Page 12 - Handbook of Materials Failure Analysis

4 CHAPTER 1 Progressive failures of components

In petroleum and chemical processing industry, gas transportation systems suffer
from CO 2 corrosion manifested by the formation of a protective scale consisting
2+ 2
mainly of FeCO 3 ,once Fe and CO 3 concentrations are above solubility limit [18]:
2 2+
CO 3 +Fe ! FeCO 3 (1.10)

2HCO 3 +Fe ! Fe HCO 3 Þ (1.11)
ð
2
ð
Fe HCO 3 Þ ! CO 2 +H 2 O + FeCO 3 (1.12)
2
FeCO 3 scale constitutes a diffusion barrier which impedes ion transfer and minimizes
steel surface exposure, hindering therefore the evolution of corrosion reactions.
The presence of hydrogen sulfide (H 2 S) in gas and oil industries induces the for-
mation of atomic hydrogen which diffused-in and trapped-in atomic scale defects,
dislocations and nonmetallic inclusion sites leading to hydrogen-induced cracking
(HIC) incidents in pipelines [19]. Although HIC-type failure is a subject of major
importance in relevant industries and numerous studies are focused on this topic,
it is out of the scope of this chapter.
A great variety of failure mechanisms is related to chemical process industries,
some of them are summarized in Table 1.1.

2 ANALYTICAL TECHNIQUES
Macroscopic observations of the failed components were performed with a high-
definition camera and a stereomicroscope. In order to reveal the main failure mech-
anisms, microscopic observations were carried out using an SFEG scanning electron
microscope (SEM) coupled with an energy dispersive X-ray spectrometry (EDS)
detector for elemental analysis of selected areas. Metallographic examination was
conducted using an inverted optical microscope. Prior to that, cross-sections of
the failed components were prepared using hot-mounting, wet grinding up to
1200 grit SiC paper and polishing with diamond and silica suspensions. Immersion
etching was performed to plain carbon steel sections using Nital 2% reagent. Hard-
ness testing was performed on polished sections using Vickers indentation technique
with a 10 kg applied load.

3 CASE STUDIES
3.1 CREEP FRACTURE OF A CARBON STEEL SUPERHEATER TUBE
INSTALLED IN A POWER-PLANT UNIT
3.1.1 Summary
The present case study is focused on the metallurgical investigation pertaining to the
high-temperature-induced failure of a pipeline after short-time service [20]. The
pipeline circuit was utilized as superheater in a lignite power-plant unit and consisted