E-Journal of Advanced Maintenance Vol.12 No.1 (2020) 14-25
Japan Society of Maintenology

The effect of Dissolved Hydrogen for PWSCC Initiation test
by Reverse U-Bend specimen of Alloy 600
Takaaki Kobayashi1,* and Wataru Sugino 2
1
2

Mitsubishi Heavy Industries, LTD, 1-1-1, Wadasaki-cho, Hyogo-ku, Kobe-city, JapanThe Japan Atomic Power Campany, 5-2-1, Ueno, Taitou-ku, Tokyo, Japan

ABSTRACT
In order to apply a more positive countermeasure against Primary Water Stress Corrosion Cracking (PWSCC)
of Alloy 600, an optimizing Dissolved Hydrogen (DH) concentration in primary water are expected in addition
to a replacing to Alloy 690 or a stress improvement by water jet peening. PWSCC initiation test data dependingon DH concentration are few. PWSCC initiation tests by Reverse U-Bend (RUB) specimens with the different
pre-strain setting had been performed. The PWSCC initiation test by 25% pre-strained RUB specimen also
showed the same dependency on DH concentration as PWSCC crack growth test by CT specimen in also thisstudy at 345°C as same as the previous study at 320°C by the authors. It’s considered due to the high stressapplied to the material surface and the crevasse of oxide film on 25% pre-strained RUB specimen. On the other
hand, the test by 10% pre-strained RUB specimen showed the different dependency on DH from the test by 25%
pre-strained RUB specimen. It’s considered that the test by lower pre-strained RUB shows the result based on
the protectable Cr enriched oxide film. The cold working rate to Alloy600 parts in the actual PWR is basicallyless than 10%. In order to evaluate the correct susceptibility of PWSCC initiation depended on DH
concentration for the actual PWR, it’s necessary to be performed PWSCC initiation test by a specimen applied alower pre-strain or stress even if it need a long-term SCC test.
KEYWORDS

ARTICLE INFORMATION

Dissolved Hydrogen, Stress Corrosion Cracking, Nickel based alloy, Alloy
600, Nickel Oxide, PWSCC, Crack Initiation time, Crack Propagation Rate

Article history:
Received 21 December 2019
Accepted 16 May 2020

1.

Introduction

The materials of Nickel based alloy are used for a steam generator and a reactor vessel as importantcomponents on pressurized water reactor (PWR). Primary Water Stress Corrosion Cracking (PWSCC)
of Ni based alloy such as alloy 600 had occurred frequently in the worldwide PWRs since 1990’s. It isvery important to maintain their susceptibility of the parts using Ni based alloy.
In general, the parts using alloy 600 replaced to alloy 690 which has a higher component ratio of Cr.
However, alloy 600 is still remained at the several parts which are difficult to upgrade to alloy 690
such as around the Bottom Mounted Instrumentation (BMI) tubes. They are also remained in restarted
PWRs which will be likely to continue the operation for a long time in the future. They are removed
the residual tensile stress on the surface by a water jet peening method. In order to apply a morepositive countermeasure against PWSCC, the techniques about environment factors are anticipated forexample: Optimizing Dissolved Hydrogen (DH) concentration in primary water in addition to thetechniques about a material factor or a stress factor [1]. Their multiple countermeasures afford a highreliability to the PWRs and must be the safer impression to public acceptance.
PWSCC crack initiation test data which show a dependency on DH are much fewer than crack
propagation rate test data. There is no fixed theory about a mechanism of PWSCC initiation depended
on DH concentration. The main purpose of this study is to have correct data about PWSCC initiationof alloy 600 depending on DH concentration. Their data will lead to be optimizing the primary water*

Corresponding author, E-mail: takaaki_kobayashi@mhi.co.jp

ISSN-1883-9894/10 © 2010 – JSM and the authors. All rights reserved.

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T. Kobayashi, et al./ The effect of Dissolved Hydrogenfor PWSCC Initiation test by Reverse U-Bend specimen of Alloy 600

chemistry of actual PWRs.

2.

