Corrosion behavior of AFA steel in lead-bismuth eutectic alloy with saturated oxygen at 500°C

Alumina-forming Austenitic (AFA) steel is one of the candidate corrosion-resistant structural materials for lead-cooled fast reactors, exhibiting excellent high-temperature corrosion resistance. However, its corrosion mechanisms in lead-bismuth eutectic (LBE) environment at the design operating temperature of lead-cooled fast reactors have not been fully elucidated. In this study, AFA steel was immersed in static LBE with saturated oxygen at 500 °C for 3000 hours. The corrosion behavior of AFA samples was obtained by analyzing the morphology of corrosion cross section and the distribution of elements. The results showed that the AFA steel did not form an oxide film that could effectively resist LBE. Dissolution corrosion of the matrix becomes more severe the longer the exposure time. When the NiAl phase in the matrix is located at the surface, oxide nodules are generated. It has a double-layer oxide structure, which has better resistance to LBE corrosion. Nb undergoes oxidative rupture and has a tendency to detach from the surface of the substrate when it is located on the surface. Cracks occur on the surface of the matrix after 3000 hours of corrosion, and LBE penetrates into the matrix along the cracks.


Introduction
Generation IV (Gen-IV) reactors offer higher safety, better economics, and longer operating lifetimes [1]   .Among them, lead-cooled fast reactors (LFR) that utilize lead or lead-bismuth eutectic (LBE) as coolant are highly regarded for their excellent neutron physics characteristics, thermal-hydraulic properties, and safety features [2,3] .However, for lead-cooled fast reactors, extreme operating conditions must be overcome, including high temperature, large density, and fast velocity of liquid metal within the reactor core.Therefore, compatibility between the materials and liquid metal is an important issue [4][5][6] .Addition of aluminum-rich phase of AFA steel is one of the current options to improve performance.The Cr2O3 oxide layer formed by conventional austenitic heat-resistant steel does not perform well enough to resist high-temperature liquid metal corrosion in lead-cooled fast reactors [7] .In order to solve the issue of insufficient denseness in the Cr2O3 oxide layer of traditional austenitic heat-resistant steels, AFA steel is being considered.Because of the addition of the Al-rich phase, AFA steel can form a dense Al2O3 oxide layer that possesses better thermodynamic stability than Cr2O3 at high temperatures.
Existing studies [8,9] have found that AFA steel can form a dense Al2O3 protective film within the temperature range of 750-900 °C, exhibiting excellent high-temperature corrosion resistance.However, lead-cooled fast reactors are typically designed to operate at temperatures between 400 to 600°C in the first stage.Subsequently it will reach 1000°C.It is necessary to verify whether the AFA steel can form a dense Al2O3 oxide layer under LBE at this temperature, and then resist the hightemperature liquid metal corrosion.Currently, AFA steel has been experimented in LBE under a variety of conditions, and there are still some problems.One of them is the effect of the composition of AFA steel on the ability to resist LBE corrosion.T. Kim et al. [12] corroded AFA steel with Ti addition (AFATi) into LBE and produced a thin oxide layer (about 10 nm).It was also found that the AFATi alloy had tiny TiC precipitates, which could increase the diffusion rate of the oxide elements.The amount of Ti and C was also optimized to reduce the number of oxide nodules generated in the AFATi alloy, thus reducing the thickness fluctuation of the oxide layer and obtaining a more uniform and dense oxide layer.H.R. Wang et al. [10] found that high Al AFA steels exhibited enhanced corrosion resistance in a 450°C LBE environment.Secondly, which phases in AFA steel are more resistant to LBE corrosion is also one of the questions.L.Z.Chen et al. [11] conducted corrosion experiments on duplex AFA steels containing austenite and ferrite at 550 °C for 1000 h in static LBE with different oxygen contents.The preferential dissolution of nickel and the deepest penetration of LBE in AFA steels were observed at oxygen concentrations of 10 -12 to 10 -8 wt.%.The austenite fraction degrades before the ferrite.At 10 -6 wt.% oxygen, a thin aluminum-rich oxide layer is formed, which protects the steel from LBE attack.The precipitation of NiAl in the steel at this oxygen concentration also did not affect the formation of aluminum-rich oxides.Besides, the corrosion mechanism of LBE for AFA steel is different at different temperature and oxygen concentration.A continuous, aluminum-rich oxide layer was formed on AFA steel in an LBE environment with low oxygen concentration at 450°C by H.R. Wang [10] .In contrast, in saturated oxygen LBE environment, unprotected layered oxide layers were formed.I.P. Serre et al. [13] found that AFA steels are sensitive to LME at 500°C.Liquid lead promotes intergranular extension of cracks on the metal surface at 500°C.The LME sensitivity and intergranular brittle fracture at high temperatures suggest that the LME mechanism of AFA steel at 500°C is dominated by grain boundary wetting.In order to further clarify the question of whether AFA steel can form a dense oxide film at the application temperature of lead-cooled fast reactors, a static corrosion experiment of AFA steel in saturated oxygen LBE at 500°C for 3000 h was designed and the experimental samples were characterized.

