E-Journal of Advanced Maintenance Vol.2 (2010) 65-76
Japan Society of Maintenology

Effects of Dissolved Hydrogen Content in PWR Primary Water on PWSCC
Initiation Property
Koji DOZAKI1,*, Daisuke AKUTAGAWA1, Nobuaki NAGATA2, Hideki TAKIGUCHI1 andKjell NORRING3
1

The Japan Atomic Power Company, 1-1 Kanda-Mitoshiro-Cho, Chiyoda-ku, Tokyo 101-0053, Japan
Tsuruga Nuclear Power Station, JAPC, 1 Myojin-Cho, Tsuruga-shi, Fukui 914-8555, Japan
3
Studsvik Nuclear AB, SE-611 82, Nyköping, Sweden
2

ABSTRACT
Environmental mitigation is being looked forward to as one of the promising preventive measures forbetter plant maintenance. Especially, it would be much more effective, when environmental measures are
introduced in combination with mechanical measures, such as stress improvement, material replacement, and soon.
Dissolved hydrogen (DH) concentration in the primary cooling system has been controlled at the level of
25~35 ml STP/kg H2O in all PWR plants in Japan, maintaining a sufficient margin to the utilities’ self-regulatedband 15~50 ml STP/kg H2O. The lower limit of this self-regulated band was determined to suppress reactor
coolant radiolysis based on experiments using a test reactor over 50 years ago, though the test was performed atmuch lower than typical operational temperature and is considered over-conservative at present.
On the other hand, primary water stress corrosion cracking (PWSCC) events have occurred on Dissimilar
Metal Welds (DMW) at vessel nozzles or at vessel head penetration in some PWR plants in Japan in recent
years. PWSCC growth test data have been showing that the current management band of DH content is aroundthe peak of crack growth rate (CGR) at about 340ºC, corresponding to the operating temperature of thepressurizer. To avoid the higher CGR levels, the DH content band should be shifted to the higher side, or to thelower side. The authors suggest much lower DH content than that at present as an alternative control, since someexperimental data on PWSCC initiation show monotonically decreasing dependency of initiation time withincreasing DH content.
In this paper, we show the effects of DH content on PWSCC initiation property. Reverse-u-bend (RUB)
tests and three-point-bend (3PB) tests were conducted for Alloy 600 base metal and Alloy 182, respectively, toexamine the dependency of the lower DH content than currently controlled band on PWSCC initiation time.
Test results showed the advantage of lowering DH content down to around 5 ml STP/kg H2O compared to15~25 ml STP/kg H2O. Analyses results of oxide and deposits on the surface of the test specimens were also
shown to try to clarify how hydrogen interacts with nickel-base alloys and influences the propensity to PWSCC.
Based on the analyses results of surface oxide and deposits, an outline of the interface region was drawn at low
and intermediate to high DH content.
Current status of the total studies on DH optimization and challenges left were summarized.
KEYWORDS

ARTICLE INFORMATION

Environmental Mitigation, Dissolved Hydrogen, PWSCC

Article history:
Received 12 May 2010
Accepted 18 August 2010

1.

Introduction

Environmental mitigation is considered to be one of the effective preventive measures against
stress corrosion cracking (SCC) in nuclear power plants (NPP). Although development of morecorrosion-resistant material and stress improvement such as peening are also effective preventive
measures for SCC, the effect of those mechanical preventive measures is limited to the position of thecomponents where those measures are applied. Environmental mitigation affects much broader
areas of the system to which it is applied than do mechanical measures. Moreover, the*Corresponding author, E-mail: kouji-douzaki@japc.co.jp

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

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K. Dozaki et al./ Effects of Dissolved Hydrogen Content in PWR Primary Water on
PWSCC Initiation Property

