简介概要

Corrosion behavior of Zr–Nb–Cr cladding alloys

来源期刊：Rare Metals2013年第5期

论文作者：Wei-Jia Gong Cong-Feng Wu Hang Tian Xiao-Dong Ni Hai-Long Zhang Xi-Tao Wang

文章页码：480 - 485

摘要：Zr–Nb–Cr alloys were used to evaluate the effects of alloying elements Nb and Cr on corrosion behavior of zirconium alloys.The microstructures of both Zr substrates and oxide films formed on zirconium alloys were characterized.Corrosion tests reveal that the corrosion resistance of ZrxNb0.1Cr（x=0.2,0.5,0.8,1.1;wt%）alloys is first improved and then decreased with the increase of the Nb content.The best corrosion resistance can be obtained when the Nb concentration in the Zr matrix is nearly at the equilibrium solution,which is closely responsible for the formation of columnar oxide grains with protective characteristics.The Cr addition degrades the corrosion resistance of the Zr1.1Nb alloy,which is ascribed to Zr（Cr,Fe,Nb）2precipitates with a much larger size than b-Nb.

稀有金属 (英文版) 2013,32(05),480-485

Corrosion behavior of Zr–Nb–Cr cladding alloys

State Key Laboratory for Advanced Metals and Materials, University of Science and Technology Beijing

School of Mathematics and Physics, University of Science and Technology Beijing

摘要：

Zr–Nb–Cr alloys were used to evaluate the effects of alloying elements Nb and Cr on corrosion behavior of zirconium alloys.The microstructures of both Zr substrates and oxide films formed on zirconium alloys were characterized.Corrosion tests reveal that the corrosion resistance of ZrxNb0.1Cr (x=0.2, 0.5, 0.8, 1.1;wt%) alloys is first improved and then decreased with the increase of the Nb content.The best corrosion resistance can be obtained when the Nb concentration in the Zr matrix is nearly at the equilibrium solution, which is closely responsible for the formation of columnar oxide grains with protective characteristics.The Cr addition degrades the corrosion resistance of the Zr1.1Nb alloy, which is ascribed to Zr (Cr, Fe, Nb) 2precipitates with a much larger size than b-Nb.

收稿日期：16 June 2013

基金：supported by the State Key Laboratory for Advanced Metals and Materials (No. 2011Z-06);the Fundamental Research Funds for the Central Universities (No. FRF-TP-11-002A);the National High-Tech R&D Program of China (No. 2012AA03A507);

Corrosion behavior of Zr–Nb–Cr cladding alloys

Wei-Jia Gong Cong-Feng Wu Hang Tian Xiao-Dong Ni Hai-Long Zhang Xi-Tao Wang

State Key Laboratory for Advanced Metals and Materials, University of Science and Technology Beijing

School of Mathematics and Physics, University of Science and Technology Beijing

Abstract：

Zr–Nb–Cr alloys were used to evaluate the effects of alloying elements Nb and Cr on corrosion behavior of zirconium alloys. The microstructures of both Zr substrates and oxide films formed on zirconium alloys were characterized. Corrosion tests reveal that the corrosion resistance of ZrxNb0.1Cr (x = 0.2, 0.5, 0.8, 1.1; wt%) alloys is first improved and then decreased with the increase of the Nb content. The best corrosion resistance can be obtained when the Nb concentration in the Zr matrix is nearly at the equilibrium solution, which is closely responsible for the formation of columnar oxide grains with protective characteristics. The Cr addition degrades the corrosion resistance of the Zr1.1Nb alloy, which is ascribed to Zr (Cr, Fe, Nb) 2 precipitates with a much larger size than b-Nb.

Keyword：

Zirconium alloys; Corrosion; Precipitates; Alloying elements; Transmission electron microscope;

Author： Xi-Tao Wang e-mail: xtwang@ustb.edu.cn ;

Received： 16 June 2013

1 Introduction

Zirconium alloys are used as fuel-cladding materials in nuclear power plants, mainly due to their low thermal neutron absorption and good corrosion resistance.In the last 20 years, waterside corrosion has become a major limit for their applications in high burning up and extended fuel cycle conditions[1–3].Several new zirconium alloys including ZIRLO^? (Zr1.0Sn1.0Nb0.1Fe, wt%) [4], optimized ZIRLO (Zr0.67Sn1.0Nb0.1Fe) [5], M5^? (Zr1.0Nb0.12O) [6], and HANA-6 (Zr1.1Nb0.05Cu) [7]are accordingly developed.Compared with the ZIRLO^?alloy, the Sn-content is reduced in the improved version to produce a higher corrosion resistance[5].Moreover, Sn is absent in the M5^?and HANA-6 alloys, in which Nb predominates.Therefore, Nb becomes the principal alloying element in the advanced zirconium alloys instead of Sn.