Materials
The composition of the AFA steel used in this study is shown in Table 1.Before corrosion, all samples were cut to 15×7.5×1 mm³ in size.Firstly, the sample surfaces were mechanically polished by using sandpapers of grit sizes 400#, 1000#, 1500#, and 2000#.Subsequently, they were mirror polished by using a diamond spray polish.After polishing, the samples were cleaned with C2H6O and CH3COCH3.
In order to protect the corrosion surface of the material, the corrosion specimen is removed directly into the cold mounting for observation without cleaning.The processed samples were analyzed using a scanning electron microscope (SEM; Helios 5 CX) and an energy-dispersive X-ray spectrometer (EDS) to study the microstructure and elemental distribution of the samples.
The corrosion experiments were conducted on the pot-type multifunctional LBE corrosion platform of North China Electric Power University.(Fig. 1) The experimental temperature was controlled at 500 °C.Samples were taken every 500 h.The oxygen concentration in the liquid LBE is saturated and it was calculated to be 6.3×10 -4 wt.% using the following equation [14]: where C is the oxygen concentration; T is the measured temperature.

Corrosion behaviors in AFA steel
It can be observed that none of the samples formed a continuous and dense oxide layer.(Fig. 2) At 500 h, the surface and matrix boundary appear relatively intact, with no LBE penetrates into the matrix.At 1000 hours and 1500 hours, respectively, samples exhibited a limited amount of localized dissolution corrosion and some protrusions at the surfaces.As the corrosion time reaches 2000 h to 2500 h, the number of surface protrusions significantly increases, and the surface of the samples becomes highly irregular, indicating an increasing severity of corrosion.After a corrosion time of 3000 h, a substantial amount of matrix detachment is observed on the sample surface, with a significant infiltration of LBE into the matrix.For sample surface bumps and abnormal element diffusion phenomenon, samples with 1000 h of corrosion were further analyzed.The corrosion morphology of the sample at 1000 h is partially enlarged as shown in Fig. 3.The surface of the samples exhibits phenomena such as dissolution of matrix elements, diffusion of matrix elements into LBE, and infiltration of LBE into the matrix.EDS point scanning (Table 2) at the specified position in Fig. 3 confirms the enrichment of Nb in that area, with Nb being precisely located on the sample surface.Nb is added because Nb has excellent mechanical properties, but the high temperature oxidation resistance of Nb is poor, undergoing intense oxidation at temperatures above 350 °C [15] .The primary oxidation product of Nb is Nb2O5, and NbO and NbO2 can also form under lower pressures.The increased volume of Nb2O5 generates significant internal stress within the oxide layer [16] , resulting in both transverse stress parallel to the surface of the matrix and longitudinal stress perpendicular to the surface of the matrix.When the internal stress exceeds the strength of the oxide film itself, cracks appear in Nb2O5, leading to fragmentation and detachment.Fig. 3 shows localized swelling and the formation of oxide nodules [17] , with the approximate size of the oxide nodules being around 1.5 μm.The line scanning results (Fig. 4) indicate that elements such as Ni and Cr, due to their high solubility in LBE, have diffused from the matrix into LBE.The occurrence of line scanning and point scanning reveals that the localized swelling is associated with the presence of the NiAl phase, where no oxide layer formation is observed, and the dissolution and diffusion of Ni and Cr into LBE are more pronounced in that region.The diffusion of matrix elements in the vicinity of the sample surface extends approximately 2-3 μm away from the matrix surface, and a limited penetration of LBE into the matrix is observed at locations where dissolution corrosion of the matrix surface occurs.Fig. 3 represents the region of dissolution and diffusion of matrix elements.

