Mechanism Of Corrosion And Plugging On Hollow Copper Conductor Of Generator And Its Prevention And Treatment

For the water-cooled stator system of the large generator,the copper-water interface is comprised of the hollow copper conductor with strong current and extremely narrow water flow channel and the generator internal cooling water(GICW)with high flow rate and of low conductivity.On this interface,the generation,release and selective redeposition of copper corrosion product would plug the hollow copper conductor,which can result in system abnormality or even accident such as the local overheating of the copper stator,the damage of the insulation layer,the unplanned unit strip and so on.The engineering failure case concerned in this thesis is that the hollow copper conductor of a 980 MW generator was failed and totally scrapped due to its severe corrosion and plugging in a nuclear power plant in our country.Herein,to reveal the mechanism of the corrosion and plugging,and further to propose the corresponding prevention and treatment processes,the specific engineering failure analysis,chemical thermodynamics analysis and dynamic simulation test were conducted.The main research work and results are summarized as follows.(1)By the engineering failure analysis and the chemical thermodynamics analysis,the mechanism of the release and the selective re-deposition of copper oxidation product was revealed under the low oxygen/neutral water chemistry condition.The failed hollow copper conductor that disassembled from the field was carefully characterized.Optical microscope,3D surface profiler,SEM and tube endoscope were used to characterize the corrosion product layer on the inner surface of the hollow copper conductor and the plug within the outlet water chamber.XRD,XPS,EDS and To FSIMS were used to identify the chemical composition of the corrosion product layer and the plug.The results indicated that the copper conductor was heavily corroded and plugged,and that O2 and CO2 inleakage had occurred for this system.Based on the failure analysis results,the chemical thermodynamic analysis was specifically carried out on this Cu-H2O-O2-CO2 multicomponent system.It was found that the copper conductor is extremely sensitive to the change of GICW water quality under the low oxygen/neutral water chemistry condition that adopted in this failure case.The copper oxidation layer would be subjected to the stress induced by the oxidation of the leaked O2 and the CO2 erosion induced by the leaked CO2,and then be released into GICW.Further,the released copper corrosion product tends to be selectively re-deposited and accumulated on the outlet of the copper conductor with higher temperature due to the temperature rise of GICW along the conductor.Worse,the coupling effect of the CO2 erosion and the temperature rise of GICW would significantly aggravate the local accumulation of copper corrosion product and result in the final plug of the hollow copper conductor.(2)By the chemical thermodynamic analysis and the dynamic simulation test,the underlying relationships between the corrosion,release,and selective re-deposition of copper conductor,and water chemistry conditions of GICW were revealed.Further,an intelligent conditioning process for GICW slight-alkalization by ion exchange reaction was proposed to prevent the copper corrosion and plugging.The quantitative relationships between the p H value as well as the potential of oxygen reduction(EOR)of GICW and the amount of CO2 inleakage were derived.It was revealed that the slightly alkaline water chemistry condition can effectively alleviate the impacts of CO2 inleakage on the system compared with the neutral water chemistry condition,and thereby inhibit the generation and release of copper oxidation product.Besides,the quantitative relationships between the solubility of Cu O as well as its temperature coefficient and the p H value of GICW were derived.It was demonstrated that in the slightly alkaline GICW,the solubility of Cu O is much lower than that in the neutral GICW,and its temperature coefficient is of positive value.Thereby,the temperature rise of GICW would not induce the selective re-deposition of copper oxides on the place with high temperature under the slightly alkaline water chemistry condition.Moreover,this water chemistry condition can effectively alleviate the negative impacts of the temperature rise of GICW coupled with CO2 erosion due to its buffer capacity against CO2 inleakage.Further,an integrated device for the dynamic simulation test of the cooling water of low conductivity flowing through the pure copper conductor was designed and constructed.Upon which,the oxidation properties of the GICW as well as the copper oxidation behaviors under the neutral and the slightly alkaline water chemistry conditions were studied.The Oxidation-Reduction Potential(ORP)value of GICW was proposed to characterize its oxidation property.The results demonstrated that the copper conductor locates at the stable passivation region of Cu O within the slightly alkaline GICW,while at the sensitive region to copper corrosion within the neutral GICW.In the dynamic simulation test of the slightly alkaline water chemistry,an intelligent conditioning process for GICW slight-alkalization by ion exchange reaction was proposed.By automatically controlling the operation of the parallel ion exchange reactors in the bypass for GICW slight-alkalization,the GICW of low conductivity for copper stator cooling and of high p H value for the copper corrosion inhibition can be synergistically controlled.During the self-passivation of copper conductor,the microscopic characteristics of the formation of the passive film were described.Whereas,in the dynamic simulation test of the neutral water chemistry,the oxidation layer was formed slowly and of poor quality due to its high solubility in the neutral GICW.It was also confirmed that the dissolved copper species would be locally re-deposited within the system due to its concentration exceeding the saturation.(3)By the dynamic simulation test,the EDTA complexing clean kinetics of the corroded layer within the hollow copper conductor was revealed.Further,an EDTA complexing clean process with a constant EDTA concentration by the H2 Y formed anion exchange resin was proposed to gently treat and remove copper corrosion product.The dynamic simulation test method for the EDTA complexing clean of the real corroded copper conductor was established.The effects of H2O2 addition,temperature,EDTA concentration and p H value on the complexing dissolution rate of the corroded layer were studied by the single factor test.Thereby,the clean conditions were optimized.Moreover,a novel EDTA complexing clean process with a constant EDTA concentration by the H2 Y formed anion exchange resin was put forward.By the ion exchange reaction between the H2 Y formed anion exchange resin and the Cu-EDTA complex anion that generated by the complexing dissolution of the corroded layer,the Cu-EDTA complex can be removed while exchanging out the equimolar clean agent of EDTA.By this way,a constant concentration of EDTA can be maintained during the clean.By the comparative study of this novel EDTA complexing clean and regular cyclic clean,it was approved that the optimized process boosts the complexing dissolution rate under the premise of gentle chemical clean without adding copper corrosion inhibitor.During this optimized complex clean,the microscopic characteristics of the dissolution of the corroded layer were described.