The Zr–Nb alloys attract a lot of interest due to the excellent corrosion resistance.The corrosion behavior of Zr–Nb binary alloys with respect to the Nb-content is clarified[8–11].It is reported that the corrosion resistance is improved by the equilibrium Nb concentration in the aZr matrix[8].When the Nb content is higher than its equilibrium solubility, the corrosion resistance first decreases and then increases.A good corrosion resistance can also be achieved when fine b-Nb precipitates are formed by annealing below the monotectoid temperature[9].Based on the Zr–Nb binary alloys, several alloying elements are added to form ternary alloys, which are potential for commercial applications.In the case of Zr–Nb–Cu, the addition of Cu is found to be beneficial for improving the corrosion resistance, especially at a lower level by contributing to the fine distribution of Nb-containing precipitates[12].Since the existence of Fe in zirconium alloys is inevitable due to*500 9 10^-6in sponge zirconium, phase diagrams and corrosion behavior of Zr–Nb–Fe alloys are thus studied[13, 14].The fcc- (Zr Nb) ₂Fe precipitates formed in low Nb/Fe ratio alloys are more beneficial for the corrosion resistance than hcp-Zr (Nb, Fe) ₂in high Nb/Fe ratio alloys[14].

Although Cr is considered to be an important alloying element in the advanced zirconium alloys[15], the synergistic effect of Nb and Cr on corrosion behaviors is not evaluated in Zr–Nb–Cr ternary alloys.To this end, Zr–Nb–Cr alloys were fabricated in this study.The corrosion behavior was examined in a static autoclave.The microstructures of Zr substrates and oxides formed on zirconium alloys were characterized by scanning electron microscope (SEM) and transmission electron microscope (TEM) .

2 Experimental

Zirconiumalloyswithnominalcompositionsof Zrx Nb0.1Cr (x=0.2, 0.5, 0.8, 1.1;wt%) and Zr1.1Nb were used for experiments.The manufacturing process of Zr–Nb–Cr sheets is shown in Fig.1.The button ingots were prepared by the vacuum arc re-melting method and remelted five times to promote the chemical homogeneity.The arc-remelted ingots were hot-forged after pre-heating to 900°C for 20 min, b-quenched at 1, 050°C for 30 min in an argon atmosphere and then cold-rolled three times to a final thickness of*1 mm.The intermediate-annealing was performed during rolling intervals at 580°C for 11 h in total.The final cold-rolled sheets were annealed at 580°C for 3 h to obtain a fully-recrystallized microstructure.

The microstructures of Zr substrates were examined by SEM (Zeiss AURIGA, Germany) .Prior to SEM observations, the specimens were mechanically ground with Si C papers (2000 grid in the final step) and then electropolished at-25°C in a solution of 5 vol%HCl O₄and95 vol%C₂H₅OH.Precipitates were characterized by TEM (FEI Tecnai F20, Netherlands) and energy dispersive spectroscope (EDS) .Specimens for TEM observation were prepared by twin-jet polishing at-25°C with a solution of10 vol%HCl O₄and 90 vol%C₂H₅OH.

Fig.1 Manufacturing process of Zr–Nb–Cr sheets

Specimens of 20 mm 9 15 mm 9 1 mm in size were cut from the alloy sheets, mechanically ground with 2000grit Si C papers, and then pickled in a solution of 40 vol%H₂O, 30 vol%HNO₃, 25 vol%HCl, and 5 vol%HF.The corrosion tests were performed in pure steam at 400°C for70 days according to ASTM G2-88.The corrosion behavior of the specimens was evaluated by measuring the weight gain with the exposure time.

After the corrosion tests, the phase structure of the oxide films was analyzed by X-ray diffraction (XRD, Rigaku TTRIII, Japan) .Cross-sectional morphologies of the oxide films were characterized by SEM with an acceleration voltage of 10 k V.In order to obtain the cross-sections of oxide films, corroded specimens were etched in a mixed solution of 10 vol%HF, 45 vol%HNO₃, and 45 vol%H₂O for 20 min to dissolve metal substrates.Then the oxide films jutting beyond metal substrates were gently fractured using a tweezer.Before SEM observation, the cross-sectional fractures were covered with gold films to improve the electrical conductivity.The fractured cross-sections can reveal the layered structure and grain morphology of oxide films.

