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Research on the corrosion mechanisms of new Zirconium alloys containing Niobium

Research on the corrosion mechanisms of new Zirconium alloys containing Niobium. Student: Zhang Haixia Supervisors: Professor Zhou Lian Doctor Daniel Fruchart Professor El Kébir Hlil

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Research on the corrosion mechanisms of new Zirconium alloys containing Niobium

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  1. Research on the corrosion mechanisms ofnew Zirconium alloys containing Niobium Student: Zhang Haixia Supervisors: Professor Zhou Lian Doctor Daniel Fruchart Professor El Kébir Hlil Reporters: Professor Li Zhongkui Professor Daniel Chateigner Examiners: Professor Sun Jun Doctor Luc Ortega 2009. 11. 18

  2. Contents Introduction Research methods Corrosion resistance of Zirconium alloys Relationship between the matrix microstructure and corrosion resistance of new Zirconium alloys The effect of the crystal structure oxide film on corrosion resistance Relationships of the residual stress, crystal structure in oxide film and corrosion resistance Corrosion mechanism of new Zirconium alloys Conclusions

  3. 1. Introduction Development of nuclear power 1954 the first nuclear power plant in USSR 1957 first commercial nuclear power plant in USA At present there are more than 440 nuclear power plants in the World

  4. Fig. 1-1 Fuel assemblies

  5. 1. Introduction • Zirconium alloys used in nuclear reactor • Zr-Sn system alloy ★ Zr-1, Zr-2, Zr-4, improved Zr-4 • Zr-Nb sytem alloy ★ E110, M5, Zr-2.5Nb •Zr-Sn-Nb system alloy ★ Zirlo, E635, NDA, HANA, NZ2, NZ8

  6. Research summary of the water-side corrosion of Zirconium alloy • Water chemical effect on corrosion behavior • Heat treatment effect on corrosion behavior • Alloy composition effect on corrosion behavior ★ Matrix microstructure (alloying elements content, precipitate characteristic); ★ Characteristic of oxide film (crystal structure, stress). 1. Introduction

  7. Composition (Nb addition) Matrix microstructure Structure of oxide film Stresses in the oxide film Corrosion resistance 1. Introduction

  8. Nb content in the matrix Low Nb contents in the matrix is better Precipitate characteristic Small and well-distributed β-Nb can improve the corrosion resistance Crystal structure of oxide film t-ZrO2 and m-ZrO2; Is a high t-ZrO2 content good to improve corrosion resistance ? On the stabilization mechanism of t-ZrO2 ? Stress distribution in the oxide film High compressive stresses in oxide film How do compressive stresses affect phase transition and corrosion resistance ? 1. Introduction

  9. •Corrosion mechanisms • Diffuse hypothesis • Dissolving hypothesis • O-Li group cumbering hypothesis • Barrier hypothesis • Phase transformation hypothesis So far, there is no clear understanding of the corrosion mechanisms of mature alloys. 1. Introduction

  10. Research route NZ2, NZ8 alloys Blank samplesCorroded in static autoclave Samples corroded XRD and Raman spectroscopy of crystal structure, of phase content, of internal stress in oxide films SEM analysis of oxides morphologies TEM analysis of size, amount, distribution and composition of precipitates Structure of precipitates confirmed by neutron diffraction To find the relationships of Nb addition, t-ZrO2 content on stress change and corrosion resistance, to confirm the corrosion mechanism of Zirconium alloys

  11. 2. Research methods Experimental materials Elemental compositions of alloys (wt.%)

  12. Techniques flow of the plates •3 vacuum melting - β forging - β quenching - α hot rolling (<600℃) • 3 intermediate annealing and 30-50% cold process after every annealing - plates (δ=1mm) • final re-crystallization annealing (580℃/2h). Intermediate annealing parameters are respectively 650℃/2h, 590℃/2h and 590℃/2h. 2. Research methods

  13. The autoclave experiments • Corrosion conditions ★ 360℃/18.6MPa in pure water; ★ 360℃/18.6MPa in lithiated water; ★ 400℃/10.3MPa in steam. • Method indicating the corrosion degree ★wt=10000•(Wt-W0)/S, W0is the weight of un-corroded sample, Wtis the weight of corroded sample, S is the area of sample, wtis the weight gain. 2. Research methods

