136

高温水中で生成したステンレス鋼の表面皮膜の3 次元アトムプローブトモグラフィー
3-D atom-probe tomography of surface oxides in stainless steels formed underhigh-Temperature water
Sergio Lozano-Perez ＊ 1, David Saxey ＊ 1,
Takuyo Yamada ＊ 2 and Takumi Terachi ＊ 2

要約 加圧水型一次冷却材環境下のステンレス鋼の酸化は，高温高圧水中で生じる複雑な現象として知られる．本報告では，アトムプローブ トモグラフィーが，原子スケールで酸化物の化学的性
質を理解するために必要な高空間分解能で且つ高い分析感度を併せ持つことを実証する．表面酸化に及ぼす冷間加工の影響についても調べ，また議論する．
キーワード

加圧水型原子力発電所，表面酸化，ステンレス鋼，3 次元アトムプローブ トモグラフィー，冷間加工

Abstract
The oxidation of stainless steels under Pressurized Water Reactor（PWR）primarywater conditions is a complex phenomenon since it occurs at high pressure, high temperature and in
the presence of a complex water chemistry. In this paper we demonstrate that atom-probe
tomography combines the high spatial resolution and chemical sensitivity required to understand thechemistry of oxidation at the atomic scale. Effects of cold work in surface oxidation have also beencharacterized and will be discussed.
Keywords

pressurized water reactor, surface oxidation, stainless steels, 3-D atom-probe tomography,
cold-work

1.

Introduction

traditionally performed with X-ray Photoemission
Spectroscopy (XPS) (3) 〜 (5), Auger Electron

Low potential stress corrosion cracking（LPSCC）

Spectroscopy (AES) (6) 〜 (8), Secondary Ion Massof stainless steels is a very important degradation

Spectroscopy (SIMS) (9) 〜 (13), direct current glow

phenomenon in pressurized water reactors（PWRs).

discharge

Autoclave testing under controlled environments

spectroscopy

confirmed that highly cold-worked stainless steels

Microscopy(TEM)(16)〜(19). When lateral resolution

are susceptible to SCC even in PWR primary water

is the critical parameter, TEM or Scanning TEM

conditions (1) and, recently, it was reported in(STEM)can extract information from areas smaller(15)

spectrometry
or

(14),

Transmission

Raman
Electron

In order to clarify thethan 1 nm, however, the microanalytical techniques

corrosion mechanisms and the role of cold work, it isavailable, Energy Dispersive Spectroscopy(EDS)or

important to characterize the surface oxides formed,

Electron Energy Loss Spectroscopy (EELS) , haveas well as the oxide-matrix interface, with thelimited sensitivities and struggle to detect minor

highest level of detail.

impurities

operating PWR

plants (2) .

mass

in

the

ppm

concentration.

Other

techniques, such as SIMS, can perform better in
Chemical characterization of oxide layers has been
＊1
＊2

terms of detectability limits, but they lack lateral

Department of Materials, University of Oxford, Parks Road, Oxford, OX1 3PH, UK
INSS, 64 Sata, Mihama-cho, Mikata-gun, Fukui 919-1205, Japan

137

resolution. Three-dimensional(3-D)information caninner oxide of ~200nm in thickness. The composition

be obtained from some techniques such as XPS,of the 316 alloy was, in weight ％ , 0.051 C, 0.51 Si,

AES and SIMS in the form of depth-profiling

1.48 Mn, 0.028 P, 0.001 S, 13.2 Ni, 2.34 Mo,

combined with mapping, or STEM using electron

16.47 Cr and balance of Fe.

tomography. In the case of electron tomography,
the best 3-D spatial resolution can be achieved with

3.

Methodology

voxels as little as 1nm3(20)(21). The only reference tothe use of atom-probe tomography to understandIn order to extract needle-shaped specimens

surface oxidation in stainless steels in the literature iscontaining the oxide-metal interface, the coupon

based on high-temperature oxidation in air of

specimens

polished needles(22).

cross-sectional surface was subsequently mirror

werecross-sectioned.

The

The characterization of surface oxidation inpolished mechanically, down to colloidal silica. A

cold-worked samples requires high lateral and depth

FEI 200 single-beam FIB equipped with an in-situ

resolution, in order to reveal how the defects

micromanipulator

created by the CW interact with the oxidation

extraction and a Zeiss NVision 40 FIB/SEM wasprocess, as well as high chemical sensitivity, in

used for the tip sharpening. When the bulk sample

order to investigate the role of minor impurities.

was first put in the FIB, a layer of Pt was depositedModerndetectover the surface oxide where the sample is going toconcentrations of 10ppm, with equal sensitivity

be extracted from, using a current of 100pA Ga+.

across all species, including light elements such as

Next, the cross-sectional surface next to theoxygen and lithium. Under suitable conditions thisdeposited Pt is further polished with the Ga+beam

chemicalwith(down to 100pA Ga+) . This helped to reveal the

sub-nanometre spatial resolution, making it the

microstructure and confirm that the oxide layer was

idealpresent under the deposited Pt, as can be observed

atom-probe

instruments

sensitivity

technique

for

is

can

combined

providing

this

type

of

information.

