Corrosion Behavior Of Zirconium Alloys In Supercritical Water At 550℃/25MPa

Super critical water cooled reactor(SCWR) is one of the Generationâ…£reactors, and which is highly expected in the application.The choice,research and development of its fuel cladding material is a challenging task so far.As a light water nuclear reactor fuel cladding material,Zr alloy has been wide used and its application experience accumulated,hence it is the potential materials for SCWR.In SCWR,under the working condition of temperature between 500～550℃and 25MPa, its corrosion behaviour is the key question of whether Zr alloy is applicable to SCWR.In this thesis,six Zr alloys,Zr-4(Zr-1.5Sn-0.2Fe-0.1Cr),N18 (Zr-1Sn-0.35Nb-0.3Fe-0.1Cr),N36(Zr-1Sn-1Nb-0.3Fe),Zr-1Nb(MS),Zr-2.5Nb and Zr-2.5Nb-0.5Cu were chosen to investigate the corrosion behavior in SCW at 550℃/25MPa by static autoclave testing.Before which,the alloys went through different heat treatment to have second phase particles in different size and distribution.To study the corrosion behaviour of different Zr alloys;to study the effect of alloying element,microstructure,composition of second phase particles,it's size and distribution on corrosion behaviour;to study the change of microstructure of metal matrix during corrosion;to focus on the effect of the alloying elements and second phase particles on the growth of oxide film during the corrosion,and to explore the mechanism of how alloying elements and second phase particles influencing the alloy's corrosion resistance,which would help to develop new Zr alloy against high temperature corrosion and provide experimental and theoretical basis for evaluation of Zr alley's application prosperity in SCWR projects.The main results are as following:1.Some lamellarβ-Zr(bcc,a=0.362 nm) riched in Fe and Cr was detected in Zircaloy-4 samples quenched after 1020℃-20 min.The contents of Fe,Cr and Sn inβ-Zr are about 3.65 wt%(5.82 at%),0.61 wt%(1.05 at%) and 1.57 wt%(1.18 at%) respetively.Thisβ-Zr would decompose intoα-Zr and fine second phase Zr(Cr,Fe)₂ particles after deformation and heat treatment at 580℃,this fine second phase particles were distributed ribbon like.So the Zircaloy-4 samples could get fine and uniformly distributed second phase particles through water quenched fromβphase,deformation and heat treatment.In the Zr-2.5Nb-0.5Cu alloy,Cu exists mainly in tetragonal second phase CuZr₂,rich in impurity element Fe.2.The weight gain of all the chosen Zr alloys increased fast by static autoclave testing,even for the best performance Zr-2.5Nb-0.5Cu samples,the weight gain after corrosion 800 h was over 600 mg/dm².Zircaloy-4 samples were found to have nodular corrosion and hence the weight gain increased extremely fast.With the addition of Nb,Zr alloy shows uniformed corrosion instead and its corrosion resistance is superior significantly to that of Zircaloy-4.Further,in the Zr-2.5Nb with the addition of 0.5 wt%Cu,its corrosion resistance is remarkably improved.3.The existence of uniformly distributed nanoscale second phase particles is good for improving the corrosion resistance of Zr alloys in the SCW at 550℃/25MPa. However,the effect is not as remarkable as the addition of alloying elements.In Zr alloy with the addition of Nb,the corrosion resistance would be rather poor ifβ-Zr stabilized with Nb existing.The effect of deformation and heat treatment on Zr alloy's corrosion behavior increased with the increase of Nb content.4.The corrosion process of Zr alloy is highly sensitive to corrosion temperature,also, higher pressure of super-heated steam will speed up the alloy's corrosion and increase the possibility of nodular corrosion in Zircaloy-4.Alloys with different composition have different weight gain behaviour in SCW,which is quite similar to the difference they show in 500℃/10.3 MPa super-heated steam and in 360℃/18.6MPa water.Although SCW has unique property,Zr alloy's corrosion behaviour in it has no distinct difference from that in super-heated steam.5.The corrosion testing of Zr-Nb alloy in SCW at 550℃/25MPa,corrosion process is accompanied by the hydrogen absorption process,there is aα→βphase transformation induced by hydrogen in alloy matrix.The H-stablizedβ-Zr were formed during corrosion testing at 550℃,at the same time the