Failure analysis of the water-wall tube in KIVCET waste heat boiler

The corrosion mechanism of the water-wall tube in KIVCET waste heat boiler was investigated. A short length of water-wall tube with a 'fish-mouth' crevasse was obtained from the boiler, and tested by visual inspection, microstructural examination, and SEM/EDS analysis. The results showed that high temperature sulfide corrosion on the fireside of the tube was attributed to the formation of H2S and free S; while on the waterside, inappropriate caustic treatment led to the caustic corrosion. Hydrogen attack observed on both sides of the failed tube should be responsible for the formation of cracking and decarburization. The root cause of the tube failure was the localized wall thinning on the fireside where high temperature corrosion occurred.