Study on zirconium hydride high temperature hydrogen permeation resistant coating

Zhang Huafeng, Yang Qifa, Wang Zhendong, Liu Xiaozhou



(Institute of Reactor Engineering, China Academy of Atomic Energy Sciences, Beijing 102413)



Abstract: The thickness of 5~20 is prepared on the surface of zirconium and zirconium hydride by gas-solid reaction μ M hydrogen permeation resistant coating. The morphology of the coating was observed by optical microscope and scanning electron microscope (SEM); The composition was analyzed by EDS; The phase of the coating was analyzed by X-ray diffractometer. The analysis results show that the coating surface is uniform and dense, with a thickness of 20 μ About m; The coating contains Zr, C, O, P and other elements, and O has obvious diffusion; There are zirconia, zirconium phosphide and other phases with dense structure in the coating, and there may also be zirconium carbide phase. Key words: zirconium hydride; Anti-hydrogen permeation coating; Gas-solid reaction





Zirconium hydride with high hydrogen content or high hydrogen to zirconium atomic ratio has high neutron moderating ability and low neutron absorption cross section, and is an ideal solid moderator material. Some small reactors use zirconium hydride as moderator, and the operating temperature is as high as 873~923 K. When zirconium hydride moderator is in this temperature range, the hydrogen balance depresses its neutron moderator efficiency. Therefore, it is necessary to prepare anti-hydrogen permeation coating on zirconium hydride surface to reduce hydrogen leakage. The commonly used anti-hydrogen permeation coatings at home and abroad include metal anti-hydrogen permeation coatings, oxide and carbon (nitrogen) compound coatings [1]. Zirconium hydride moderator coating has special requirements different from general materials. First of all, zirconium hydride itself is very brittle. In addition to the irradiation in the reactor, the temperature of the moderator itself is uneven and the thermal shock of shutdown and shutdown make zirconium hydride very easy to form cracks. Once zirconium hydride cracks, the decomposed hydrogen will escape from the fresh surface, and the coating on the surface of zirconium hydride will lose its function. According to this actual situation, the coating applied to the surface of zirconium hydride should have the self-healing function, that is, once cracks appear, the new film will form quickly. Secondly, zirconium hydride is used as reactor moderator, which requires a small neutron absorption cross section of the coating. Thirdly, it is required that the irradiation stability of the coating is good and that it is firmly combined with zirconium hydride. Based on these particularities, the coatings that can be considered include: aluminized or chromized coatings and coatings formed by subsequent oxidation and ZrO2 coatings formed by direct oxidation. The data [1] shows that by heating zirconium hydride, high-purity carbon dioxide and phosphorus, the reaction gas is adsorbed and dissociated on the surface of zirconium hydride at a set temperature, and then the dense and uniform coating is formed through element diffusion, dissolution and matrix reaction, which is used to prevent the permeation of hydrogen.

Fig. 1 Optical microstructure of zirconium hydride

Fig. 2 X-ray diffraction spectrum of zirconium hydride



1 Experiment

1.1 Test sample

In this work, Zr24 and zirconium hydride samples (provided by Beijing General Research Institute of Nonferrous Metals) are selected to verify whether the preliminarily determined process is feasible. The hydrogen-zirconium ratio is estimated to be about 1 76 。 Figure 1 shows the original structure and morphology of zirconium hydride. The melted Zr21Nb alloy completely absorbs hydrogen to form lamellar hydride. From a single grain, the Zr21Nb grain has been transformed into lamellar martensite. According to the analysis of microstructure characteristics of Zr2H binary phase diagram combined phase, the lamellar hydride phase in the original sample is ε Phase, with high hydrogen zirconium ratio, n (H)/n (Zr)>1 7 。 Figure 2 shows the XRD spectrum of zirconium hydride, and the results are similar to Zr H1 The curve of 801 is in good agreement, indicating that the hydrogen zirconium ratio is close to 1 80 。 This is consistent with the results of metallographic analysis and the data of the sample provider [2, 3].

1.2 Experimental conditions

The predetermined experimental atmosphere of Zr24 and zirconium hydride is CO2 mixed with a small amount of phosphorus vapor, the reaction temperature is 673-1073 K, and the reaction time is 30-180 h. Two planes of the sample, one side is polished and the other side is in the original wire cutting state.

2 Test results and discussion


2.1 Analysis of coating surface morphology

Figure 3 shows the surface morphology of the coating. It can be seen from Fig. 3a, b and e that the coating grown on the surface of zirconium is dense and uniform with fewer pores. However, some sample coatings have cracks, as shown in Fig. 3f. Comparing Fig. 3c and d, it can be seen that there are many large holes on the surface of zirconium hydride coating grown without P, while the coating is relatively dense with P, so it can be considered that the coating with high density can be obtained in the presence of P. The reasons for the cracks in the coating are analyzed as follows: 1) The microcracks in the original substrate expanded during the coating preparation process, so that cracks appeared locally on the sample surface; 2) The original sample reacts with the gas used to prepare the coating, resulting in a certain lattice distortion, resulting in a certain phase change stress in the coating; 3) The thermal expansion coefficient of the coating and the substrate is quite different. As long as the total stress generated exceeds the critical stress of coating cracking


Force can cause cracks in the coating [4]. It can be seen from Fig. 3d that the zirconium hydride matrix has a lamellar structure. This is because the coating on the zirconium hydride surface is thin, so the above structural characteristics similar to those of the original zirconium hydride sample after etching appear.


