J. Plasma Fusion Res. SERIES, Vol. 9 (2010)

Erosion-corrosion of RAFM JLF-1 steel in Lithium flow induced by impeller
M. Kondo1, V. Tsisar2, T. Muroga1, T. Nagasaka1, O. Yeliseyeva2
12

Fusion Engineering Research Center, National Institute for Fusion Science, Toki, Japan
Physical-Mechanical Institute of National Academy of Sciences of Ukraine, L`viv, Ukraine(Received: 26 October 2009 / Accepted: 9 March 2010)

The purpose of the present study is to investigate erosion-corrosion characteristics of reduced activation ferritic
martensitic steel, JLF-1 (Fe-9Cr-2W) in flowing Li. The erosion-corrosion characteristics were investigated by means ofcorrosion tests with rectangular plate type specimen of JLF-1 in Li stirring pot, where the Li flow was induced by impellerin the pot. The test temperature was 500ÛC and 600ÛC, and there was no temperature difference in the Li stirring pot. The
erosion-corrosion characteristics were evaluated by the weight loss measurement of the specimens and metallurgicalanalysis for the corroded surface. The results were compared with those obtained from the tests at static condition tofeature the effect of Li flow on the corrosion. The weight loss of the specimens tested in Li flow was larger than that tested
in the static tests. The corroded surface showed the local corrosion along lath boundary and grain boundary of JLF-1 as thesame as that after the test at static condition. There were defects which were not observed in the specimen tested at thestatic condition. The defects were the trace of the erosion-corrosion. The corrosion attack to the boundaries made weakbonding between the subgrains, and the subgrains were peeled from the surface by the Li flow.
Keywords: Corrosion, Erosion, Erosion-corrosion, Lithium, Blanket, RAFM

1. Introduction

The erosion-corrosion in flowing lead-bismuth(Pb-Bi) at high flow velocity of 2m/s was reported in ref.
[8]. The corroded surface was mechanically eroded by thehigh density fluid flow. However, the data oferosion-corrosion by liquid Li was limited.
The stirred pot type corrosion test apparatus wasdeveloped to investigate the effect of mass transfer andshear stress by the Li flow on the corrosion. The Li flow
was induced by the circulating impeller. There was notemperature difference in the Li. The corrosion tests by thestirred pot were performed and the corrosion
characteristics were compared with that obtained from thetest at static condition. The purpose of the present study is
to investigate the erosion-corrosion characteristics in the Liflow induced by impeller.

Blanket concepts with liquid metal lithium (Li)
breeder/coolant provide attractive options for high tritium
breeding ratio, high efficiency and simplicity of thesystem [1]. Fe-Cr-W based RAFM (Reduced Activation
Ferritic/Martensitic) steels are widely regarded aspromising structural materials of the blanket system,
because of its low activation properties, radiation
resistance and industrial maturity. JLF-1 (Japanese Low
activation Ferritic steel 1, Fe-9Cr-2W) has excellent
resistance against neutron irradiation in both
microstructural and mechanical properties [2], and is apotential candidate for the structural material of blanket.
The corrosion characteristics of JLF-1 in Li havebeen investigated by authors. The authors found that the
JLF-1 showed phase transformation from martensite toferrite due to the carbon depletion from the steel surface
in Li [3]. It was found that the phase transformation wascaused by the low carbon potential in Li [4]. The carbide
in the grain boundary, block boundary and lath boundary
was dissolved by the Li. These boundaries wereselectively corroded. Cr in the steel matrix was also
depleted by the Li with nitrogen via the formation of
unstable nitride of Li-N-Cr [5].
The corrosion test at flowing condition is necessary toinvestigate the effect of mass transfer on the corrosion [6].
The corrosion test by thermal convection Li loop has been
performed to investigate the mass transfer due to thetemperature difference in the system. In the convection Li
loop, the flow velocity was small, i.e. 5cm/s, and the effectof the shear stress by the flow on the corrosion is small [7].

