Considerations for metallographic observation of ... · INTERGRANULAR ATTACK IN ALLOY 600 STEAM GENERATOR TUBES DO HAENG HUR*, MYUNG SIK CHOI, DEOK HYUN LEE, and JUNG HO HAN Korea
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Technical Note
CONSIDERATIONS FOR METALLOGRAPHIC OBSERVATION OFINTERGRANULAR ATTACK IN ALLOY 600 STEAM GENERATORTUBES
DO HAENG HUR*, MYUNG SIK CHOI, DEOK HYUN LEE, and JUNG HO HAN
Korea Atomic Energy Research Institute, 150 Deokjin-dong, Yuseong-gu, Daejeon, 305-353, Republic of Korea
a r t i c l e i n f o
Article history:
Received 6 April 2015
Received in revised form
13 July 2015
Accepted 9 August 2015
Available online 19 October 2015
Keywords:
Crazing
Deformation
Intergranular attack
Steam generator tube
Stress corrosion crack
* Corresponding author.E-mail address: dhhur@kaeri.re.kr (D.H. H
This is an Open Access article distributecreativecommons.org/licenses/by-nc/3.0) whdium, provided the original work is properlyhttp://dx.doi.org/10.1016/j.net.2015.09.0031738-5733/Copyright © 2015, Published by El
a b s t r a c t
This technical note provides some considerations for the metallographic observation of
intergranular attack (IGA) in Alloy 600 steam generator tubes. The IGA region was crazed
along the grain boundaries through a deformation by an applied stress. The direction and
extent of the crazing depended on those of the applied stress. It was found that an IGA
defect can bemisevaluated as a stress corrosion crack. Therefore, special caution should be
taken during the destructive examination of the pulled-out tubes from operating steam
generators.
Copyright © 2015, Published by Elsevier Korea LLC on behalf of Korean Nuclear Society.
1. Introduction
Intergranular corrosion of nuclear steam generator tubes can
be divided into at least three forms: intergranular stress
corrosion cracking (IGSCC), intergranular attack (IGA), and
intergranular penetration (IGP) [1]. In the case of IGSCC, the
corrosion morphology consists of single or multiple major
cracks with minor to moderate amounts of branching. The
morphology of IGA is characterized by a relatively uniform
attack of numerous grain boundaries to a uniform depth over
ur).
d under the terms of theich permits unrestrictedcited.
sevier Korea LLC on beha
the surface of themetallicmaterials. This is because corrosion
is localized at and adjacent to grain boundaries with relatively
little corrosion of the grains. Finally, IGP can be described as a
mixture between the other two forms.
IGA has been one of the major corrosion degradation
modes in steam generator tubes. It has been observed
mainly on the outer diameter (OD) side of the tubes in the
sludge piles on top of the tubesheet or in the deposits
adjacent the tube support structures [2e5]. However, acci-
dental ingress of thiosulfate into the primary water led to
Creative Commons Attribution Non-Commercial License (http://non-commercial use, distribution, and reproduction in any me-
lf of Korean Nuclear Society.
Nu c l E n g T e c h n o l 4 7 ( 2 0 1 5 ) 9 3 4e9 3 8 935
extensive IGA on the inner diameter (ID) side of the sensi-
tized tubes [6].
We have found that steam generator tubes with IGA are
easily crazed along the grain boundaries when under plastic
deformation. Corroded grain boundaries would lose fracture
toughness and become brittle. Therefore, a region with IGA/
IGP defects is expected to be susceptible to cracking by an
external stress. This article provides the metallographic
characteristics of IGA in Alloy 600 steam generator tubes. In
addition, the effect of the applied stress on the morphology
change of the IGA region is discussed.
2. Materials and methods
The Alloy 600 tubing material used in this study was supplied
by a commercial vendor. The tubes were mill-annealed in a
temperature range of 1,024~1,070�C for 3 minutes and then
cooled down to 500�C within 7 minutes. The nominal OD of
the tube was 19.05 mm and the nominal wall thickness was
1.07 mm. The chemical composition is listed in Table 1. To
accelerate intergranular corrosion, the tubes were addition-
ally sensitized at 590�C for 10 hours in a vacuum furnace
under about 5 � 10�6 torr.
Samples were prepared by cutting the tube circum-
ferentially into 6-cm-long pieces. For manufacturing IGA on
the inner side of the tubes, one end of each tube specimenwas
plugged with a Teflon cap so that the solution inside the tube
did not leak out. Next, the tube specimen was filled with an
oxidized solution of 0.1M sodium tetrathionate (Na2S4O6).
Solutions containing sulfur oxyanions has been known to
accelerate the corrosion of nickel-based alloys and stainless
steels along the grain boundaries [7,8]. By contrast, to produce
IGA on the OD side of the tube, both ends of each tube spec-
imen were plugged with Teflon caps. Next, the tube speci-
mens were immersed in 0.1M Na2S4O6 solution. In this way,
IGA was grown on the ID or OD side of the tube at room
temperature for 5 days.
The IGA tubes were deformed by applying several types
of stress, such as hoop stress, three-axes stress, hard roll-
ing, and indentation. If necessary, the tubes with IGA were
cut into pieces of appropriate size. The subsequent
morphology changes of the IGA area were observed using
scanning electron microscopy. The detailed information
about how stress or deformation was applied to specimens
and where the morphology was observed in the specimen is
described in the results and discussion section. Because this
work is focused on the morphology change of the IGA
specimen by an applied stress, the magnitude of the applied
stress and the corresponding deformation extent are not
quantified.
Table 1 e Chemical composition of Ally 600 tube (wt %).
C Cr Fe Ni Si Mn Ti Al S
0.025 15.52 9.30 Bal. 0.19 0.21 0.29 0.22 <0.001
Al, aluminum; C, carbon; Cr, chromium; Fe, iron; Mn, manganese;
Ni, nickel, S, sulfur; Ti, titanium.