Objective

PWSCC initiation test at 320℃ was performed in Japan (2007～2010) by the authors. The previous
test results showed the clearly different dependency on DH concentration from the test results by
Akutagawa et al. as Figure 1. PWSCC initiation time decreased with increasing of DH concentrationin the study by Akutagawa. The authors’ previous study showed that PWSCC initiation time became
longer with higher DH concentration. The range of PWSCC initiation time between their tests wasclearly different although the material of specimen, the test method, the specimen type and thetemperature conditions were almost same between their tests.
It was considered that the difference was by the tensile stress applied to the top of the RUB
specimen. In the authors’ study, the test material was processed into the RUB shape after 25%
pre-strain by a cold working was applied, and the tensile stress was approximately 9×102 MPa at the
top. On the other hand, In Akutagawa’s study, the test material was processed into the RUB shapewithout a cold working, and the tensile stress was approximately 5×102 MPa at the top. The object of
this study is to verify the following viewpoint and to consider the mechanism.

1 / crack detection time (1/h)

・ The difference of the pre-strain percentage which applied to the RUB specimens gives the clear
different dependency on DH concentration about PWSCC initiation time of 600 alloys.

1E-03

MA600 320℃ RUB : Previous study[2]MA600 320℃ RUB : Literature data[3]

1E-04

5

15

25

30

DH (ml/kg)
Figure 1 The comparison of PWSCC initiation test results between
the previous study[2] and the literature data[3]3. Experimental Procedures
3.1. Material and Specimen
Mill Annealed (MA) alloy 600 was used as PWSCC test material in this study. This MA alloy 600
was cut from the material which was used for steam generator heat transfer tubes of actual domestic
PWR power plants. Table1 shows the composition and heat treatment conditions of the MA 600 which
was used in this study.

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T. Kobayashi, et al./ The effect of Dissolved Hydrogenfor PWSCC Initiation test by Reverse U-Bend specimen of Alloy 600

One of the important purposes of this study is to have the reproduce, compare and verify with theprevious study. Therefore, PWSCC initiation test was performed with Reverse U-Bend (RUB)
specimens. The RUB specimens were prepared two kinds about the pre-strain percentage. One wasreverse U-bended after 25% pre-strained to the original material by cold working. The other one wasreverse U-bended after 10% pre-strained to it. The tightening force is reduced due to the relaxation
effect of the material particularly in the beginning term of exposure in high temperature water.
Therefore, the RUB specimen had the preventive mechanism by hard spring from its relaxation oftightened force. Figure 2 shows an appearance of the specimen. A strong tensile stress was applied tothe top of U-bend.

Table 1 Chemical composition and Heat treatment condition of the test material
Chemical composition (wt%)

MA 600

C

Si

Mn

P

S

Fe

Cr

Ni

Cu

Ladle

0.029

0.28

0.28

0.009

0.001

9.53

15.57

73.54

0.014

Check

0.030

0.29

0.28

0.010

0.001

9.62

15.60

73.36

0.015

Mechanical property (at 20℃)

Heat treatment condition
Mill annealing

975℃×2h → Air Cooling

Figure 2

Yield strength (σ0.2)
Tensile strength (σB)
Elongation

345 MPa
681 MPa
46 %Appearance of the Reverse U-Bend Specimen(with preventive mechanism from relaxation of tightened force)

3.2. Test conditions
16 Specimens; 8 specimens of the 25% pre-strained RUB and 8 specimens of the 10% pre-strained
RUB were loaded into the water circulative autoclave filled with PWR primary simulated water thecoordinated to 5, 15 or 30ml/kg DH concentration. 48 Specimens were prepared totally.
Test temperature was set at 345°C. It is the maximum temperature of PWR and effective to have a
result in relative shorter term.
The specimens were taken out from the autoclave at the exposure timing of 200, 400, 600, 900 and
1200 hours, and the specimens were observed whether SCC had occurred or not by an optical
microscope at each exposure timing. The specimens which any cracks were not detected returned tothe autoclave and kept on exposure again.
The water chemistry conditions except for DH concentration were as follows: Boron concentration
was 1800ppm, lithium concentration was 3.5ppm and Dissolved Oxygen concentration was less than
5ppb. Table 2 shows the PWSCC test conditions.

