Defects in boiler evaporator tubes—A detailed case study concerning defect initiation and extension processes

The present report attempts to describe a detailed failure analysis that was conducted on a series of full and partial through-thickness tube wall defects. Essentially two different defects were observed. One was a full through-thickness defect, the shape of which strongly suggests that it was driven by an active corrosion-dominated process. The second type of defect was partial through-thickness in nature and exhibited a common development route inasmuch as it extended through the evaporator tube wall by at least four discrete events or defect extension processes, viz.

1. (i) semi-elliptical pit formation, 0·3 mm deep;

2. (ii) thick porous-like magnetite growth (up to 400 μm thick) at the base of the pit;

3. (iii) heavy or active corrosion tunnelling, 1.3 mm in diameter; and

4. (iv) an environmentally assisted crack (EAC) fatigue crack propagation region which varied between 0.5 and 1.4 mm in size.

In some instances, a fifth defect extension event, which was corrosion dominated, was observed at the end of the fatigue-dominated event (iv). This event was similar to event (iii). Detailed fractography demonstrated that pit formation was the result of a mixed transgranular and intergranular corrosion process while the corrosion tunnelling was caused by a predominantly intergranular corrosion activity. The fatigue-dominated (event (iv)) fracture surface region contained a mixture of flat, cleavage-like facets which are indicative of an EAC growth process, and a fissured, ductile striated, failure mode which is commonplace in the case of pure, mechanically driven, fatigue processes. In the case of the full through-thickness defect, only an intense intergranular corrosion process was evidenced. The likelihood of these discrete events being caused by on-load or off-load process is discussed, while the presence of localised concentrations of sulphur and chlorine were detected on the corrosion-tunnelled and EAC fatigue growth surface regions.