Characterization Of Interface Reaction Properties And Stress Corrosion Cracking Behavior Of Pre-deformed Stainless Steel In Simulated PWR Primary Water

Austenitic stainless steels(SS)are mainly used to manufacte of primary circuit pipes,main pump structural components and core structure components in pressurized water reactor(PWR)nuclear power plants(NPPs).These components long-term contact with high temperature and high pressure primary circuit coolant,and there is a certain degree of stress corrosion cracking(SCC)problems in high temperature water.Artificial or unavoidable pre-deformation would affect the mechanical properties and reaction characteristics of austenitic stainless steel,thus affects the SCC behavior in high temperature water.Material properties,stress state and water chemistry would affect the stress corrosion cracking behavior of structural materials in NPPs in high temperature water.Studying the environmental degradation of structural materials in NPPs in high temperature water under the interaction of materials,mechanics and chemistry is an important aspect of aging and life management of NPPs,and is of great significance to ensure the long-term,safe and stable operation of NPPs.It would provide support for our independent research and development and going global of nuclear power materials.In this work,the effect of pre-deformation on SCC behaviors and interface reactions of 316 L SS in simulated PWR primary water was investigated by mainly using SCC crack propagation tests in high temperature water,electrochemical tests,microstructure and morphology tests(including optical microscope,scanning electron microscope,electron backscatter diffraction,transmission electron microscopy,etc.).The interaction of materials properties,applied load and chemistry was analyzed.The main results were listed as below.(1)Pre-deformation decreased the ?3 CSL grain boundaries proportion and increased the local misorientation inside the original grains of 316 L SS.The enhancing effect of pre-deformation on the anodic dissolution of 316 L SS in 0.05 mol/L H2SO4 + 0.3 mol/L NaCl solution was significant in the potential range from-0.135 to 0.700 V(SCE).The effect of pre-deformation was more significant in sulfuric acid solution with chlorides than in chloride-free solution.For both solution annealed and pre-deformed(20% cold rolled)316L SS,the current densities under potentiostatic polarization,pits numbers and open pit diameters increased with increasing applied potentials.Predeformation increased the number of pits and decreased the pit size at a certain applied potential.Both the ICP results of the tested solutions after the potentiostatic polarization and the results of current vs.time curves showed that pre-deformation accelerated the anodic dissolution of 316 L SS.(2)In hydrogenated PWR primay water,there was no obvious difference between the EIS results of solution annealed and those of pre-deformed 316 L SS.In oxygenated PWR primary water,the EIS and anodic polarization curves results indicated that predeformation accelerated the anodic process of 316 L SS.The enhancing effect of predeformation on the anodic process of 316 L SS in oxygenated PWR primary water was significant in the potential range from open circuit potential(OCP)to 0.400 V(SHE).(3)Both in hydrogenated and deaerated PWR primary water environment,Transgranular fatigue cracking was observed in the pre-cracking procedure for onedirectionally cold-rolled(1DCR)316L SS specimens in the T-L(the notch direction of CT specimens were parallel to the rolling direction)and L-T(the notch direction of CT specimens were perpendicular to the rolling direction)orientations.The SCC crack growth rate was almost not affected by the specimen orientation.More extensive intergranular SCC were found in the T-L orientation with a higher crack growth rate than that in the L-T orientation with a lower crack growth rate in the hydrogenated and deaerated environments.The crack branching phenomenon found in the L-T orientation in deaerated PWR primary water was synergistically affected by the applied stress as well as the preferential oxidation path along the elongated grain boundaries,of which the latter factor was dominant.(4)A significant retarding effect of prior single tensile overloading was observed on the SCC growth of pre-deformed(20% cold rolled)316L SS specimens in T-L and L-T orientation in simulated PWR water environment.Extensive intergranular SCC was observed on the fracture surface of the non-overloaded T-L specimen,while intermittent intergranular SCC was found on limited regions of the fracture surface of the non-loaded L-T specimen.There was no significant indication of SCC on the fracture surface of the prior-overloaded T-L and L-T specimens.The stretch zone found near the crack front in the specimens with overloading affected the SCC propagation.The interaction between the compressive residual stress caused by overloading and the applied load lowered the effective driving force for SCC propagation,thus reduced the SCC growth rate of the specimens with overloading.The negative dK/da(the gradient of stress intensity factor K with crack length)region in front of the overloaded crack tip would play an important role in slowing SCC growth.The effect of overloading on SCC growth is dependent on water chemistry and the stress condition.(5)In hydrogenated and oxygenated PWR primary water,pre-deformation accelerated the SCC propagation of 316 L SS.The effect of sensitization treatment on the SCC of 316 L SS in simulated PWR primary water depended on water chemistry.In hydrogenated water,the sensitization treatment lowered the SCC crack growth.In oxygenated water,the sensitization treatment accelerated the SCC crack growth.The TEM analysis of crack tip and fracture morphologies indicated that the carbide precipitated on grain boundary played an important role in affecting the SCC behaviors of 316 L SS in high temperature water with different water chemistry.The SCC crack growth rates of sensitized,sensitized with pre-deforming and pre-deformed 316 L SS in oxygenated water were higher than those in hydrogenated water.(6)Post welding heat treatment(PWHT)eliminated part of the residual strain caused by welding,and had no significant effect on chemical compositions variation near the fusion line of 52M-316 L dissimilar metal welding joint.The 316 L SS heat affected zone(HAZ)specimens with and without PWHT and stainless steel base metal specimen without PWHT exhibited no significant SCC in normal hydrogenated PWR primary water.The SCC crack growth rates were significantly lower than those in oxygenated high temperature water.Comparing to the SCC growth rate of stainless steel with different stress/strain state caused by different pre-deformation modes in hydrogenated and oxygenated high temperature water,the low SCC crack growths of stainless steel HAZ specimens in hydrogenated water were related to the interaction of water chemistry,materials microstructure and stress/strain state.(7)In simulated PWR primary water with different water chemistry,the OCP of 316 L SS in the deaerated water was higher than that in the hydrogenated water,and lower than that in the oxygenated water.Dissolved oxygen(DO)and dissolved hydrogen(DH)in high temperature water significantly affect the EIS parameters.The Rct and Rf were the highest in the oxygenated water and the lowest in the hydrogenated water.The Cdl and CPEf were the highest in the hydrogenated water and the lowest in the oxygenated water.The oxide film grew with immersion time in various hightemperature water environments.The Rct and Rf increased with increasing immersion time.The Cdl and CPEf decreased with increasing immersion time.Oxide films formed in the hydrogenated water and in the deaerated water were similar in chemical composition but very different in morphology.The outer layer was composed of spinel oxides,and the inner layer was compact and Cr-rich.Outer oxide particles formed in the deaerated water were larger and more loosely distributed than those formed in the hydrogenated water.High DO in high temperature water results in a large change in the composition and morphology of the oxide film.In oxygenated water,an oxide film with hematite outer particles and Ni-rich inner layer was formed.Equiaxial outer oxide particles with curving edges and blunt angles covered the surface of 316 L SS in the oxygenated water,while outer oxide particles with straight edges and planar faces were found in the hydrogenated and deaerated water environments.The results of electrochemical measurements and oxidation tests could evaluate the time-dependent evolution of reaction rate of materials under various high temperature water chemistry conditions.The variation of electrochemical parameters and oxidation behaviors relating to the steady or quasi-steady state oxidation at the time-period of measurements could provide the fundamental information for understanding the SCC processes.