Research Article |
Corresponding author: Rafael Sh. Isayev ( rsisayev@mephi.ru ) Academic editor: Yury Kazansky
© 2024 Rafael Sh. Isayev, Pavel S. Dzhumaev, Irina A. Naumenko, Maria V. Leontieva-Smirnova.
This is an open access article distributed under the terms of the Creative Commons Attribution License (CC BY 4.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited.
Citation:
Isayev RSh, Dzhumaev PS, Naumenko IA, Leontieva-Smirnova MV (2024) Corrosion resistance of chromium coating on the inner surface of EP823-Sh steel cladding. Nuclear Energy and Technology 10(2): 81-88. https://doi.org/10.3897/nucet.10.119642
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The processes of corrosion damage of the inner surface of the cladding are determined by corrosive reagents aggressive with respect to the cladding and the type of fuel used. Reactor irradiation of cladding made of EP823-Sh steel with mixed nitride fuel planned for use in the BREST-OD-300 reactor revealed non-uniform corrosion of the inner surface of the cladding.
In this paper, the use of the chromium coating is proposed to prevent the corrosion of the inner surface of the steel fuel cladding. The results of corrosion tests of chromium coating applied to the inner surface of cladding made of EP823-Sh steel by electrolytic deposition are presented. Electron-microscopic studies of the chromium coating on EP823-Sh steel showed no significant signs of corrosion damage when tested in the environment of simulant fission products (CsI+Te) and in liquid lead at 650 °C.
corrosion, ferritic-martensitic steel, EP823-Sh, chromium coating
In reactor experiments of steel fuel claddings with mixed nitride uranium-plutonium nuclear fuel (hereinafter – MNUP fuel), along with fuel-cladding contact, corrosion of the inner surface of the cladding was detected (
Corrosion products consist of two phases differing in composition and location in relation to the base metal. The first phase adjacent to the base metal of the steel cladding, and it was characterized by an increased content of chromium, oxygen and reduced iron. The second phase, filling the fuel-cladding gap, consisted predominantly of iron and oxygen, which corresponds to Fe3O4 oxide.
For the BREST-OD-300 reactor, fuel cladding made of EP823-Sh steel with a lead heat-transfer sublayer is being developed. In samples with lead heat-transfer sublayer with mass fraction of oxygen up to 10-5%, irradiated in the BOR-60 reactor (
To reduce the degree of fuel-cladding corrosion interaction, it is necessary to take additional measures to increase the corrosion resistance of cladding. One of the options for solving this problem may be the use of protective coatings.
The choice of the coating material for EP823-Sh steel can be based on the experience of developing barrier coatings for the outer cladding side of similar ferritic-martensitic steels (
Cr 2 O 3 → SiO 2 → Al 2 O 3 → ZrO 2
Chromium oxide Cr2O3 is considered as an effective diffusion barrier for oxygen. Silicon dioxide, aluminium oxide and zirconium dioxide have remarkable protective properties. The effectiveness of the oxygen diffusion barrier depends not only on the oxygen concentration in the lead, but also on the steel/oxide/microstructure system. Elements that can form stable, strong and dense oxides which should reduce the oxidation rate.
The purpose of this work is to provide experimental data of corrosion tests of chromium coating on the inner surface of cladding made of EP823-Sh steel. The corrosion resistance of the coating was evaluated by testing in liquid lead and fission product simulants (CsI+Te) at 650 °C for 100 hours, followed by SEM and EDS analyses.
In this work, fuel cladding tube fragments with length 50 mm and diameter 10.5 mm (cladding thickness 0.4-0.5 mm), made of 12% chromium steel EP823-Sh (12Cr–1SiMoWVNb) were used for coating deposition and further corrosion tests. The chromium coatings were obtained by electroplating using direct current with different durations of deposition process. Aqueous solution of chromium anhydride (250 g/l) and sulfuric (2.5 g/l) acid was used as an electrolyte. During the deposition process, the electrolyte was maintained in isothermal equilibrium at different temperatures. A lead wire was used as an insoluble anode, which was passed through the fuel cladding. The anode-equipped cladding was immersed in a bath with electrolyte. The thickness of the Cr layer increased linearly with increasing plating time. The temperature range for the electrolyte was chosen as 10-45 °C, and the electrodeposition rate was ~0.35 μm/min. The thickness of the obtained chromium coating on the studied samples was ~10 μm. Fig.
The surface microstructure and cross sections of the samples were studied using an EVO 50 XVP scanning electron microscope manufactured by Carl Zeiss (Germany). SEM images were obtained in backscattered electrons during the study of microstructure. The elemental composition of the samples was analyzed by X-ray microanalysis using energy dispersive (EDS, INCA 350x-Act) and wave dispersive (WDS, INCA Wave 500) spectrometers manufactured by Oxford Instruments, combined with an EVO 50 scanning electron microscope. Measurements were performed at an accelerating voltage of 10–20 kV and a probe current of 5–50 nA.
For corrosion tests in lead and simulant fission products (CsI+Te), fuel claddings with and without chromium coating were washed in an ultrasonic bath in acetone, then in deionized water, dried and placed in an argon box. For tests in lead, C0 grade lead (Pb>99.992%) and niobium shavings as an oxygen getter were loaded into the washed samples in a ratio of 25:1, respectively. According to our estimates, the oxygen concentration in molten lead was ~10-5 wt.%, with the initial oxygen concentration in C0 grade lead being ~10-4 wt.%. For testing in a simulant fission products environment, an oxygen getter and a CsI+Te mixture was loaded into the washed samples. The samples were sealed, then moved to a muffle furnace and held at a temperature of 650 °C for 100 hours. After the corrosion tests were completed, the fuel claddings were cut into fragments. After cleaning from corrosive components (Pb, CsI+Te, getter), cladding fragments were submitted for metallographic analysis.
