Oil-Ash Corrosion Resistance of Dissimilar T22/T91 Welded Joint of Super Heater Tubes

The studies on the high temperature corrosion of the dissimilar metal weldment are necessary for longer service of the weldments in corrosive medium. This paper reports the performance of microstructurally different regions, namely heat-affected zone (HAZ), weld metal (WM), and base metal (BM) of dissimilar metal weldment of T22/T91 in the molten salt (Na₂SO₄-60%V₂O₅) environment under cyclic studies. The T22 HAZ, WM, and T91 HAZ were observed to oxidize at higher rates and develop more scale thickness than other regions in the weldment. Microstructures and elemental analysis indicate lesser availability of Cr in T22 HAZ and T91 HAZ due to formation of Cr-rich phases, which ultimately causes the difference in oxidation behavior of different regions. The presence of chromium carbides and intermetallics in un-oxidized T22 HAZ region and martensitic structure with the presence of delta ferrites in un-oxidized T91 HAZ region was observed to be the major cause behind the weak corrosion resistance of the respective HAZs. The higher oxidation rate of T22 HAZ may be attributed to the absence of protective scale of Cr₂O₃ and presence of Fe₃O₄ phases. Similarly higher oxidation rate of T91 HAZ region can be attributed to lesser availability of Cr due to the propensity of development of delta ferrite in martensitic structure.