Corrosion–fatigue coupled failure mechanism of a TP347 loose air duct in an external heat exchanger: EBSD-based microstructural insights

This study investigates the failure of a TP347 loose air duct in an FCC external heat exchanger operating under high-temperature, corrosive, and vibrational conditions. Cracks initiated at folded edges and propagated from the outer to the inner wall under combined corrosion–fatigue loading. SEM observations revealed predominantly transgranular cracking, while EDS detected O and S enrichment near the crack, confirming oxidation and sulphidation. Mechanical testing, metallography, hardness measurements, and composition analysis excluded intrinsic material defects and phase transformation as primary causes. EBSD analysis showed that 98% of grains in the crack region had Schmid factors of 0.4–0.5, indicating strong slip activation. KAM results further demonstrated severe strain localisation, with misorientation values above 0.8° accounting for 43.8% near the crack. GND analysis showed localised dislocation accumulation along the crack path, although 97% of the matrix retained a low dislocation density below 1.6 × 1014 m−2. The failure was therefore identified as corrosion-assisted transgranular fatigue driven by localised plastic deformation and dislocation pile-up. By integrating field evidence, fracture tracing, conventional materials characterisation, and EBSD-derived deformation metrics, this work provides a mechanistic explanation for service-induced corrosion–fatigue failure in loose air ducts of FCC external heat exchangers.