J. Plasma Fusion Res. SERIES, Vol. 11 (2015)

Retention and release behaviors of plasma-loaded hydrogen in ferritic steels
M. HIGAKI*, T. OTSUKA and K. HASHIZUMEInterdisciplinary Graduate School of Engineering Sciences, Kyushu University, Fukuoka, Japan(Received: 24 September 2014 / Accepted: 19 January 2015)

In order to understand retention and release behaviors of plasma-loaded hydrogen in ferritic steels, hydrogen
depth profiles were examined by means of a tritium imaging plate technique (TIPT). Hydrogen release was strongly
affected by trapping effect in bulk and/or barrier effect of surface oxide layers during long term storage in air atroom temperature (RT) and even heating in vacuum. Helium (He)-DC glow discharge (DCGD) treatment at hightemperature played a role in removing surface oxide layers and made quick hydrogen release by normal diffusion.
Keywords: tritium imaging plate technique, ferritic steel, hydrogen, diffusion, He-DC glow discharge

1. Introduction

Ferritic/martensitic steels are candidate materials in aJapanese DEMO reactor [1]. Tritium removal from thesteels is one of safety issues in the reactor. Tritium removal
methods such as thermal heating and inert-gas DC glow
discharge (DCGD) have been proposed [2-4]. However,hydrogen release mechanism during these removal
processes is not fully clarified due to lack of information
on hydrogen behaviors in bulk of the steels. In the previouswork, hydrogen depth profiling method using a tritium
imaging plate technique (TIPT) has been proposed [5] toexamine hydrogen penetration and retention behavior in a
reduced activation ferritic/martensitic steel (F82H). In thepresent study, TIPT has been applied to investigate changesof hydrogen depth profiles after removal processes tounderstand release behavior of plasma-loaded hydrogenfrom the ferritic steels.

Fig. 1 Schematics of the experimental apparatus forhydrogen loading by DC glow discharge method andHe-DCGD method.
After the loading, three removal processes wereconducted to release plasma-loaded hydrogen from thesample. The first method was long term storage in air at
RT for 1 year. The second one was heating treatment invacuum at 473 K for 1 h. The third one was helium(He)-DCGD treatment at 473 K for 1h in which theexperimental apparatus was used as shown in Fig. 1.
After these removal processes, hydrogen concentration
profiles on the surface and in the bulk of the sample wereobserved by TIPT [6]. Surface profiling was conductedon the plasma-loaded surface side at RT for 1 h as
schematically shown in Fig. 2 (a). Subsequently, thesample was cut into two halves by a diamond wire saw to
examine a cross-section surface perpendicular to theplasma-loaded surface. The depth profiling wasconducted on the cross-section surface for 24 h as shownin Fig. 2 (b).
The resolution in the hydrogen depth profile was 50μm, corresponding to a minimum reading size of the TIP
scanning apparatus (BAS2500).

2. Experimental
The sample used here was a disc of a ferritic steel,SS430, with a diameter of 10 mm and a thickness of 3~6
mm. The compositions of SS430 were 16Cr, 0.4Mn,0.09Ni, 0.04C in wt-% and balanced Fe. The sample
surfaces were mirror finished by 0.06 μm colloidal silica
to remove working stains. Fig. 1 shows a schematic
experimental apparatus for hydrogen loading by DCGD
method [6, 7]. Hydrogen (H) including a trace amount oftritium (T, T/H = 1.3×10-4) was loaded to one edge
surface of the sample through a hole (I= 5 mm) in amask made of molybdenum. The operating gas pressureof DCGD was about 80 Pa and the applied DC voltage
was set to be 400 V.
The loading was conducted at 373 K for 2~3 h toallow hydrogen diffusion into the bulk. After the loading,the sample was rapidly cooled down to RT.
author’s e-mail: higaki@aees.kyushu-u.ac.jp

6©2015 by The Japan Society of Plasma
Science and Nuclear Fusion ResearchM. Higaki et al., Retention and Release Behaviors of Plasma-Loaded Hydrogen in Ferritic Steels

SampleHydrogen concentration (arb. unit)

120

Just after loading at 373 K for 2 h100

Long term storage in air at RT

80

numerical solution

60
40
20001

2 3 4 5Depth (mm)67Fig. 3 Hydrogen depth profiles loaded at 373 K for 2 hand long term storage in air at RT for 1 year.

Hydrogen concentration (arb. unit)

profiles after the loading at 373 K for 3 h and subsequentheating in vacuum at 473 K for 1 h. The figure indicatesthat hydrogen concentration at the surface was still highand most of hydrogen remained in the bulk to penetrate up
to 3 mm in the bulk.

Fig. 2 (a) Schematic of surface profiling and(b) depth profiling on the cross-section surface.

