However, nickel-based alloys undergo stress corrosion cracking (SCC) in high temperature and high pressure environments. Under these conditions, the oxidation of nickel-based alloys can be considered as the origin of the occurrence of SCC, the main degradation process causing the failure of nuclear power plants. 14 Figure 72-4 Schematic diagram of the interface region (left: lower DH level, right: medium and high DH level) [25].
INTRODUCTION
Background
Goal and approach
Figure 1-4 illustrates the approaches and scope to investigate the oxidation process in nickel-based alloy in simulated environments.
![Figure 1-1 The applications of nickel based alloy in pressurized water reactor primary system [2]](https://thumb-ap.123doks.com/thumbv2/123dokinfo/10497773.0/19.892.196.695.380.760/figure-applications-nickel-based-alloy-pressurized-reactor-primary.webp)
Basic theory and literature study
Primary Water Stress Corrosion Cracking of Ni-based Alloys
- Historical background
- Hydrogen based mechanism
- Internal oxidation mechanism
The internal oxidation model is currently considered one of the leading mechanisms to account for the SCC generation of alloy 600 in PWR. In addition, the structure of the crack tip of IGSCC in alloy 600 is found to be similar to the internal oxidation model. Finally, the internal oxidation model is the only mechanism that can explain both initiation and propagation processes of cracks.
Computational Science Methodology
We show that the length of time until cracks are formed calculated through equations 2.1 and 2.2 and distinct cracks are observed is consistent with experimentally obtained results [57]. As such, internal oxidation models account for most of the key thermodynamic and kinetic behaviors observed experimentally, but have some unexplained drawbacks, and studies are underway to compensate for them. This calculation is determined by numerically interpreting Newton's equations of motion for systems of interacting particles, where the forces between particles are usually accounted for through interatomic potentials or molecular mechanics force fields.
X-ray experimental Methodology
The study of metal-water interfaces is one of the most important factors in terms of metal corrosion, since oxidation reactions start at metal and water surfaces. The basic principle of X-ray reflection is the direct reflection of the beam on a surface and the measurement of the intensity of the reflected X-ray in a certain direction. Figure 72-4 Schematic diagram of the interface region (left: lower DH level, right: intermediate and high DH level) [25].
Methods and materials
- First principles calculation
- Ni/O & Ni/H 2 O system using Molecular dynamics
- Ni-15Cr-8Fe/H 2 O system using Molecular dynamics
- X-ray experiments
- X-ray reflectivity of Ni single crystal
- X-ray diffraction of Ni-15Cr-8Fe single crystal
In the field of corrosion, developments in ReaxFF potentials, originally developed to study the hydrocarbon and catalytic properties of organic compounds, have enabled the investigation of metal and water reactivity. The upper part of the nickel substrate was divided into three sublayers to calculate the radial distribution function (RDF), and the lower substrate was fixed because nickel atoms contained in the substrate are not affected by oxygen atoms. To investigate the effect of alloying elements and the factors that can cause crack initiation in the early stage of oxidation of Ni-15Cr-8Fe/H2O at the atomic scale, molecular dynamics calculation was performed using LAMMPS.
The pretreatment process was carried out in the order of mechanical grinding – Ar Sputtering – Annealing – formation of passivity film, among which Ar sputtering and annealing processes were carried out four times. In the mechanical grinding phase, the grinding phase using SiC paper and polishing phase using Diamond Suspension solution and Alumina Suspension solution were performed as in the normal metal sample treatment. In the process of execution, the pressure applied was minimized to minimize residual stress caused by surface friction, and each step was performed for more than 30 minutes.
The surface was etched using argon ions with an energy of 0.5 kV and 10 mA in the loaded sample and the process was carried out for 10 minutes. The beamline consists of a 6-axis goniometer, a Pilatus 100K area detector, and so on, which used X-rays with 18 keV energy ratio in the experiment. The detail of the sample is shown in Table 3-2, and the surface pretreatment process was carried out in the same way as the Ni single crystal sample.
Figure 143-3 Top view of a Ni(1 1 1) surface with one oxygen atom (red) located in hcp (left) and fcc (right) hollow sites.
Results
First principles calculation
- Partial density of states
- Adsorption energy
- Barrier energy
Previous results computationally show that oxygen is adsorbed in selectively stable positions during the initial oxidation process. For corrosion to occur at the metal and water interfaces, oxygen atoms must penetrate the substrate. To determine the behavior of these oxygens, the diffusion direction of oxygen was calculated after the oxygen was adsorbed to the surface by deriving the barrier energy from the transition state calculation.
These results show that oxygen needs the most energy when diffusion occurs on the surface of Ni(111), followed by Ni(100) and Ni(110).
Ni/O & Ni/H 2 O system
- Structural changes of nickel surface
- Radial distribution function
Modeling the reactants and products can calculate relative values of how much energy is needed to move the oxygen. In addition, this pattern can show that structural changes have occurred not only on the surface, but also within the substrate. As the oxidation process progressed, the Ni (111) form of the HCP crystal structure was created within the nickel as shown in Figure 4-9.
Constant volume molecular dynamic simulations were performed to evaluate the reaction process of the phenomenon and the mechanism of oxygen consumption. RDF is a statistical mathematical function used to describe a cluster or grouping of objects. As shown in Figure 4-10, the RDF is calculated for each sublayer to investigate the structural changes and effects of oxygen permeability to the nickel substrate.
Substrate 1 cannot retain the basic characteristic of the nickel substrate and forms an amorphous nickel oxide layer. Also, substrate 2 (green line) is under the influence of the oxygen atom, therefore, a change in the radial distribution is shown.
