| In order to maintain the continuity and stability of solar thermal power generation equipment under sufficient solar radiation conditions (at night, clouds blocking. etc), thermal storage system is indispensable. Inorganic salt, as a large class of high temperature latent thermal energy storage material, has the merits of wide range of melting point, abundant alternative material and high phase change latent heat value, which makes it a promising candidate for heart storage. However, under the operating conditions, high-temperature molten salts have strong corrosion effects on its container material.For the molten halide salt high-temperature phase change thermal-storage medium, each kind representative Fe-based, Cr-based, Ni-based alloys as well as Fe-wt.24.4%Al alloy were prepared as the thermal-storage cavity material. The corrosion kinetics curves were measured and drawn through a long term corrosion experiment in molten NaCl, binary and ternary eutectic salt NaF-60KF and NaF-22CaF2-13MgF2. The cross section characteristics, elements distribution features and corrosion products of the samples were analyzed by SEM, EPMA, EDS and XRD after corrosion, and the corrosion mechanism was studied.The results showed that:a. when the corrosion medium is pure molten NaCl, the corrosion kinetics curves of three kinds of alloys followed the linear law; the conventional protective Cr2O3produed during the corrosion proccese did not play a protective role, but reacted with the molten NaCl and accelerated the corrosion rate; grain boundaries provide channels for the infiltration of molten NaCl to alloy matrix, hence the corrosion process was accelerated. A thicker corrosion layer formed on the surface of nickel based and iron based alloy, which provide a foundation for the adherence of Cr2O3. The adherence of Cr2O3can lower the driving force of the Cr diffusion, which improved the corrosion resistance of the alloy. In addition, a spinel structure layer of corrosion product formed on the surface of iron based alloy, which improved the iron based alloy corrosion resistance ulteriorly, and made it perform the best corrosion resistance in the three. The corrosion mechanism can be explan with the activative oxidation of high temperature chloride theory, high temperature chloride reacted with metal oxide and produced Cl2, Cl2or Cl-go through the oxide film and reach the oxide film/metal matrix interface, then reacte with alloy generate volatile chlorides, the chloride will be oxidized during the continuous diffusion outward process, the new generated oxide will grow on the oxide film and destroyed the integrity of the oxide film, the unreacted chlorine diffuse to the oxide film/metal matrix interface until exhausted. The screened Fe-75wt%Ni alloy showed good corrosion resistance, but the mechanical properties need to be further improved.b. When the corrosion medium is binary and ternary eutectic salt NaF-60KF and NaF-22CaF2-13MgF2, the corrosion resistance of alloy remains substantially consistent, Ni-base alloy showed the best corrosion resistance; Fe-based and Cr-based Fe-wt.24.4%A1samples exhibited uniform corrosion; therein, Fe-based and Cr-based samples showed intergranular corrosion characteristics; the corrosion resistance improved with the increase of Ni. Cr reacted with fluoride under high temperature leading to the increase mass loss rate of Fe, Cr, Ni-based samples. Fe-wt.24.4%A1alloy exhibited serious corrosion traces and desolution of Al from alloys matrix uniformly; after long-term corrosion, corrosion channels formed at grain boundaries and cracks emerged at the end of the channels due to thermal stress, the cracks will become the triggers of the further corrosion. Consequently, A1should be avoided in thermal storage cavity material of fluoride salt. |
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