environmental mitigation costs much less than mechanical measures in most cases. We could reap agreat deal of merit if we were to introduce environmental mitigation in combination with themechanical preventive measures.
For example, hydrogen water chemistry (HWC), noble metalchemical addition (NMCA) and so on have been developed and applied to many BWR plants all over
the world, in combination with stress improvement like water jet peening.
Recently there have been many examples of PWSCC on dissimilar metal welds (DMW) of
Vessel Head Penetration and Vessel Nozzles. Repairs such as weld inlay, or spool piece replacement
have been applied to the damaged DMW, using Alloy 690 for repair/replacement as a
PWSCC-resistant material. Stress improvement such as ultrasonic shot peening (USP) and laser
stress improvement process (L-SIP) are applied. However, environmental mitigative measures for
PWSCC are under development.
Dissolved hydrogen (DH) concentration in PWR primary water has been controlled sufficiently
to suppress the reactor water radiolysis to prevent structural material from SCC due to the oxidizing
species generated by water radiolysis. The lower limit of current utilities’ self-regulated band of DH
content, 15ml STP/kgH2O, was established based on the results of Hart’s experiment in the 1950’s [1]from the viewpoint of suppressing the oxidizing species such as Hydrogen Peroxide (H2O2).
However, this experiment was carried out under the temperature condition of 60°C, which was much
lower than the actual operating temperature in a real plant. It is expected that re-combination ofoxidizing species with hydrogen becomes more dominant than the generation of oxidizing species athigher operating temperature resulting in a well-suppressed condition of oxidizing species even with asmaller amount of DH content than that necessary at 60°C. To examine the suppression effect ofwater radiolysis by DH in high temperatures occurring during operation quantitatively, a calculationof critical hydrogen concentration (CHC), which is defined as the minimum necessary DH content tosuppress the oxidizing species concentration below 50% increase compared to a saturated value
assuming infinite DH supply, was performed using a radiolysis computing model developed byStudsvik Nuclear AB. The calculation result supported the expectation that the current lower limit ofself-regulated band of DH content was too conservative [2]. To confirm the calculation results in areal plant, electrochemical corrosion potential (ECP) was measured in Tsuruga Unit 2, changing the
DH content within the range of the self-regulated band. The measurement results of ECP supported
the calculation results of water radiolysis [3].
Some reports say that the experimental data of PWSCC growth rate has a convex upward
dependency for DH content, that is, CGR peaks around a DH content position, which depends on
temperatures tested [4][5][6][7]. According to those reports, for example, the current management
band of DH content in Japan, 25~35ml STP/kg H2O, causes peak CGR at just around 340°C, which is
the operating temperature of the pressurizer. This CGR dependency on DH content suggests to us
the current management band of DH content is in the worst position from the viewpoint of PWSCC
propagation. Since CGR dependency on DH content has a peak, it is possible for us to select two
different orientations for improvement. One is moving to higher DH content, and the otherpossibility is moving to lower DH content.
On the other hand, it is reported that the experimental data of PWSCC initiation shows amonotonic decrease of initiation time for the increasing DH content [8][9][10]. From the utilities'
point of view, improvement of PWSCC initiation resistance yields more advantages than that ofpropagation resistance only. Because most of DMW has relatively higher weld residual stresses in
the circumferential direction than in the axial direction, the crack tends to grow in the axial direction
without stationary tendency and will penetrate sooner or later. Therefore, it is necessary for theutilities to perform successive inspections frequently. It would be better for us utilities to have aperspective on PWSCC initiation possibility in the operating period, so that we could prepare
preventive measures or repair/replacement activities in a more proactive manner. This is the major
reason why the authors consider an orientation for lower DH content more promising. Data anddiscussion on the effects of DH content on PWSCC initiation property are given in section 2 in detail.
Influences of lowering DH content on PWSCC growth, the structural integrity of the othermaterials and the other degradation modes such as IASCC or IGSCC of stainless steels, fuel integrity
and the source of radiation dose must be taken into account. Current status and challenges arebriefly summarized in section 3.

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K. Dozaki et al./ Effects of Dissolved Hydrogen Content in PWR Primary Water on
PWSCC Initiation Property

2.

