Phosphate induced stress corrosion cracking in a waterwall tube from a coal fired boiler

Abstract

The failure of a waterwall tube in a coal-fired boiler from a thermal power station was analyzed. Mild whitish deposit formation in the water-side of the tube with multiple fine cracks on the internal surface (water-side) of the tube was observed in the failure zone. This was associated with localized wall thinning at the failed section. Swelling with a visible thin-lipped crack at the fireside surface of the failure zone was also observed. The failure zone was studied using stereo and optical microscopy, micro-hardness measurements and scanning electron microscopy in the secondary electron imaging and back scattered electron imaging mode. Further, the deposit was analyzed by X-ray diffraction and chemical analysis and the composition at the fracture surface was determined by EDAX. It was observed that the failure occurred because of localized caustic corrosion in the internal surface of the water wall tube followed by inter-granular stress corrosion cracking. The corrosion products at the grain boundaries were found to be mainly iron and sodium phosphates.