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T. Kobayashi, et al./ The effect of Dissolved Hydrogenfor PWSCC Initiation test by Reverse U-Bend specimen of Alloy 600

Table 2 PWSCC test conditions
Material of specimen

Mill Annealed Alloy 600

Specimen type for SCC test

RUB*
Pre-tensile strain before reverse U-bending
10, 25Pre-Strain Rate (%)
Number of specimens

Each 8 per 2 Strains×3 DH conditions ; Totally 48

Temperature (℃)

345

Simulatedor modified
primary waterchemistry

B (ppm)1800 as B

Li (ppm)

3.5 as Li

DH (ml/kg)

5, 15, 30

Exposure Time (h)200, 400, 600, 900, 1200

* with preventive mechanism from relaxation of tightened force

3.3. Observation and Analysis
Whether a SCC crack occurred or not was judged by an optical microscope. Some specimens wereobserved by Scanning Electron Microscope (SEM): HITACHI S-3400N and analyzed by X-ray
Photoelectron Spectroscopy (XPS): PHYSICAL ELECTRONICS QUANTUM 2000. The setting of
XPS analysis were as follows:
X-ray spot size; 100μm<Ar sputtering> Acceleration Voltage; 2kV, Sputtering Area; 2mm×2mm,
Sputtering Rate; 3.25nm/min (for SiO2)

4. Result
Table 3 shows the PWSCC Initiation test results. The numerator in the table means the number of
specimens in which PWSCC cracks were confirmed. The denominator means the number of test
pieces for each test condition. The reason of decrease in the denominator was the removal of
specimens for the observations in detail by SEM and the analysis of oxide film by XPS.
The sensitivity of PWSCC initiation depending on DH concentration in this study were as follows.
[ 25% Pre-strained RUB ]DH: 15ml/kg => 30ml/kg >> 5ml/kg[ 10% Pre-strained RUB ]DH: 30ml/kg > 15ml/kg > 5ml/kg

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T. Kobayashi, et al./ The effect of Dissolved Hydrogenfor PWSCC Initiation test by Reverse U-Bend specimen of Alloy 600

Table 3Temp.
(℃)

SCC detected time on RUB specimens for MA Alloy 600
in simulated or modified PWR primary water.

Pre-Strain
(%)

25
345
10

DH
(ml/kg)200

Exposure Time (h)
400
600
900

1200

5

0/8

0/8

0/7*

3/7

3/7

15

0/8

3/8

8/8

-

-

30

0/8

2/8

7/7*

-

-

5

0/8

0/8

0/7*

0/7

0/7

15

0/8

0/8

0/7*

0/7

2/7

30

0/8

0/8

0/7*

2/7

5/7

Example: "3/8" means "SCC was detected in 3 specimens per 8 tested specimens": Nothing cracked,: Some specimen cracked,: All specimen cracked* The reason of decrease in denominator was the removal of specimens for XPS analysis etc.

One of the cracked specimens was observed the fracture surface. Figure 3 shows the observation of
the fracture surface. The fracture surface had an aspect of a typical inter-granular SCC.
Figure 4 shows the SEM images of oxide films formed on the top of RUB specimens. All SEM
images were taken of the surfaces of specimens for which SCC cracks could not be determined by
optical microscopy. On 25% pre-strained RUB specimens, the crevasses of the oxide film weredetected about them in each 15 and 30ml/kg DH condition. The crevasses of the oxide film were not afew. On the other hand, the crevasse of the oxide film was not detected about it in 5ml/kg DH
condition. On 10% pre-strained RUB specimens, the crevasses of the oxide film were not detected
about them in all DH conditions.
Figure 5 shows the depth profiles by XPS analysis to the oxide films of the specimens which were
picked out from the autoclave at 400 hours. For example, a blue solid line shows an atomic
concentration of Cr as Cr oxide, a blue broken line shows it as Cr metal. In each profile, there were no
significant differences about the thickness of the oxide films such as the increasing shoulders of eachatomic concentration as metal were indicated when their sputtering times were around 50 minutes. On
the other hand, there were differences in their atomic concentrations of Cr as Cr oxide for example.
The enriched Cr oxide layer was existed at each depth positions where each profile of Cr atomic
concentration as Cr oxide shows the peak. The peak values of each Cr atomic concentration as Croxide on each XPS profile were plotted in figure 6. The Cr concentrations as Cr oxide in the oxide
films of 30ml/kg DH condition were clearly lower than the others. The absolute values by 25%
pre-strained RUB were lower than by 10% pre-strained RUB, However, the overall tendencies are
similar between the data by 25% pre-strained RUB and the data by 10% pre-strained RUB.