Effects of DH Content on PWSCC Initiation Property

2.1 PWSCC Initiation Tests
To investigate the influence of DH content on PWSCC initiation property of nickel-based alloys,
long-term constant load tests in simulated PWR primary water have been carried out. Test
temperature was set at 320°C which is identical to reactor coolant outlet temperature in a PWR,
without acceleration in temperature of SCC susceptibility.
According to the previous studies, crack initiation time of Alloy 600MA increased significantly at
lower DH levels, from the results up to about 9,000 hours of exposure [9]. In this study, the latest test
results for over 25,000 hours of exposure were obtained [11]. The characterization of oxides anddeposits formed on test specimens tested in the 15ml STP/kgH2O condition, assumed to be close to the
Ni/NiO phase boundary, has been investigated to discuss possible mechanisms for PWSCC initiation.
The crack initiation time for Alloy 600 was evaluated using RUB specimens made from 3/4” and7/8” 600MA tube and from 3/4” 600TT tube. Surface finish condition of RUB specimens was assame as that of real tubes. The stress in the axial direction (σ1) of the 7/8” RUB specimen was about
700MPa, and the stress in the perpendicular direction (σ2) was about 530MPa. For the 3/4” RUB
specimen, σ1 and σ2 were about 570MPa and 440MPa, respectively.
The Alloy 182 has not been tested for crack initiation before, although it has been shown to besusceptible to PWSCC growth. Because of the difficulty in applying the RUB specimen as the weld
metal, the 3PB specimen was chosen for Alloy 182. Surface of 3PB specimens were ground andpolished, to remove the very thin layer due to EDM cutting, taking care not to introduce awork-hardened layer. 3PB specimens were loaded up to the yield stress at room temperature,according to the standards SS-ISO-7539-2 or ASTM G39. 1.25 and 1.5 times deflection of the firstcase were also loaded. Table 1 shows the number of each type of specimen in each autoclave in detail.
Three different DH content levels were selected to watch DH dependency on initiation time, that is,25, 15 and 5ml STP/kgH2O. As shown in Table 2, a separate autoclave was used for each conditionof DH content. 12 RUB and 24 3PB specimens were put in each autoclave. Chemical compositionof test materials and the schematic of RUB and 3PB specimens were shown in literature [11].
Table 1 Test Specimen Types and the Number of Specimens for Each Autoclave [11]Material of Test Specimen
Alloy 600MA

3/4"

Alloy 600TT

3/4"

RUB*
RUB

Alloy 600MA

7/8"

RUB

Alloy 182

Δ→(σ=Rp0.2)

Alloy 182
Alloy 182

1.25Δ
1.50Δ

Autoclave Autoclave
1
2
5 specimens
5

**

3PB
3PB
3PB

Autoclave
3
5

5

5

2

2

2

8

8

8

8
8

8
8

8
8

5

*RUB: Reverse-U-Bend**3PB: Three-Point-Bend

Table 2 Water Conditions of Each Autoclave Tested [11]Autoclave1
Dissolved Hydrogen (cm3STP/kgH2O)
25
Parameter

Autoclave215320

Temperature
Pressure(degree C)(MPa)

Conductivity
Boron(μS/cm)
(ppm)

1200

Lithium

(ppm)

2.2

15.2
21

67

Autoclave35

K. Dozaki et al./ Effects of Dissolved Hydrogen Content in PWR Primary Water on
PWSCC Initiation Property

Fig.1, 2 and 3 show the latest results of RUB and 3PB tests up to 32,600 hours of exposure with
DH content of 25, 15 and 5ml STP/kgH2O, respectively.
Each data plot shows the ratio of cracked specimens to the total at the exposure time at eachinspection chance.
DH:25cm3/kg

Cracked Specimens ( % )100

80600MA 3/4"
600MA 7/8"
Alloy182 ∆σ=Rp0.2
Alloy182 1.25∆
Alloy182 1.5∆

60

40

20

0
0

5000

10000

15000

20000

25000

30000

35000

Exposure Time ( h )Fig.1 Change in Cracked Specimens Ratio with Exposure Time (Autoclave-1, 25ml STP/kgH2O)
600MA 3/4"

100

Cracked Specimens ( % )DH:15cm3/kg

600MA 7/8"
Alloy182 1.5∆

80

60

40

20

0
0

5000

10000

15000

20000

25000

30000

Exposure Time ( h )Fig.2 Change in Cracked Specimens Ratio with Exposure Time (Autoclave-2, 15ml STP/kgH2O)

Cracked Specimens ( % )100600MA 3/4"
600MA 7/8"
Alloy182 ∆σ=Rp0.2
Alloy182 1.25∆
Alloy182 1.5∆

DH:5cm3/kg

80

60

40

20

0
0

5000

10000

15000

20000

25000

30000

Exposure Time ( h )Fig.3 Change in Cracked Specimens Ratio with Exposure Time (Autoclave-3, 5ml STP/kgH2O)

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K. Dozaki et al./ Effects of Dissolved Hydrogen Content in PWR Primary Water on
PWSCC Initiation Property