2. Experimental condition
2.1 Experimental apparatus

The static test was performed using the test
apparatus shown in Fig. 1. The detail was described in theprevious reports of ref. [3, 4 and 9]
The stirred pot (Fig. 2) was designed to investigate
the effect of flow conditions on the corrosion without
temperature gradient. The higher flow velocity than thatin thermal convection loop can be given in the test. The
pot was assembled in glove box as the same as the test in
Li pot. The specimens were fixed using the specimen
holder as the same as the simple immersion. The crucible
made of 9Cr steel was used in place of Mo crucible,
though the Mo was more corrosion resistant than the
Fe-9Cr in the Li. The Mo crucible was not used toremove the effect of carbon trapping on the corrosion andsome precipitation of Mo dissolved in flowing Li on theauthor’s e-mail: kondo.masatoshi@nifs.ac.jp294©2010 by The Japan Society of Plasma
Science and Nuclear Fusion Research

M. Kondo et al., Erosion-Corrosion of RAFM JLF-1 Steel in Lithium Flow Induced by Impeller

weight loss measurement for specimens [4, 6 and 9].
The flow of Li in the pot was induced byrotating impeller. The baffle plate for decrease therotating flow was not placed in the pot, and the pot was
so-called unbaffled stirred pot [10].The rotating flow withsmall upward and down ward flow of Li was made by therotating impeller. Then, the Li was mixed well and theconcentration of impurity between upper part and lower
one became the same. The flow velocity was roughly
estimated as inertial Li flow as the flow velocity became
the same to the speed of impeller. The velocity is
obtained by the following equation of

VLi

ʌdn

After the corrosion test, specimens were taken out fromLi, and the adhered Li was removed in water.
Pot(Made of
SUS316L)
HeaterSpecimen holder(SUS410)
Li Specimens
I.D. 48mm

Crucible(9Cr steel)Heat
insulation

Pot (316)

Motor for rotate
impeller(1)

HeaterMagnetcoupling
Impeller(Mo)Ar

Li

Thermo couple

Specimenholder(SUS410)
Crucible(9Crsteel)Specimens
36mm 16mm

Und 2
PSpecimens

Thermo couple

Fig. 1 Static test pot

where VLi is flow velocity on the specimen surface in unitof m/s, d is width of impeller in unit of m and n is
rotating speed in unit of s-1. The width of impeller d was
0.033m and the rotating speed was 100rpm, whichcorresponded to 1.66s-1 in the present work. Then, theflow velocity on the specimen surface was estimated as
0.17m/s. Reynolds number for mixing in the pot wasestimated by

Re mixing

Covergas: Ar

33mm
48mm

Fig. 2 Stirring Li test pot
The nitrogen content in Li was analyzed before
and after the corrosion tests by ammonia extraction
titration method to check the increase of the nitrogencontent. The analysis results indicated no change ofnitrogen content in Li by the test. The weight change ofthe specimens before and after the test was measured
using electro reading balance with the accuracy of 0.1mg.
The corroded surface was analyzed by using
scanning electron microscope and energy dispersive x-ray
spectroscopy (SEM/EDX). The initial surface of thespecimen was analyzed by SEM/EDX after the electro
etching at 30V in the pool of 13%HClO4- 13%
Ethyleneglycol monobutyl ether – 10% ethanol- 64%
acetic acid. The other specimen was analyzed by
SEM/EDX without etching procedure after the test.

(2)

where ȡ is the density of Li in unit of kg/m3 and μ is
viscosity of Li in unit of Pa s. The data for thermal
properties of liquid Li were obtained in ref. [11].
Reynolds number was 2538 at 500ÛC and 2859 at 600ÛC,this indicates that the flow in the pot was possibly
turbulent flow. The flowing condition was determined
due to the investigation on the erosion-corrosion
characteristics of JLF-1 steels in flowing Li with higher
shear stress on the surface at turbulent flow conditions.

2.2 Experimental condition

Chemical component of JLF-1 (JOYO-HEAT)
is 8.93Cr- 0.49Ni- 1.96W- 0.64Mn- 0.21V- 0.01C0.015N- Fe balanced. The size of specimen was 10mm x
15mm x 2mm. The specimens were heat-treated at
1050ºC /3.6ks/air cooled (normalizing) and 780ºC /3.6
ks/air cooled (tempering). The specimens were polished
to remove the native oxide layer and then cleaned in theacetone and ethanol. The surface was mechanicallypolished
using
SiC
papersgritorder:
180ĺ400/P800ĺ800/P1200 followed by finishing withalumina with particle size 0.3μm and 0.05μm.
The experimental conditions are presented inTable 1. The ratio of the volume of Li to specimen
surface area was almost the same, though the volume of
Li and the specimen number were different in each test.

