Plasma and Fusion Research: Regular Articles

Volume 11, 2405064 (2016)

Hydrogen Gas-Driven Permeation through F82H Steel Coatedwith Vacuum Plasma-Sprayed Tungsten∗)
Yue XU1) , Yoshi HIROOKA1,2) , Takuya NAGASAKA1,2) and Juro YAGI1,2)
1)

The Graduate University for Advanced Studies, 322-6 Oroshi, Toki 509-5292, Japan
2)
National Institute for Fusion Science, 322-6 Oroshi, Toki 509-5292, Japan(Received 30 November 2015 / Accepted 14 March 2016)

The subject of hydrogen isotopes transport through tungsten coated reduced activation ferritic steels suchas F82H has attracted increasing interest in the fusion engineering research community. This paper reports onlaboratory-scale studies that have been done to assess the hydrogen permeation properties of vacuum plasmasprayed tungsten (VPS-W) coatings at the temperature range of 200 - 500 ◦ C. W coatings with thicknesses of46 µm and 90 µm have been investigated. It has been found that the observed permeation rates through composite
VPS-W/F82H specimen are reduced to ∼7% compared to that of pure F82 H. VPS-W coating is porous and has
an open system of connected pores, which density is evaluated to be ∼7%. The main eﬀect of the W coatingon hydrogen permeation is to reduce the incoming flux at the W/F82H interface owing to pore diﬀusion in thecoating and to reduce the eﬀective surface area for hydrogen dissolution in the substrate.
c 2016 The Japan Society of Plasma Science and Nuclear Fusion Research

Keywords: gas-driven permeation, tungsten coating, reduced activation ferritic steel
DOI: 10.1585/pfr.11.2405064

1. Introduction
Reduced activation ferritic steels such as F82H are thecandidate first wall materials for DEMO reactors [1]. For
the blankets employing self-cooled breeder, the first wallis supposed to be subjected to bi-directional hydrogen isotopes permeation by two diﬀerent mechanisms: in one direction by edge plasma-driven permeation (PDP) and in
the other direction by bred tritium gas-driven permeation(GDP) [2]. In the previous work, hydrogen PDP and GDP
through pure F82H were studied [3, 4]. It was found thatfor a DEMO reactor, with a first wall area of several thousand square meters, the uncontrollable gas fueling rate isto be of the order of 1000 Torr liter/s (133.3 Pa m3 /s) [3],which is undesirable from the viewpoint of achieving highconfinement via plasma-wall boundary control. Therefore,
there is a need for surface coating as hydrogen permeation
barrier. W has been proposed as a candidate plasma-facing
material for advanced fusion devices because of its beneficial properties, such as high melting point, low sputtering
yield and low hydrogen retention [5]. Often, the easiest
way to produce a W armor is to deposit a W coating onsome feasible substrate. This avoids the problems of manufacturing plasma-facing components from bulk W, which
is diﬃcult to machine due to its hardness and brittleness.
Vacuum plasma spraying is an eﬃcient technique toget W coatings. As it has been investigated by Golubeva
et al. [6], that the mechanism of hydrogen permeation
through VPS-W coated graphite is molecular hydrogen gas

flow through the system of connected porosity. The purpose of this paper is to report on measurements and analyses that have been made to determine the hydrogen permeation properties of VPS-W. Such information is importantto evaluations of hydrogen isotopes retention and permeation safety issues for fusion devices that utilize VPS-W
components.

2. Experimental
Hydrogen GDP experiments are carried out using theVEHICLE-1 facility [7]. VPS-W coated F82H specimens
are prepared in the same dimensions as those commerciallyavailable conflat flanges with an outer diameter of 70 mm
and the permeation membrane surface area is ∼35 mm indiameter. Shown in Table 1 are the specimens with diﬀerent thicknesses of W coating and F82H substrate. The W
coatings are deposited at ∼600 ◦ C and blast treatment has
been carried out for the F82H steel surface before VPS process. The average size of W powder particles is ∼25 µm.
Figure 1 shows a scanning electron microscopy (SEM) image of VPS-W coating surface. It is shown that VPS-W
coating has an inhomogeneous microstructure, i.e., a mixTable 1 Specimen list.

author’s e-mail: xu.yue@LHD.nifs.ac.jp∗) This article is based on the presentation at the 25th International Toki
Conference (ITC25).