3 Results and discussion

3.1 Microstructures of Zr–Nb–Cr alloys

Figure 2 shows SEM images of the Zr–Nb–Cr alloys with various compositions.In the Zr0.2Nb0.1Cr alloy, recrystallized equiaxed grains are observed.With the increase of Nb content, the grain size reduces, as shown in Fig.2b, c.The deformed microstructure is still observed in the Zr1.1Nb0.1Cr alloy, whereas the Zr1.1Nb alloy shows a fully-recrystallized microstructure.Therefore, the alloy element Cr inhibits the recrystallization of the Zr1.1Nb alloy during the annealing.

Figure 3 shows the type and distribution of precipitates characterized by TEM and EDS.The precipitate type varies with chemical compositions of alloys.The Zr0.2Nb0.1Cr alloy is found to have two types of precipitates, Zr (Cr, Fe, Nb) ₂with a size of 200–300 nm and Zr (Cr, Fe) ₂with a size of 100–200 nm.It is reported that Zr (Cr, Fe, Nb) ₂is a Nb-containing Zr (Cr, Fe) ₂-type precipitate[16].It implies that Nb would dissolve in Zr (Cr, Fe) ₂to form Zr (Cr, Fe, Nb) ₂, although the Nb content in Zr0.2Nb0.1Cr is lower than the solubility limit of*0.37wt%in a-Zr matrix[13].With the increase of Nb content to 0.5 wt%, the amount of precipitates obviously increases, as shown in Fig.3b, c.These precipitates are identified as irregularly-shaped Zr (Cr, Fe, Nb) ₂, as well as spherical b-Nb with a small size of*50 nm.Figure 3b shows that the area fraction of Zr (Cr, Fe, Nb) ₂precipitates is obviously larger than that of b-Nb.The presence of b-Nb indicates the saturated solution of Nb in the Zr0.5Nb0.1Cr alloy.It is reasonably inferred that the amount of b-Nb would increase with the increase of Nb content to 1.1 wt%.Figure 3d reveals that b-Nb is the major precipitates in the Zr1.1Nb alloy.Besides, Zr (Nb, Fe) ₂as minor precipitates are observed at grain boundaries.Based on analyses on precipitates, it is demonstrated that the Cr addition in the Zrx Nb alloys introduces Zr (Cr, Fe, Nb) ₂precipitates with a much larger size than b-Nb.

Fig.2 SEM images of Zr–Nb–Cr alloys with various compositions:a Zr0.2Nb0.1Cr, b Zr0.5Nb0.1Cr, c Zr0.8Nb0.1Cr, d Zr1.1Nb0.1Cr, and e Zr1.1Nb

3.2 Corrosion behavior and oxide characteristics

Figure 4 shows the weight gain of alloys as a function of exposure time.The Zr0.2Nb0.1Cr alloy exhibits an accelerated corrosion, showing a spalling phenomenon of oxide films after 3 days.The lowest weight gain is observed in the Zr0.5Nb0.1Cr alloy.However, in the range of Nb content more than 0.5 wt%, the corrosion resistance continuously decreases with the increase of Nb content.The comparison of Zr1.1Nb and Zr1.1Nb0.1Cr alloys reveals that the Cr addition reduces the corrosion resistance of zirconium alloys.

After the corrosion tests, XRD was employed to investigate the phase structure of oxide films formed on all specimens.Figure 5 shows a typical XRD pattern of oxides formed on the Zr1.1Nb alloy.The oxide films mainly consist of m-Zr O₂ (monoclinic symmetry, JCPDS#65-1025) .Figure 6shows the cross-sectional morphology of oxide films formed on all specimens.Based on SEM observations, the thickness of oxide films forming after 70 days was measured.As shown in Table 1, the measured thickness of oxide films follows theorder:Zr0.5Nb0.1Cr\Zr1.1Nb\Zr0.8Nb0.1Cr\Zr1.1Nb0.1Cr, which is consistent with the estimated results from weight gain.

In Fig.6a, the oxide films formed on the Zr0.2Nb0.1Cr alloy mainly consist of equiaxed grains.It is reported that the oxidation process of zirconium alloys is controlled by the oxygen diffusion through the oxides to the oxide/metal interface[1].Owing to a large amount of grain boundaries and porous microstructure, the region of equiaxed grains provides short circuits for the oxygen diffusion, which induces the spalling phenomena of oxide films on the Zr0.2Nb0.1Cr alloy.Whereas, the oxide films formed on other four alloys exhibit a complex morphology of outer equiaxed grains and inner columnar grains, as shown in Fig.6b–e.The columnar grains, aligned tightly in a direction parallel to the oxide growth, constitute the majority of the oxide film, while the equiaxed grains cover the outermost part*0.5 lm from the surface.The complex morphology of oxide films identified by SEM is in good agreement with TEM observations in our previous study[17].Compared with the equiaxed grains, dense columnar grains are much more protective for Zr substrates against further oxidation.