  14. • Analyses and measurements •JEM-200CX transmission electron microscope • Grazing XRD diffractometer - PW3830 (Fe, λKα=1.9364Å) •Normal XRD diffractometer - PW3830 (Cu, λKα=1.5444Å) • JY-T64000 laser Raman spectrometer • D1B neutron PSD diffractometer (n0, λ=2.42Å) •JSM-840A scanning electron microscope 2. Research methods

  15. 3. Corrosion resistance of Zirconium alloys Corrosion resistance in 360oC lithiated water Fig. 3-1 Corrosion kinetics of NZ2, NZ8 and Zr-4 alloys in 360oC lithiated water

  16. Corrosion resistance in 400oC steam 3. Corrosion resistance of Zirconium alloys Fig. 3-2 Corrosion kinetics of NZ2, NZ8 alloys investigated in 400oC steam

  17. Corrosion kinetics of NZ2 alloy in different media 3. Corrosion resistance of Zirconium alloys Fig. 3-3 Corrosion kinetics of NZ2 alloy investigated in different mediums

  18. Summary • Nb addition reveals good to improve corrosion resistance of Zirconium alloys • In both media, corrosion resistance of NZ2 alloy is better than of NZ8 alloy • Oxide thickness at transition point is 2~3μm 3. Corrosion resistance of Zirconium alloys

  19. 4. Relationship of the matrix microstructure and corrosion resistance of new Zirconium alloys Matrix microstructure of NZ2 Element at% Cr 8.76 Fe 35.75 Zr 55.49 Element at% Cr 8.47 Fe 36.76 Zr 43.42 Nb 11.35 (a) (b) 200nm 200nm (c) (d) Fig. 4-1 TEM images of NZ2 alloy matrix and the EDS result of the precipitates (（b）is the dark image)

  20. 4. Relationship of the matrix microstructure and corrosion resistance of new Zirconium alloys • Matrix microstructure of NZ8 (b) (a) 500nm 500nm (c) Element at% Fe 9.05 Zr 82.34 Nb 8.60 Fig. 4-2 (a) TEM images of NZ8 alloy matrix, (b) corresponding dark image, (c) EDS analysis of precipitates

  21. Fig. 4-3 Neutron diffraction pattern of NZ2 alloy matrix Fig. 4-4 Neutron diffraction pattern of NZ8 alloy matrix 4. Relationship of the matrix microstructure and corrosion resistance of new Zirconium alloys

  22. Summary • Nb content in the matrix • Oxidation characteristics • Type and volume fraction of precipitates. 4. Relationship of the matrix microstructure and corrosion resistance of new Zirconium alloys

  23. 5. Oxide film crystal structure effecton corrosion resistance Crystal structure of NZ2 alloy oxide film Fig. 5-1 Grazing incidence XRD patterns of oxide films surface of NZ2 alloys exposed to 360oC lithiated water for 3 d Fig. 5-2 Grazing incidence XRD patterns of oxide films surface of NZ2 alloys exposed to 400oC steam for 3 d

  24. 5. Oxide film crystal structure effect on corrosion resistance Fig. 5-3 Normal XRD spectrum of oxide films of NZ2 alloys exposed to 360oC lithiated water for different times Fig. 5-4 Normal XRD spectrum of oxide films of NZ2 alloys exposed to 400oC steam for different times

  25. 5. Oxide film crystal structure effect on corrosion resistance T-ZrO2 content obtained from XRD data Fig. 5-5 Relation of corrosion time and t-ZrO2 content in oxide films of NZ2 alloys corroded in 360oC lithiated water and 400oC steam

  26. 5. Oxide film crystal structure effect on corrosion resistance Fig. 5-6 Raman spectra of oxidized films at difference distances from surface, which results exposing NZ2 alloys to 360oC lithiated water for 70 d Fig. 5-7 Raman spectra of oxidized films at difference distances from surface, which results exposing NZ2 alloys to 400oC steam for 70 d

