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Corrosion behavior of 304 and 316H stainless steel in LiF-NaF-KF molten salt

The corrosion behavior of 304 and 316H stainless steel in LiF NaF KF (flinak) molten salt at 700 ℃ was studied by means of static corrosion, SEM / EDS and EPMA. The results show that the main corrosion form of the two stainless steels in flinak molten salt is the selective loss of CR at the surface and near surface grain boundaries. The corrosion depth and weight loss of 316H stainless steel are lower than that of 304 stainless steel due to Mo content. After corrosion, Ni and Fe rich corrosion layers appear on the surface of both stainless steels, while a large number of nano scale precipitates appear near the surface. EDS analysis results show that these precipitates are nitrides or carbonitrides of Cr and Al, which significantly improve the hardness of the material.

Molten salt reactor (MSR) is one of the six advanced reactor types proposed in the “fourth generation nuclear energy international forum”, which has the advantages of inherent safety, economy, sustainability, non-proliferation and efficient use of nuclear fuel [1,2,3]. Fluoride of alkali metal or alkaline earth metal is selected as the nuclear fuel carrier and coolant in MSR, and the ternary eutectic molten salt LIF NaF KF (46.5% – 11.5% – 42%, mole fraction, flinak) is one of the candidate molten salts, which has the advantages of high specific heat, high heat conduction, high boiling point, low saturated vapor pressure, etc. [4].
The corrosion behavior of metal materials in molten salt is the key factor to determine the service life and safety of materials. Previous studies [5,6,7] have shown that nickel based alloys have good corrosion resistance to fluoride coolant, and hastelloy-n alloy is the only alloy material used in the construction of molten salt reactor so far. However, the alloy is expensive and not included in the ASME nuclear material standard, so it can not be directly used to build commercial molten salt reactor. 304 and 316 austenitic stainless steels have good high temperature mechanical properties and economy, and have the qualification of nuclear materials for ASME high temperature reactor, so they are important candidate materials for molten salt reactor [8]. However, compared with Ni based alloy, the corrosion resistance of stainless steel to fluoride molten salt is weak, and the related research is also less.
Sellers et al. [9] studied the corrosion behavior of 316L stainless steel, hastelloy-n and graphite in flinak salt at 850 ℃. The results showed that graphite accelerated the corrosion of CR. Zheng [10] compared the corrosion behavior of 316L and hastelloy-n in FLIBE molten salt at 700 ℃, and showed that the annual corrosion rates of 316L and hastelloy-n were 12.0 and 17.2 μ M / a respectively without graphite. Ding Xiangbin et al. [11] showed that hastelloy-n and 316L stainless steel showed obvious uniform corrosion in flinak molten salt, while intergranular corrosion occurred in the latter; no obvious corrosion was found in hastelloy-n alloy after 1000 h, while the corrosion depth of 316L stainless steel reached 40 μ M. Researchers from Oak Ridge National Laboratory (ORNL) corroded 304L stainless steel in lif-bef2-zrf4-thf4-uf4 (70% – 23% – 5% – 1% – 1%, mole fraction) dynamic molten salt for 5 years. Due to the fuel contained in the molten salt, it was found that the corrosion was very serious, and the corrosion hole depth reached 250 μ m [12]. Most of the above researches are focused on 316L stainless steel and hastelloy-n alloy, while the research on 304 stainless steel is relatively lacking, especially the comparative study on the corrosion performance of 304 and 316 in coolant molten salt (FLIBE or flinak). Kondo et al. [13] compared the corrosion behavior of 304L and 316L in FLIBE molten salt at 600 ℃. Due to the poor selection of test system and short test time, it is unable to draw a clear conclusion. The test results of ORNL researchers [14] show that flinak has the characteristics of Lewis base, which is slightly more corrosive than FLIBE. Therefore, this study compared the corrosion behavior of 304 and 316H stainless steels in flinak, in order to get the clear difference of corrosion resistance between the two stainless steels.