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Science and Nuclear Fusion Research

Plasma and Fusion Research: Regular Articles

Volume 11, 2405064 (2016)

Fig. 1 SEM image of VPS-W coating surface.

ture of disorganized areas composed of large unmelted W
particles, fine randomly melted W areas and void regions(or porosity) [8]. Void regions or pores are observed primarily next to the unmelted particles. The average density
of VPS-W coatings is ∼90% of bulk W.
The background pressures of GDP upstream anddownstream are ∼10−5 Pa and 10−6 Pa, respectively. Hydrogen GDP flux can be measured by a quadrupole mass
spectrometer on the downstream side. A resistive heater
is positioned beneath the membrane to control the sample
temperature, which is measured by a thermocouple. All thesamples have been in-situ degassed at ∼500 ◦ C for 12 hours
before experiments.

Fig. 2 (a) Curve fitting of the transient GDP flux and (b) time
lag from time-integrated flux Q(t). In this case, the thicknesses of W coating and F82H substrate are 0.09 mm and0.5 mm, respectively and the temperature is 420 ◦ C. The
upstream hydrogen pressure is 760 Pa. The lag time ismeasured to be ∼21 s.

3. Results and Discussion
The eﬀective diﬀusivity Deﬀ can be obtained by fitting
the transient permeation flux [9], as shown in Fig. 2 (a), orfrom the so-called time lag method tl = L2 /6D [10], asshown in Fig. 2 (b).
The hydrogen Deﬀ for VPS-W coated F82H has been
evaluated from 200 ◦ C to 500 ◦ C, as shown in Fig. 3. Literature data are shown for comparison. The evaluated hydrogen Deﬀ can be expressed as:−0.20 eVDeﬀ = 7.9 × 10−4 exp(1)
cm2 s−1 .
kTShown in Fig. 4 are the steady-state hydrogen permeation fluxes through VPS-W coated F82H at the temperature range of 200 - 500 ◦ C. A linear relationship between
the GDP fluxes and the square-root of upstream pressure
has been found, indicating that hydrogen GDP through theVPS-W coated F82H is diﬀusion-limited [13].
To investigate the eﬀective upstream surface area effects, hydrogen permeation in both directions has been carried out. Figure 5 shows the hydrogen permeation rates
through composite VPS-W/F82H specimen in both directions compared to that of pure F82 H. The upstream hydrogen pressure is set at 104 Pa. It shows that the GDP fluxes
with the W side on the upstream are reduced to ∼7% compared to that of pure F82 H. However, the GDP fluxes with
the F82H side on the upstream are comparable with that ofpure F82 H.

Fig. 3 Temperature dependence of hydrogen eﬀective diﬀusivity for VPS-W coated F82 H. The thicknesses of W coating and F82H substrate are 0.09 mm and 0.5 mm, respectively. Zhou’s [4], Serra’s [11] and Frauenfelder’s [12]data are shown for comparison.

Figure 6 shows the hydrogen permeation fluxes
through 1 mm F82H coated with VPS-W coatings of different thicknesses. It is shown that the permeation fluxesfor these two specimens are comparable with each other.
Usually, hydrogen permeation through materials is affected by both bulk and surface processes. In this study,
F82H steel surface is covered by a layer of VPS-W coating. As it has been mentioned before, the typical VPSW coating consists of partially melted W particles, melted
W and substantial surface-connected porosity [8]. Even a2405064-2

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Fig. 4 Steady-state hydrogen GDP fluxes through a VPS-W
coated F82H at the temperature range of 200 - 500 ◦ C.
The thicknesses of W coating and F82H substrate are0.09 mm and 0.5 mm, respectively.

Fig. 5 Steady-state hydrogen permeation fluxes through VPSW coated F82H (in both directions) and pure F82 H. The
upstream hydrogen pressure is set at 104 Pa. The thicknesses of W coating and F82H substrate are 0.09 mm and0.5 mm, respectively. A pure F82H specimen with thickness of 0.5 mm is used for comparison.

