Hot corrosion mechanism in transient liquid phase bonded HX superalloy: Effect of bonding time

This study investigates the hot corrosion behavior of transient liquid phase (TLP) bonding in Hastelloy X (HX) subjected to a molten salt environment of Na₂SO₄–V₂O₅ at 900°C, examining various bonding times of 5, 20, 80, 320, and 640 minutes. The samples were bonded at 1070°C, and their corrosion products along with microstructural features were examined. The microstructural analysis confirmed the presence of primary eutectic phases in the joints, including Ni-rich borides and silicides, Ni-Si eutectics, and several chromium-rich borides. Samples bonded for 20 and 80 min showed inferior hot corrosion resistance. Conversely, the sample that was bonded for 320 minutes exhibited improved resistance because of a more uniform distribution of alloy elements and lower boride concentrations at the interface. During the hot corrosion tests, initially, the TLP surface is covered by a dense Cr₂O₃ and NiO layer. After 20 h of hot corrosion, due to the reaction of oxide layers with vanadium, NaVO₃ forms, while sulfur diffusion leads to the evolution of internal sulfides based on Ni, Cr, and Mo. The presence of NaVO₃ and SO₃, along with the reduction of Cr₂O₃, significantly affects the hot corrosion resistance over prolonged exposure.