Experimental method

The materials used in the experiment are 304 and 316H stainless steel hot rolled plates, and the composition is shown in Table 1. A 15 mm × 10 mm × 2 mm sheet corrosion sample was cut from the original stainless steel plate by wire cutting, and each sample was marked by laser marking instrument. Before the corrosion test, the flake samples were ground to 1500? With water abrasive paper step by step to remove the possible oxide impurities on the surface of the samples. Then, the samples were put into deionized water and anhydrous ethanol successively for ultrasonic cleaning, and the surface of the samples was dried by electric blower. Finally, the actual size and weight of the sample were measured by vernier caliper and precision balance. The flinak salt used in the experiment was purified by h2-hf to remove the impurities in the raw material. The main impurities (mg / kg) of the purified salt were as follows: Fe 8.04, Cr 0.94, Ni 218.3, Mo 0.13, Cl – 16.4, no – 31.4, PO43 – 12.1, SO42 – 16.4, O 276. Metal elements were measured by inductively coupled plasma atomic emission spectrometry (ICP-OES), anions by ion chromatography, and oxygen content by oxygen analyzer (LECO ro600). The salt was always stored in an AR filled glove box with water and oxygen content less than 10 mg / L from purification to experiment to avoid pollution.

Table.1 Chemical compositions of 304 and 316H stainless steels (mass fraction / %)
Steel C Mn S Si P Cr Ni Mo Fe
304 0.06 1.13 0.002 0.40 <0.001 18.58 7.74 Bal.
316H 0.06 1.08 0.004 0.47 0.035 16.24 10.04 2.07 Bal.

The corrosion test adopts the static immersion method, and the container for immersion is shown in Figure 1. The vessel is composed of double-layer Crucible: the inner layer is graphite crucible, which is used to hold samples and molten salt; the outer layer is stainless steel crucible, which is used to seal and isolate the external atmosphere. Before the experiment, the graphite crucible and the graphite crucible cover were cleaned with absolute ethanol, and then transferred to a vacuum furnace at 700 ℃ for drying for 24 h to remove the water and oxygen adsorbed in the graphite as much as possible. After the vacuum furnace is cooled to room temperature, the graphite crucible is quickly transferred to the glove box for standby. The stainless steel crucible was cleaned with deionized water and anhydrous ethanol, and then transferred to a 120 ℃ vacuum drying oven for drying for 2 h. after cooling, the crucible was transferred to a glove box for standby.

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Fig.1 Schematic illustration of the static corrosion capsule

In the glove box, 304 and 316H stainless steel sheet corrosion samples were placed in two containers respectively, and three parallel contrast samples were placed in each container. In order to avoid galvanic corrosion [15,16,17] caused by the connection between the sample and graphite, an insulating ceramic rod was placed on the top of the graphite crucible, and the sample was suspended on the insulating ceramic rod through 316 stainless steel wire to maintain the insulation between the sample and graphite. After 220 g of flinak salt was filled into each graphite crucible, the graphite crucible was covered and put into the outer stainless steel crucible, and the outer crucible was welded and sealed in the glove box to avoid being polluted by the external atmosphere during the experiment. Finally, the welded corrosion container was taken out from the glove box and put into the high temperature furnace for 400 h at 700 ℃.
At the end of the corrosion test, 1 mol / L Al (NO3) 3 aqueous solution and deionized water were used for ultrasonic cleaning to remove the residual salt on the sample surface. A precision balance (sensitivity 0.01 mg) was used for weight test after corrosion to calculate the corrosion weight loss per unit area. Merlin compact field emission scanning electron microscope / energy dispersive X-ray spectrometer (FE-SEM / EDS) and epma-1720 electron probe microanalyzer (EPMA) were used to study the morphology, microstructure and element distribution of the corroded samples. The main alloying elements in flinak salt after corrosion were analyzed by spectro Arcos ICP-OES.

