Influence of Nb on the Hot Corrosion Behaviors of a Fe–Cr–Ni Heat-Resistant Steel Weld Metal in the Na 2 SO 4 –NaCl Molten Salt at 700 °C

A multiple gas tungsten arc welding was performed on the fabrication of the Fe–Cr–Ni-based weld metal. Microstructures and hot corrosion behaviors of the non-equilibrium weld metals with different Nb contents in 75 wt%Na 2 SO 4 –25 wt%NaCl molten salt at 700 °C have been investigated. A bilayer oxide film composed of an inner Cr 2 O 3 layer and an outer Fe 2 O 3 layer formed on the surface of the weld metals. The mass gain rate and the thickness of the surface oxide film were enlarged with Nb addition, which could be attributed to the rapid inward diffusion of O through the cracks resulting from the Nb-oxide. Oxidation and sulfidation occur in the matrix, resulting in a large internal oxidation zone (IOZ) in the weld metal. Nb seemly promoted the sulfidation reaction due to a short-circuit diffusion of S through the interface of the Nb(C, N) and matrix and made the internal corrosion products to be distributed along the interdendritic zone of the weld metal. An apparent Cr-dealloyed layer appears in the IOZ of the weld metal. Dealloying was the detrimental form of hot corrosion of Fe–Cr–Ni-based weld metal exposed to molten salts containing chloride salts. Alloying with 0.8 wt% of Nb further accelerated the dealloying and resulted in a profound attack. Graphical Abstract