Fig.4 and Fig.5 show the data re-plot for each DH Content level of Alloy 600MA (Fig.4) andAlloy 182 (Fig.5). All specimens of Alloy 600MA have cracked but no specimens of Alloy 600TT
have cracked at all. Note that some of the tests are still on-going for Alloy 182, though the clear
difference of initiation property between 15~25 and 5 ml STP/kgH2O is manifest.
100

Cracked Specimens ( % )600MA 3/4"
80

60
DH=25 cm3STP/kgH2O

40

DH=15 cm3STP/kgH2O
DH=5 cm3STP/kgH2O

20

0
0

5000

10000

15000

20000

Exposure Time ( h )Fig.4 Change in Cracked Specimens Ratio for Each DH Content Level (Alloy 600MA)

Cracked Specimens ( % )10080Specimen :Alloy 182
loading level:High (1.5Δ)
60
DH=25 cm3STP/kgH2O

40

DH=15 cm3STP/kgH2O
DH=5 cm3STP/kgH2O

20

0
0

5000

10000

15000

20000

25000

30000

35000

Exposure Time ( h )Fig.5 Change in Cracked Specimens Ratio for Each DH Content Level (Alloy 182)

Fig.6 shows a curve fitting result to have the dependency of DH content on the initiation propertyof Alloy 600 based on the data plots in Fig.4 using the tanh-function. Fig.7 gives the similar one forAlloy 182 based on Fig.5.
It is shown by Fig.6 that the difference between the environments with 25 and 15ml STP/kgH2O
is small, less than 10%, in time to crack initiation of Alloy 600MA. In the environment with thelowest hydrogen content, 5ml STP/kgH2O, the initiation time for Alloy 600 MA is about 50% longer
than in the other two environments.
In Fig.7, the curve for the environment with the highest hydrogen content (25ml STP/kgH2O) has agood statistical basis (all eight specimens). For the other two environments (15 and 5ml STP/kgH2O
respectively) the curves are based on a few cracked specimens only. These few cracked specimens
make it possible to estimate the position of the curve time-wise. The shape of the curve is assumed tobe the same as for the curve related to the environment with the highest hydrogen level. For theenvironment with the lowest hydrogen content (5ml STP/kgH2O), the difference in crack initiation time
is about 50% compared to the other two environments, i.e. the same difference as for Alloy 600 MA.

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K. Dozaki et al./ Effects of Dissolved Hydrogen Content in PWR Primary Water on
PWSCC Initiation Property

Fig.6 Dependency of DH Content on the Initiation Property of Alloy 600 [11]Fig.7 Dependency of DH Content on the Initiation Property of Alloy 182

2.2 Analyses of oxide and deposits
Selected specimens exposed within this project have been analyzed to try to clarify howhydrogen interacts with nickel-base alloys and influences the propensity to PWSCC. To get a betterview of the surface appearance of different specimens, some were analyzed under FEG SEM (Field
Emission Gun Scanning Electron Microscope). The main objectives of these analyses were:・ To demonstrate differences in oxides and deposits formed in the three environments withdifferent hydrogen levels
・ To demonstrate differences in oxide and deposits formed on Alloy 600 and Alloy 182
Typical appearances of the surface oxide, formed in environments with three levels of DH
contents respectively, on Alloy 600 RUB-specimens and polished Alloy 182 3pb-specimens areshown in Fig.8.

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K. Dozaki et al./ Effects of Dissolved Hydrogen Content in PWR Primary Water on
PWSCC Initiation Property

Exposure time > 17 500 h.
A: crystallites
B: planar oxides
C: wiresD: external oxide
Fig.8 Typical surface oxides formed on Alloy 600 and Alloy 182 with three levels of DH content

A summary of the EDS analyses of the different types of oxides found on the surface of these
specimens is shown in Fig.9.
・ Crystallites – high Fe content. Probably of type Ni(1-x)Fe(2-y)Cr(x+y)O4.
・ Planar oxide – bridging between crystallites and wire-like oxide
・ Wire-like oxide – high Ni content
・ External oxide – wide scatter

Fig.9 Summary of EDS-analyses of oxides formed on Alloy 600 and Alloy 182

With regard to the protectiveness of the inner chromium-rich oxide layer, not only the thickness
but also the homogeneity and continuity are important issues. Fig.10 summarizes the differenceswith respect to all these properties of the oxides formed on Alloy 600 MA (8000~9000h of exposure),TT (18,000~22,000h of exposure) and polished weld metal 182 (22,500~30,000h of exposure) in theenvironments with 25 and 5ml STP/kgH2O. The inner Cr-rich oxide layer formed on Alloy 600 in
the environment with the lower hydrogen content is both homogeneous and continuous. The
corresponding layer formed at the higher hydrogen level contains a number of Ni-ferrite crystals;

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K. Dozaki et al./ Effects of Dissolved Hydrogen Content in PWR Primary Water on
PWSCC Initiation Property

some of them bridging the entire inner oxide layer.
layer appears not to be continuous in this case.