Experimental result 

Analysis of weight loss and composition of molten salt after corrosion

After corrosion in flinak molten salt at 700 ℃ for 400 h, the weight loss per unit area of 304 and 316H stainless steels was (4.97 ± 0.18) mg / cm2 and (2.5 ± 0.1) mg / cm2, respectively. It can be seen that the weight loss per unit area of 304 stainless steel is about 1 times that of 316H stainless steel containing Mo, which means that 304 stainless steel has suffered more serious corrosion.
The concentration of main alloy elements in molten salt after corrosion is shown in Table 2. Compared with the concentration of the main alloy elements in molten salt before and after corrosion, it can be seen that the content of the main elements in molten salt increases after corrosion, which indicates that these elements migrate from the alloy to the salt under the action of molten salt corrosion. The total amount of alloy elements in the salt after corrosion of 304 stainless steel is more than that of 316H stainless steel, which indicates that the corrosion of 304 stainless steel is more serious. Among them, Fe increases the most. After corrosion of 304 stainless steel, Fe in salt increases about 1200 mg / L, while that of 316H stainless steel increases about 480 mg / L. The Cr content of 304 stainless steel increased by 240 mg / L, while that of 316H stainless steel only increased by 160 mg / L. After corrosion of 304 and 316H stainless steel, the increase of Ni in molten salt is 177 mg / L and 72 mg / L respectively. Therefore, the increase of Mo in the salt after corrosion of 304 stainless steel comes from 316 stainless steel wire used for hanging sample.

Table.2 Element contents of Cr, Fe and Ni in FLiNaK salt after corrosion (mg/L)
Steel Cr Fe Ni Mo
304 241 1235 395 1.1
316H 162 484 290 14.8

Surface morphology of stainless steel after corrosion

Figure 2 shows the surface SEM images of two kinds of stainless steels after corrosion in flinak molten salt at 700 ℃ for 400 H. It can be seen that there are a large number of corrosion holes on the surface of 304 stainless steel, and in some areas there are also surface spalling due to the interconnection of corrosion holes. There are also corrosion holes on the surface of 316H stainless steel, but the density of holes is lower, and there is no obvious surface spalling.

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Fig.2 SEM images of the surfaces of 304 (a) and 316H (b) stainless steels after corrosion at 700 ℃ in FLiNaK salt for 400 h

Element and microstructure analysis of stainless steel section after corrosion

The results of EPMA scanning analysis of Cr, Fe and Ni in 304 and 316H stainless steel after corrosion are shown in Fig. 3. It can be seen that the corrosion holes of 304 stainless steel are more than that of 316H stainless steel, and the distribution depth of corrosion holes to the interior of the alloy is deeper. After the corrosion of the two alloys, Cr depletion occurs at the surface and near the surface grain boundary, and Ni and Fe enrichment occurs in the CR depletion region. Among them, the CR deficient depth of 304 stainless steel is about 90 μ m, while that of 316H stainless steel is about 40 μ M. It is obvious that the change of element distribution near the surface is caused by molten salt corrosion. The CR poor depth of 304 stainless steel is much greater than that of 316H stainless steel, which indicates that the corrosion of 304 stainless steel is more serious, which is consistent with the results of weight loss and molten salt analysis.
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Fig.3 Cross sections (a, e) and EPMA elemental distribution mappings of Cr (b, f), Fe (c, g) and Ni (d, h) of 304 (a~d) and 316H (e~h) stainless steels after corrosion in FLiNaK salt for 400 h

Figure 4 shows the backscattered electron images of 304 and 316H stainless steel sections after corrosion and the EDS line scan results of the inner unaffected area. It can be seen that there are a layer of gray white areas on the surface of 304 and 316H stainless steel after corrosion in flinak molten salt at 700 ℃ for 400 h, and these areas extend to the interior along the grain boundary of the alloy. There are a lot of black precipitates in the grains near the gray white region. These precipitates are distributed from the gray white area near the alloy surface to the interior of the alloy; with the change of depth, the distribution of precipitates is less and less, until the position about 350 μ m away from the alloy surface (as shown in Fig. 4), the black precipitates can not be observed. From the high multiple backscattered electron image in Figure 4, it can be seen that the microstructure of the corrosion affected area of 304 and 316H stainless steel is similar, which is composed of gray white corrosion layer and black precipitates. Although there is no obvious black precipitates in the grain boundary of the two materials, the morphology of precipitates on the grain boundary is slightly different. The number of precipitates on the grain boundary of 304 stainless steel is lower than that of 316H stainless steel, but the size is larger.