200 µm thick layer of VPS-W has an open system of connected pores, which connects the front and rear surfaces ofthe deposited layer [6]. The density of the prepared VPSW coating is ∼90% of bulk W, which means the porosityis ∼10%, including open (or connected) pores and closed
pores. Thus, the results obtained in this study can be explained by molecular hydrogen permeation through open
pores of VPS-W coating combined with permeation in a
dissociated form through F82H substrate. That means, theopen pores allow hydrogen gas reach to the W/F82H interface. Consequently, hydrogen dissolves at the interface
and diﬀuses into the F82H substrate to allow hydrogen permeation after a certain period of time [14].
It is well known that the solubility of hydrogen in Wis quite low compared to, for example, steels [15]. So onemight assume that the bulk of W is practically impermeable for hydrogen under the given conditions. As it isshown in Fig. 5 that the permeation rates with the F82H
side on the upstream are almost same with that of pure
F82 H, which could be attributed to the same upstream surface area. Because the permeation rate is limited by thediﬀusion process in the F82H substrate, one might assume
the hydrogen pressure at the W/F82H interface is ∼0, i.e.,hydrogen concentration at the interface is ∼0. As a result,hydrogen GDP fluxes are not aﬀected significantly by the90 µm-thick porous VPS-W coating. But, the permeation
rates with the W side on the upstream are ∼7% comparedto that of pure F82 H, which means the eﬀective surface
area of the F82H steel is reduced to ∼7% by a porous
W. In other words, the density of open pores is ∼7% and∼93% of steel surface is covered by W (including closedpores). The main eﬀect of the W coating on hydrogen permeation is to reduce the incoming flux at the W/F82H interface and the eﬀective area for hydrogen dissolution in
the substrate [16]. Therefore, the permeation rates throughcomposite VPS-W/F82H specimen are independent of theW coating thickness, which is illustrated in Fig. 6.
Although the square-root dependence of permeation rates against driving pressure (Fig. 4) suggests thediﬀusion-limited permeation under the given conditions,the eﬀective diﬀusivity shown in Fig. 3 is smaller than thevalues reported for F82H steel [4, 11]. These observations
indicate that the permeation in transient states could be affected by also other factors such as surface contamination
or trapping.

4. Conclusion
Fig. 6 Steady-state hydrogen permeation fluxes through 1 mm
F82H coated with VPS-W coatings of diﬀerent thicknesses (with the W side on the upstream).

Hydrogen GDP experiments have been conducted for
VPS-W coated F82 H. It has been found that the permeation rates with the W side on the upstream are reduced to∼7% compared to that of pure F82 H. However, the permeation rates with the F82H side on the upstream are comparable with that of pure F82 H. VPS-W coating has an open
system of connected pores and the density is evaluated tobe ∼7%. For a fusion reactor, based on these data one

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might predict that it is infeasible for VPS-W coatings to
be used as hydrogen isotopes permeation barrier from theviewpoint of bred tritium gas-driven permeation throughthe first wall. In the future work, hydrogen permeation
properties of physical vapour deposition W coatings will
be studied.

Acknowledgement

The authors would like to thank Mr. H. Bi and Dr. N.
Ashikawa for their assistance during experimental operation.
[1] H. Tanigawa, K. Shiba, H. Sakasegawa et al., Fusion Eng.
Des. 86, 2549 (2011).
[2] Y. Hirooka, H. Zhou, N. Ashikawa et al., Fusion Sci. Technol. 64, 345 (2013).
[3] Y. Hirooka and H. Zhou, Fusion Sci. Technol. 66, 63(2014).
[4] H. Zhou, Y. Hirooka, N. Ashikawa et al., J. Nucl. Mater.
455, 470 (2014).

[5] M. Rieth, J.L. Boutard, S.L. Dudarev et al., J. Nucl. Mater.
417, 463 (2011).
[6] A. Golubeva, A. Spitsyn, M. Mayer et al., J. Nucl. Mater.
415, S688 (2011).
[7] Y. Hirooka, H. Ohgaki, Y. Ohtsuka and M. Nishikawa, J.
Nucl. Mater. 337-339, 585 (2005).
[8] R. Anderl, R. Pawelko, M. Hankins et al., J. Nucl. Mater.
212-215, 1416 (1994).
[9] J. Crank, The Mathematics of Diﬀusion, 2nd Edition.
(Clarendon Press Oxford: London, 1975).
[10] M.A.V. Devanathan and Z. Stachurski, Proc. R. Soc. Lond.
A 270, 90 (1962).
[11] E. Serra and G. Benamati, Mater. Sci. Technol. 14, 573(1998).
[12] R. Frauenfelder, J. Vac. Sci. Technol. 6, 388 (1969).
[13] B.L. Doyle, J. Nucl. Mater. 111-112, 628 (1982).
[14] T. Otsuka, T. Tanabe and K. Tokunaga, J. Nucl. Mater. 438,
S1048 (2013).
[15] E. Serra, G. Benamati and O. Ogorodnikova, J. Nucl.
Mater. 255, 105 (1998).
[16] T. Otsuka, T. Tanabe and K. Tokunaga, Phys. Scr. T145,
014035 (2011).

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