Anti-molten salt corrosion coating is widely used because of its own advantages, so what is the research progress of anti-molten salt corrosion coating? Let's find out together!

Fluoride molten salt has a series of advantages such as low melting point, high boiling point, high heat capacity, low vapor pressure, high thermal conductivity, low viscosity and excellent chemical stability, which can be used as a new type of heat transfer or heat storage medium. It can be used as a reactor coolant, and can be used in power fields such as high-temperature hydrogen production, solar heat transfer and storage, and fuel cells. However, high-temperature fluoride salts are corrosive, and the corrosion of materials in contact with fluoride salts limits the widespread use of fluoride salts.

Nickel is the No. 28 element in the periodic table of elements. It has a face-centered cubic structure (fcc) and has no allotropy changes from room temperature to high temperature. It is a very outstanding matrix metal. At the same time, because the third electron shell of Ni atoms is basically filled, it can dissolve more modifying elements for alloying, while still maintaining the stability of the γ austenite phase. In different high temperature environments, Ni-based superalloys are the most widely used.

Nickel-based alloys are based on metal nickel, and corresponding alloying elements are added according to different uses to improve the performance of the alloy. Nickel-based alloys are widely used in various high-temperature molten salt environments based on their better mechanical and processing functions, as well as their excellent resistance to molten salt corrosion. Primary candidate structural data.

In recent years, with the increasing research on molten salt reactors and the potential application value of fluoride salts in the field of power, many international research institutions have also conducted a lot of corrosion research on high-temperature nickel-based alloys in molten salt environments. Summarize almost all nickel-based superalloy molten salt corrosion tests in recent years. The characteristics of corrosion rules of nickel-based alloys in molten salt environment are summarized from the following aspects: the corrosion sequence of metal elements, the effect of impurities on corrosion, the effect of time and temperature on corrosion, the effect of Cr content in the alloy on corrosion, and the effect of Cr in the molten salt. Corrosion products in salt, etc.

1. Corrosion sequence of different metal elements

In the high temperature molten salt environment, the active elements are prone to selective preferential corrosion. Research by the National Aeronautics and Space Administration (NASA) shows that in high-temperature fluoride salts, the ease of corrosion of some common metal elements is in the following order from easy to difficult: The elements given by AlNASA are in molten salt The order of corrosion difficulty is consistent with the Gibbs formation of metal fluorides. In Ni-based superalloys, in order to improve the high-temperature oxidation resistance of the alloy, a certain amount of metal element Cr is generally added to form a Cr2O3-based oxide film with good protection function on the surface of the alloy, and to improve the high-temperature oxidation resistance of nickel-based superalloys , Nickel-based superalloy is actually an alloy based on Ni-Cr binary system. However, the oxide film of Cr is unstable in high-temperature molten salt, and it is easy to react with the molten salt and be corroded and dissolved, so that the effect of maintaining the alloy cannot be achieved. According to the corrosion sequence of metal elements, the selective corrosion of Cr exists in the nickel-based superalloy in the molten salt environment.

2. The influence of water impurities on corrosion

Because some components in the fluoride salt have extremely strong water absorption, such as KF, etc., it is difficult to completely remove the water impurities in the molten salt by drying and other methods. Moisture in molten salt, molten salt forms HF at high temperature, HF is a strong acid that can cause corrosion of almost all metal materials. Ouyang et al. performed corrosion tests on two nickel-based superalloys, Hastelloy N and Hastelloy B3, in FLiNaK salts with different water contents. The corrosion weight loss of the alloy in the salt containing 3.19 wt.% moisture is approximately three times that in the salt containing 1.91 wt.% moisture. The corrosion loss of the samples in pure FLiNaK salt is much less than that in FLiNaK salt with water separation. In conclusion, water impurities in molten salts can accelerate the corrosion of nickel-based alloys. In order to reduce the corrosion of materials, the content of water impurities in the molten salt should be strictly controlled during the corrosion test or the use of molten salt.

3. The influence of time and temperature on corrosion

Nickel-based superalloys, as candidate structural materials for molten salt reactors, need to operate at different temperatures for a long time. It is necessary to study the regular characteristics of nickel-based alloys in different temperatures in molten salt corrosion. Ouyang et al. studied the different corrosion characteristics of Hastelloy N and Hastelloy B3 two nickel-based superalloys in FLiNaK salts at different temperatures. The results show that with the increase of temperature, the corrosion of the alloy is aggravated; at the same corrosion temperature, with the extension of the corrosion time of the alloy in the molten salt, the corrosion of the alloy is aggravated, and the corrosion weight loss increases. Ouyang calculated the corrosion rate of the sample through the corrosion weight loss calculation. The results show that the corrosion rate of the sample is relatively large in the first 100 hours, and the corrosion rate of the sample gradually decreases with the extension of the corrosion time.