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Fig.4 Back scattered electron images (a, b) and the magnified images of areas I (c), II (e) in Fig.4a and III (d), IV (f) in Fig.4b of the cross-sections and EDS line scanning results (g, h) across the grain boundaries in the unaffected zones (along the marked lines) for 304 (a, c, e, g) and 316H (b, d, f, h) stainless steels after corrosion in FLiNaK salt at 700 ℃ for 400 h

In order to further study the possible phase composition, EDS was used to analyze the composition of gray white corrosion layer on the surface, black precipitated phase near the surface, grain interior and grain boundary in the area not affected by corrosion. The results are summarized in Table 3 and Figure 4. It can be seen from table 3 that the composition changes of the two materials are similar. The gray white corrosion layer on the surface is obviously enriched in Fe and Ni, while Cr is lost, compared with the area not affected by corrosion inside. The absolute values of Fe and Ni in the corrosion layer increase by about 13% and 4% compared with those in the non corrosion affected zone, while the loss of Cr is more serious, from about 17% in the non corrosion affected zone to about 2%. The content of N in the gray white corrosion layer and internal non corrosion affected zone of 316H and 304 stainless steels is almost 0, while the content of N in the black precipitated zone is about 3% and 5%, respectively. A little Al is also enriched in the black precipitate area, while the content of Cr is almost the same as that in the area not affected by corrosion, indicating that the black precipitate may be the nitride of Cr and al. It can be seen from the line scan analysis at the grain boundary of the uncorrosion zone in Fig. 4 that Cr and C are enriched in the grain boundary precipitates of the two alloys, and there is obvious Mo enrichment on the grain boundary of 316H stainless steel containing Mo, indicating that the grain boundary precipitates should be carbides of these alloy elements.

The change of hardness in the alloy after corrosion

In order to confirm whether the near surface black precipitates caused by corrosion will affect the mechanical properties of the alloy, the Vickers hardness of the near surface black precipitates area and the internal non corrosion affected area of the two kinds of alloy cross-section specimens after corrosion were measured by micro Vickers hardness tester. The results are as follows: the Vickers hardness of 304 stainless steel precipitates area and non corrosion affected area are respectively 0 The Vickers hardness of the precipitated phase zone and the non corrosion zone of 316H stainless steel were (536.67 ± 8.08) and (206 ± 1.73), respectively. It can be seen that the hardness of the black precipitates near the surface of the two alloys is significantly higher than that of the non corrosion affected area, indicating that the precipitates have obvious hardening effect on the alloy.

Analysis and discussion

Early research [18] believed that in the fluoride molten salt system, the protective passive oxide film cannot exist stably, so the corrosion of the alloy is determined by the thermodynamic stability of the main element fluoride relative to the molten salt component or impurities. The fluorides CrF3, FeF2, FeF3, NiF2 and MoF3, which are the main elements in stainless steel, form Gibbs free energies at 700 ℃, respectively, about -880, -820, -790, -780 and -700 kJ/mol (equivalent to 1 mol In the case of F2) [11], the degree of corrosion of these elements in fluoride molten salt is Cr>Fe>Ni>Mo. The Cr in the alloy is most prone to selective dissolution corrosion in FLiNaK molten salt. . Because the thermodynamic stability of the FLiNaK salt itself is much higher than the fluoride of the alloying element, the corrosion of the molten salt to the alloy is mainly caused by the impurities in it [18]. When there are impurity metal ions such as Fe and Ni in the molten salt that are inert than Cr, the Cr in the alloy will be corroded through the reaction formulas (1)~(3). In addition, the presence of impurity ions Ni2+ will also corrode Fe in the alloy through reaction equations (4) and (5). These Fe2+ or Fe3+ dissolved into the molten salt due to corrosion will cause secondary corrosion to the Cr in the alloy. Ni2+ in molten salt will cause severe corrosion to stainless steel.