Fig.3 TEM analysis on precipitates in Zr–Nb–Cr alloys:a Zr0.2Nb0.1Cr, b, c Zr0.5Nb0.1Cr, and d Zr1.1Nb

Fig.4 Weight gain versus exposure time for Zr–Nb–Cr alloys in400°C pure steam

Fig.5 XRD pattern of oxides formed on Zr1.1Nb alloy

Fig.6 Cross-sectional SEM images of oxides formed on Zr–Nb–Cr alloys:a Zr0.2Nb0.1Cr, 3 days;b Zr0.5Nb0.1Cr, 70 days;c Zr0.8Nb0.1Cr, 70 days;d Zr1.1Nb0.1Cr, 70 days;e Zr1.1Nb, 70 days

Table 1 Corrosion results following 70 days in 400°C pure steam 下载原图

^aEstimated from weight gain using the relationship of1 lm=15 mg?dm^-2[3]

Table 1 Corrosion results following 70 days in 400°C pure steam

As a result, the oxide films forming on the four alloys retain the structural integrity.Therefore, it is deduced that the absence of columnar oxide grains is responsible for the degradation of corrosion resistance of the Zr0.2Nb0.1Cr alloy.

3.3 Influence of Nb and Cr on corrosion behavior

Figure 7 shows weight gain versus the Nb content, where the types of precipitates identified by TEM are included.As a result, the influences of alloying elements Nb and Cr on corrosion behavior of Zr–Nb–Cr alloys can be discussed.

For the Zrx Nb0.1Cr alloys, the weight gain first decreases and then goes up with increasing the Nb content.Owing to the consumption of Nb in Zr (Cr, Fe, Nb) ₂, the precipitation of b-Nb requires more Nb addition than the solubility limit*0.37 wt%[13].As shown in Fig.4c, bNb exists in the Zr0.5Nb0.1Cr alloy.So the actual value (x_sa) of Nb content corresponding to the equilibrium solution of Nb in a-Zr matrix is in the interval from 0.37 wt%to 0.5 wt%, as approximately indicated by the arrow in Fig.7.When the Nb content is much smaller than x_sa, e.g., in the Zr0.2Nb0.1Cr alloy, the weight gain is large.With the Nb content increasing to x_sa, the concentration of Nb in a-Zr matrix increases and the corrosion resistance is improved.When the Nb content is larger than x_sa, the amount of precipitates increases with the increase of the Nb content, which results in a decline in corrosion resistance.Therefore, the equilibrium solution of Nb in a-Zr matrix favors the best corrosion resistance of Zr–Nb–Cr ternary alloys, which is in good agreement with the result in Zr–Nb binary alloys[8].Compared with the equilibrium solution in other four alloys, the Nb concentration in Zr0.2Nb0.1Cr matrix is much less.Correspondingly, only equiaxed grains are formed in oxide films on the Zr0.2Nb0.1Cr alloy.It is inferred that the equilibrium solution of Nb in a-Zr matrix is closely responsible for the formation of columnar oxide grains with protective characteristics.

Fig.7 Influence of alloying elements Nb and Cr on corrosion behavior

In the Zr1.1Nb alloy, b-Nb is the major precipitates as well as a very small amount of Zr (Nb, Fe) ₂at grain boundaries.The addition of 0.1 wt%Cr decreases the corrosion resistance of the Zr1.1Nb alloy, as shown in Fig.7.The degradation is ascribed to Zr (Cr, Fe, Nb) ₂precipitates induced by the Cr addition.Owing to the large size, Zr (Cr, Fe, Nb) ₂precipitates readily introduce microcracks after incorporation into oxide films and thus decrease the corrosion resistance.

4 Conclusion

In this study, the corrosion behavior, precipitates, and oxide characteristics of the Zr–Nb–Cr alloys with various chemicalcompositionswereinvestigated.Inthe Zrx Nb0.1Cr alloys, the weight gain first decreases and then goes up with the increase of the Nb content.The best corrosion resistance can be obtained when the Nb concentration in Zr matrix is nearly at its equilibrium solution.The equilibrium solution of Nb in Zr matrix is closely responsible for the formation of columnar oxide grains with protective characteristics.The Cr addition introduces Zr (Cr, Fe, Nb) ₂precipitates with a large size, and thus degrades the corrosion resistance of the Zr1.1Nb alloy.