The aim of this work is to characterize the oxide layer structure of Alloy 690TT in high-temperature water with different dissolved hydrogen (DH) contents by using an X-ray photoelectron spectroscopy. Under the low DH contents (0.4494–0.8988 mg/kg), the oxide layers were composed of an outermost layer of Ni(OH)2 and Cr(OH)3 enriched in Ni, an intermediate layer of hydroxides and oxides enriched in Cr, and an inner Cr2O3 layer. Outermost NiO coexists with small amount of Cr2O3 layer, while in the inner oxide only Cr2O3 remains. The oxide layers at medium and high DH contents (3.1458– 8.9880 mg/kg) consisted of an outermost layer of Ni(OH)2 and Cr(OH)3 enriched in Cr, an intermediate layer of metallic Ni, hydroxides and oxides enriched in Cr, and an inner Cr2O3 layer. In addition, boron compounds containing B3+ ions were accumulated in the thick and porous NiO layer formed at low DH contents, whereas the accumulation of boron compounds did not occur in the thin and dense polyhedral oxide layer formed at medium and high DH contents.
Alloy 690, a nickel-based alloy having high chromium content (27–31 wt.%), was developed as a replacement for Alloy 600 steam generator (SG) tubes owing to its excellent resistance to stress corrosion cracking (SCC) in a pressurized water reactor (PWR) [
The effects of various water chemistry parameters such as dissolved oxygen (DO) [
There are many researches related to hydrogen contents in the primary water of PWRs. For the fuel cladding, Kawamura [
Meanwhile, boron accumulation may occur within not only the crud formed on the fuel cladding but also the oxide layer on Alloy 690TT at low DH content because both oxide layers have a thick and porous morphology. However, the effects of the hydrogen contents on the oxide structure and associated boron accumulation within oxide layer of Alloy 690TT have not yet been investigated. Furthermore, the detailed depth profile of the chemical state of the oxide layers formed on Alloy 690TT has not been reported under the broad range of DH contents by using X-ray photoelectron spectroscopy (XPS).
Therefore, the purpose of this study is to characterize the oxide layer structure of Alloy 690TT and associated boron accumulation within its oxide layers in primary water at 330°C and 150 bars in the broad range of DH contents (0.4494–8.9880 mg/kg) by using XPS. To observe the boron accumulation within the oxide layers, the depth profile and chemical species of the oxide layers were analyzed by XPS analysis.
Alloy 690TT tubes, which are commonly used as a SG tubing material, were used in this study. Alloy 690TT tubes have a dimension of a 19.07 mm outer diameter (OD) and a 16.93 mm inner diameter (ID). The specimens were cut into a size of 10 mm × 10 mm × 1.07 mm for the oxidation tests and various oxide layer analyses. The specimens were cleaned in acetone and distilled water. After the specimens had been cleaned, the piece specimens were directly dried in a vacuum oven at 60°C. While the preparing for the oxidation tests, the samples were stored in a vacuum desiccator at room temperature to prevent the oxidation reaction in the air. In addition, because Alloy 690TT is a high-alloyed steel having excellent corrosion resistance, the surface composition of the Alloy 690TT before the oxidation test could be considered to the same as that of the matrix. The chemical composition of Alloy 690TT was analyzed in accordance with the standard of ASTM E353-14 for method for chemical analysis of stainless, heat-resisting, maraging, and other similar chromium-nickel-iron alloys [
Chemical composition of Alloy 690TT (wt. %).
C | Cr | Fe | Si | Mn | Ti | Al | Ni |
---|---|---|---|---|---|---|---|
0.02 | 28.0 | 10.2 | 0.1 | 0.3 | 0.1 | 0.1 | Bal. |
Oxidation tests for the oxide formation were performed in an autoclave connected to a high-temperature water circulating system. The circulation system consisted of the following main components: a primary solution tank, high-pressure pump, preheater for solution inlet temperature control, back pressure regulator for pressure control, heat exchanger, and an autoclave for placing the specimens. The test solution was preheated and entered into the autoclave where the piece specimens were hung on a specimen holder.
The test solution was maintained consistently at 330°C under 150 bars in the autoclave because the saturation pressure of the high-temperature water at 330°C is about 128 bars. The simulated primary water was made using the demineralized water with the resistivity above 18 M
The simulated primary water was stored in the solution tank with a capacity of 150 L. The DH contents were selected to 0.4494, 0.8988, 3.1458, 4.4940, 5.8422, and 8.9880 mg/kg (5, 10, 35, 50, 65, and 100 cc/kg H2O) at standard temperature and pressure (STP), which is a temperature of 0°C and a pressure of 1 bar by controlling the hydrogen overpressure of the solution tank. DO content was controlled in the range of 2–3 ppb to eliminate the oxygen effect on the corrosion potential and oxidation reaction. After the oxidation tests were finished, the specimens were directly cleaned in distilled water and dried in a vacuum oven at 60°C. Then, before analyzing the oxide layer by various analyses, the specimens were quickly stored in a vacuum desiccator at room temperature to avoid the variation on the oxide layer structure. In a previous study, the oxidation tests were conducted to Alloy 690TT under the same experimental conditions [
The outer surface of the oxide layer formed on Alloy 690TT specimens was observed by using a SEM with Hitachi S-4800, of which acceleration voltage was 10 kV. The cross-section of oxide layer was also examined by using focused ion beam FIB (FEI company QUANTA 3D FEG)-SEM. The thickness of oxide layers was measured from the cross-sectional images obtained by FIB-SEM analysis. The thickness measurement was done at least at three different positions.