2Cr+3NiF2→3Ni+2CrF3
2Cr+3FeF2→3Fe+2CrF3
Cr+FeF3→Fe+CrF3
Fe+NiF2→Ni+FeF2
2Fe+3NiF2→3Ni+2FeF3
2MF+H2O→2HF+M2O(M=Li,Na,K)
xM+yHF→MxFy+y2H2(M is alloying element)

As shown in Table 2, the initial Ni content in the FLiNaK salt used in this experiment is relatively high, so it corrodes the Cr and Fe in the two stainless steels. With the progress of corrosion, the selective dissolution corrosion of Cr leads to the formation of Cr-poor areas in the alloy, and Ni and Fe are relatively enriched in the Cr-poor areas. At the same time, the Ni and Fe atoms generated by the corrosion reaction of the alloy in the molten salt will partially deposit on the surface of the sample and diffuse into the alloy. Therefore, the off-white corrosion layer as shown in FIG. 4 is formed under the interaction of the two. It can be seen from Table 3 that, compared with the unaffected matrix, the content of Fe and Ni in the corroded layer both increase, while the concentration of Cr is greatly reduced, which verifies the occurrence of the reaction of formulas (1)~(5). In addition to the inert metal ions, the water impurities contained in the salt will generate HF through reaction formula (6), which will cause corrosion to the alloy through reaction formula (7) [19]. The fluoride produced by the corrosion reaction is soluble in FLiNaK, and the metal atoms produced by the substitution reactions (1)~(5) cannot be deposited and bonded on the surface of stainless steel, and the reaction (7) produces H2, so with With the progress of the corrosion reaction, the alloy surface will have corrosion holes due to the loss of a large amount of elements. Compared with 304 stainless steel, 316H stainless steel has a higher Ni content and contains Mo. These two elements have strong resistance to fluoride corrosion. Therefore, the number of corrosion holes on the surface of 316H stainless steel is less than that of 304 stainless steel. As the density of corrosion holes increases, some holes will be connected, causing local spalling of the corrosion layer on the alloy surface. From the analysis results of the molten salt after the experiment (Table 2), it can be seen that the contents of Cr, Fe, and Ni in the molten salt increased after corrosion. In the two iron-based alloys targeted by the experiment, Fe occupies a large proportion, so the dissolution of Fe occupies the main part of the alloy corrosion weight loss. After corrosion, the Ni content in the salt increases, which may be due to a part of the residual water remaining in the graphite crucible (although the crucible has been rigorously treated as required, there will still be some residual water that has not been removed). Under the action of metal impurity ions such as Ni accelerating the corrosion of the alloy, although part of Fe and Ni is deposited on the surface of the alloy during the corrosion process, the amount of Fe and Ni dissolved is much greater than the amount deposited on the surface of the alloy. Therefore, the content of Fe and Ni in the molten salt increased after corrosion, and the increase of Cr was mainly caused by the corrosion reaction itself; and the increase of Fe and Ni not only originated from the corrosion reaction itself, but also partly from the surface corrosion layer. Exfoliation, under the combined action of self-corrosion and corrosion layer exfoliation, the content of Fe and Ni in the salt after corrosion is significantly increased.

Table.3 EDS analysis results of different areas of 304 and 316H stainless steels after corrosion (the numbers in the brackets correspond to the reference signs of the areas in Fig.4) (mass fraction / %)

Position N Al Cr Fe Ni Mo
304 Unaffected zone (1) 0 0.1 18.9 72.4 8.6 0
304 Precipitation zone (dark) (2) 5.9 0.2 18.6 67.2 8 0.1
304 Corrosion zone (gray) (3) 0 0.1 2.2 85.2 12.2 0.3
316H Unaffected zone (4) 0 0 17.5 69.9 10.5 2.1
316H Precipitation zone (dark) (5) 3.4 0.7 17.7 65.4 10.2 2.6
316H Corrosion zone (gray) (6) 0 0 1.6 81.5 16.2 0.7