ORNL has conducted many long-term corrosion tests on the primary candidate nickel-based alloys, and concluded that the same corrosion rules are found: the corrosion rate of the alloy is faster in the early stage, and with the extension of the corrosion time, the corrosion rate in the later stage gradually slows down. The corrosion of the alloy in the early stage of the molten salt is mainly caused by impurities. With the extension of the corrosion time, the impurities in the molten salt are depleted, and the corrosion rate gradually slows down. The later stage is mainly controlled by the dispersion rate of the elements in the alloy. Therefore, the corrosion depth of the sample in the molten salt for a long time can be predicted by the dispersion coefficient. DeVan et al. tested the dispersion coefficient of Cr in Hastelloy N alloy by element tracing method. Using Fick's second rule, they could briefly predict the element dispersion of Hastelloy N alloy after 30 years of service in molten salt environment. Although it is known that the corrosion of alloys is controlled by the thermal dispersion mechanism, the corrosion of alloys in molten salts is affected by interface contact conditions, alloy defects, corrosion reactions, etc. The dispersion behavior of elements in alloys cannot be easily predicted using Fick's rule. The dispersion rules of Cr of nickel-based alloys in molten salt environment have important implications for predicting the corrosion of alloys, but the research on this aspect is not sufficient, and in-depth research is urgently needed.

4. The effect of Cr content in the alloy on corrosion

The corrosion of nickel-based superalloys in molten salt is mainly the selective corrosion of Cr, and the corrosion behavior of nickel-based alloys in molten salt is greatly affected by its Cr content. Olson et al. conducted corrosion tests on six nickel-based alloys Hastelloy-X, Hastelloy-N, Ni-201, Incoloy-800H, Haynes-230 and Inconel-617 in FLinaK salt at 850 °C for 500 hours, and the results showed that the alloys were Corrosion increases with the increase of Cr content in the alloy, and the corrosion weight loss is almost linearly related to the Cr content in the alloy. Ouyang et al. carried out high temperature molten salt corrosion on Hastelloy-B3, Hastelloy-N, Haynes-242, Haynes-263 and TZM alloys, and found that after corrosion in molten salt containing a few water impurities, the corrosion weight loss of the alloy followed that of the alloy. With the addition of Cr content, the corrosion weight loss has a linear relationship with the sample (Cr 1/3Mo wt.%). Sona et al. and Min Liu et al. conducted corrosion tests on different nickel-based alloys and found that with the addition of Cr content in the alloy, the corrosion of the alloy increased.

Exploring the corrosion characteristics of nickel-based alloys with different Cr contents has important implications for the research and the selection and development of molten salt reactor structural materials. However, regarding the corrosion rules of nickel-based alloys with different Cr contents in molten salt, the data is relatively small and the system is not available, and it is necessary to conduct in-depth research.

5. Corrosion products of Cr in molten salt

The corrosion of nickel-based alloys in molten salt is mainly the selective corrosion of Cr. Studying the corrosion products of Cr in molten salt is helpful to predict the corrosion reaction of Cr and deepen the understanding of the corrosion mechanism of Cr in molten salt. In the nuclear powered aircraft project at Oak Ridge, a lot of research was done on the corrosion products of Cr in fuel salts, and it was found that in FLiNaK fuel salts, Cr mainly exists in the form of Cr(III), while in NaF-ZrF4 salts In and FLiBe salts, the primary valence state of Cr is divalent. Koger et al. studied Hastelloy-N alloy and found that the main corrosion product of metallic Cr in NaBF4-NaF (92-8 molewt.%) salt is Na3CrF6.

Brinnt et al. found that some common stainless steels, Inconel alloys, etc. have Cr corrosion in NaF-KF-LiF-UF4 salt, and some green crystalline substances are formed in molten salt. Yes, it is found that Cr mainly constitutes K2NaCrF6 in NaF-KF-LiF-UF4 salt. The research on the corrosion products of Cr in molten salt is relatively concentrated in the Oak Ridge National Laboratory in the last century. The research on the corrosion products in China is basically suspended, and the research on this aspect is not sufficient, and it is necessary to continue the in-depth research.