3. Results and Discussion
3.1 Weight loss of the specimens

The results of weight loss measurement are
shown in Fig.3. In the static tests, the weight loss waslarger at higher temperature. This indicated the corrosion
was larger at higher temperature. In the tests at 500°C, theloss and scattering was larger at the flowing condition thanthat at static condition at 500°C. The scattering of theweight loss data was large not only after the corrosion test
but flowing test at 600°C.
The larger weight loss by the Li flow was due to(1) accelerated dissolution of steel alloying elements bymass transfer in Li flow and (2) effect of mechanical shear
stress on the surface by the flow.

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M. Kondo et al., Erosion-Corrosion of RAFM JLF-1 Steel in Lithium Flow Induced by Impeller

Table 1 Corrosion test conditions

Weight loss per unit area (g/m2)

Test atstatic condition
Test instirred condition
0.9
0.8
0.7
0.6
0.5
0.4
0.3
0.2
0.1
0

Temp.
(°C)

Time
(hour)

Volumeof Li (cc)500
600 [4]
500
600

283
250
250
250

80
92
65
65

Average
0.382g/m2

Flow velocity
on specimen
surface (m/s)
0
0
0.17
0.17

Nitrogencontent
in Li (wppm)
65
65
65
65

Specimennumber

Li volume/surface area [m]

3
10
3
3

0.008
0.007
0.008
0.008

surface before and after the corrosion tests, and Table 2
presents the chemical components of surface analyzed by
EDS. The specimen surfaces before the corrosion test
(Fig.4 (a)) showed precipitated carbides on the surface,
and the carbides outlined the boundaries. The boundaries
were grain boundary, block boundary and lath boundary.
In the static test at 500°C (Fig.4 (b)), the carbide was
not observed though the surface showed martensite
structure. There was gap along the boundaries. This gas
might be caused by the dissolution of carbides Li. Afterthe static tests at 600°C, the clear boundary of ferrite wasobserved due to the phase transformation in Li by carbon
depletion as reported in ref. [3, 4] (Fig.4 (c)). The
precipitated carbide was dissolved by Li and the Li
penetrated into the grain boundary.

Average0.424 g/m2

Average
0.202g/m2
Average
0.581 g/m2
Static test Test in Static test Test in
at 500°C stirring at 600°C stirring
Li at 500°
Li at 600°C

Fig. 3 Weight loss of specimen by corrosion in Li at staticand stirring condition

3.2 Observation of corroded surface

Fig. 4 shows the SEM image of specimen
5μm

5μm

5μm

gap
(a)

Initial surface (afteretching)

10μm

Defect (all the surface)(d) After stirring test at 500㰛C(low mag.)(b) After static test at 500㰛C(c) After static test at 600㰛C

5μm

5μm

Subgrain

Defect(e) After stirring test at 500㰛C

Subgrain(f) After stirring test at 500㰛C(high mag.)
5μm

5μm
Resin

10μm

Ref. [8]subgrain(g) After stirring test
at 500㰛C (low mag.)

Rough surface(h) After stirring test at 600㰛C(i) Surface cross section afterstirring test at 600 㰛C

Fig.4 Corroded surface (a) initial surface after etching, (b) after static test at 500°C, (c) after static test at 600°C(d) after stirring test at 500°C, (e) defect after stirring test at 500°C, (f) after stirring test at 500°C(high magnification), (g)after stirring test at 600°C (low mag.), (h) rough surface after stirring test at 600°C and (i) surface cross section afterstirring test at 600°C (electro etching)296

M. Kondo et al., Erosion-Corrosion of RAFM JLF-1 Steel in Lithium Flow Induced by Impeller

Table 2 Chemical composition of surface analyzed by
EDX after corrosion test (wt%)
Temperature
Initial steel
500
600

Condition
StaticStirring
StaticStirring

Cr and W from the surface by the corrosion. The result of
surface cross sectional analysis after electro etchingprocedure was shown in Fig. 4 (i). The loss of the carbides
on the surface as reported in [3,4 and 9] was not observed.

Chemical composition
Cr
W
Fe
8.9
2.0
8.8
2.0
Balance
7.2
1.1
7.9
2.0
6.4
1.1

3.3 Erosion-corrosion mechanism

The mechanism of erosion-corrosion in stirring
Li is summarized in Fig. 5 based on the experimental
results. The Li attacked to the boundary of subgrains,though the Li was not detected because the penetrated Li
was removed in the cleaning procedure of the specimens.
In the same time, the carbides were dissolved by the Li.
By the penetration of the Li to the boundary, the bonding
between the subgrains became weak. The shear stressand/or normal stress, which worked on the steel surface,peeled off the subgrains. The defects shown in Figs. 4(d-f) were the trace of the erosion-corrosion in Li. The
rough surface shown in Figs.4 (g, h) is caused by theerosion-corrosion, which was more severe than that at
500°C.
The corrosion in static Li may promote thecoarsening of the subgrains. Fig. 4(c) showed larger sizeof the grains. On the contrary, the coarsening was notobserved after the test at flowing condition. The
subgrains were peeled off by the Li flow, and then, thesubgrains were not able to be coarsened.

