Page 403 - Handbook of Materials Failure Analysis

3 Stress-Corrosion Induced Failures 401

3.3 ROLE OF MICROSTRUCTURE ON SCC FAILURES
OF AUSTENITIC SS
Corrosion attack is usually attributed to the difference in the composition and micro-
structures of the weld and parent metals caused by the high temperature and cooling
rate; these in turn are being affected by the welding procedure used [42]. The welding
defects which cause corrosion include poor adhesion, slag inclusions, crevice forma-
tion, macro and microfissures, liquid embrittlement, galvanic coupling, etc. Any of
these defects can act as initiation points for corrosion attack on weld [26]. Under con-
ditions of high chloride concentration and stagnancy, pits are developed at the vicin-
ity of weld zone forming crevices which act subsequently as initiation centers for
corrosion. It was reported that autogeneous welding has a detrimental effect on pit-
ting resistance, as the pitting potential and critical pitting temperatures were lower
for welded than unwelded steel [43]. The role of nonmetallic inclusions and left-over
slag should be also emphasized. For deeper understanding of pitting corrosion, the
reader may be referred to Frankel [44].
Almost, all austenitic SS grades, especially types 304 and 316, are susceptible to
SCC. Other localized forms such as pitting and crevice corrosion probably precedes
SCC and may act as origins where the cracks can nucleate and then propagate into the
metal. These stresses may be originating from external sources (applied) or internal
sources (residual stresses). Nearly, all metals are susceptible to SCC in the presence
of tensile stresses in specific environments. There are three necessary conditions for
SCC to occur; one factor is susceptible microstructure, the second is stress conditions
either resulting from intrinsic or extrinsic sources and the third is the presence of a
corrodant environment. Complete lists of environments that may cause SCC in many
common metals are available in the literature [45].
In many austenitic SSs, the heat of welding causes sensitization (depletion of
chromium from the matrix due to the precipitation of chromium carbides along
the grain boundaries) [46]. Stress-corrosion cracks are reported to extend in the
chromium-depleted regions. This has been explained in view of Cr being the element
responsible for giving SS its corrosion resistance [45]. Other microstructural changes
associated with sensitization are sigma phase precipitation and grain growth. Sensi-
tization promotes intergranular corrosion [47].
SCC has been found to be related to microstructure. A study [48] showed that
SCC happens in almost all microstructures: austenitic, martensitic, and ferritic-
pearlitic, in discerningly order, where each microstructure produced a unique shape
for SCC. The initiation point was pitting type cracks for austenite, corroded pearlite
for ferrite-pearlite and corroded carbides for martensite structures. In all cases, prop-
agation was intergranular, associated with grain boundary thickening in austenite.
A recent approach has been handled in literature [49] where correlations are made
between intergranular stress-corrosion cracking (IGSCC) resistance and grain
boundary character and crystallographic texture for pipeline steels, however, no
work could be found related to SSs. These studies have linked SSC susceptibility
to crystallographic texture and grain boundary features resulting from the recrystal-
lization and thermomechanical effects during rolling of these steels.