Degradation Mechanism on Corrosion Resistance of High Nb-Containing Zirconium Alloys in Oxygen-Containing Steam
HUANG Jiansong, PEI Wen, XU Shitong, BAI Yong, YAO Meiyi(), HU Lijuan, XIE Yaoping, ZHOU Bangxin
Institute of Materials, Shanghai University, Shanghai 200072, China
Cite this article:
HUANG Jiansong, PEI Wen, XU Shitong, BAI Yong, YAO Meiyi, HU Lijuan, XIE Yaoping, ZHOU Bangxin. Degradation Mechanism on Corrosion Resistance of High Nb-Containing Zirconium Alloys in Oxygen-Containing Steam. Acta Metall Sin, 2024, 60(4): 509-521.
In some water-cooled nuclear power reactors, a hydrogenation-deoxygenation device is generally not used to simplify the system and save space, which can increase dissolved oxygen (DO) concentration in primary loop water. The increase in DO concentration will inevitably affect the corrosion resistance of zirconium alloy cladding materials. In particular, DO will accelerate the corrosion of Nb-containing zirconium alloys, and the corrosion rate of zirconium alloys with high Nb content is sensitive to DO concentration. In exploring the deterioration mechanism of the corrosion resistance of high-Nb-containing zirconium alloys in oxygen-containing steam, the corrosion behavior of the Zr-0.75Sn-0.35Fe-0.15Cr-1.0Nb (mass fraction, %) alloy was studied in superheated steam through deoxygenation, with 300 μg/kg of DO and 1000 μg/kg of DO at 400oC and 10.3 MPa. The corrosion behavior was characterized by measuring the mass gain per unit area. SEM was used to observe the fracture morphology of the oxide film; TEM-EDS was used to observe and analyze the morphology, elemental distribution, and crystal structure of the alloy, second-phase particles (SPPs), and oxide film. The elemental distribution on the outer surface of the oxide film was analyzed by XPS, and the valence state of Nb was determined in accordance with the binding energy to analyze the influence of DO on the oxidation behavior of Nb in the oxide film. Results show that the corrosion resistance of the Zr-0.75Sn-0.35Fe-0.15Cr-1.0Nb alloy in superheated steam at 400oC deteriorates with the increase of DO concentration. The deterioration mechanism of the corrosion resistance of the alloy in oxygen-containing steam is proposed. On the one hand, DO accelerates the oxidation of Nb in the oxide film and promotes the conversion of Nb2+ to Nb5+. On the other hand, DO promotes the oxidation of SPPs to m-Nb2O5 (monoclinic) and amorphous phase, thereby promoting the initiation and growth of cracks. These newly generated cracks provide more channels for the diffusion of O2- and other oxidizing ions to accelerate the oxidation of SPPs near the cracks and the microstructural evolution of the oxide film, thereby accelerating the corrosion of the alloy.
Fig.1 TEM image (a) and HAADF image (b) of the Zr-0.75Sn-0.35Fe-0.15Cr-1.0Nb alloy, selected area electron diffraction(SAED) patterns of typical second phase particles (SPPs) (a1, a2), and size distributions of SPPs (c)
Fig.2 Corrosion mass gain curves in the form of natural number coordinates (a) and double logarithmic coordinates (b) of Zr-0.75Sn-0.35Fe-0.15Cr-1.0Nb alloy in superheated steam with DE, 300 μg/kg DO, and 1000 μg/kg DO at 400oC and 10.3 MPa (DE—deoxygenation, DO—dissolved oxygen)
Environment
Transition
Pre-transition
Post-transition
/ d
/ (mg·dm-2)
/ (mg·dm-2·d)
/ (mg·dm-2·d)
DE
72
57.39
4.78
0.46
1.38
0.86
300 μg·kg-1 DO
40
48.73
7.58
0.43
1.62
0.88
1000 μg·kg-1 DO
40
51.84
4.67
0.55
1.73
0.89
Table 1 Oxidation kinetic parameters of the alloy corroded in superheated steam with DE, 300 μg/kg DO, and 1000 μg/kg DO at 400oC and 10.3 MPa
Fig.3 SEM images of the fracture morphologies of oxide films on Zr-0.75Sn-0.35Fe-0.15Cr-1.0Nb alloy corroded in superheated steam with DE (a), 300 μg/kg DO (b), and 1000 μg/kg DO (c) at 400oC and 10.3 MPa for 42 d
Fig.4 TEM bright-field images of the cross-sectional microstructure of oxide films on Zr-0.75Sn-0.35Fe-0.15Cr-1.0Nb alloy corroded in superheated steam with DE (a), 300 μg/kg DO (b), and 1000 μg/kg DO (c) at 400oC and 10.3 MPa for 42 d
Fig.5 HAADF images and corresponding EDS mapping of the complete region (a) and region 1 magnification (b) of cross-sectional oxide film on the alloy corroded in superheated steam with DE at 400oC and 10.3 MPa for 42 d
Fig.6 HRTEM images and FFT analyses (insets) of the SPPs marked in Fig.5b, including SPP1 (a-e), SPP2 (f-h), and SPP3 (i), wherein magnifications of regions B-E (b-e) and G, H (g, h)
Fig.7 HAADF images and corresponding EDS mappings of the complete region (a), region 1 (b), and region 2 (c) of cross-sectional oxide film on the alloy corroded in superheated steam with 300 μg/kg DO at 400oC and 10.3 MPa for 42 d
Fig.8 HRTEM images and FFT analyses (insets) of SPPs marked in Figs.7b and c, including SPP1 (a-c), SPP2 (d-f), and SPP3 (g-i), wherein magnifications of regions B, C (b, c), E, F (e, f), and H, I (h, i)
Fig.9 HAADF images and corresponding EDS mappings of complete region (a), region 1 (b), and region 2 (c) of cross-sectional oxide film on the alloy corroded in superheated steam with 1000 μg/kg DO at 400oC and 10.3 MPa for 42 d
Fig.10 HRTEM images and FFT analyses (insets) of SPPs marked in Figs.9b and c, including SPP1 (a-c), SPP2 (d-f), and SPP3 (g-h1), wherein magnifications of regions B, C (b, c), E, F (e, f), H (h), and FFT analysis corresponding to the region in Fig.10h (h1)
Environment
SPPs
D / μm
Oxidation product
DE
SPP1
0.1
t-ZrO2, m-Nb2O5
SPP2
0.7
m-Nb2O5
SPP3
2.0
Amorphous
300 μg·kg-1
SPP1
1.0
-
SPP2
0.4
Amorphous, m-Nb2O5
SPP3
2.3
m-ZrO2, m-Nb2O5
1000 μg·kg-1
SPP1
0.1
m-Nb2O5
SPP2
0.3
m-Nb2O5
SPP3
2.8
Amorphous
Table 2 Oxidation products of SPPs in the oxide film on the alloy corroded in superheated steam with DE, 300 μg/kg DO, and 1000 μg/kg DO at 400oC and 10.3 MPa for 42 d
Fig.11 XPS fine spectra and peak fitting of Nb element on the oxide film surface formed on the 300 DO/160 d (a) and 1000 DO/130 d (b) specimens after Ar+ sputtered for 180 s
Time
300 DO / 160 d
1000 DO / 130 d
s
Nb2+
Nb5+
Nb2+
Nb5+
0
4.98
95.05
2.39
97.61
90
9.32
90.68
1.75
98.25
180
14.29
85.71
0.95
99.05
Table 3 Proportions of different valence states of Nb element in the oxide film on 300 DO/160 d and 1000 DO/130 d specimens
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