Ni-15Cr-8Fe/H 2 O system
- Oxidation behavior analysis
- Analysis of oxide layer
- X-ray experiments of Ni-15Cr-8Fe/H 2 O single crystal sample
The preceding results can be more clearly observed from the results of the trajectory oxygen atom method. At the top of the surface, oxygen atoms combine with metal atoms that appear on the surface early in the oxidation process to form an oxide layer. However, it can be seen that the position of the atoms changes as surface atoms repeat bonding and debonding due to the continuous oxidation process.
Therefore, oxygen atoms bonded to the Nickel atom move to the Chromium atom to form an oxide as shown in Figure 4-13. In Ni-15Cr-8Fe systems, chromium atoms can be seen to migrate to the surface through oxidation over time. Chromium atoms migrating to the surface react with oxygen and are found to be oxidized.
This phenomenon occurred by adsorbed oxygen atoms located on the surface diffusing into the nickel substrate, as the results of the first principles calculation. When the oxygen atom diffuses within the nickel substrate, it binds to the surrounding metal atom and causes an oxidation process. As the oxidation process continues, it was found that chromium atoms having relatively more oxygen affinity than the nickel atoms move to the part where oxidation has taken place within the nickel substrate.
Figure 4-15 shows that chromium atoms migrate to the surface and form an oxide layer. In addition, there is a possibility that the peak may not appear to distinguish it from the background due to the different energy of X-rays. Figure 394-14 The results of oxidation layer analysis and atomic distribution diagram in the z-axis direction.
Discussion
Atomistic structure of interfacial layer between nickel oxide and water
Consistent with the XRR experiments, the simulation result shows a highly deformed interface, specifically as a result of the oxidation process. While previous studies only focused on the changes in the lateral structure due to the limited measurement tools, our simulations show that there are also significant deformations in the surface normal direction. Amorphous and crystalline phases coexist at the interface, suggesting complex growth processes caused by an inherent mismatch between the rectangular lattice of the NiO surface and the stable hexagonal packing of the NiO(111) epitaxial layer.
This structural change can provide a diffusion path because it is an unstable state in which continuous phase change occurs. This provides an environment in which cracks can develop in the oxide layer, as shown in Figure 5-3.
The role of Cr atoms on oxidation characteristic of Ni based alloy
It has been observed that exposure to PWR primary coolant in the absence of applied stress causes irreversible embrittlement of Alloy 600 grain boundaries near the surface. Some intergranular oxidation is also methodically observed in Alloy 600 and affects grain boundary embrittlement, which is generally accepted as the cause of primary water stress corrosion cracking. The properties of the oxide layer remain uncertain in certain cases and selective oxidation does not occur at the oxide/metal interface in alloy 600 where the presence and role of Cr2O3 is unclear.
Therefore, it is important to determine the role of the chromium atom in the oxidation process at an early stage. Cr atoms can be concentrated on the surface due to the above phenomenon and Cr atom prevents oxygen atoms from spreading inside (oxygen affinity: Ni < Cr). In addition, this phenomenon could create a Cr-deficient phase under the surface oxide layer, as shown in Figure 4-9 ~ 4-11. Consequently, two or more mismatches on the surface are expected to produce mutual stresses, as shown in Figure 5-10.
Figure 475-5 Transmission electron microscope cross-section of the oxide shell formed on the 5% Cr alloy in a simulated primary water environment [43]. Figure 485-6 Transmission electron microscope cross-section of the oxide shell formed on the 15% Cr alloy in a simulated primary water environment [43]. Figure 495-7 Transmission electron microscope cross-section of the oxide shell formed on the 30% Cr alloy in a simulated primary water environment [43].
Figure 515-9 Stability diagram of the oxides likely to form during the exposure of alloy 600 in media of primary water type PWR as a function of temperature [52].
Conclusion
Vettier, Stress corrosion cracking of Inconel in high temperature water, in the Third Colloquium of Metallurgy. Scott, Contribution to the understanding of stress corrosion cracking of alloy 600 in PWR primary water, Energy Materials: Materials Science and Engineering for Energy Systems. Insights from crack-tip exams, at an international conference on degradation of plant materials - Application to stress corrosion cracking of Ni-base alloys.
Andrieu, Influence of oxide films on primary water stress corrosion crack initiation of alloy 600, Journal of Nuclear Materials, 2006, vol. 34] Xiaolong Liu et al., Atomistic investigation on stress corrosion crack initiation of polycrystalline Ni60Cr30Fe10 alloys under high temperature water by reactive molecular dynamics simulation, Computational Materials Science, 2021, vol. Effect of chemical compositions and microstructure on the resistance to stress corrosion cracking of nickel-based alloys in high-temperature water.
Influence of carbide precipitation and rolling direction on intergranular stress corrosion cracking of austenitic stainless steel in hydrogenated high temperature water. Some Possible Mechanisms of Intergranular Stress Corrosion of Alloy 600 in PWR Primary Water, Proc. Surface layers of alloys 600 and 690 in PWR primary water: possible influence on stress corrosion initiation.
In International Conference on Degradation of Plant Materials - Application to Stress Corrosion Cracking of Ni Base Alloys, EDF R&D Center of Les Renardières, Moret-Sur-Loing, France, 2008.
![Figure 1-2 Factors affecting stress corrosion cracking [4]](https://thumb-ap.123doks.com/thumbv2/123dokinfo/10497773.0/20.892.228.661.376.783/figure-1-2-factors-affecting-stress-corrosion-cracking.webp)