The surface after the test in stirring Li at 500°C
showed small grains and defects (Figs.4 (d) to (f)), whichwere not detected in the static test (Fig. 4 (b)). The size ofsmall grains was less than 1μm, and these small grains
might be martensite lath subgrains [12]. The size of thedefects was 1-5μm. The defects were possible due to thedepletion of subgrains in the stirring Li. The EDX analysis
showed the depletion of W, which was not caused in static
Li. Phase transformation from martensite to ferrite was notobserved after the test in static and flowing Li at 600°C.
Large number of the subgrains was observed on thespecimen surface after the test in stirring Li at 600°C. The
defects, which were observed after the test in stirring Li at
500°C, were not clearly detected. The phase transformation
was not detected after the test in flowing Li at 600°C. The
EDX analysis for the surface indicated that the depletion ofSurfaceLathmartensite
subgrain

Prior austenite
grain boundaryBlockboundary

Packetboundary
Penetration toboundaries
Cross section

Peel off of
sub grainsCr
depletion
Carbondepletion [4]Li flow

Li flow(1)

Li flow(2)

Corrosion

Corrosion-erosion(3)

Static corrosion athigh temperature (2)’Coarsening of laths
and/or ferrite grain

5μmRef. [4]Fig. 5 Mechanism of erosion-corrosion in stirring Li and coarsening of subgrain in static corrosion

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M. Kondo et al., Erosion-Corrosion of RAFM JLF-1 Steel in Lithium Flow Induced by Impeller

3.4 Mass transfer

The coefficient of mass transfer wasdetermined by Sherwood number and Schmidt number as

The corrosion of steels in the stirring Li wascaused by mass transfer. The corrosion was caused by the
Cr depletion as mentioned in previous chapter. Therefore,
the mass transfer of Cr was discussed here. The
concentration difference of Cr in the boundary layer on
the steel surface was assumed to be the model of the masstransfer. Then, the mass flux from the steel to Li, J in unitof g/m2s in the corrosion is expressed asJ

h D,s ȡ(Cs  C)

Sh
Sc

dC
dt

(3)

JS

(4)

where V is the volume of Li in unit of m3 and S is total
surface area of specimens. The corrosion of inner surface
crucible made of 9Cr steel was taken into account as thesame dissolution per unit area might be caused as the
specimen. The equation for the concentration is obtainedby eqs. (3) and (4) using the boundary condition t=0 and
C=0 asC

Cs(1  exp(

 h D,static St

V

))

(5)

where t is the exposure time in unit of s, Then, the massbalance between the weight loss of specimens and theincrease of Cr concentration in Li is given from eq. (5) asǻmUVCs(1  exp(h D,static St
V

))

D me
Ȟ
D Cr

K(Re mixing ) 2/3 (Sc)1/3

(7)

(8)

where K is coefficient depends on the shape of impeller
and varied between 0.6 to 0.3 [14]. K was selected as
0.35 from the empirical data, which was correlated withthe ratio of inner diameter of crucible to the width ofimpeller. Ȟ is kinematic viscosity of Li at 500ÛC in unit of
m2/s, DCr is diffusion coefficient of Cr in Li at 500ÛC. The
data of DCr 2.20x10-8m2/s at 570ÛC [13] was used for theevaluation of 500ÛC and 600ÛC. Sc number was 32.5 at
500ÛC and 28.9 at 600ÛC. Sh number was 207 at 500ÛC
and 200 at 600ÛC. Then, hD mixing was obtained as
1.39x10-4m/s at 500ÛC and 1.33x10-4 m/s at 600ÛC.
The weight loss of specimens for 250 hours
exposure was obtained by eq. (7) using the above
mentioned mass transfer coefficient. The effect of thecorrosion of crucible made of Fe-9Cr steel was taken into
account for the estimation based on the assumption the
corrosion loss of the Fe-9Cr was the same to that of
JLF-1 specimens. The specimen loss theoreticallyobtained form eq. (6) was estimated as 0.178g/m2 at
500ÛC and 0.349g/m2 at 600ÛC.
The averaged weight loss of the steel in thestatic Li was 0.202 at 500ÛC and 0.424 g/m2 at 600ÛC.
The averaged weight loss of the steel in the stirring Li
flow was 0.382 at 500ÛC and 0.581 g/m2 at 600ÛC. The
weight loss data obtained by the corrosion experiment at
static condition was slightly larger than that of theoreticalvalue. The saturation of the Cr in the Li [13] wasindicated even at the static condition. The weight loss at
flowing condition was larger than that of theoretical value.
The reason was the erosion-corrosion, which was causedby physical stress of Li flow.
In the case of erosion-corrosion of steels in theflowing Pb-Bi [8], the weight loss was much larger thanthat without erosion occurrence. That was because ofcontinuous corrosion occurrence without the saturation in
the non-isothermal flowing system, and the erosion was
continuously caused by the corrosion. Further study isnecessary for the detail estimation of erosion-corrosion in
the Li flow using non-isothermal flowing system.

