Study of the Initial Stage and an Anisotropic Growth of Oxide Layers Formed on Zircaloy-4

Transcription

1 16 th International Symposium on Zirconium in the Nuclear Industry, Chengdu, P. R. China, May 10-13, 2010 Study of the Initial Stage and an Anisotropic Growth of Oxide Layers Formed on Zircaloy-4 B. X. Zhou 1, J. C. Peng 2, M. Y. Yao 1, Q. Li 2, S. Xia 1, C. X. Du 1, G. Xu 1 1. Institute of Materials, Shanghai University, Shanghai , P. R. China 2. The Key Laboratory for Advanced Micro-Analysis, Shanghai University, Shanghai , P. R. China

2 Introduction 1. Zirconium alloys of a hexagonal close-packed crystal structure have prominently anisotropic characteristic in comparison with the metals of a cubic structure. The anisotropic characteristic is bound to reflect on the corrosion behavior of zirconium alloys. 2. The most existing research on the relationship between the growth of epitaxial oxide and the crystal orientation of metal matrix was carried out with small single crystals or coarse grains of pure zirconium. The results were inconsistent with each other. The reasons might be attributed to the fact that the corrosion tests were carried out in different conditions, at different temperatures, and for different exposure time. 3. It is worthwhile to make further investigation of the formation of epitaxial oxide and the anisotropic growth of oxide layer, because this is one of the essential issues for better understanding of the corrosion mechanism of zirconium alloys. 4. The goal of this work is to highlight the role of the anisotropic growth and the microstructural evolution of oxide layer during the corrosion process at different temperatures and in different water chemistry for long-term 2 exposure.

3 Part 1: the initial stage of the formation of oxide layers Part 2: the anisotropic growth of oxide layers 3

4 Part 1: the initial stage of the formation of oxide layers 4

5 In-situ investigation of epitaxial oxide layer formed on Zircaloy-4 1. A thin oxide film formed with different interference-colour to outline the original metal grains, when a thin specimen was heated in TEM column (~10-3 Pa) for 2 days at In-situ investigation of epitaxial oxide formed on zirconium alloys by TEM is possible. 5

6 The different morphology and different crystal structures of epitaxial oxide layer 6 μm Dot-like grains Strip-like grains A thin specimen was heated in TEM column at 400 for 2 hours. Dot-like grains of epitaxial oxide were observed around the penetrated hole of the thin specimen and many strip-like grains grown at relatively thicker areas on the thin specimen at a certain distance from the edge of the hole. 6

7 The crystal structure of the dot-like grains A monoclinic ZrO 2 and an orientation relationship of (001) m //(0-111) α-zr, (111) m //(1-101) α-zr with α-zr matrix were identified. 7

8 The crystal structure of the strip-like grains These strip-like grains formed on the (0001) plane of α-zr matrix crossed each other to form 60 degree angle, and preferentially grew up along <11-20> direction of α-zr matrix. A bcc structure (a = 0.66 nm) and the orientation relationship of (110) bcc //(10-10) α-zr, [1-10] bcc //[0001] α-zr with α- Zr matrix were identified. This is probably a new kind of zirconium suboxide. 8

9 Conclusions (part 1) In-situ investigation of epitaxial oxide layer formed on Zircaloy-4 was carried out by heating the thin specimens within TEM column. Several conclusions were obtained as follows: 1. Dot-like grains with monoclinic oxide (m-zro 2 ), about 10 nm in size, were formed on the surface of relatively thinner area around the hole of a thin specimen, and have an orientation relationship of (001) m //(0-111) α-zr, (111) m //(1-101) α-zr with α-zr matrix. 2. Strip-like grains with bcc structure of a = 0.66 nm, probably a new kind of zirconium suboxide, were formed on the surface of relatively thicker area at a certain distance from the edge of the hole of a thin specimen, and have an orientation relationship of (110) bcc //(10-10) α-zr, [1-10] bcc //[0001] α-zr with α-zr matrix. 9

10 Part 2: the anisotropic growth of oxide layers 10

11 Experimental procedures In order to investigate the anisotropic growth of oxide layers, the preparation of coarse grain specimens is needed to avoid the interference of the grain boundaries. The preparation of coarse grain specimens :specimens in 1 mm thickness were quenched from β phase in vacuum, then annealed at 800 for 10 hours. Specimens with coarse grains in mm were obtained. Coarse grain specimens were finally heat treated at h. Corrosion tests:autoclave tests were carried out at 500 /10.3 MPa steam, 400 /10.3 MPa steam, 360 /18.6 MPa lithiated water (0.01M LiOH) and 360 /18.6 MPa deionized water. 11

12 Weight gains versus exposure time 8.9μm 14μm 9.3μm 42.5μm 12

13 Surface morphology of the specimens corroded at different temperatures for different exposure time The anisotropic growth of oxide layer was the most prominent when the specimens were corroded at 500 steam in comparison with at 400 steam, 13 at 360 deionized water and at 360 lithiated water.

