| Lead-cooled fast reactor(LFR)using lead bismuth eutectic(LBE)as coolant is one of the typical reactors of the fourth generation advanced nuclear power systems.The compatibility of candidate structural materials with LBE is one of the main scientific issues that restrict the engineering application of LFR.The Chinese irradiation-resistant steel,that is China low activation martensitic(CLAM)steel,can be used as one of the candidate structural materials for LFRs due to its good thermophysical and mechanical properties such as low radiation swelling,low thermal expansion coefficient and high thermal conductivity.However,its LBE corrosion resistance also needs to be further studied.Previous studies show that the main paths of LBE corrosion in ferritic/martensitic(F/M)steels are grain boundaries,and the addition of rare earth Ce can improve grain boundaries,optimize and regulate the material,so Ce is expected to improve the corrosion resistance of CLAM in LBE.In this dissertation,the microstructure and hardness changes of three kinds of specimens(i.e.CLAM,0.02Ce-CLAM and 0.51Ce-CLAM)steels with different Ce additions(i.e.0wt%,0.02wt%and 0.51wt%)were studied and analyzed.Corrosion experiments were carried out in static LBE at 500℃ with the oxygen concentration of 10-6wt%for 250h,500h,750h,1000h and 1500h respectively to study the effect and mechanism of Ce on the corrosion behavior of CLAM steel.The main contents and conclusions are as follows:1.Effect of Ce on the microstructure and microhardness for CLAM steel1)Ce mainly existed at grain boundaries,especially at triple junction and had three forms i.e.refractory Ce-containing inclusions,Ce-containing precipitates and Ce solidly soluble in grains.In 0.02Ce-CLAM,Ce mainly existed at grain boundaries in the form of Ce-containing precipitates with the sizes of 0.5-1μm,including elements such as Ce,O,Ta and W;In 0.51Ce-CLAM,Ce existed mainly as Ce-containing inclusions at grain boundaries with the large sizes of 1-20μm,including elements such as Ce,O,P,S and W.2)The average grain sizes of CLAM,0.02Ce-CLAM and 0.51Ce-CLAM were 9.5μm,8.1μm and 8.5μm,respectively.The addition of 0.02wt%Ce obviously reduced the amount of M23C6 precipitates at grain boundaries,while the addition of 0.51wt%Ce had no obvious inhibitory effect on M23C6 precipitates.3)Ce reduces the microhardness of CLAM steel,probably due to the inhibition of M23C6 precipitates by Ce,which reduces the density and dispersion of these precipitated phases,weakens the precipitation strengthening effect,and ultimately leads to a slight reduction in hardness.2.Oxidation corrosion behavior of CLAM,0.02Ce-CLAM and 0.51Ce-CLAM specimens in LBE1)The addition of 0.02wt%Ce did not affect the phase composition of the corrosion products,but the addition of 0.51 wt%Ce resulted in the creation of a new phase of CrO.87 in the corrosion products.2)For CLAM:At 250h,the oxide scale of CLAM specimens was thin and flat.At 500h,the approximately circular island-shaped magnetite with large spacings between the islands generated.There were local peeling and holes in these magnetite islands,resulting in a double layer magnetite structure;The oxide scale thickness increased with exposure time and reached a maximum thickness of about 18μm at 1500h.At this time,the extensive peeling of oxide scale occurred,exposing large areas of spinel regions,and many pits were found in spinel regions.3)For 0.02Ce-CLAM:At 250h,the oxide scale of 0.02Ce-CLAM specimens was thin and flat.At 500h,the lath-shaped magnetite islands generated and almost no magnetite bulges or holes were found.At 1000h and 1500h,the surface morphology of oxide scale was relatively continuous and complete.Although partial detachment of the magnetite layer was also found at 1500h,the exposed regions were still magnetite structure,indicating that it also formed a double layer magnetite structure.The continuity and integrity of oxide scale were relatively good.From the perspective of cross-section,the oxide scale thickness also basically increased with time,reaching a thickness of about 13μm after 1500h.4)For 0.51Ce-CLAM:At 250h,the obvious magnetite pellet on the surface were observed,and there were also many Ce-containing inclusions(containing CeO2 and other phases)that were semi-exposed outside spinel layer,which inhibited the growth of magnetite.At 500h,the thickness of magnetite layer increased obviously,and there were large areas of exfoliation,local exfoliation and pores,and local pitting in spinel area.At 500h and 1500h,the oxide scale was peeled off in a large area.At 750h and 1000h,the oxide scale surface was more complete.At 250h,the oxide scale thickness was about 7μm,which was bigger than those of CLAM and 0.02Ce-CLAM(i.e.about 4μm),and reached 16μm at 500h.Between 500h and 750h,the oxide layer peeled off extensively and the thickness decreased sharply to 9μm,and then reached 21μm at 1500h.3.Mechanism of Ce affecting the LBE corrosion resistance of CLAM steel and the evolution of oxide scale1)Within 1500h,the growth of oxide scale thickness for the three specimens generally meet the parabolic growth law.In the fitting results of parabolic laws without constants,the relationship between the parabolic rate constants for the three specimens is:kp,0.51Ce-CLAM>kp,CLAM>kp,0.02Ce-CLAM.Based on the analysis of the degree of continuity,integrity and some special morphologies of oxide scale surface in Section 5.1.1,the order of LBE corrosion resistance for the three specimens was evaluated as follows:0.02 Ce-CLAM>0.51Ce-CLAM>CLAM.2)The addition of an appropriate amount(i.e.0.02wt%)of Ce improves the LBE corrosion resistance of CLAM steel by improving the adhesion of oxide scale,enhancing the stability of grain boundary and preventing the outward diffusion of Fe in matrix at grain boundary.3)Excessive Ce addition(i.e.0.51wt%)accelerated the growth of oxide scale,especially in the magnetite layer.In addition,it generated many large size Ce-containing inclusions in 0.51Ce-CLAM specimen,and there was local enrichment,making the Ce-containing inclusions into spherical or striped shape.Since there is a tendency to increase the size of Ce-containing inclusions(Ce2O3)when they are further oxidized to CeO2 in LBE,this localized enrichment of Ce-containing inclusions may seriously damage the integrity of oxide scale and matrix once oxidized.The striped Ce-containing inclusions are mainly formed by the linear arrangement of spherical Ce-containing inclusions,and it will probably initiate larger cracks and induce pitting.(4)In Chapter 5,the typical and special morphologies of oxide scales for the three specimens(i.e.CLAM、0.02Ce-CLAM and 0.51Ce-CLAM)were discussed and the possible evolution processes of oxide scales were summarized.In a word,Ce addition can improve the corrosion resistance of CLAM steel to LBE in oxidizing environment,but the way and content of Ce addition need to be reasonably controlled.The related studies are expected to provide experimental support and explore a new direction for improving the LBE corrosion resistance of candidate structural materials for LFRs. |
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