Fig.
The coating obtained at 10 °C has poor adhesion to the substrate and flakes off (Fig.
Fig.
For ferritic-martensitic steels, the most dangerous factor in terms of corrosion interaction with fission products is Cs with the formation of localized corrosion areas of the inner surface to a depth much greater than the depth of uniform corrosion. Under normal conditions, Fe-Cr alloys are covered with a passive protective oxide film Cr2O3, which, according to thermodynamic data, does not react with Cs, Te or caesium tellurides. The authors of works (
4Te+2/23 Cr23C6 →Cr3Te4 + 18/23 C
4Te+3Cr→Cr3Te4
It is accepted that the corrosion enhancement and intergranular corrosion observed in oxide fuel cladding because of volatile fission products (Cs, I, Te). Ferritic-martensitic steels under normal conditions are covered by a protective oxide layer, the probability of direct dissolution of steel components along grain boundaries under the action of Te and Se should be negligible if the protective film retains its integrity. The most dangerous for this type of steel is the formation of chromates under the action of Cs, penetrating to depths much greater than the depth of uniform corrosion. Thus, one of the main tasks is to prevent the penetration of caesium into the cladding and the formation of chromates, which can be successfully achieved by creating a uniform dense protective film on the surface of the steel. The natural oxide film present on the surface of ferritic-martensitic steels is not sufficient.
Fig.
After corrosion tests at 650 °C for 100 h, no uniform corrosion is observed on the inner surface of the uncoated fuel cladding (Fig.
After corrosion testing, no change in coating morphology was observed in the chromium-coated samples (Fig.
Fig.
One of the key parameters affecting the oxidation rate in liquid lead is the oxygen content. Its content determines the corrosion mechanism of the material. For a temperature of 650 °C, at low oxygen concentration in the lead melt (С[O]<10-7 wt.%), the steel components can undergo selective dissolution, and this leads to penetration of lead deep into the material at grain boundaries. At high oxygen concentration (С[O]=10-5-10-3 wt.%) there is intensive formation of oxide layer, in this case the corrosion products of EP823-Sh steel are external oxide layer of magnetite, chromium spinel of non-stoichiometric composition of Fe(Fe1-x,Crx)2O4. The formation of a porous layer of Fe3O4 and plumboferrites (nPbO*mFe2O3) formed at the boundary of the outer oxide layer and lead is also observed.
Samples with internal chromium coating do not show any significant changes of microstructure after corrosion tests (Fig.
SEM-image analysis of the microstructure of the cross-section of the samples after oxidation for 100 h shows the presence of a uniform chromium layer on the steel substrate (green layer in Fig.
Corrosion tests in liquid lead at 650 °C for 100 h allow us to conclude that, under these test conditions, the chromium coating effectively blocks the diffusion of oxygen and lead into the cladding material. High corrosion resistance of fuel element cladding is necessary to exclude penetration of fuel fission products into the coolant, which may result in the spread of radioactive elements into the core and the first reactor cooling circuit. A more resistant and safer cladding, in turn, leads to longer fuel assembly life, longer fuel company duration and deeper heavy atoms burnup. The application of a 10 µm thick chromium coating does not result in a noticeable change in the geometric dimensions of the fuel element cladding. EP823-Sh steel contains ~12 wt.% of chromium, presumably chromium coating of such thickness should not significantly affect thermal-hydraulic and neutron-physical parameters of the reactor core. However, this circumstance requires a detailed study to confirm the assumption made.
Corrosion tests in liquid lead with oxygen concentration ~10-5 wt.% and simulant fission products (CsI+Te) at 650 °C for 100 h have presented that:
Uncoated EP823-Sh steel cladding is subjected to corrosion. Interaction of simulant fission products (CsI+Te) with steel leads to the local corrosion damage with depth up to ~3 microns and width ~6 microns. After liquid lead corrosion tests an oxide film of non-uniform thickness and structure is formed on the steel surface. It has a two-layer (duplex) structure. The internal layer directly adjacent to the steel surface with thickness of ~150 nm is Cr2O3 oxide and the external loose layer with thickness up to ~1.5 µm of mixed iron and chromium oxide.
The chromium coated samples are resistant to the corrosion in the simulant fission products (CsI+Te) and liquid lead. In liquid lead, a dense and uniform Cr2O3 oxide film up to ~600 nm thick with good adhesion is formed on the surface of the chromium coating. There is no penetration of oxygen and lead into the steel under the coating.
Thus, the electrolytic chromium coating acts as a diffusion barrier preventing deep interaction of fission product simulants, liquid lead and oxygen with steel. Short-term corrosion tests allowed to justify the choice of a single-layer heat-resistant chromium coating with thickness of ~10 microns for fuel element cladding made of EP823-Sh alloy. The technology of electrolytic deposition of chromium coating provides the necessary dimensional and physical-mechanical characteristics. However, to fully reveal the corrosion-resistant properties of such a coating, long-term high-temperature studies are required, which is the purpose of our further research.
R. Sh. Isayev: Conceptualization, Investigation, Visualization, Writing – original draft, Writing – review & editing; P. S. Dzhumaev: Conceptualization, Supervision, Investigation, Writing – original draft, Writing – review & editing; I. A. Naumenko: Resources, Writing – review & editing; M. V. Leontieva-Smirnova: Resources, Writing – review & editing.
The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.