At least for the MA material, the inner oxideFig.10 Average thickness and homogeneity of inner Cr-rich oxide layer

It should be mentioned that no pronounced nickel enriched or chromium depleted zone wasobserved beneath the metal/oxide interface in any of these Alloy 600 samples, Fig.11 and 12. It is
possible that the nickel content was increased slightly. Oxygen was detected at grain boundaries
without any visible oxide precipitation at least down to 1 μm beneath the metal/oxide interface.
Fig.12 shows quantitative point analyses (EDS/TEM) through the base material and the inner oxide
layer on Alloy 600 MA exposed for 7,400 hours in an environment with 5ml STP/kgH2O.

Fig.11 Line-Scan Analyses on Alloy 600 MA exposed for 7,400h with 25ml STP/kgH2O DH Content

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K. Dozaki et al./ Effects of Dissolved Hydrogen Content in PWR Primary Water on
PWSCC Initiation Property

Fig.12 Quantitative Point Analyses (EDS/TEM) through Base Material and Inner Oxide
Layer on Alloy 600 MA Exposed for 7,400h with 5ml STP/kgH2O DH Content

EDS-line scans and point analyses are shown in Fig.13. The inner Cr-rich oxide layer has, inthis case, very low Ni-content16 (<5% Ni and 35~40% Cr). Grain or dendrite boundaries a few
hundred nanometers below the metal-oxide interface show some enrichment in Ni and depletion in Cr.

Fig.13 Line-Scan Analysis and Quantitative Point Analysis Made on Alloy 182 Exposed for 22,650h with
25 ml STP/kgH2O DH Content

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K. Dozaki et al./ Effects of Dissolved Hydrogen Content in PWR Primary Water on
PWSCC Initiation Property

In recent years much attention has been paid to the Ni/NiO-line i.e. the oxidation of nickel. In
the system illustrated in Figure 14 as a schematic summary of previous knowledge, the nickel
containing oxides and mixed oxides with considerable Ni-content are found in the non-protective
layers facing the aqueous environment. The only protective layer in this system is the inner
chromium-rich oxide layer. The composition and thickness of this layer are dependent on suchfactors as the alloy composition and the aqueous environment. Connected to this layer is also thesource of the chromium, which is the Cr-depleted layer just beneath the oxide/ metal interface. If thechromium oxide layer is discontinuous for some reason, the metal surface will be directly exposed tothe aqueous environment. Thus, the protectiveness of the chromium oxide layer is of crucial
importance to the influence of, among others, hydrogen on PWSCC. The importance of thechromium oxide layer is also supported by the fact that the Cr-content of the nickel-base alloy isknown to be of vital importance for the PWSCC resistance.

Fig.14 Outline of the interface region at lower (left) and intermediate to upper (right) DH levels

3.

Current Status of Studies and Challenges for DH Optimization

3.1 PWSCC Initiation and Growth Property
Experimental data of the effect of DH on PWSCC initiation showed that lowering DH content
has advantage to realize longer initiation time, but the test temperature was limited to around the hot
leg temperature, 320°C or higher, because of the need for acceleration in laboratories. From theutilities’ viewpoint, bottom mounted instrument (BMI) is the most important aspect in mitigating theenvironmental condition for PWSCC, because it is hard to repair or replace the penetrating part of
BMI through the reactor vessel. The temperature of BMI in operation is about 290°C. Since it has
not been clearly shown experimentally that PWSCC initiation is still DH content dependent at
temperature as low as 290°C, the PWR Owners Group (PWROG) in Japan, Central Research Instituteof Electric Power Industries (CRIEPI) and Mitsubishi Heavy Industries (MHI) are jointly conducting
PWSCC initiation tests at 290°C.
Currently, the disposition lines of PWSCC CGR versus Stress Intensity Factor K for Alloy 600
and Alloy 182 are given in the Code Cases of the Japan Society of Mechanical Engineers (JSME)
code for nuclear power generation facilities, Rules on Fitness-For-Service for nuclear power plants(JSME FFS Code). These disposition lines are based on the data obtained on the condition ofenvironmental parameters of primary water currently controlled. Since some reports say that the
PWSCC growth property has a convex upward dependency for DH content and the current control
band of DH content is around the peak at hot leg temperature, it is expected to revise the dispositionlines of PWSCCGR with new ones that are DH content dependent. Based on such new dispositionlines, FFS Code rules can be improved so that utilities may gain advantages as far as flaw evaluationis concerned, and possibly inspection intervals as the benefit of introducing DH optimization as one of
the environmental mitigation measures.