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T. Kobayashi, et al./ The effect of Dissolved Hydrogenfor PWSCC Initiation test by Reverse U-Bend specimen of Alloy 600

RUB Top Surface→①
25% Pre-Strain, 15ml/kg DH, 400h Exposure①②②
Figure 3

SEM images of the fracture surface for a cracked specimen

Figure 4

400h

400h

400h

1200h

1200h

1200h

SEM images of oxide films on material surfaces(No SCC for all based on decision by optical microscope)

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T. Kobayashi, et al./ The effect of Dissolved Hydrogenfor PWSCC Initiation test by Reverse U-Bend specimen of Alloy 600

Sputtering Time (min.)Atomic Concentration (%)

Sputtering Time (min.)Sputtering Time (min.)Sputtering Time (min.)

Depth profiles by XPS analysis of oxide films from surface to the base metal (400h)

Atomic Concentration of Cr as Cr Oxide (%)

Figure 5

Sputtering Time (min.)Atomic Concentration (%)

Atomic Concentration (%)

10% Pre-Strained RUB

Sputtering Time (min.)DH: 30 ml/kg

Atomic Concentration (%)

Atomic Concentration (%)

DH: 15 ml/kg

Atomic Concentration (%)

25% Pre-Strained RUB

DH: 5 ml/kg

16
14
12
10
8

10% Pre Strain
25% Pre Strain

6
4
2
0

5

15

30

DH (ml/Kg)

Figure 6

The peak values of Cr atomic concentration as Cr oxide in the oxide films by XPS

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T. Kobayashi, et al./ The effect of Dissolved Hydrogenfor PWSCC Initiation test by Reverse U-Bend specimen of Alloy 600

5. Discussion
The dependencies of PWSCC initiation on each DH concentration were deferent between the resultsby 25% pre-strained RUB specimen and them by 10% pre-strained RUB specimen.
The dependencies of DH concentration by 25% pre-strained RUB specimen showed the peek of the
PWSCC sensitivity at a point at chemical equilibrium Ni/NiO as evaluated in the next section. And it
by 10% pre-strained RUB specimen didn’t follow a chemical equilibrium Ni/NiO.
5.1 PWSCC test by 25% pre-strained RUB
In the previous study at 320°C of the authors, it was reported that the dependency on DH
concentration of PWSCC initiation test by 25% pre-strained RUB specimen matched to it of PWSCC
crack growth test by Compact Tension (CT) specimen.
PWSCC initiation tests data depending on DH concentration are few. On the other hand, PWSCC
crack growth rate tests data depended on DH concentration are a lot in the world. The mechanism of adependency on DH concentration is evaluated by a chemical equilibrium Ni/NiO in DH concentration
and at temperature [4]. Andresen et al. showed that PWSCC crack growth rate depending on DH
concentration was capable to be expressed as the function based on a lot of test data and theoreticalvalues. According to that, DH concentration based on a chemical equilibrium Ni/NiO at eachtemperature was written by the following equation (1).
Ni/NiO Phase Boundary = 10(0.011×T -2.59）ml/kg(1)[5]

T：Temperature(℃)

Moreover, PWSCC crack growth rate was capable to draw an upward convex curve with thisNi/NiO chemical equilibrium point as the apex. The curve was written by the following equation (2).
Vp = p×exp(-0.5×(Ni/NiO Phase Boundary)/(λ・0.46)1/p)2+1(2)[5]

Vp : Crack Propagation rate at the peak (mm/s)
λ: Width depended on any materials: input 35.6 on the case of MA600
p : Hight depended on any materials: input 1 for the relative comparison

Figure 7 shows the curves based on the function of PWSCC crack growth rate at 320°C and 345°C.
As a result, the relative superiority or inferiority of the PWSCC crack growth rate at each temperatureand each DH concentration condition could be compared.