21.2 Analysis of coating thickness and bonding state between coating and substrate

Figure 4 shows the sectional morphology of four different coatings. It can be seen from Fig. 4a and b that the thickness of the coating prepared with Zr as the substrate has exceeded 20 μ M, the coating is uniform, dense and well combined with the substrate. Figure 4d is the backscattered electron image of zirconium surface coating, coating

Fig. 3 Surface morphology

Fig. 4 Cross-sectional morphology of coating



The thickness is relatively uniform. It can be determined from Fig. 4 b and c that the process parameters shall be strictly controlled during the coating preparation process, and the thickness of zirconium surface can be more than 20 μ M coating. Figure 4e and f are the cross section photos of the coating with Zr H x as the substrate and CO2 in the absence of P and the presence of P, compared with the Zr surface coating, the coating on the Zr H x surface is thinner, with a thickness of 5~7 μ M, the possible reason is that when zirconium hydride is at a high reaction temperature, the surface hydrogen atom is heated and activated, and after separating from zirconium hydride, it recombines into hydrogen molecules, resulting in a large number of hydrogen vacancies on the surface, and forms a certain concentration gradient with the matrix. Then, the hydrogen atom in the body diffuses to the surface under the action of the concentration field and the thermal field, causing a certain obstruction to the dissociation and diffusion of CO2, C, O, P, etc, It makes the coating growth more difficult, resulting in a thinner coating prepared under the same experimental parameters, which is about 1/3 [5] of the Zr surface coating. Compared with Fig. 4e and f, it can be seen that there is a big difference between the coating with P and without P, there are more microcracks in the coating without P, and there are fewer coatings with P, which proves that P can improve the coating quality. However, the specific mechanism needs to be demonstrated by relevant experiments. Fig. 4c is the SEM photo of the Zr surface coating after etching. It can be seen that the coating structure is accumulated along the longitudinal direction, indicating that the grain grows along the direction parallel to the substrate surface during the coating growth process. It can be seen from Fig. 4c and d that there is a narrow transition layer between the interface of the coating and the substrate. This shows that the coating growth is formed by the uniform reaction between the atmosphere substrate and the substrate on the surface, and then the reaction gas diffuses through the coating to the area below the coating and contacts with the substrate, and then further reacts to form the coating. The steps in Fig. 4c, e and f are caused by the inconsistent etching rate of the etchant on the coating and substrate.

Figure 5 is the secondary electronic image of Zr H x unpolished surface coating. It can be seen from Figure 5 that the coating on the unpolished surface is easy to nucleate and grow at the surface defects, thus resulting in uneven coating thickness and loose structure. In addition, there are wide cracks in the area with thick coating. Therefore, to obtain a crack-free coating, the sample surface



Must undergo certain mechanical or chemical treatment.



3 Composition analysis



1) Fixed-point scanning



The surface and cross section of Zr and CO2 generated coating and Zr, CO2 and P generated coating were scanned by scanning electron microscope at fixed points. The results of energy spectrum analysis showed that the coating contained Zr, C, O and P.



2) Line scan diagram 6 is the line scan spectrum of Zr H x, CO2 and P generated coatings. There is an obvious diffusion of O in the coating, which is consistent with the results of the section morphology analysis of the coating. Due to the existence of heavy peaks in C, P and Zr, the presence of a large amount of Zr has certain interference with the determination of C and P, and more effective component analysis methods must be sought to determine C, P.



2.4 Phase analysis of coating



The above four coatings were analyzed by XRD, and the results showed that there were tetragonal zirconia in the four coatings, which was consistent with the results of energy spectrum analysis. There was zirconium phosphide in the coating formed by adding P, and there was matrix zirconium hydride in the zirconium hydride coating, while the carbide phase was not detected, which may be due to the relatively small content of this phase. The phase detected in the coating has a dense structure. Whether this coating has a strong barrier to the diffusion of hydrogen and can really play a role in preventing hydrogen permeation needs to be demonstrated by hydrogen release experiments. Figure 7 shows the XRD curve of Zr and CO2 coating.

Conclusion:





1) The thickness of 5~20 can be prepared by using this experimental method and process parameters μ M. Dense and uniform coating.



2) The growth rate of the coating on the zirconium hydride surface is slower than that on the zirconium surface. Therefore, the coating obtained in the same experimental time is slightly thinner, about 1/3 of the zirconium surface coating.



3) From the analysis of the surface and section morphology of the coating with and without P, it can be seen that P can improve the quality of the surface coating of zirconium and zirconium hydride, make the coating more dense and prevent the formation of cracks in the coating.



4) XRD analysis showed that the coating contained zirconia and zirconium phosphide; The results of composition line scanning show that oxygen has obvious diffusion.



5) From the coating morphology of polished surface and unpolished surface, the defects on the surface are conducive to the growth of the coating, but the thickness of the coating is uneven and the structure is loose. I would like to thank the Laboratory for Post-irradiation Inspection of Reactor Materials of the Chinese Academy of Atomic Energy for its assistance in metallographic experiments. Thank you for your help, Mr. Miao of the X-ray diffraction group of Tsinghua University and Mr. Cui of the scanning electron microscope group of Beijing University of Science and Technology.