After the Cr on the alloy surface is dissolved into the molten salt through the corrosion reaction, the internal Cr will diffuse to the surface, so that the corrosion continues, and at the same time, the Cr depletion of the alloy near the surface area. It can be seen from Figure 3 that the Cr depletion in the near surface area of 304 and 316H stainless steel is concentrated on the grain boundary, indicating that obvious intergranular corrosion has occurred in both stainless steels. Since the experimental temperature of this experiment is in the sensitization temperature range of austenitic stainless steel, both materials have sensitized after the experiment, and a large number of carbides precipitated on the grain boundaries, as shown in Figure 4. The Cr-rich grain boundary carbides generated by the sensitization will lead to the formation of Cr-poor regions around the grain boundaries, which will cause the grain-grain boundaries to form a passivation-activated micro galvanic structure. This “Cr-poor theory” is about the Austrian The most widely accepted mechanism of intergranular corrosion in stainless steel [20]. However, there is no Cr2O3 passivation film in the oxygen-poor high-temperature fluoride molten salt environment, and the high-Cr area is prone to corrosion, so the traditional “Cr-poor theory” is not applicable. Considering that the corrosion process of alloy in fluoride salt is controlled by the diffusion of Cr [18], and the diffusion rate of Cr at the grain boundary in stainless steel at 700 ℃ is much higher than that inside the grain [21], the intergranular corrosion may be Because the grain boundary promotes the rapid diffusion of Cr. In addition to the intergranular diffusion of Cr itself, the influence of the grain boundary carbides produced by sensitization on the intergranular corrosion of stainless steel in molten fluoride salt is currently unclear. Olson et al. [22] compared the corrosion behavior of several superalloys in FLiNaK molten salt at 850 ℃, and believed that for alloys with a Cr content of 20% to 23% (mass fraction), the corrosion degree is proportional to the C content of the alloy. Therefore, it is inferred that the Cr-rich carbides at the grain boundary will promote the intergranular corrosion of the alloy in the molten fluoride salt. Zheng [10] compared the corrosion behavior of model alloy without C and 316 stainless steel in FLiBe molten salt at 700 ℃, and believed that the corrosion depth of model alloy was much higher than that of 316 stainless steel. Cr carbide can slow down the rate of outward diffusion of Cr along the grain boundary. Although the Cr content and C content of the two stainless steels in this study are very similar, the intergranular corrosion depth of 304 stainless steel is more than twice that of 316H stainless steel, which is caused by the difference in grain boundary carbides of the two materials. The grain boundary carbides of 304 and 316H stainless steel are all M23C6 type, but for 304, M is mainly Fe and Cr, and M in 316H stainless steel also includes Mo[23]. The EDS line scan results of grain boundaries in areas not affected by corrosion in this experiment (Figure 4) also support this result. Since these carbides have a much higher Cr content than the matrix, they may themselves be susceptible to molten salt corrosion. Since Mo itself is not easily corroded by molten salt, the enrichment of Mo in grain boundary carbides may increase the resistance of these carbides to molten salt corrosion, thereby reducing the degree of intergranular corrosion of 316H stainless steel.
In addition to the surface corrosion layer and the near-surface intergranular depletion of Cr, molten salt corrosion also causes the formation of a large number of nano-scale precipitates in the near-surface area of the two stainless steels, as shown in Figure 4. Similar precipitation phases are also reported in Zheng et al. [24] on the corrosion of 316 stainless steel in FLiBe molten salt. It is believed that these near-surface precipitation phases are due to the Cr7C3 and Al4C3 formed by the diffusion of C in the graphite crucible into the alloy. From the EDS analysis results of the precipitation phase region in this experiment, it can be seen that the precipitation phase region of the two alloys has a significant increase in N content and a decrease in Fe content relative to the alloy matrix, while the content of Cr and Ni does not change much. . In addition, the Al content in the precipitation phase region of 316H stainless steel increased significantly, the Mo content increased slightly, and the Al content in the precipitation phase region of 304 stainless steel also increased slightly. The significant N enrichment in the precipitated phase region indicates that in addition to carbides, these precipitated phases may also contain nitrides or carbonitrides. Since Al and Cr in stainless steel are strong nitride forming elements, and Cr and Mo are strong carbide forming elements, N and C that diffuse into the alloy from the outside are easily combined with these alloying elements to form precipitates in the near surface area . The source of C in this experiment may be the graphite crucible used in the experiment, and the source of N may be the NO3- initially contained in the FLiNaK salt or the N2 introduced during the sealing process of the crucible in the glove box (because the glove box has a certain leakage rate , And its circulating purification device can only remove H2O and O2 leaking into the air, so it will inevitably contain a certain amount of N2). In addition to the internal diffusion of elements in the corrosive environment, the rapid diffusion and dissolution of Cr in the alloy along the grain boundary can also change the composition and microstructure of the alloy grain boundary, which may lead to the formation of internal carbides and nitrides along the local area of the alloy. . Therefore, the precipitates formed on the near surface after corrosion of the alloy may be affected by the internal diffusion of external elements and the loss of the alloy itself. Compared with the corrosion depth of the material (about 90 μm for 304 stainless steel and about 40 μm for 316H stainless steel), the distribution depth (about 350 μm) of the precipitates formed during the corrosion process in the two types of stainless steel is deeper. It can be seen from the results of the hardness analysis that these precipitated phases significantly increase the hardness of the two materials, which may affect the performance of the materials during service. Therefore, in the actual use of fluoride molten salt in the related engineering design process, in addition to evaluating the direct corrosion of molten salt to stainless steel, the changes in the microstructure of the material caused by molten salt immersion must also be considered. The latter may be possible under certain conditions. It has a greater impact on the service performance of the material. Limited by the space and element resolution of SEM (the content of light element C cannot be accurately measured), this study has not been able to determine the specific composition and structure of these precipitates. More in-depth analysis will be carried out by TEM in the future.