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T. Kobayashi, et al./ The effect of Dissolved Hydrogenfor PWSCC Initiation test by Reverse U-Bend specimen of Alloy 600
9.2

The number on the curvemeans DH concentration
at the peak.

Arbitrary SCC crack growth rate

320℃

17.4

345℃

1

10

5

15

30

50

DH (ml/kg)

Figure 7

The simulated curves of arbitrary SCC crack growth rate depending on DH concentrationand temperature for MA Alloy 600 based on a chemical boundary of Ni/NiO

The data shown in Figure1 on the previous study by 25% pre-strained RUB matched to the curve at
320°C on Figure 7. The data shown on this study by 25% pre-strained RUB also matched to the curveat 345°C on Figure 7. The mechanism about that is considered as follows.
A tensile stress at a top of RUB specimen is capable to be set proportionally with pre-strain to abase material. A top of 25% pre-strained RUB specimen had a high tensile stress slightly lower than atensile strength of a cold worked alloy 600. It’s reported that a pre-crack tip of the CT specimen
applied 30 MPa・m1/2 which is in general as a stress intensity factor for a crack growth rate test has
also a high tensile stress slightly lower than a tensile strength of the base material [6].
An oxide film is less ductile than a base metal basically. There is a possibility that crevasses of onan oxide film may be occurred by a high tensile stress on the surface at a lower density area of anoxide film. It is well known that Cr7C3 continuously precipitates at grain boundaries of alloy MA600,
and a shortage of Cr concentration is occurred around Cr carbides [7,8]. Since a Ni based alloy hasmuch more Ni component unlike a stainless steel, any Ni oxides supplementary grow on grainboundaries even if Cr oxides are insufficient. However, Ni component is hard to form to Ni oxide in awater condition at chemical boundary of Ni/NiO or the higher DH concentration. Both Cr oxide andNi oxide are insufficient on grain boundaries, and only low density oxide film is formed. An oxide
film such like this is probably easy to have crevasses at a high stressed surface. The crevasses of anoxide film in a water condition at chemical boundary of Ni/NiO were also detected in the other study
by the author. Figure 8 shows the example of the crevasses of an oxide film in the other study. The
crevasse was observed above Cr oxides based on Cr7C3 which continuously precipitates at grainboundaries of alloy MA600.
In addition, a material surface under a clevis is contact with test water directly. As a result, both agrain boundary at under a crevasse of oxide film on a surface of 25% pre-strained RUB specimen anda pre-crack tip of a CT specimen are in a similar situation. Therefore, It’s considered that a SCC test
method by a 25% pre-strained RUB specimen apply almost same as it by a CT specimen. Ni/NiO
equilibrium points are defined by electro potential and pH in the water. The facts that the SCC test
results follow the Ni/NiO equilibrium theory further support the assumption; the material surface