Sample30

Just after loading at 373 K for 3 h25

After heating in vacuumat 473 K for 1 h20

numerical solution

15105001

2 3 4 5Depth (mm)67Fig. 4 Hydrogen depth profiles after heating in vacuum at473 K for 1h

3. Results

Figure 3 shows the hydrogen depth profiles just afterthe loading at 373 K for 2 h and how the hydrogen depth
profile changed after long term storage in air at RT. In thefigure, one can clearly see that the hydrogen penetration
reached a maximum around 0.25 mm in depth anddecreased towards the loading surface and penetrated
towards a deeper region (2 mm). After isothermal storagein air at RT, the amount of hydrogen in the bulk wasreduced by release at RT, but some fraction of hydrogenremained and penetrated up to 3 mm in the bulk.
Figure 4 shows changes of the hydrogen depth

Figure 5 shows changes of the hydrogen depth
profiles after the loading at 373 K for 3 h and subsequent
He-DCGD treatment at 473 K for 1 h. The data for theHe-DCGD treatment are multiplied by 10. The hydrogenconcentration at the surface became almost zero and theamount of hydrogen in the bulk significantly decreased incomparison with that only by heating in vacuum (Fig. 4) inspite of the same treatment temperature.

7

Hydrogen concentration (arb. unit)

M. Higaki et al., Retention and Release Behaviors of Plasma-Loaded Hydrogen in Ferritic Steels

isothermal storage in air at RT was much lower than that
determined from the depth profiles just after the loading.
Although the microstructures are different between F82H
and SS430 except for the main DFe content, hydrogen
release from the ferritic steels would be much influenced
by the surface effect and/or the trapping effect due to low
driving force.
The value obtained from the profile changes during
thermal heating in vacuum was two orders of magnitude
smaller than that obtained from the profile changes during
the He-DCGD treatment. At high temperatures, SS430
would be easily oxidized even in vacuum due to higher
Chromium contents [11]. Comparing these profiles inFigs. 4 and 5, it was found that hydrogen release byannealing at higher temperature was strongly affected bytrapping effect in the bulk and/or barrier effect of surface
oxide layers during long term storage in air at RT and evenheating in vacuum [12]. The diffusion coefficient for
SS430 by the He-DCGD treatment was almost the same asthat by normal diffusion of hydrogen in F82H. Thisindicates that the He-DCGD treatment would play a majorrole in making quick hydrogen release by normal diffusion
by removal of surface oxide layers.

Sample30

Just after loading at 373 K for 3 h25

After He-DCGD treatment
at 473 K for 1 h (10)20

numerical solution (10)

1510
500

12 3 4 5
Depth (mm)67Fig. 5 Hydrogen depth profiles after He-DCGD treatment
at 473 K for 1 h.

4. Discussion

Fig. 6 summarizes the hydrogen diffusion
coefficients for F82H determined in our previous works
compared with the literature data [5, 6, 8, 9]. The
literature data were examined by means of the hydrogengas permeation method and the hydrogen release method.
These data in its Arrhenius plot were likely to reduce thediffusion coefficient as temperature is decreased bend
downwards than expected from the extrapolated values
from higher temperature data to lower temperatures below
473 K as shown Fig.6. The reasons of the bend, which
could be attributed to a surface effect and/or a trapping
effect in the bulk as known in pure Fe [10]. Our results
were in good agreement with extrapolated curve from thehigher temperature data of literature as shown in Fig.6.
This indicates that the surface effect and/or trapping effectis negligible on hydrogen diffusion even at lower
temperatures due to excess injection of energetic hydrogenby the DCGD resulting in higher driving force to saturatehydrogen trapping sites at the surface and even in the bulkof F82H such as lattice imperfections, dislocations andgrain boundaries. Hydrogen loaded by plasma exposure
penetrated into the bulk by normal interstitial diffusion.
In Figs. 3-5, assuming that changes of hydrogen
depth profiles were caused by hydrogen diffusion during
long term storage in air at RT, heating in vacuum and
He-DCGD treatment, the hydrogen concentration profiles
were numerically analyzed on a basis of the Fick’s
diffusion equation (hereinafter referred as a numericalsolution). The hydrogen diffusion coefficient for SS430
was determined as a fitting parameter. The best fit results
were shown in Figs. 3-5 as a broken line. The obtained
diffusion coefficients for SS430 are summarized in Fig. 6.
The value obtained from the profile changes during

2 -1

Hydrogen diffusion coefficient (m s )Temperature (K)573
10-810-9

10-10

10-11

10-1210-1310-14

10-15

373

473

323 298

Serra [8], Schliefer [9]previous work (F82H)
He-DCGD treatment

Heating in vacuum
Otsuka [6]Long term storage in air at RT

1.5

2

2.5

3-1

3.5

4

1000/T (K )Fig. 6 Comparison of hydrogen diffusion coefficients inferritic steels determined in the present study together
with literature data [5, 6, 8, 9].

5. Conclusions

Hydrogen depth profiles in the ferritic steels were
examined by TIPT to understand retention behavior ofplasma-loaded hydrogen and its release behavior during
long term storage in air at RT, heating in vacuum and8

M. Higaki et al., Retention and Release Behaviors of Plasma-Loaded Hydrogen in Ferritic Steels

He-DCGD treatment.
Hydrogen loaded by plasma exposure at higher
temperature penetrated into the bulk by normal interstitial
diffusion and remained for a long time by trapping in the
microstructures at RT. Such hydrogen was not easily
released by diffusion due to trapping effect in the bulk
and/or barrier effect of surface oxide layers during longterm storage in air at RT and even heating in vacuum.
He-DCGD treatment at higher temperature played a majorrole in removing surface oxide layers and made hydrogeneasy to release by normal diffusion.

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