Study of the hot corrosion behavior of S30432 and FCN austenitic steels in mixed molten salts

The hot corrosion behavior of Super304H (S30432) steel and a new type of Fe-Cr-Ni (FCN) steel optimized for melting the composition of S30432 steel in mixed molten salts of 15% KCl+15% K₂SO₄+70% Na₂SO₄ (wt. %) has been investigated at two temperatures, 650 ℃ and 700 ℃. The samples were completely corroded in the mixed salt melt, and their weights were measured intermittently to obtain hot corrosion kinetic curves. X-ray diffraction, scanning electron microscopy (SEM), and energy-dispersive X-ray spectrometry (EDS) were used to analyze the physical phase composition, morphology, and composition of the corrosion products. At 650 ℃, both S30432 and FCN steels increase, with S30432 steel losing approximately twice as much weight as FCN steel. At 700 ℃, S30432 steel experienced severe corrosion, resulting in spalling of corrosion products. FCN steel exhibited a ‘weight loss-weight gain-weight loss’ trend, with a lower degree of corrosion compared to S30432 steel. In contrast to FCN steel, S30432 steel showed serious intergranular corrosion. The corrosion products of S30432 steel mainly consisted of Fe₃O₄, Fe₂O₃, and FeCr₂O₄, while those of FCN steel included Fe₃O₄, Fe₂O₃, FeCr₂O₄, Cr₂O₃, and a small amount of FeS. In the mixed molten salt, FCN steel exhibits greater corrosion resistance than S30432 steel. The corrosion product layer is susceptible to erosion by composite sulfate and other corrosive media, leading to erosion of both the corrosion layer and substrate. The sulfur partial pressure required for the predominant sulfurization reaction to occur was calculated from a thermodynamic perspective. Internal oxidation and internal sulfidation may occur, and Cr₂O₃ can easily dissolve in chloride salts, generating the volatile compound CrO₂Cl₂ that accelerates corrosion.