22

T. Kobayashi, et al./ The effect of Dissolved Hydrogenfor PWSCC Initiation test by Reverse U-Bend specimen of Alloy 600

under the oxide film crevasses directly contacts with the test water.
SEM

O

C

Ni

Fe

10μm
Cr

345°C, 15ml/kg DH, approx. 1200h
360°C, 30ml/kg DH, approx. 2000h

Figure 8

An example of crevasse of an oxide film in the other study[9](3 pointed bending specimen of MA Alloy 600 in the same heat treatment, No SCC for each it based on decision by optical microscope )5.2 PWSCC test by 10% pre-strained RUB
The PWSCC test of the reference [3] was operated by RUB without pre-strain. The dependency of
DH concentration on the test result by 10% pre-strain RUB in this study was similar to it of thereference [3]. Since the tensile stress on material surfaces of their specimens are not high enough tocause crevasses of their oxide films, their PWSCC sensitivity are determined not by Ni/NiO
equilibrium points, but by protective capabilities of oxide films for the material on a model in which
SCC occurs on a material surface uniformly covered with an oxide film.
It is well known that a Cr oxide layer on a material containing Cr component contributes to preventa material surface from a corrosion or SCC. It is also known that a Cr oxide layer is formed at a deeparea from a surface of an oxide film and close to a base material [10,11,12]. In general, an oxide film is
initially formed on a material surface in contact with test water according to pH-potential diagram.
During a growth (depositing) process of oxides, main constituent elements ; Fe, Ni and Cr are
migrating through an oxide film according to the solubility and diffusion rate between a base metal
and test water. As a result, Cr was concentrated in a region close to a base metal, and an inner oxide
layer is formed. Combrade, Scott, et al. reported that Cr2O3 is formed in an additional inner oxide
layer below FeCr2O4 [12], and these layers Cr2O3 and FeCr2O4 are contribute to suppress a progress ofan inter-granular oxidation which is a first step before an initiation of SCC.
In figure 6, the enriched maximum Cr concentration of the inner oxide layer formed in 30ml/kg DH
test water were clearly lower than the others. The PWSCC test results by 10% pre-strained RUB in
this study and by no pre-strained RUB in the reference [3] was that their specimens in higher DH test
water were faster cracked. It’s considered that the protective capability of the oxide film formed inhigher DH test water is lower the others.
DH concentration affects an electro potential in test water, and the electro potential affects theamount of Fe oxide etc.. Fe oxide is also contribute to a protectivity for a surface of a material. It will
be expected to evaluate in more detail in the future.

6. Summery and Conclusion
In order to apply a more positive countermeasure against PWSCC of Alloy 600 which will be usedin the future, the techniques about an environment factors such as optimizing DH concentration inprimary water are expected in addition to the techniques about a material factor or a stress factor.
PWSCC initiation test data depending on DH concentration are important and few. In this study,23

T. Kobayashi, et al./ The effect of Dissolved Hydrogenfor PWSCC Initiation test by Reverse U-Bend specimen of Alloy 600

PWSCC initiation test by RUB specimens with the different pre-strain setting had been performed.
In the previous study at 320°C, it was reported that the dependency on DH concentration of
PWSCC initiation test by 25% pre-strained RUB specimen matched to it of PWSCC crack growth test
by CT specimen. In this study at 345°C, PWSCC initiation test by 25% pre-strained RUB specimen
also showed the same dependency on DH concentration as PWSCC crack growth test by CT specimen.
The reason was considered as follows.
・ Both the top surface of 25% pre-strained RUB specimen and the pre-crack tip of CT specimen
have a high tensile stress slightly lower than a tensile strength of the base material.
・ Cr oxides are not capable to grow adequately due to precipitate Cr carbides on grainboundaries. And more, Ni oxides are also insufficient in the water condition at chemicalboundary Ni/NiO ; near 17.4 ml/kg DH at 345°C. The ductile oxide film such like this is easy
to have crevasses above grain boundaries.
・ A part of grain boundaries with crevasses of oxide film on the material surface is not covered
with an enough protective oxide layer and is in a similar situation to be directly contacted withtest water.
・ As the result of these situations, 25% pre-strained RUB specimen shows the same dependency
on DH concentration as PWSCC crack growth test by CT specimen.
On the other hand, PWSCC initiation test by 10% pre-strained RUB specimen showed the different
dependency on DH concentration from PWSCC crack growth test by CT specimen and PWSCC
initiation test by 25% pre-strained RUB specimen. It’s considered that PWSCC initiation test by thelower pre-strained RUB shows the result based on the protectable ability of the oxide film. For
example, PWSCC initiation time become probably longer in DH concentration which forms the much
more Cr enriched oxide layer (film).
The degree of a machining or a cold working to Ni based Alloy parts in the actual PWR is not high.
It is basically less than 10% at all parts of the actual PWR if it converts to the pre-strain level applied
to the specimen in this study. It’s considered that PWSCC initiation test by the lower pre-strained
RUB such as below 10% is capable to reflect the process in which a crack is gradually initiated on thematerial surface without any peculiar points. The test by the lower pre-strained RUB is possible toreflect correctly whether the protective oxide layer is good or not.
In order to evaluate the correct susceptibility of PWSCC initiation depended on DH concentration
for the actual PWR, it’s necessary to be performed PWSCC initiation test by a specimen applied alower pre-strain or stress even if it need a long-term SCC test.

24

T. Kobayashi, et al./ The effect of Dissolved Hydrogenfor PWSCC Initiation test by Reverse U-Bend specimen of Alloy 600

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