Effects Of Simulated Hot Functional Test Water Chemistry On The Corrosion Behaviors Of Nuclear-grade Austenitic Alloys In High-temperature And High-pressure Water

The corrosion behavior of the key equipment in primary coolant of pressurized water reactor(PWR)nuclear power plants exposed to high-temperature and high-pressure(288-340 ℃,12-16 MPa)water involves the processes of formation of the oxide films and release of corrosion products or metal ions from reactive base metal.The released corrosion products or the metal ions,especially Ni ions,carried by the reactor coolant system(RCS)can deposit on the surface of fuel assemblies as crud and become activated.These activated corrosion products may again be released and transported through the RCS and incorporated into the ex-core oxide films,causing an increase of dose rate which greatly affects the personnel radiation exposure during shutdown maintenance and periodical inspection.Therefore,a protective and stable pre-film can significantly reduce the release rate of metal ions especially the Ni ions and the uniform corrosion rate of the key equipment during the subsequently normal operations.Hot functional test(HFT)is the key equipment of the primary circuit in PWR exposed to high temperature and designed water chemistry conditions without fuel for the first time.An ideal HFT water chemistry should pre-form an oxide film on the surface of the base metals which can protect the substrate from further corrosion and greatly reduce the release of metal ions and the incorporation of activated corrosion products during the subsequently normal operations.However,the constraints of schedule or budget may prevent the plants to perform passivation processes as part of HFT or as part of the initial startup after SG replacement and an ex-situ process may be considered to passivate the surfaces of selected components prior to HFT.The present work investigated two kinds of passivation processes.On the one hand,the effects of HFT water chemistry parameters on the characteristic of the oxide films formed on 316LN stainless steel(SS)and Alloy 690 were investigated by in-situ high-temperature electrochemical testing methods,ex-situ oxide film characterization methods combined with high-temperature solubility curves and E-pH diagrams of the corrosion products calculated from high-temperature thermodynamic parameters.A protective and stable pre-film can be formed on the key equipment by regulating and optimizing HFT water chemistry.On the other hand,in order to found the ex-situ surface treatment process that can decrease the oxidation rate of Alloy 690 in high-temperature water,the synergetic effects of surface state and Zn injection on the corrosion behavior of Alloy 690 in borated and lithiated high-temperature water were investigated by ex-situ oxide film characterization methods combined with high-temperature solubility curves of the corrosion products.The effects of boric acid and lithium hydroxide on the corrosion behaviors of 316LN SS in simulated HFT high-temperature water at 300℃ were investigated.The electrochemical testing results show that the resistance of oxide films formed on 316LN SS in low concentration of boric acid solutions is the largest among all testing solutions and the protective ability of the oxide films with pre-exposure to low concentration of boric acid solutions is higher than that with or without pre-exposure to low concentration of lithium hydroxide solutions when subsequently exposed to primary coolant.According to the high-temperature solubility curves,the massive dissolution of Fe promotes the formation of Cr-rich layer.Defects as a result of dissolution of Fe would permit the formation of fast paths for oxidant to access to the metal/oxides interface and react with the Cr-rich layer to form a protective inner oxide film rich in Cr3+.The effects of Zn injection into boric acid,lithium hydroxide and borated and lithiated solutions on the corrosion behaviors of Alloy 690 and the optimal concentration of Zn injection into simulated HFT water chemistry were investigated.The electrochemical testing results show that Zn injection into boric acid solutions decreases the oxide films resistance while Zn injection into lithium hydroxide solutions increases the oxide films resistance.20-100 ppb Zn injection into low concentration of lithium hydroxide solutions can significantly improve the oxide films resistance of 316LN SS.The resistance of the oxide film with pre-exposure to Zn-injected HFT water chemistry in Zn-injected primary coolant has been significantly improved.According to the high-temperature solubility curves,Zn injection into boric acid solutions ihibits the oxidation and dissolution of metallic Ni0 and increases the incorporation of metallic Ni0 into the oxide films.While Zn injection into lithium hydroxide solutions increases the oxide film resistance as the formation of thermodynamically stable ZnCr2O4 with higher Cr3+content and fewer defects.The effects of boric acid and lithium hydroxide with or without Zn injection on the corrosion behaviors of Alloy 690 in simulated HFT high-temperature water at 300℃ were investigated.The electrochemical testing results show that Zn injection into boric acid or lithium hydroxide solutions decreases the passivation current density of Alloy 690.The passivation current density of Alloy 690 in low concentration of lithium hydroxide solutions is the minimum among all testing solutions with or without Zn injection.20-100 ppb Zn injection into low concentration of lithium hydroxide solutions can significantly decrease the passivation current density and broaden the passivation potential range.The content of Ni in the oxide films formed on Alloy 690 in 0.5 ppm Li solutions with or without 50 ppb Zn injection is low.According to the high-temperature solubility curves and E-pH diagram,NiCr2O4 with relatively high solubility could form on Alloy 690 in boric acid solutions without Zn injection,resulting in a relatively high dissolution rate of the oxide films and oxidation rates at the alloy/oxides interface.This may increase the defect density of the oxide film and lead to the formation of a thick oxide film on Alloy 690.FeCr2O4 and Cr2O3 with relatively low solubility could form on Alloy 690 in lithium hydroxide solutions without Zn injection,which may lead to the formation of a thin oxide film.Zn injection into boric acid or lithium hydroxide solutions promotes the formation of thermodynamically stable ZnCr2O4 with the lowest solubility and defect density,which significantly improve the protective properties of the oxide films.The synergetic effects of surface state and Zn injection on the corrosion behavior of Alloy 690 in borated and lithiated high-temperature water were investigated.At the early stage of corrosion,the cold-worked layer on Alloy 690 can provide fast path for oxidant or Zn2+ to diffuse inward which promotes the selective oxidation of Cr.Especially initial exposure to Zn-injected primary water chemistry promotes the formation of thermodynamically stable ZnCr2O4 which decreases the defects of the oxide films and the oxidation rate at the alloy/oxides interface.While mechanical polishing effectively removes the cold-worked layer on Alloy 690 which inhibits the selective oxidation of Cr and the enrichment of Cr3+ in the inner oxide films.In addition,Zn-injected water chemistry can significantly decrease the oxidation rate of Alloy 690 with different surface states in high-temperature water.To sum up,in view of the protective properties of the oxide films and the content of Ni in the oxide films formed on key equipment in PWR,low concentration of lithium hydroxide solutions are recommended for HFT water chemistry and low concentration of lithium hydroxide solutions with 20-100 ppb Zn are recommended for Zn-injected HFT water chemistry.In addition,it is recommended that primary coolant and HFT water chemistry adopt Zn injection.