was

used

for

the

samplein Figure 1. Finally, a 25μm long specimen was
extracted from the bulk sample as shown in Figure

It is well known that the oxide films formed in

2. The outer end of the specimen contained thesteels under PWR primary water conditions consist

oxide-metal interface. A detailed description of thisof a double-layer structure, with an inner Cr-richprocedure can be found in(27)〜(29). The specimen isspinel and an outer Fe-rich spinel layer(23). Recent

then welded to a dedicated Cu post made bystudiesFischione(See Figure 3).

byTerachiandco-workers

havecharacterized the influence of cold-work andCr-content amongst other parameters (1) (6) (19) (24) 〜(26).

At this point, the sample is moved to the Zeiss FIB
for the final polishing. A series of annular maskswith decreasing current and inner radii is placed

2.

Materials

around the top of the sample, gradually milling the
tip to a needle-shape with a final diameter smaller

In this research, the oxidation behaviour of 20％

than 200nm. In Figure 4, an image taken near the

CW 316 SS was characterized. Coupon specimens

end of the polishing procedure reveals the level of

weredetail that can be observed with the electron gun andcold-worked

beforeautoclave

testing.

Specimens were miller-polished and degreased with

the in-lens detector in the NVision FIB/SEM.

acetone and then immersed in the autoclave undersimulated PWR primary water(500ppm B ＋ 2ppm

The surface-oxide specimens were analysed in a

Li ＋ 30cc-STP/kg H2 O)for 1500h at 340℃ . After

LEAP® 3000X HR atom probe using〜10ps laser

the test, the specimen developed a Cr-rich spinel

pulses to stimulate field evaporation. The laser spot

138

FiguresPt-layerSurface oxides

Protective Pt-layer
Surface oxides

10µm
5µm
Figure 3. FIB SE image showing thespecimen post welded on a Fischione
post ready for the final sharpening
Figure 1. FIB SE image showing the cross-sectional area of the sample witha Pt layer deposited on top to preserve the surface oxides during sample
preparation.

Pt-layerSurface oxides

Cross-sectionalsurfaceAtom-probe sample

20µm

Surface oxides

Figure 2. FIB SE image showing how the post containing thesurface oxides is milled from the sample

200 nmFigure 4. SEM in-lens image showing
the atom-probe sample at the latest
stages of polishing. The Cr-rich oxideappears as a darker band below theremaining Pt-layer at the top.

139

a)

b)

Figure 5. Atom maps showing the distribution of（a）Cr and Fe atoms within oxides（green-blue dots）and（b）
lithium atoms（blue spheres). The isoconcentration surface in（a）corresponds to a concentration of approximately
10％ oxide species. The cylinder in（a）shows the region from which the 1-D concentration profile（below）was
generated.

was approximately 5-10μm in diameter, with a

Micrograph.

wavelength of 532nm, a pulse rate of 160kHz, and
0.6-0.7nJ per pulse. Ions were field-evaporated

4.

Results

from the specimens to give an average detection rate
of

0.01

ions

per

pulse.

The

specimenbase-temperature was 65K during the analyses.

316SS oxidized under simulated PWR primarywater conditions developed a dual oxide layer withan inner layer of Cr-rich spinel and an outer layer of

TEM samples containing the surface oxides wereFe-rich spinel (33) (34). With the addition of the

prepared in the FEI 200 FIB from the same bulk

microstructure caused by the cold-work, thematerials following the procedure described in (30)〜oxidation process is more complex and enhanced(32).

EELS characterization was performed in aoxidation of twin deformation bands has been

JEOL 3000F equipped with a Gatan GIF andobserved (13) (26). Since the total width of the oxide

operated at 297kV. The microscope was operated inlayers (Fe-rich plus Cr-rich) exceeds the depth of

STEM mode using a 150 pA probe with a 12mrad

analysis that can be realistically achieved with thehalf-convergence angle and 9mrad half-collection

atom-probe, only the Cr-rich oxide layer wasangle. EELS quantification was performed using theanalyzed. For this reason, the oxide-metal interface

inelastic

was intentionally left close to the apex of the tip so it

cross-section

values(withtheHartree-Slater approximation) provided by the
EELS quantification routine in Gatanʼs Digitalcan also be analyzed.

140

The reconstructed dataset after atom-probe
evaporation can be seen in Figure 5. In order to

In order to ensure that quantitative data wassimplify the representation, since more than 10

consistent,

million atoms were collected, only the oxide has

containing the surface oxides and the metal-oxide

been represented in Figure 5a. The inner Cr-richinterface was prepared from the same specimens

surface oxide is clearly visible at the top of theand characterized using EELS. The region chosen

sample,intrusions,for the 316SS sample is shown in Fig 7, together

following microstructural features created by thewith the line profile across the oxide-metal

cold work, have been rendered as isosurfaces forinterface.

ease of view. In Figure 5b, a 3-D reconstruction

processing the EELS data are shown in Figure 8. As

revealing the location of the Li atoms can be seen. A

can be seen, the atom-probe concentrations shown

1-D line profile has been extracted from the dataset

in the line profile in Figure 6 match very well thoseat the location marked by the cylinder in Figure 5a.

obtained by electron energy loss spectroscopy. The

while

the

Cr-richoxideacross-sectional

Quantitative

results

TEM

obtained

sampleafter

only inconsistency occurs when following the Mn

5.

