Corrosion resistance of chromium coating on the inner surface of EP823-Sh steel cladding

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.


Introduction
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 (Pukari 2013;Guéneau et al. 2015).Potential risks of cladding thickness reduction make it urgent to develop measures to increase its corrosion resistance.According to (Grachev et al. 2017;Grachev et al. 2019;Grachev et al. 2021;Zvir et al. 2017;Zvir et al. 2019;Grin et al. 2020), post-radiation tests of experimental assemblies with MNUP fuel irradiated in BN-600 and BOR-60 reactors showed the presence of corrosion damage on the inner side of the cladding made of EP823-Sh (12Cr-1SiMoWVNb) ferritic-martensitic steel (Dvoriashin et al. 2007).The analysis of interaction of oxide and MNUP fuel with cladding (Grachev et al. 2017;Grachev et al. 2021) showed that the corrosion damage of claddings with MNUP fuel in comparison with oxide fuel had greater depth and localized character.The corrosion depth at the achieved fuel burnup of 7.3% h.a. was ~150 μm, whereas the depth of corrosion damage of claddings with oxide fuel did not exceed ~50 μm.Fig. 1 presents a typical type of corrosion damage of EP823-Sh cladding with MNUP fuel and helium heat transfer sublayer.The corrosion damage of the cladding is localized in small areas along the perimeter of its inner surface and does not exceed 150 µm (Grin et al. 2020).
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 Fe 3 O 4 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 (Kryukov et al. 2022), cladding corrosion is caused by selective dissolution of steel components.The depth of the achieved fuel burnup was 7.3% h.a.Along with the dissolution zone in fuel rods with lead sublayer, the zones of deposition of steel components on the inner surface of the cladding due to mass transfer were also detected.The main components forming the deposition zone are iron and chromium.The first signs of physical and chemical interaction between the cladding material and the lead sublayer are found at a height of 300-312 mm from the bottom of the fuel core, corresponding to the maximum cladding temperature of 590 °C.The nominal length of the fuel element is 650 mm.The maximum depth of the cladding dissolution zone is 50 µm (Fig. 2a).Selective dissolution of cladding components in the lead sublayer at some parts of its inner surface was accompanied by formation of layers of deposits of dissolved elements at the neighboring sites (Fig. 2b).
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 (Zhang 2009).To increase the corrosion resistance  of ferritic-martensitic chromium steels, coating or alloying of the steel surface with such elements or compositions as Al, Si, Cr are used.Known effective protective coatings are metal oxides having high affinity for oxygen.The degree of affinity is usually evaluated through the free energy values of oxide formation.The most resistant and technologically achievable are the following oxides, in order of increasing free energy (per 1 atom in the molecule): Chromium oxide Cr 2 O 3 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.

Electrolytic deposition of coatings
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-1Si-MoWVNb) 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. 3 presents a sample of fuel cladding with an internal chromium coating.

Methodology of electron microscopic studies
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.

Corrosion tests in liquid lead and simulant fission products (CsI+Te)
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.The coating obtained at 10 °C has poor adhesion to the substrate and flakes off (Fig. 3a).The coatings are porous and consist of submicron-sized grains.When the electrolyte temperature is increased to 25 °C (Fig. 3b), the adhesion of the coating improves significantly and no delamination occurs, the grain size on the surface increases to 1-2 µm, and pores are also observed in the coating structure.At an electrolyte temperature of 35 °C (Fig. 3c), dense chromium coatings are formed, the transverse grain size on the coating surface changes from 1 to 5 µm.A further increase in electrolyte temperature to 45 °C (Fig. 3d) results in the formation of smoother coatings, however, there are micro cracks in the coatings.The deposition rate drops by a factor of five compared to the regime at an electrolyte temperature of 35 °C.At electrolyte temperatures above 60 °C the coating on the inner surface of the sample is not deposited, the surface of the steel at the grain boundaries is etched.Thus, as the electrolyte temperature increases from 10 °C to 45 °C, the number of pores in the coatings decreases while the thickness of the chromium coating formed on the surface decreases.The structure of the coating is represented by small equiaxed grains with the size of 0.5-5 μm.The coatings obtained in the temperature range of 25-35 °C have good adhesion to the surface, without pores and cracks and they were selected for corrosion tests.

