Effect of Nb on high-temperature corrosion resistance of 304 austenitic stainless steel in molten salt

ZHANG Kaixin, XIAO Guizhi, WANG Fan, ZOU Yutianqi, ZOU Dening, ZHANG Wei

Iron and Steel ›› 2025, Vol. 60 ›› Issue (3) : 147-155.

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Iron and Steel ›› 2025, Vol. 60 ›› Issue (3) : 147-155. DOI: 10.13228/j.boyuan.issn0449-749x.20240520
Materials

Effect of Nb on high-temperature corrosion resistance of 304 austenitic stainless steel in molten salt

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Abstract

Concentrated solar power plants usually use a mixture of NaNO3 and KNO3 molten salt as the heat transfer fluid for the thermal storage system. The high-temperature corrosion resistance of 304 austenitic stainless steel used for the thermal storage system structure in the molten salt environment is particularly important. The influence of microalloying element Nb on the steel mainly lies in the refinement of grains, precipitation strengthening, and improvement of toughness. However, the mechanism of 304 austenitic stainless steel's high-temperature corrosion in molten salt under Nb is not clear, which is worth further research. Two groups of test steels containing 0.49Nb and 0Nb were subjected to a 0-200 h molten salt corrosion test at 565 ℃ under a constant temperature molten salt immersion method, and the corrosion mass loss was measured and the corrosion rate was calculated. The corrosion layer and base material organization were characterized using scanning electron microscopy (SEM), X-ray diffraction (XRD), and energy dispersive spectrometry (EDS). The research results show that when the corrosion time is 200 h, the corrosion rates of 0Nb and 0.49Nb are 178.1 μm/a and 133.9 μm/a, respectively, indicating that adding Nb element can improve the resistance to molten salt corrosion. Both samples form a double corrosion product layer on the surface, with the inner layer mainly being FeCr2O4 and the outer layer being Fe2O3 and Fe3O4. The internal stress generated by Fe oxides is easy to produce defects or cause the corrosion layer to peel off, leading to the intrusion of oxidizing ions in the molten salt and the deterioration of the protective properties of the Fe oxide layer. Nb has a low content in the corrosion layer and forms uniform and dispersed NbC in the steel matrix, inhibiting the precipitation of Cr carbide at grain boundaries, while the precipitation of Cr carbide at grain boundaries is prone to causing intergranular corrosion, and intergranular corrosion will introduce more stress into the corrosion layer. The corrosion layer of the steel sample without Nb is thicker and has more cracks, indicating that the addition of Nb element can improve the high-temperature corrosion resistance of 304 austenitic stainless steel in molten salt.

Key words

niobium / 304 austenitic stainless steel / molten salt corrosion / NbC precipitated phase / high-temperature corrosion / concentrated solar power / solar salt / intergranular corrosion

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ZHANG Kaixin, XIAO Guizhi, WANG Fan, et al. Effect of Nb on high-temperature corrosion resistance of 304 austenitic stainless steel in molten salt[J]. Iron and Steel, 2025, 60(3): 147-155 https://doi.org/10.13228/j.boyuan.issn0449-749x.20240520

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