Crevice Corrosion Behaviors Of Nickel-based Alloy 690 And 405 Stainless Steel In High Temperature High Pressure Water

Steam generator(SG)tube is the key equipment connecting the primary and secondary circuits in pressurized water reactor(PWR)nuclear power plants(NNPs).Long-term operation experience of PWR NNPs shows that crevice corrosion between SG tube and support plate is one of main degradation mode of SG tube materials.However,very little work has been done to investigate crevice corrosion behaviors of SG tube and support plate materials in high temperature and high pressure water due to the difficulties of simulation experiments.Currently,SG tube and support plate is usually made of Alloy 690 and 405 stainless steel(SS),respectively.Therefore,studying crevice corrosion behaviors of Alloy 690 and 405 SS in high temperature and high pressure water and elucidating the related mechanism and influencing factors of crevice corrosion,are much helpful for finding the effective paths to resist crevice corrosion in high temperature and high pressure water and ensuring the safe service of SG tube in PWR NNPs.In the present work,crevice corrosion behaviors of Alloy 690 and 405 SS and its influencing factors were systematically investigated using a high temperature crevice corrosion exposure device.The characteristics of oxide films formed on Alloy 690 and 405 SS during crevice corrosion were investigated and the mechanism of crevice corrosion in high temperature water is also discussed.In addition,a crevice corrosion electrochemical tests device has been designed to investigate the effect of concentration ratio of Cl-and S2O32-on crevice corrosion electrochemical behavior of Alloy 690,and crevice corrosion electrochemical behavior between Alloy 690 and 405 SS dissimilar metals was also investigated.Crevice corrosion behavior of Alloy 690 in 290? pure water containing 3 ppm dissolved oxygen(DO)was investigated.Crevice geometry restricts the mass transport between the bulk and crevice solutions,resulting in a decrease in the DO concentration and an increase in the H+concentration within the crevice,thereby affecting the formation of oxide films within the crevice.The oxide film at crevice mouth mainly consists of compact NiO and NiFe2O4,while it consists of NiCr2O4,NiO,Cr2O3,Ni(OH)2 and Cr(OH)3 at the site within the crevice near the crevice mouth.The oxide film at deeper site within the crevice consists of an outer layer of Cr(OH)3 and FeCr2O4,and a continuous inner layer of Cr2O3.The formation of Cr(OH)3 results from the Cr3+hydrolysis within the crevice.With increasing the crevice depth,the Cr3+hydrolysis becomes increasingly serious,and thus the content of Cr(OH)3 in oxide films gradually increases.The thin oxide film within the crevice is due to the formation of a protective inner layer of Cr2O3,while the thick oxide film at crevice mouth is due to the precipitation of Ni2+and Fe2+.Effect of 405 SS on crevice corrosion behavior of Alloy 690 in 290? pure water containing 3 ppm DO was investigated.405 SS affects the composition,structure and thickness of oxide film formed within the crevice of Alloy 690.The outer layer of oxide film mainly consisted of Ni-Fe spinel and Fe2O3 at crevice mouth,and Fe2O3 at the site within the crevice near crevice mouth,and Fe3O4 at the deeper site within the crevice.The inner layer of oxide film mainly consisted of Fe-Cr spinel and Cr2O3 within the crevice,and porous NiO at crevice mouth.A thin inner layer of oxide film formed within the crevice of Alloy 690 is attributed to the formation of continuous Cr-rich protective inner layer and the consumption of most of oxygen by 405 SS,while the outer layer of oxide film formed within the crevice of Alloy 690 became thick due to the precipitation of Fe2+ dissolved from 405 SS.Crevice corrosion behavior of Alloy 690 in 290? deaerated chloride solution was investigated.Cl-promotes the dissolution of Ni2+from matrix and then Ni2+is deposited on the surface,causing the oxide film formed outside the crevice to be rich in Ni and depleted in Cr.However,the oxide film formed inside the crevice is rich in Cr and depleted in Ni,which results from that Cr-rich oxide is stable and Ni-rich oxide is not stable in acidic crevice solution.The Cr-rich oxide film inside the cievice is thinner than Ni-rich oxide film outside the crevice,which is caused by the fact that the Cr-rich oxide has better protection than Ni-rich oxide and a large amount dissolved Ni2+ cannot precipitate in acidic crevice solution.In addition,the combind effect of Cl-and H+leads to nodular corrosion.The surrounded oxide of corrosion nodule is rich in Ni and the center oxide is rich in Cr.The center oxide with many defects provides diffusion paths for Cl-and O2,which is benefical to the development of nodular corrosion.The autocatalytic effect resulting from the limitation of metallic ions diffusion caused by the crevice geometry promotes the development of nodular corrosion.Effects of S2O32-and DO concentrations on crevice corrosion of Alloy 690 in 290? chloride solutions were investigated.In aerated chloride solutions,S2O32-consumes oxygen to form SO42-and inhibits corrosion of Alloy 690.In addition,the hydrolysis of Cr3+ causes the pH of solution to decrease inside the crevice,and the concentrated H+and Cl-lead to the active dissolution of metal inside the crevice.In aerated chloride solutions,S2O32-is easily reduced to S0,S2-and H2S in acidic crevice solution.These reduction products can promote the dissolution of metal or enter into oxide film to form sulfide to hinder the re-repair of the passive film,thereby stabilizing the metastable pitting corrosion induced by Cl-,leading to steady-state pitting.Design a crevice corrosion electrochemical tests device to investigate the effect of concentration ratio of Cl-and S2O32-on crevice corrosion electrochemical behavior of Alloy 690.The critical concentration ratio(Rc)of Cl-and S2O32-to jointly promote pitting corrosion is about 500.Since the electromigration rate of S2O32-into the crevice than that of Cl-during crevice corrosion,when R?500,S2O32-promotes crevice corrosion,while when R?500,S2O3-inhibits crevice corrosion.The reason why S2O3-promotes crevice corrosion is that it is easily reduced to H2S(aq)in a low-potential and acidic crevice solution and H2S(aq)can promote the active dissolution of metal.In addition,crevice corrosion electrochemical behavior between Alloy 690 and 405 SS dissimilar metals was also investigated.405 SS releases a large amount of metal ions due to active dissolution inside the crevice.The hydrolysis of metal ions leads to a significant increase in the H+concentration inside the crevice,thereby resulting in reaching the conditions for the hydrogen evolution reaction and the S2O32-reduction reaction.Finally,Alloy 690 outside the crevice acts as a cathode for oxygen absorption reaction,and Alloy 690 inside the crevice acts as a cathode for hydrogen evolution reaction and the S2O32-reduction reaction,both of which promote the anodic dissolution of 405 SS inside the crevice.