3. Results and Discussion
Fig. 1 shows themorphology change of the IGA tube surface by
an applied stress. No defects were observed on the ID surface
of the tube without any applied stress conditions, as shown in
Fig. 1A. However, some crazing occurred along the tube axial
direction when applying hoop stress by bending the specimen
along the circumferential direction of the tube, as shown in
Fig. 1B. The arrows indicate the same location before and after
deformation. They just look like axial stress corrosion cracks
(SCCs). Similarly, some crazing occurred along the circum-
ferential direction of the tube by applying tensile stress.
Therefore, they seem to be circumferential SCCs.When three-
axes stress was applied to the IGA tube specimen, the surface
was crazed into a radial crack-like morphology (Fig. 1C).
Finally, the numerous attacked grains were apparently
revealed through a distorted deformation (Fig. 1D). These re-
sults indicate that an IGA can be misunderstood as a SCC by a
directional deformation. Similar behaviors were also observed
on the tubes with IGA on the OD side. In the IGA region, the
attacked grain boundaries become brittle, although they are
extremely tight in nature. Therefore, the corroded grain
boundaries are easily opened through a deformation by an
externally applied stress.
Fig. 2A shows a circumferential cross section of the IGA
tube. There was no evidence of IGA on the as-polished metal-
lographic sample. However, when the same tube was
expanded outward by hard rolling, IGAwas clearly revealed by
a crazing of the IGA region, as shown in Fig. 2B. The scratch and
arrow indicate the same location before and after deformation.
Fig. 3A shows a circumferential cross-section of the IGA
tube. There is no doubt that the feature of the defect type is a
single SCC. In this case, it is reasonable to call this flaw a
primary water stress corrosion crack (PWSCC) because it was
initiated from the ID surface of the tube. However, when the
same area was forced by a Vickers hardness indenter (Mitu-
toyo, model HM-122, Japan) at a load of 1 kg, abundant crazing
along the grain boundaries occurred, as shown in Fig. 3B. The
white arrows indicate the same location before and after
indentation. Consequently, this result clearly indicates that
this defect is IGA, not PWSCC.
Fig. 4 shows the fracture surface of the laboratory-grown
IGA tube and PWSCC in a tube pulled from an operating
plant. The fracture surface of the IGA tube specimen showed
the same appearance as that of typical intergranular SCCs.
Therefore, the intergranular nature of the fracture surface
cannot be proof of SCCs. Among somemechanisms of PWSCC,
the internal oxidation mechanism is related to oxygen pene-
tration at the grain boundary, resulting in the formation of a
brittle intergranular oxide [9]. The model predicts a strong
dependence on the potential. When the potential is too low,
oxidation is not possible; when it is too high, a compact oxide
grows and prevents further oxygen diffusion and oxidation
[9]. However, intergranular crazing observed in this work de-
pends on the degree of intergranular corrosion before
deformation.
The degree of IGA depends on not only the depth andwidth
of the chromium depletion along the grain boundaries [10e12]
but also the corrosive environmental factors [13,14]. Therefore,
Fig. 1 eMorphology change of intergranular attack (IGA) region: (A) before deformation, after deformation (B) by hoop stress,
(C) by three-axes stress, and (D) by distortion.
Fig. 2 e Morphology change of intergranular attack region on the transverse cross section of the tube by a hard roll
expansion: (A) before expansion and (B) after expansion.
Nu c l E n g T e c h n o l 4 7 ( 2 0 1 5 ) 9 3 4e9 3 8936
grain drops are not always identified on the IGA surface.
Similarly, IGA can sometimes be observed on an as polished
cross-section [5], whereas in some cases it can be viewed only
using proper etching methods [15]. Therefore, special caution
should be taken to evaluate the degradation type of an IGA-
affected sample.
The results obtained in this work clearly show that the
presence of IGA can be revealed by an applied deformation to
the tube sample. A region with IGA can easily be crazed along
the grain boundaries by an externally applied stress. The
extent and direction of the crazing depend on the type and
direction of the applied stress to the corroded tube. Such a
stress could be applied to the tubes during the tube-pulling
process in a steam generator for destructive examination.
Depending on the direction and extent of the stress, the
invisible IGA can be apparently revealed, whereas the IGA can
also be misevaluated as an SCC. In addition, etching tech-
niques may fail to reveal the IGA defect. Therefore, it seems
that the best way to evaluate the IGA is to expand the tube by
internal pressurization. Thereby, some of the errors described
can effectively be eliminated. It is recommended that a sec-
tion of a pulled tube be hydraulically or pneumatically
Fig. 3 e Morphology change of intergranular attack region on the transverse cross section of tube by forcing with a Vickers
hardness indenter. (A) Before indentation and (B) after indentation.
Fig. 4 e Fracture surface. (A) Intergranular attack and (B) primary water stress corrosion crack.
Nu c l E n g T e c h n o l 4 7 ( 2 0 1 5 ) 9 3 4e9 3 8 937
expanded before sectioning to the axial direction for metal-
lographic examination.
4. Conclusions
It was found that an IGA tube was crazed along the grain
boundaries into various types and directions through a
deformation from an applied stress. The direction and extent
of the crazing depended on those of the applied stress. It was
clearly shown that an IGA could be misevaluated as an SCC.
Therefore, it is recommended that a section of pulled tube be
hydraulically or pneumatically expanded for an exact evalu-
ation of the IGA defect in steam generator tubes.
Conflicts of interest
All authors have no conflicts of interest to declare.
Acknowledgments
Thisworkwas supportedby theNational Research Foundation
of Korea (NRF) grant funded by the Korea government (MSIP).
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