SEM analysis after electrodeposition of the coating
Fig. 5 shows the distribution of the main elements of 10 µm thick coating obtained in the temperature range of 25-35 °C.The distribution of chromium over the coating thickness is homogeneous.Within the sensitivity of the energy dispersive detector no impurities were detected in the chromium coating.

SEM analysis of samples after corrosion tests in the environment of fission product simulators (CsI+Te)
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 Cr 2 O 3 , which, according to thermodynamic data, does not react with Cs, Te or caesium tellurides.The authors of works (Sasaki 2015(Sasaki , 2017) ) propose in the case 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. 6 presented SEM images of microstructure of coated and uncoated specimens after corrosion tests in CsI+Te at 650 °C for 100 h.
After corrosion tests at 650 °C for 100 h, no uniform corrosion is observed on the inner surface of the uncoated fuel cladding (Fig. 6a).No signs of intergranular corrosion were found.Analysis of the microstructure of the inner surface of the initial fuel cladding after corrosion tests showed the presence of local corrosion damage with After corrosion testing, no change in coating morphology was observed in the chromium-coated samples (Fig. 6b).No evidence of either uniform or intergranular corrosion was observed.Localized corrosion areas are also absent.The investigated coatings do not tend to sensitize and effectively block diffusion of fission products and oxygen into the material of the fuel cladding.

SEM analysis of samples after corrosion tests in liquid lead
Fig. 7 presents a SEM image of the inner surface of uncoated (a) and coated (b) samples after corrosion tests in liquid lead at a temperature of 650 °C for 100 h.The oxide film has a two-layer structure: an external oxide layer with a light grey contrast with a thickness of ~0.6-1.4 µm and an internal oxide layer with a dark grey contrast with a thickness of ~150 nm (Fig. 7a).There are areas with no adhesion of the oxide film to the steel surface, the oxide film has a loose structure.EDS analysis of the film cross-section showed that the internal oxide layer with a thickness of ~150 nm is Cr 2 O 3 type oxide, while the external oxide layer is a mixed iron and chromium oxide.
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(Fe 1-x ,Cr x ) 2 O 4. The formation of a porous layer of Fe 3 O 4 and plumboferrites ( n PbO* m Fe 2 O 3 ) 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. 7b).The integrity of the coating is preserved over the entire cross-section of the fuel element cladding fragment.Adhesion of the coating to the base material is not disturbed, no cracks are detected.On the surface of chromium coatings, a typical picture of uniform corrosion development is observed.The formation of a dense Cr 2 O 3 oxide film with a thickness of ~500-600 nm takes place.The main protective mechanism of metal coatings is the formation of protective oxide films, which do not chemically interact with lead and are resistant to dissolution.The continuous oxide film Cr 2 O 3 film, impedes the penetration of reagents to each other and their growth is accompanied by self-inhibition of the process.Oxidant accumulation on the inner surface of the oxide film does not occur due to kinetic inhibition of the oxide film growth process along with inhibition at the mass transfer stage.The absence of mass transfer in the oxide layer of the chromium-coated sample leads to the fact that its thickness is ~3 times less than that of the sample without chromium coating.Fig. 8 presents the X-ray distribution maps of iron, chromium, and oxygen at the chromium coating/steel interface after corrosion tests in liquid lead.
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. 8).The chromium coating has a thin oxygen layer on its surface, indicating the presence of Cr 2 O 3 oxide (blue layer in Fig. 8).No voids are observed between the Cr 2 O 3 oxide layer and the residual chromium, which could be formed due to outward directed cationic diffusion of chromium from the coating and reverse diffusion of vacancies in the oxide-coating system.No cracking of the coating was identified.Cracks may occur due to delamination of the oxide film, as well as at the Cr/steel interface during polishing of microscopy specimens after corrosion testing.The absence of cracks indicates that the adhesion of both the coating and the oxide film on the chromium is high and this prevents deep penetration of corrosive components.However, there are pinholes in the chrome coating.They are formed in the process of electrodeposition of chromium, which can be seen in Fig. 4b.X-ray mapping shows the presence of oxygen (blue layer in Fig. 8) in these voids.It is most likely that chromium electrolyte remains in the pitting voids, which reacts with the chromium coating  during the high temperature test at 650 °C to form pitting oxide inclusions.As stated above, the pitting oxides in the chrome coating during the 650 °C test for 100 h did not cause any cracking or discontinuity and therefore did not affect its corrosion resistance properties.
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.

