Metallurgical Failure Analysis of Economizer Tube of Thermal Power Plant

In present investigation, failure investigation of
failed economizer tube of boiler of the thermal power plant
was analyzed through visual inspection, dimensional
measurement, bulk chemical analysis, mechanical properties evolution and metallurgical analysis. Failure of the
economizer tube was occurred in the form of cavity having
one longitudinal hairline crack passing through the cavity
on the external surface of economizer tube. Metallurgical
failure analysis of an economizer tube was carried out by
visual observation, chemical analysis, optical microscopy
(OM), scanning electron microscopy, SEM-EDS analysis,
x-ray diffraction (XRD) and mechanical properties evaluation. Ash deposit and surface corrosion were on outer
surface were observed. Severe corrosion and thinning were
observed on the external surface of economizer tube. SEMEDS analysis on the fractographic samples near failure
zone showed good amount of Cu deposition on the inner
surface of the tube and ash-related corrosion products on
the outer surface of the tube. Localized reduction/thinning
of wall thickness of the economizer tube happened due to
combined effect of ash corrosion-erosion from outside and
pitting/galvanic corrosion from inner surface due to Cu
deposition and finally resulted in failure of the tube.
However, outer corrosion on the tube is significantly
dominant due to ash corrosion. Therefore, ash corrosion is
major cause of failure of economizer tube. Ash corrosion
was due to use of poor-quality coal. To mitigate the
problem, removal of ash deposition on outer surface and
Cu deposition on inner surface of tube should be carried
out by periodically by proper cleaning and use of good
quality coal is suggested.