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K. Dozaki et al./ Effects of Dissolved Hydrogen Content in PWR Primary Water on
PWSCC Initiation Property

Flexibility and the feedback system in Code rules would be much more beneficial for the utilities,
and for all stakeholders including regulatory bodies and the general public in the end than a fixed ruleto have an incentive for improvement. Therefore, cumulative experimental data acquisition on DH
content effect on PWSCC initiation and CGR is needed at industries, research institutes, universities
and administrative bodies.
3.2 Other Possible Influences of DH Content Change
Possible influences, when DH content is changed, are given as follows,・ IASCC of stainless steel. Baffle former bolts (BFB) are managed to be inspected andreplaced as necessary considering IASCC at below the bolt head. When DH content ischanged, influence on IASCC behavior should be taken into account.
・ IGSCC of cold-worked stainless steel. Work-hardened stainless steel by, for example,
machining should be checked for its IGSCC property by changing DH content.
・ IGSCC of stainless steel in contact with stagnant water. When DH is changed and thestainless steel is significantly irradiated, the reduction of oxidant content may be influenced
and may change IGSCC properties.
・ Integrity of fuel cladding material and fuel assembly material. When changing DH
content, depending on the extent of the change, it is necessary to demonstrate the integrityof these materials. In-core tests using a test reactor is needed in such cases.
・ Influence on the source of radiation dose. Since lower DH content may change the stableform of nickel, attaching behavior of nickel to fuel cladding may be changed and result in achange of source of radiation dose.
PWROG in Japan, CRIEPI and MHI are examining the previous studies and extracting thechallenges left on those issues mentioned above.

4. Conclusion
Alloy 600 MA, Alloy 600 TT, and weld metal Alloy 182 specimens have been exposed for 2.5 to
4.5 years (17 500 to 32 600 h) in simulated primary PWR environments with 25, 15 and 5 ml STP/kg
H2O respectively. Inspections were performed to examine the ratio of cracked specimens to the total
with suitable intervals. The following results were obtained.
・ The difference in crack initiation time for Alloy 600MA RUB specimens in the twoenvironments with the highest hydrogen levels, 25 and 15 ml STP/kg H2O, is less than 10%.
In the environment with the lowest hydrogen content, 5 ml STP/kg H2O, the PWSCC
initiation time is about 50% longer than in the other two environments.
・ The behavior of Alloy 182 indicates that, as with Alloy 600 MA, the difference caused by areduction of hydrogen from 25 to 15 ml STP/kg H2O is small. For the environment with thelowest hydrogen content (5 ml STP/kg H2O), the crack initiation time is about 50% longer
than in the other two environments, i.e. the same difference as for Alloy 600 MA. Thus, theeffect of hydrogen on the initiation of PWSCC is the same for Alloy 600MA and weld metal
Alloy 182.
・ No cracks have been detected in the Alloy 600 TT RUB specimens.
・ The oxides visible on the surface of the specimens are made up of multilayered structure.
・ The inner, chromium-rich oxide layer formed on Alloy 600 and Alloy 182 at 5ml
STP/kgH2O is thicker than the corresponding oxide formed at 25ml STP/kgH2O.
・ The inner chromium-rich oxide layer formed on Alloy 600 at 5ml STP/kgH2O is bothhomogeneous and continuous, while the corresponding layer formed at 25ml STP/kgH2O
contains a number of nickel-ferrite crystals, some of them bridging the entire inner oxide
layer. This difference seems to be of great importance when considering the PWSCC
susceptibility.
Current status of studies and challenges for DH optimization were summarized. Further
investigation on this issue is encouraged not only for industries but also for research laboratories andadministrative bodies.

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PWSCC Initiation Property

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