To elucidate the effect of oxide layer structure on the boron accumulation within the oxide, the depth profiles and chemical species of the oxide layers were analyzed by using XPS. The XPS analysis was carried out by using a Thermo Fisher Scientific (Theta Probe AR-XPS) X-ray photoelectron spectrometer with an Al K
The removal (ex situ) of Alloy 690TT specimen from the PWR primary water environment may result in a modification of the oxide layer structure. However, in previous studies [
Figure
SEM micrographs of the oxide layers formed on Alloy 690TT in simulated primary water with different DH contents at 330°C for 500 h: (a) 0.4494 mg/kg, (b) 0.8988 mg/kg, (c) 3.1458 mg/kg, (d) 4.4940 mg/kg, (e) 5.8422 mg/kg, and (f) 8.9880 mg/kg.
Figure
FIB-SEM micrographs of the oxide layers formed on Alloy 690TT in simulated primary water with different DH contents at 330°C for 500 h: (a) 0.4494 mg/kg and (b) 3.1458 mg/kg.
Figure
Depth profiles of the oxide layers formed on Alloy 690TT in simulated primary water with different DH contents at 330°C for 500 h: (a) 0.4494 mg/kg, (b) 3.1458 mg/kg, (c) 4.4940 mg/kg, and (d) 5.8422 mg/kg.
In this work, the thickness of oxide layers could be associated with the thermodynamics and kinetics of corrosion and incorporation of boric oxide. In the viewpoint of corrosion thermodynamics, Cr2O3 and Fe3O4 are stable phases under all DH contents handled in this work. However, Ni/NiO phase transition is expected to be characterized by the effect of DH content on the electrochemical potential (ECP). The ECP of Alloy 690TT with various DH contents was calculated using Nernst equation for hydrogen-hydrogen ion exchange reaction, and the ECP gradually decreased with increasing DH content [
However, the oxidative dissolution reaction of Ni is reduced in the Ni stable region because Ni is no longer a thermodynamically oxidized state. Second, the corrosion kinetics is not only dependent on the microstructure and chemical composition but also dependent on the electronic property of oxide layer. These parameters are crucial to the stability of the oxide layers. Peng et al. [
The XPS depth profile of the three major elements (Ni, Cr, and Fe) normalized to 100% is also shown in Figure
Normalization of the oxide films formed on Alloy 690TT in simulated primary water with different DH contents at 330°C for 500 h: (a) 0.4494 mg/kg, (b) 3.1458 mg/kg, (c) 4.4940 mg/kg, and (d) 5.8422 mg/kg.
Figure
XPS spectra of Ni, Cr, Fe, and O for the oxide layers formed on Alloy 690TT in simulated primary water with DH = 0.4494 mg/kg at 330°C for 500 h.
The binding energies of some chemical species for the XPS analysis.
Chemical species | Binding energies ( |
Chemical species | Binding energies ( |
---|---|---|---|
Nio2p3/2 | 852.9 [ |
Feo2p3/2 | 706.8 [ |
Niosat 2p3/2 | 858.9 [ |
Fe2+2p3/2 | 708.5 [ |
NiO 2p3/2 | 854.0 [ |
Fe3+2p3/2 | 712.0 [ |
Ni(OH)2 2p3/2 | 856.0 [ | ||
Ni(OH)2 sat 2p3/2 | 862.0 [ |
O2−1s | 530.0 [ |
OH−1s | 531.5 [ | ||
Cro2p3/2 | 574.0 [ | ||
Cr2O3 2p3/2 | 576.1 [ |
B3+1s | 192.0 [ |
Cr(OH)3 2p3/2 | 577.3 [ |
Figure
XPS spectra of Ni, Cr, Fe, and O for the oxide layers formed on Alloy 690TT in simulated primary water with DH = 4.4940 mg/kg at 330°C for 500 h.
Figure
XPS spectra of B1s for the oxide layers formed on Alloy 690TT in simulated primary water with DH = 0.4494 mg/kg at 330°C for 500 h (a) without sputtering and (b) with sputtering time for 300 s.
