Interfacial microstructure evolution of P91&304 graded composite transition joint during thermal aging and its effect on corrosion behavior

The novel graded composite transition joint (GCTJ) between AISI 304 stainless steel and ASTM A335 P91 steel offers a promising strategy for dissimilar metal integration in high-temperature applications. However, the long-term thermal stability and corrosion resistance of its interfacial zone remain poorly understood. This study investigates the microstructural evolution of the interfacial zone under 700 °C and its influence on hot corrosion behavior. Thermal aging progressively develops three characteristic interfacial subzones: a coarsened carbides zone (CCZ), a lathy zone (LZ) and a Cr-depleted zone (CrDZ). This aging-induced heterogeneity results in distinct zone-dependent hot corrosion behavior at 700°C. The CrDZ shows limited intergranular corrosion associated with preferential grain-boundary attack, whereas the LZ and CCZ become susceptible to early-stage localized corrosion due to local chemical heterogeneity and their reduced capability to establish and sustain protective oxide scales. In long-term thermally aged specimens, preferential corrosion propagation along carbide/matrix regions is observed within the CCZ. The work provides mechanistic insight essential for predicting long-term durability and guiding the design of dissimilar metal joints in high-temperature service environments.