Failure Analysis of a Water Wall Boiler Tube for Power Generation in a District Heating System

Corrosion failure of materials often exposed to the corrosive environment in district heating systems is critical for operation and maintenance of the facility. It is important to establish a prevention method from an accurate prognosis on the cause of the corrosion failure. In this study, a failure analysis on the water wall boiler tube in a district heating system was carried out. The fracture occurred in an open window shape with a significant internal wall thinning that formed Na- and Fe-oxide by caustic corrosion. Tensile residual stress in the hoop direction developed inside the tube weakened the protective magnetite layer by accelerating the caustic corrosion, from which hydrogen atoms diffused into the alloy forming cavities or micro-voids at the grain boundaries. Furthermore, high temperature creep promoted the evolution of the cavities only inside the tube where there were tensile residual stresses. Consequently, a combination of hydrogen embrittlement and thermal creep initiated the cracks at the grain boundaries inside the tube, and the cracks propagated at the grain boundaries along the axial direction exhibiting an intergranular feature on the inside. Then, it gradually propagated toward the outside, finally exhibiting a transgranular fracture mode on the outside.