Oxide nodule analysis
For the samples with corrosion time of 1500 h and 2000 h, the number of oxide nodules generated by LBE corrosion on the surface of the substrate gradually increased with the growth of corrosion time, and their depth remained at about 1.5 μm.The diffusion zone of the matrix elements at the oxide nodules became larger, about 4-5 μm.From the analysis of Fig. 5, it can be obtained that when the NiAl phase is located at the sample boundary, an oxide layer containing Fe, Cr and Al is generated, while the area is accompanied by a large amount of diffusion of Cr and Ni elements to LBE.It is presumed that a structure similar to the internal oxidation zone (IOZ) spinel layer rich in Fe and Cr was formed inside [18] , while the outward diffusion of Al elements formed a discontinuous trace Al2O3 oxide layer.And an EDS surface scan of a local oxide nodule in 2000 h (Fig. 6), the element distribution in this region also confirms the speculation.O elements are enriched in the surface layer, and the Fe and Cr elements in the surface layer have been dissolved and diffused into the LBE in large quantities, and the Fe and Cr element content at the oxide nodule has been significantly lower than that in the matrix.Al elements and Ni elements are enriched at the nodule.The solubility of Ni in LBE is high, but because of the presence of NiAl phase, it is the relative content of Ni that is stable and prevents the penetration of LBE into the matrix.When the corrosion time reaches 3000 h, the corrosion layer on the surface of the substrate has completely fallen off, and the oxide nodules generated at the location where the NiAl phase exists have been dissolved.(Fig. 8) Due to the invasion of LBE into the lower matrix, the NiAl phase which is not dissolved loses its dependence on the matrix.Then it detached from the surface of the matrix and loses its ability to resist LBE erosion.

The role of Nb elements in AFA steel
It has been showed that increasing the Nb content increases the precipitation of the Laves phase in AFA steels, which will precipitate together with the NiAl phase in AFA steels [19] .An increase of Nb content also increases the volume percentage of the NiAl phase in the matrix, providing a constant source of Al elements for the formation of the alumina film.NbC particles were observed in the AFA steel used for the experiments (Fig. 9).Since Nb is a strongly biased element, it is easily enriched during the curing process and combines with the enriched C elements during the curing process to generate NbC particles [16] .

Fig. 9. BSE image containing NbC
Since the experiments were conducted in a saturated oxygen environment, which provided sufficient O for the oxidation of Nb.The oxidation mechanism of niobium is that during oxidation, oxygen ions migrate from the outside to the inside, while niobium ions migrate from the inside to the outside.At a distance of about 40 nm from the surface, the concentration of both oxygen and niobium is high, providing conditions of Nb2O5 [20,21] .When the NbC formed located on the surface of the substrate, Nb comes into direct contact with the O element dissolved in LBE toward and undergoes severe oxidation, and with the increase of oxidation corrosion time, the powdered oxide film of Nb2O5 formed keeps falling off and rupture oxidation occurs (Fig. 3).The oxidation of Nb particles in this region breaks off, which is not conducive to the resistance of the substrate surface to LBE corrosion, and LBE is easy to penetrate into the substrate along the cracks of the generated Nb2O5.Under longterm corrosion conditions, the Nb at the surface cannot resist LBE corrosion.

