Ltd., Wuhan, Hubei, P.R. China