14 The determination of metal grain orientations and the measurement of oxide thickness EBSD (Electron backscatter diffraction) was employed for the determination of grain orientations. The thickness of oxide layers formed on different grains was measured from SEM micrographs. Inversed pole figures were adopted to show the orientations of grain surface. 14

15 The relationship between the thickness of oxide layer and the surface orientation of metal grains corroded at 500 steam Exposure time (hour) Weight gains (mg/dm 2 ) Average thickness of the oxide (μm) The range of the oxide thickness (μm) max/min The total number of grains on which the oxide thickness measured ~ hours exposure 12 hours exposure 3 97 The oxide layer thickness is divided into three ranges, 3~6μm, 10~20μm and 30~60μm, then three lines with different thickness are used to link the poles having a same range of oxide layer thickness. The thickest oxide layers were detected on the surface of metal grains with the orientations distributed in the range from (01-10) to (12-10) planes. 15

16 The relationship between the thickness of oxide layer and the surface orientation of metal grains corroded at 360 lithiated water Exposure time (day) Weight gains (mg/dm 2 ) Average thickness of the oxide (μm) The range of the oxide thickness (μm) max/min The total number of grains on which the oxide thickness measured ~ The oxide layer thickness is divided into three ranges, 8~11 μm, 12~20 μm and 25~65 μm, then three lines with different thickness are used to link the poles having a same range of oxide layer thickness. The thickest oxide layers were detected on the surface of metal grains with the orientations distributed in the range from 15 to 30 degrees away from (0001) plane. 16

17 The relationship between the thickness of oxide layer and the surface orientation of metal grains corroded at 360 deionized water Exposure time (day) Weight gains (mg/dm 2 ) Average thickness of the oxide (μm) The range of the oxide thickness (μm) max/min The total number of grains on which the oxide thickness measured ~ The oxide layer thickness is divided into two ranges, 5.5~8.5μm and 9~11μm,then two lines with different thickness are used to link the poles having a same range of oxide layer thickness. The thicker oxide layers were detected on the surface of metal grains with the orientations distributed in the range of 40 degrees around the (0001) plane. 17

18 The relationship between the thickness of oxide layer and the surface orientation of metal grains corroded at 400 steam Exposure time (day) Weight gains (mg/dm 2 ) Average thickness of the oxide (μm) The range of the oxide thickness (μm) max/min The total number of grains on which the oxide thickness measured ~ The oxide layer thickness is divided into two ranges, 5~8 μm and 9~12 μm,then two lines with different thickness are used to link the poles having a same range of oxide layer thickness. The thicker oxide layers were detected on the surface of metal grains with the orientations distributed in the range of 45 degrees around the (0001) plane. 18

19 The variation of the anisotropic growth of oxide layer with corrosion temperature and water chemistry 500 steam 400 steam The relationship between the anisotropic growth of the oxide layers and the surface orientation of metal grains is variable with the corrosion tests at different temperatures and in different water chemistry. 360 lithiated water 360 deionized water 500 steam 400 steam 360 deionized water 360 lithiated water Exposure time 3 hours 280 days 476 days 340 days Weight gains (mg/dm 2 ) Average thickness of the oxide (μm) The range of the oxide thickness (μm) max/min 3~ ~ ~11 2 8~

20 Morphology difference of the oxide grains 500 steam 1 hour, the average thickness of oxide layer: 6.3μm (94.5 mg/dm 2 ) thicker thinner 400 steam 90 days, the average thickness of oxide layer: 5.4 μm (81.6 mg/dm 2 ) 360 lithiated water 90 days, the average thickness of oxide layer: 4. 1μm (61.5 mg/dm 2 ) 360 deionized water 265 days, the average thickness of oxide layer: 4.4μm (65.8 mg/dm 2 ) 20

21 The relationship among the anisotropic growth of oxide layers, the surface orientation of metal grains and the corrosion temperature and water chemistry The corrosion temperaure and water chemistry The difference of surface orientation of metal grains The microstructural evolution of oxide layers The microstructure and the crystal structure difference of oxide layers The characters of the oxide layer growth The anisotropic growth of oxide layer is variable with the corrosion temperature and water chemistry, it does not only depend on the surface orientation of metal grains. 21

22 A relationship between the corrosion resistance and the textures of the specimens When Zircaloy-4 specimens corroded in lithiated water at 360, the curves of weight gain versus exposure time are different for fine and coarse grain specimens. Explanation: The textures of the specimens bring the majority grains with their crystal planes, on which the oxide layer grows faster after the transition, to be parallel to the normal plane of the specimens. 22

23 A relationship between the corrosion resistance and the textures of the specimens The nodular corrosion resistance is improved by the increase of Fe and Cr solid solution in α-zr matrix. For the Zircaloy-4 specimens annealed at 820, there are no nodules on the normal plane but a very thick oxide layer developed from the nodules around the edges of the specimen. Explanation: The textures of the specimens bring the majority grains with their crystal planes, on which the oxide layer grows faster to develop the nodular corrosion, to be parallel to the side planes around the specimens. 23

24 Conclusions (part 2) The investigation of anisotropic growth of oxide layer formed on Zircaloy-4 was carried out with coarse grain specimens by autoclave tests. Several conclusions were obtained as follows: 1. The anisotropic growth of oxide layer was the most prominent when the corrosion tests were carried out at 500 steam in comparison with the corrosion tests at 400 steam, at 360 deionized water and lithiated water. The thickest oxide layers formed on the surface of metal grains with the orientations around the prism planes{from (01-10) to (-12-10)}. The thicker oxide layers were further developed into nodular corrosion as the exposure time prolonged. 2. When the specimens corroded in lithated water at 360, the thickest oxide layers formed on the surface of metal grains with the orientations tilted from 15 to 30 degrees away from the basal (0001) plane. 3. When the specimens corroded at 400 steam and at 360 deionized water, the difference of oxide layer thickness on different grain surfaces was less prominent. the thicker oxide layers formed on the surface of metal grains with the orientations around the basal (0001) plane within 40 degrees. 24

25 4. The anisotropic growth of oxide layers is variable with the corrosion temperature and water chemistry for long time exposure, and it dose not only depend on the surface orientation of metal grains. In order to explain this phenomenon, it is suggested that the microstructural evolution of the oxide layers plays an important role in the growth of oxide at the later stage of the oxidation. 5. There is a relationship between the corrosion resistance and the textures of the specimens. This relationship is also variable with the corrosion temperature and the water chemistry. 25

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