EDX and XRD analysis of corrosion products showed the
presence of magnetite as steam-side corrosion product and
particularly for tube #2, sulfur and vanadium were present in
the oxide scale. Microstructure study revealed trans-crystalline
crack path for both tube #1 and 2 with decarburization process.
In addition, some micrograph showed needle-shaped crack.
Crack propagation was perpendicular to the metal surface
which suggest a thermal fatigue process. Thus, the presence of
corrosion process combined with thermal fatigue seems to be
the most plausible mechanism by which the failure occurred.