where hD,s is coefficient for mass transfer in unit of m/s, ȡ
is density of Li with dissolved steel element in unit ofg/m3, Cs is the solubility of dissolved element in weightratio, C is the concentration of dissolved element in Li inweight ratio.
The information for the solubility of Cr in Li islimited. The solubility was evaluated from the weightlosses of specimens in the previous static tests, whichshows the parabolic rate law and constant weight loss
after 250 hours test [3, 4]. The solubility was determined
as 48wppm at 500ÛC and 96wppm at 600ÛC. The
solubility at 500ÛC was slightly higher than that inliterature data (26wppm) at 523ÛC [13] because thesolubility included the dissolution of other metal such as
Fe and W.
The transient of concentration of Cr in Li by
the corrosion is expressed asȡVh D, m d(6)

where ¨m is the total weight loss of all specimens in unit
of g.

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M. Kondo et al., Erosion-Corrosion of RAFM JLF-1 Steel in Lithium Flow Induced by Impeller

4. Conclusion[8]

M. Kondo, M. Takahashi, T. Suzuki, K. Ishikawa, K. Hata,
S. Qiu and H. Sekimoto, J. Nucl. Mater., 343, 349-359(2005).
[9] Q. Xu, Doctorate thesis, Department of Fusion Science,
The Graduate University for Advanced Studies (2008).
[10] T.R. Cumby, J. Agric. Engng Res, 45, 157-173 (1990).
[11] JSME Data Book : Heat transfer 4th edition, Maruzen corp.
in Japanese (1991).
[12] F. Abe, T. Noda and M. Okada, J. Nucl. Mater., 195, 51-62(1992).
[13] G.M. Griaznov, V.A. Evtikhin, L.P. Zavialskiy et al.,Interaction of structural materials with liquid metals under
the isothermal conditions, in: Material science of liquid
metal systems of thermonuclear reactors, in Russia
Moscow, Energoatomizdat, 1989, pp. 32-142.
[14] Chemical engineering, kagakukougakkai, in Japanese(1992).

Erosion-corrosion characteristics of JLF-1 steel(Fe-9Cr-2W) in flowing Li without temperature
difference was investigated by means of corrosion
experiment using stirring Li pot. The results werecompared with that of static tests to feature the effect of
Li flow. Major conclusions are follows;(1) The weight loss of the specimens tested in flowing Li
was larger than that in static Li at the same
temperature because of mass transfer and erosion
corrosion.
(2) The corroded surface was partly eroded in stirring Li
flow at 500ÛC.
(3) The specimen surface had large number of small
grains after the corrosion test in flowing Li at 600ÛC.
The grains might be martensite lath subgrain. The
boundary of subgrains was clearly revealed becauseof the corrosion. The erosion-corrosion was causedby that the peeling off of the subgrains from the steel
matrix by the Li flow.
(4) The weight loss obtained in the corrosion experimentin flowing Li was larger than that evaluated by themass transfer equation. The difference between theexperimental data and theoretical evaluation was dueto the erosion-corrosion.
(5) The erosion-corrosion was controlled by the
suppression of corrosion due to the saturation ofdissolved alloying elements from the steel into the Li
at isothermal Li system.

5. Acknowledgement

This work was supported by NIFS budget code
NIFS08UCFF004 NIFS09UCFF004 and Grant-in-Aid for
Scientific Research (A) 19206100 (2007-2009).

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