  27. 5. Oxide film crystal structure effect on corrosion resistance Fig. 5-13 SEM image of oxide films of NZ2 alloys exposed to 360oC lithiated water for 3 d Fig. 5-14 SEM image of oxide films of NZ2 alloys exposed to 360oC lithiated water for 126 d

  28. Summary • T-ZrO2, m-ZrO2 T-ZrO2 content decreases gradually • C-ZrO2 appears when the oxide film thickness reaches about 2μm • T-ZrO2→m-ZrO2, t-ZrO2→c-ZrO2→m-ZrO2 • T-ZrO2 content is the highest at the oxide/metal interface • The high t-ZrO2 content can improve the corrosion resistance 5. Oxide film crystal structure effect on corrosion resistance

  29. 6. Relationships of the residual stresses, crystal structure in oxide films and corrosion resistance Introduction The stresses mainly result from volume changes of metal and oxide, of the phase transformation from t-ZrO2 to m-ZrO2, of the oxidation of the precipitates The stresses affect the stabilization of the oxide films, and change the diffusion coefficient Then, the corrosion kinetics is changed.

  30. 6. Relationships of the residual stresses, crystal structure in oxide films and corrosion resistance • Experimental method Microstrains are given by the relation: By the ‹sin2ψ› method, the diffraction peak shift can be described as follows: We can get the formula from above two relations: So σ11is deduced from the slope p of the d-sin2ψ line:

  31. Experimental results Fig. 6-1 The d=f (sin2ψ) plots for samples corroded in 360oC lithiated water for 14 d., 70 d., 126 d. and 210 d. Fig. 6-2 The d=f (sin2ψ) plots for samples corroded in 400oC steam for 3 d., 28 d., 42 d. and 154 d.

  32. 6. Relationships of the residual stresses, crystal structure in oxide films and corrosion resistance Kinetic transition Fig. 6-3 Relationship of the oxide film thickness and compressive stresses in oxide films of NZ2 alloy corroded at 360oC lithiated water and at 400oC in steam

  33. Analysis and discussion • The kinetics transition is associated with a sudden stress release • The higher t-ZrO2 content corresponds to the higher compressive stress as a whole. • The average t-ZrO2 content decreases continuously and smoothly, independent of the kinetic transitions 6. Relationships of the residual stresses, crystal structure in oxide films and corrosion resistance

  34. The corrosion mechanism of new Zirconium alloys • Oxidation of the matrix and alloying element(s) • Differential oxidation of precipitates inside the oxide layers • Oxidation of Nb in the precipitates, formation of vacancy clusters, transformation of t-ZrO2 to c-ZrO2 • Cracks form and compressive stresses are released • Kinetics transition happens. 7. Investigation of corrosion mechanism of new Zirconium alloys containing niobium

  35. (a) Precipitated oxidized fully Precipitated oxidized partially Precipitated unoxidized C-ZrO2 T-ZrO2 (b) Oxide sub-layer rich in t-ZrO2 (c) (d) Fig. 7-4 Model of corrosion mechanism of new Zirconium alloys

  36. Conclusions Appropriate Nb addition makes benefit to improve the corrosion resistance of Zirconium alloys. The corrosion resistance of NZ2 is better than that of NZ8 Low Nb content in matrix and a small quantity of precipitates with small size are benefit to improve the corrosion resistance The oxide films are mainly composed of m-ZrO2 and t-ZrO2 mainly. When the oxide thickness reaches to 2μm, the c-ZrO2 appears

  37. There are two kinds of phase transformations during corrosion: t-ZrO2→m-ZrO2 and t-ZrO2→c-ZrO2→m-ZrO2 T-ZrO2 is stabilized by the compressive stresses and vacancies, and c-ZrO2 is stabilized by vacancies The average t-ZrO2 content decreases continuously and smoothly, independent of the kinetic transitions as the oxidation proceeded Conclusions

  38. High compressive stresses occur in oxide films Sudden release of the compressive stresses in oxide films is related to corrosion transition High t-ZrO2 content and compressive stresses in the oxide films can improve the corrosion resistance of Zirconium alloys These new alloys candidate for new generation long life nuclear power plants Conclusions

  39. Many thanks for your attention!

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