Circumferential cracking of waterwall boiler tubes is an important problem facing modern electric utilities that operate coal-fired boilers. Analysis of the problem indicated that the crack mechanism had two components, thermal stress and corrosion. The corrosion component was studied and found to be localized sulfidation corrosion. The sulfidation mechanism occurs beneath a previously existing oxide layer and attacks the fireside surface grain boundaries. These corroded grain boundaries act as surface notches, which concentrate the thermal stresses at the grain boundaries, leading to crack initiation. The corrosion mechanism also contributes to circumferential crack growth by attacking the tube steel in front of the crack tip.
NACE International
1992
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