Failure analysis of expansion joints in the inlet duct of induced draft fans in thermal power plants

This study investigates the failure of an expansion joint located at the inlet duct of an induced draft fan in a thermal power plant's #3 boiler. Through macroscopic observation, scanning electron microscopy (SEM), energy -dispersive X-ray spectroscopy (EDS), metallographic examination, and mechanical property testing, the failure mechanism was systematically analyzed. Results indicate that the leakage resulted from severe wall thinning and cracking due to dew point corrosion on the inner surface. Flue gases containing SO₂, NOx, and HCl condensed into acidic liquid when the wall temperature dropped below the dew point (140–150°C), leading to pitting and intergranular corrosion, accelerated by Cl⁻ ions. Additionally, dust particles in the flue gas caused erosive wear, further degrading the material. Under internal pressure and thermal stress, cracks initiated at corrosion pits and propagated, ultimately causing brittle fracture. The material, 06Cr19Ni10 (S30408), retained compliant mechanical and chemical properties, confirming that environmental factors dominated the failure. Recommendations include maintaining wall temperature above the dew point, using corrosion-resistant materials or coatings, optimizing flue gas treatment, and implementing regular thickness inspections.