Nb alloying element diffused into theβ-Zr to form Nb/H-enrichedβ-Zr,which is stable in 550℃,but it will decomposed to ZrHx,α-Zr and Nb-enriched Zr-Nb phases as the corrosion temperature decreased.Zr-Sn and Zr-Sn-Nb alloys have similar phase transformation during corrosion testing in SCW at 550℃/25MPa,Fe,Cr etc alloying elements will partition diffuse into the H-richβ-Zr,from which an amorphous ZrSnFe phase will be precipitated during cooling.The H-inducedβ-Zr transformation during corrosion will deteriorate the alloy's corrosion resistance.6.Nb will remarkably lower the temperature for H-inducedβphase transformation, in the content range of alloys used,this temperature will get much lower as the Nb content increases.For Zr-2.5Nb after 800h corrosion testing,the content of H is about 0.30 wt%,the starting temperature ofα→βis close to 500℃measured by DSC,which is about 50℃below the H inducedα→βtransformation temperature according to Zr-H phase diagram.Other element will also affect the temperature for H-inducedα→βtransformation.During corrosion in SCW at 500～550℃,the hydrogen absorption is unavoidable.So,even if the corrosion resistance of Zr alloys could be improved,they are not suitable for fuel cladding materials,from the fact H will induceα→βphase transformation.7.The nodular corrosion of Zircaloy-4 alloy is inclined to happen in the oxide film on some special crystallographic planes on metal surface.This kind of oxide film is quite dense,featuring few micro pores and cracks.In WQ-CR-580℃treated Zircaloy-4 samples,there exist some uniformly distributed micro cracks in the oxide film,which parallel to the oxide/metal(O/M) interface but not inter-connected to each other.These micro cracks relax the stress in the film to prevent from forming the thorough cracks across the thickness of the film, blocking the direct oxygen passage and it is difficult for nodular corrosion to happen.So the status of the film is the key to the nodular corrosion,and all the other nodular corrosion relevant factors function through their effects on the oxide film.8.The mechanism of the nodular corrosion of the Zircaloy-4 samples in SCW at 550℃/25MPa is referred as following:In the oxide film formed in the corrosion of Zircaloy-4,the film is quite dense locally with unique property.At certain thickness,arch shaped cracks paralleled to the O/M interface form under internal stress.Since the film around the cracks is too dense to relax the stress effectively, the cracks continue to grow toward the surface of the film,driven by stress.This causes the broken-up of the dense surface of the film to allow water vapor into the paralleled cracks,introducing direct oxygen-providing passages,leading to the local acceleration of film growth and hence the uneven nodular corrosion.9.In the oxide film,the early appearance of large amount of cracks paralleled to O/M interface does no good to the corrosion resistance of Zr alloys.The addition of Nb gives rise to nano scale micro pores in the oxide film formed during corrosion.In the chosen range of Nb content in the alloys,the density of the pores increases with the content of Nb.The formation of the micro pores relaxes the internal stress,so as to reduce the growth of paralleled cracks and improve the corrosion resistance to some extent.This is also the main reason that addition of Nb can refrain from forming the nodular corrosion.Cu segregates on the grain boundaries of oxide film,reducing the number of micro pores and preventing the formation of paralleled micro cracks,which may be the reason that the addition of Cu in Zr-2.5Nb alloy can improve the corrosion resistance.It also reduce,to a great extent,the amount of H absorbed in the corrosion of Zr-2.5Nb in SCW at 550℃/25MPa.10.The microstructure of Zr alloy oxide film has profound influence to the corrosion resistance of Zr alloys.The alloying elements are very important in changing the performance through their effects in the oxide film,e.g.,to form micro pores and cracks,to segregate at the oxide grain boundaries,etc.Processing and heat treatment history have effect on the existence and distribution of the alloying elements and hence affect the microstructure of the oxide film,which is the major reason of their importance to the corrosion resistance.