Microstructure and corrosion performance of X65/Inconel625 bimetallic composite pipes

X65/Inconel625 bimetallic composite pipes were fabricated by surfacing welding. The microstructural evolution and compositional distribution characteristics across the various weld cladding layers were systematically investigated. The influence on intergranular corrosion resistance of the bimetallic composite pipes in the ASTM G28 A method (ferric sulfate-sulfuric acid corrosion test) was elucidated. The results indicate that the Inconel 625 cladding layers exhibit the characteristic dendritic microstructure, with significant amount of precipitates present in the interdendritic regions. The number of interdendritic precipitates increases closer to the fusion line. Two types of precipitates are identified within the cladding layer: the small and granular precipitates identified as (Nb,Ti)C carbides, and the larger and irregularly shaped blocky precipitates identified as either M₆C phase or Laves phase. During the welding process, Fe diffuses from the X65 steel substrates into the cladding layers. This diffusion of Fe dilutes the concentration of other elements in the Inconel 625. The increase in Fe mass fraction not only leads to the decrease in the content 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, the corrosion initiates at the electrochemically active Cr-depleted zones surrounding the precipitates. At these sites, galvanic cells form, accelerating dissolution; the precipitates detach, forming the 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 as y=0.8+0.0385x, where y is the corrosion rate and x is the mass fraction of Fe.