Conclusion

  • (1) The corrosion form of 304 and 316H stainless steel in FLiNaK molten salt at 700 ℃ is mainly manifested in the selective loss of Cr at the surface and near-surface grain boundaries, and the formation of corrosion holes due to the loss of local elements.
  • (2) The corrosion depth and corrosion weight loss of 316H stainless steel are significantly lower than those of 304 stainless steel, indicating that it has better resistance to FLiNaK molten salt corrosion. The reason is that 316H stainless steel contains a small amount of Mo, which improves the corrosion resistance of the stainless steel matrix and grain boundary carbides to FLiNaK molten salt.
  • (3) After corrosion, both 304 and 316H stainless steels have a surface corrosion layer rich in Ni and Fe. It may be the preferential dissolution of Cr in the alloy to form a Cr-poor zone, so that the relative enrichment of Ni and Fe appears in the Cr-poor zone. , And the Ni and Fe impurity ions in the molten salt react with Cr and Fe in the steel on the surface of the material to cause Ni and Fe to deposit on the surface of the material and diffuse into the interior.
  • (4) A large number of nano-scale precipitates, which may be nitrides or carbonitrides of Cr and Al, appeared in the near surface area after corrosion of the two stainless steels. These precipitates significantly increased the hardness of the stainless steel.

Author: Hui LIU, Wei QIU, Bin LENG, Guojun YU (https://www.jcscp.org/CN/Y2019/V39/I1/51)

Source: Network Arrangement – China Stainless Steel Flange Manufacturer – Yaang Pipe Industry (www.epowermetals.com)

(Yaang Pipe Industry is a leading manufacturer and supplier of nickel alloy and stainless steel products, including Super Duplex Stainless Steel Flanges, Stainless Steel Flanges, Stainless Steel Pipe Fittings, Stainless Steel Pipe. Yaang products are widely used in Shipbuilding, Nuclear power, Marine engineering, Petroleum, Chemical, Mining, Sewage treatment, Natural gas and Pressure vessels and other industries.)

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