Conclusions
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 Cr 2 O 3 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.Shortterm 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.

Figure 1 .
Figure 1.Typical type of corrosion damage to cladding made of EP823-Sh steel with a helium heat transfer sublayer after irradiation in the BN-600 reactor (Grin et al. 2020).

Figure 2 .
Figure 2. Typical view of the dissolution (a) and deposition (b) zone on the inner surface of the cladding in the upper cross sections of fuel elements (Kryukov et al. 2022).

Figure 3 .
Figure 3. Sample of fuel cladding with internal chromium coating.

Fig. 4
Fig. 4 presents SEM images of the cross-section microstructure of the inner surface of cladding after deposition of coatings in the temperature range 10-45 °C.The coating obtained at 10 °C has poor adhesion to the substrate and flakes off (Fig.3a).The coatings are porous and consist of submicron-sized grains.When the electrolyte temperature is increased to 25 °C (Fig.3b), the adhesion of the coating improves significantly and no delamination occurs, the grain size on the surface increases to 1-2 µm, and pores are also observed in the coating structure.At an electrolyte temperature of 35 °C (Fig.3c), dense chromium coatings are formed, the transverse grain size on the coating surface changes from 1 to 5 µm.A further increase in electrolyte temperature to 45 °C (Fig.3d) results in the formation of smoother coatings, however, there are micro cracks in the coatings.The deposition rate drops by a factor of five compared to the regime at an electrolyte temperature of 35 °C.At electrolyte temperatures above 60 °C the coating on the inner surface of

Figure 4 .
Figure 4. SEM images of the cross-section microstructure of the inner surface of cladding after deposition of coatings in the temperature range 10-45 °C.а. 10 °C; b. 25 °C; c. 35 °C; d. 45 °C depth up to ~1-3 µm and width of ~6 µm.The presence of such corrosion damage indicates the destruction of the protective oxide film Cr 2 O 3 in local areas as a result of sensitization.The deepest local corrosion damage corresponds to the areas of large (~1 μm) M 23 C 6 carbides.If the protective oxide film Cr 2 O 3 is sensitized, Te reacts both with grain-boundary carbides M 23 C 6 and with the matrix near the grain boundary to form Cr 3 Te 4 , activating the development of intergranular corrosion.Thus, localized corrosion on uncoated samples, located near large carbides, are potentially dangerous locations for further development of intergranular corrosion.

Figure 5 .
Figure 5. X-ray maps of iron and chromium distribution in the cross section of the inner surface of the chromium-coated cladding.

Figure 6 .
Figure 6.SEM images of uncoated (a) and coated (b) samples after corrosion tests in CsI+Te at 650 °C for 100h.

Figure 7 .
Figure 7. SEM image of uncoated (a) and coated (b) samples after corrosion tests in liquid lead at 650 °C for 100 h.

Figure 8 .
Figure 8. X-ray distribution maps of iron, chromium, and oxygen in the chromium coating after corrosion tests in liquid lead at 650 °C for 100 h.