Based on the deconvoluted XPS profiles, the depth profile of chemical species on the oxide layers could be presented. Figure
Speciated chemical composition profiles of the oxide layers formed on Alloy 690TT with sputtering time in simulated primary water with various DH contents at 330°C for 500 h: (a) 0.4494 mg/kg and (b) 4.4940 mg/kg.
According to the XPS results, the oxide layers formed on Alloy 690TT were composed of oxides and hydroxides under all hydrogen conditions. The formation of Ni(OH)2 and Cr(OH)3 at the outer oxide layer is most likely due to the hydration of Ni2+ and Cr3+. The oxide layers at low DH content were composed of an outermost layer of Ni(OH)2 and Cr(OH)3 enriched in Ni, an intermediate layer of hydroxides and oxides enriched in Cr, and an inner Cr2O3 layer. Outermost NiO coexists with a small amount of Cr2O3 layer; while in the inner oxide, only Cr2O3 remains. The oxide layers at medium and high DH contents were composed of an outermost layer of Ni(OH)2 and Cr(OH)3 enriched in Cr, an intermediate layer of metallic Ni, hydroxides and oxides enriched in Cr, and an inner Cr2O3 layer.
Boron, in the form of boric acid (H3BO3), is added into primary water to control the neutron flux while lithium hydroxide (LiOH) is also dosed to control the pH of the primary water. As presented in the EPRI report [
Previous studies have reported that there are only a few different forms of boron species in fuel crud originating from PWR core. It is well known that boric acid may be thought of as hydrates of boric trioxide (B2O3). Doncel et al. [
In this study, to elucidate the relationship between the oxide layer structure and boron accumulation within the oxide layers, the depth profiles and chemical species of the oxide layers formed on Alloy 690TT were analyzed by using XPS. It was observed that the B3+ ions were detected in the thick and porous NiO layer formed at low DH contents. This indicates that the boron compounds containing B3+ ions were accumulated in the thick and porous NiO layer. The content of B3+ ions gradually decreased with the increase of sputtering time. Finally, the B3+ ions disappeared in the inner Cr2O3 layer. However, the accumulation of B3+ ions did not occur in the dense and thin polyhedral NiCrFeO4 layer formed at medium and high DH contents (DH = 3.1458–8.9880 mg/kg). Based on the results, it is confirmed that the morphology and thickness of oxide layer formed on Alloy 690TT could significantly affect the accumulation of boron compounds within its oxide layer. The effect of DH content on the oxide layer structure of Alloy 690TT and associated boron accumulation within its oxide layer is schematically shown in Figure
Schematic of the effects of DH contents on the oxide layer structure and associated boron accumulation of Alloy 690TT in simulated primary water at 330°C.
According to EPRI report, the DH content has been specified to be controlled in a range of 2.2470–4.4940 mg/kg in the primary water chemistry guidelines [
We think that boron accumulation could occur within not only the pores of crud on the fuel cladding but also porous oxide layer formed on the Alloy 690TT at low DH contents (0.4494–0.8988 mg/kg) because both oxide layers are porous and thick. Based on the results, it was clear that boron compounds containing B3+ were accumulated in the thick and porous NiO layer formed at low DH contents.
Under the operation of nuclear power plants, the accumulation of boron compounds containing B3+ ions within the oxide layer of Alloy 690TT should be considered for establishing the operation condition of hydrogen content. Accumulation of boron compounds could accelerate general corrosion and primary water stress corrosion cracking (PWSCC) due to a decrease in pH within the porous oxide layer as through the following precipitation reaction of LiBO2.
Based on the results obtained from this work, it is concluded that the medium and high DH contents located in the Ni stable region are more desirable for SG tube for prohibiting the boron accumulation within the oxide layer of Alloy 690TT because the hideout of boron compounds does not occur in the external dense and thin oxide layer.
In primary water of PWRs at 330°C, the oxide layers at low DH content were composed of an outermost layer of Ni(OH)2 and Cr(OH)3 enriched in Ni, an intermediate layer of hydroxides and oxides enriched in Cr, and an inner Cr2O3 layer. Outermost NiO coexists with a small amount of Cr2O3 layer, while in the inner oxide only Cr2O3 remains. The oxide layers at medium and high DH content were composed of an outermost layer of Ni(OH)2 and Cr(OH)3 enriched in Cr, an intermediate layer of metallic Ni, hydroxides and oxides enriched in Cr, and an inner Cr2O3 layer. By XPS analysis, it was observed that B3+ ions were accumulated in the porous NiO layer formed at low DH content, whereas the accumulation of B3+ ions did not occur in the dense polyhedral oxide layer at medium and high DH contents. Boron compound accumulation should be considered within not only crud on fuel cladding but also oxide layer on SG tube owing to corrosion problem and the content of boron in primary coolant of PWRs.
The authors declare that there is no conflict of interests regarding the publication of this paper.
This work was supported by the National Research Foundation of Korea (NRF) grant funded by the Korean government (2017M2A8A4015159).