Abstract: X65/Inconel625 bimetallic composite pipes were fabricated using the overlay welding process. The microstructural evolution and compositional distribution characteristics across the various weld cladding layers were systematically investigated, and their influence on intergranular corrosion resistance in the ASTM G28 A method (ferric sulfate-sulfuric acid corrosion test) was elucidated. The results indicate that the Inconel 625 cladding layer exhibits a characteristic dendritic microstructure, with a significant amount of precipitates present in the interdendritic regions. The number of interdendritic precipitates increases closer to the fusion line. Two types of precipitates were identified within the cladding layer: small, granular precipitates identified as (Nb,Ti)C carbides, and larger, irregularly shaped blocky precipitates identified as either M?C phase or Laves phase.During the welding process, Fe diffuses from the X65 steel substrate into the cladding layer. This diffusion of Fe dilutes the concentration of other elements in the Inconel 625. The increase in Fe content not only leads to a decrease in the mass fractions of Ni and Cr, but also promotes the segregation of Nb and Mo in the interdendritic regions, indirectly increasing the number of precipitates. A Cr-depleted zone approximately 1 μm wide forms at the interface between these precipitates and the surrounding matrix. In the ferric sulfate-sulfuric acid corrosion test, corrosion initiates at the electrochemically active Cr-depleted zones surrounding the precipitates. At these sites, galvanic cells form, accelerating dissolution. The precipitates detach, forming corrosion pits. As corrosion progresses, these pits interconnect, leading to more severe intergranular corrosion. The corrosion rate shows a positive correlation with the Fe content. Fitting the data points yielded the relationship between corrosion rate (y) and Fe mass fraction (x): y = 0.8 + 0.0385x, where y is the corrosion rate (mm/a) and x is the mass fraction (%) of Fe.