Analysis of NiAl phase in AFA steel
Since the solubility in LBE is Ni > Cr > Fe [22] , Ni and Cr are more easily dissolved into LBE by diffusion from the matrix.As observed in Fig. 7, all the locations of the non-NiAl phases, the content of Ni, Cr is significantly lower and LBE is easy to penetrate into the interior of the matrix along the channels of elemental dissolution.In the position of NiAl phase, the relative content of Fe element is elevated due to the selective dissolution of Cr and Ni elements.A discontinuous internal oxide layer rich in Fe and Cr is formed, while there is a relative enrichment of Al and O elements on the surface, but no continuous dense Al2O3 oxide film is formed.
There are two possible reasons for not forming Al2O3 oxide film, one of which is the high temperature conditions required to generate Al2O3 oxide film.Heinzel et al. [23] found that Al2O3 can only exist on the sample surface for corrosion over 10,000 h in 650 °C LBE corrosion experiments, which indicates that continuous Al2O3 protective layer at low temperature as well as short LBE corrosion time does not formation.The literature has shown that AFA steel has excellent high-temperature oxidation resistance in the range of 750-900 °C, although its research conditions are higher than the experimental temperature (500 °C) in this paper, which may also lead to the failure to form a continuous dense Al2O3 oxide film.Second, the solubility of Al in LBE is large.In LBE, the solubility of Al is only slightly lower than that of Ni and much greater than that of Cr [24] .During the corrosion of LBE at 500 ℃, Al diffuses out of the substrate along the selective dissolution channel of Cr and Ni, part of the O on the surface of the substrate combines with Cr to form Cr2O3 and part penetrates into the substrate.Only a small amount of O in the NiAl get along with Al combined to form Al2O3 oxide.And the remaining Al enriched at this place was not formed as continuous dense Al2O3 oxide film because it could not combine with O quickly.More of Al was dissolved in LBE.Meanwhile, due to the selective dissolution of Ni and Cr and the enhanced inward penetration of O, Al may undergo internal oxidation and impair the generation of Al2O3 oxide film [18] .Although the presence of NiAl phase also did not form a continuous dense Al2O3 oxide film, the presence of NiAl phase also hindered the diffusion of LBE into the matrix.The presence of NiAl phase prevented nearby areas being dissoluted due to the massive dissolution of Ni, which dissolved the substrate and caused the massive diffusion of LBE into the substrate.Moreover, the NiAl phase, which is a fine and diffuse second phase, is distributed in the matrix of the alloy and interacts with dislocations to impede the dislocation movement.Thus, NiAl phase improves the deformation resistance of the alloy and enhances the lasting strength of the alloy at high temperature.The second phase can effectively impede dislocation movement, and the degree of impediment to dislocation movement is closely related to the precipitation location, size and volume fraction of the second phase.The smaller the size, the more diffuse the distribution and the larger the volume fraction of the second phase.The more obvious the hindrance to dislocation motion, the more it can improve the high-temperature endurance strength and high-temperature creep performance of the material.In general, the presence of NiAl phase has a positive effect on the resistance of AFA steel to LBE corrosion.

Conclusion
In this paper, corrosion experiments under saturated oxygen for 3000 hours were conducted to address the question of whether AFA steel can form a stable oxide film at 500°C.The experimental samples were analyzed and the following conclusions were obtained.1. AFA steel suffered dissolution corrosion and oxidation corrosion in LBE with saturated oxygen at 500°C and produced oxide nodules, which fell off after 3000 hours.
2. The addition of Nb improves the high temperature mechanical properties of AFA steels, but its oxidation forms Nb2O5 which is prone to rupture and leads to LBE penetration into the matrix.
3. The presence of NiAl phase keeps the surrounding area away from poor Ni, protecting the integrity of the matrix.And it prevents the LBE from penetrating the material surface.

Fig. 3 .
Fig. 3. BSE image of the AFA steel sample after 1000 h of LBE corrosion.

Fig. 5 .
Fig. 5. (a) BSE image of cross section after LBE corrosion of 1500 h AFA steel sample, (b) Local enlargement and line scan area, (c) Elemental Composition.

Table 1 .
Composition table of AFA steel

Table 2 .
Element point scan information at different positions in Fig.3.