Discussion

concentration across the oxide-metal interface. In
the EELS line-profile, the Mn signal falls to almost

In recent years, the introduction of commercial

zero in the metal. This behaviour has been identified

systems capable of operating in laser-pulsed

as an artefact due to the increase in sample thickness

has enabled routine

after the interface. The extra thickness allows forcharacterization of non-conductive samples in thesome multiple scattering to occur which has a dual

atom-probe.

thedetrimental effect. First, it changes the shape of the

instrumentation (38) (39) have greatly increased both

Mn L23 pre-edge, preventing a satisfactory fitting

the lateral field-of-view and the speed at which datawith an inverse power-law curve. Second, it

can be collected, leading to a greater than 100-foldspreads out the already weak Mn signal.

evaporation

mode (35) 〜 (37) ,Also,

recentadvancesinincrease in the volume of material that can beanalysed within a day. At the same time, modern
dual-beam

FIBs

have

providedthe

The data also reveals for the first time, therequiredlocation of lithium atoms coming from the primary

precision in the sample preparation to design thewater, which are incorporated into the Cr-rich

type of specimen described in this paper. For the

spinel. Boron was not observed in this region.

first time, cold-worked samples containing real

However the local detection limit for boron was

surface oxides have been successfully prepared andapproximately 15ppm, which was higher than in thecharacterized in the atom-probe. The evaporation

metal due to excess noise during evaporation of theof samples containing oxides or interfaces, however,oxide. It should be noted that prior NanoSIMS

is not a simple phenomenon and its analysis must be

characterization of the same oxide layer (13)approached with great care. For example, thesuggested that Boron might be present only at thedifferent evaporation fields for the oxide and matrix

outer part of the Cr-rich spinel layer, which was

can lead to distortions in the reconstructed interface

intentionally milled away during FIB sample

area. The interpretation of the transition region in

preparation of the atom-probe needle.

profiles such as in Figure 6 is not straightforward.
Also, when oxidized species are evaporated, they

As can be seen in Figure 5a, the oxidation did not

can reach the detector as a single atom(e.g. metalstop at the oxide-metal interface and progressedor oxygen) or as a molecular combination of metal

beyond this point, benefiting from the highly

and oxygen atoms. It is therefore challenging to

deformed microstructure caused by the cold work.

ensure that all elements have been fully accounted

When the sample was characterized using STEM

for in the final quantification.

HAADF, similar pockets of oxidation below the141

Figure 6. 1-D concentration profile across oxide-metal interface. Region of analysis shown in previous figure. All
molecular ions are decomposed into their constituent atomic species. Si, Mo and C are not shown. The left side of the
profile corresponds to the surface oxide. The O-rich region on the right side is a sub-surface feature.

80
70
60
O
Cr
Mn
Fe
Ni

Wt. %

50
40
30
20
10
0
-50

50

150

250

350

d (nm)

Figure 8. Quantitative EELS line profile across the oxide
layers and the oxide-metal interface in the 316 sample.
Location of line profile is shown in Figure 7.
Figure 7. HAADF image showing a cross-sectional
TEM sample from the 316SS specimen and the locationof the EELS line profile

142

Surface oxides

Oxide pockets

Grain boundary

50 nm
Figure 9. STEM HAADF image showing localized oxidation at a grain boundary intersecting the surface exposed tothe environment. Cr-rich spinel（darker）is at the top and metallic matrix（brighter）is at the bottom. The darker
localized areas below the interface were identified as pockets of oxide（only a few of them have been arrowed).

oxide-metal interface were observed(see Fig. 9).

an important role in the future clarification of theUnderstandingoxidation mechanisms of these alloys.

theoxidationmechanisms

ofcold-work austenitic stainless steels will be very
valuable in order to understand SCC mechanisms,

7.

Acknowledgements

since it illustrates how the cold work locally modifies
Sergio Lozano-Perez is grateful to INSS and thethe original corrosion behaviour of the alloy.

Department of Materials (University of Oxford) for6.

Conclusions

sponsoring this research. The atom probe analysis
was conducted within the Opal UK Atom Probe

This work has demonstrated that atom-probe

Facility at the University of Oxford ― funded by the

tomography can be efficiently used to characterize

UK Engineering and Physical Sciences Researchsurface oxides in stainless steels produced underCouncil(EPSRC)under grant no. EP/077664/1.

simulated PWR primary water conditions. The highchemical sensitivity of the atom-probe allows the8.

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