211

NanoSIMS によるステンレス鋼の応力腐食割れき裂先端分析の新評価法
Novel Characterization of Stress Corrosion Cracks in Stainless Steels by NanoSIMS
S. Lozano-Perez ＊ 1, M.R. Kilburn ＊ 1, ＊ 2, M. Schroeder ＊ 1,
T. Yamada ＊ 3, T. Terachi ＊ 3, C.A. English ＊ 4 and C.R.M. Grovenor ＊ 1
要約 NanoSIMS による高分解能分析を，ステンレス鋼の粒界型応力腐食割れき裂先端部と接液表面部における元素マッピングに初めて適用し，この分析方法が，ホウ素や硫黄などの微量成分が層状酸化物に対してどのように振舞うかを明確にできるなど，この種の評価に理想的であることを立証した．また，試料準備作業および比較的広範囲の領域を分析するのに要する時間が短くてすむので，NanoSIMS は改善された統計精度で，系統的な腐食のキャラクタリゼーションに最適な手段であり，これにより現実的な時間内でき裂先端部の実用的分析が可能となる．
キーワード

高分解能２次イオン質量分析，ステンレス鋼，応力腐食割れ，き裂先端，元素マッピング

Abstract
For the first time, high resolution analysis using a NanoSIMS has been used to map
the oxide chemistry in intergranular cracks and on the exposed surfaces in stainless steels. The
technique has proven ideal for this type of samples, as it is able to discern between the different oxide
layers and clarify the role of minor segregants such as boron and sulphur. The short time required toprepare and examine multiple regions makes the NanoSIMS an optimum tool for a systematic
corrosion characterization and allows a realistic analysis of all the crack tips in a record time, withimproved statistics on the results.
Keywords

1.

NanoSIMS, stainless steel, stress corrosion craking, crack tip, elemental mapping

Introduction

ing the detailed morphology and internal structure ofcracks in SCC specimens originating from laboratory

Environmentally assisted cracking is one of thetests and in samples extracted from operating plants

least understood and most important degradation[5-8]. The primary objective of these studies is to gain

phenomena in predicting the safe lifetime of structu-

an improved understanding of the critical phe-ral steels. The complex interaction of environment,

nomena controlling SCC, and have revealed, formicrostructure and mechanical loading requires

example, characteristic narrow oxide-filled cracks inanalysis by a wide range of different techniques in

the case of nickel alloys, details of the oxide

order to obtain a complete picture of the critical

microstructure, evidence of porosity and details ofprocesses and mechanisms. In particular, theinteraction of cracks with dislocations, defects andmechanisms controlling stress corrosion cracking

precipitates in the surrounding metallic matrix.

(SCC) and irradiation assisted stress corrosion
cracking (IASCC) of austenitic steels and Ni based

It is clear that the characterization of cracks inalloys employed in nuclear reactor components needsamples subjected to SCC should provide informa-

to be properly understood if reliable predictions oftion at high spatial resolution (〜 1-10nm) on:

lifetime are to be achieved[1-4].
1. Oxide composition, including levels of minor
Considerable effort has been placed on characteris-constituents such as B, P and S. These trace＊ 1 Department of Materials, Universityof Oxford, ParksRoad, OX1 3PH Oxford, UK＊ 2 Centre for Microscopy and Microanalysis, University of Western Australia, 35 Stirling Hwy, Crawley 6009 WA, Australia＊ 3 Institute of Nuclear Safety Systems Inc, 64Sata, Mikata-gun, Fukui 919-1205, Japan＊ 4 Nexia Solutions, Harwell Business Centre, Didcot, Oxon, OX11 0RA, UK＊ Corresponding author: Sergio.lozano-perez@materials.ox.ac.uk

212

impurities are known to have a strong influence onintergranularfracture [9-13] .

In particular it isnecessary to provide chemical information to compare with that available from the TEM.

important to obtain composition profiles across thecrack flanks and crack tip and in the metal matrix
immediately ahead of the crack tip.

In this paper we will describe, for the first time, thebenefits of using a Cameca NanoSIMS (secondary
ion mass spectrometer) for characterizing stress

2. Oxide structure both at the crack flanks and at thecorrosion cracks, and demonstrate that it has a major

crack tip. Information on oxide porosity, orienta-role to play, particularly for improved insight into thetion relationships with the surrounding metaloxide composition and the influence of crystallog-

matrix, and how the oxide grows with respect toraphic features on oxide formation.

the original grain boundary is also of value. Of
importance is also the interaction of any oxide with

2.

Material

crystallographic features such as dislocations,
The materials employed in this study are a Typedeformation bands or twins.

304 stainless steel (SUS304) and two 304-like alloys
3. Information on the metal matrix containing thethat have lower Cr levels but with the levels of othercrack (s) , for example grain orientation, grainalloying elements chosen to match the original

boundary parameters, dislocation density in the

SUS304 composition (12Cr and 15Cr) . Type 304

material adjacent to the crack tip and flank.

stainless steel (SUS304) is widely used in Pressurised Water Reactors (PWRs) and Boiling Water

A schematic representation of a crack is shown in

Reactors (BWRs) . The compositions of the alloys

Figure 1 to illustrate the features described above. It

used are shown in Table 1. All alloys were solutionis also important to examine a sufficient number oftreated (SUS304 at 1060°C for 100 min and 12Cr

individual cracks to identify typical behaviour. Thisand 15Cr alloys at 1100°C for 100min) and thenhas been a particular problem in the past in studies

water-quenched. The specimens were cold-rolled to arelying heavily on transmission electron microscope

reduction of 20％ prior to testing. They will be(TEM) analysis.

referred as 20％ Cold Work (20％ CW).

Characterisation of cracks successfully addressing

The SCC test was performed using a 1/2T CT

these requirements has come predominantly from

specimen, under constant load and a simulated PWR

studies in modern TEM with the latest analyticalprimary water chemistry (500ppm B + 2ppm Li,

capabilities [5,14,15] . Such studies necessarily involve+30cc-STP/kg-H2 O DH2) at 320°C. The SCC testcomplex and time consuming specimen preparation

lasted 666h and produced intergranular cracks overto obtain electron transparent samples from the

100 μ m in length. Some samples were fatigued after

crack flank and crack tip. In addition, it is only

testing in order to complete the fracture.

possible to examine a very restricted region of
cracks within the sample. Techniques where a larger

3.

Methodology

volume and more statistically significant number ofcracks may be examined have been employed; for

Samples with a suitable geometry for NanoSIMS

characterization

analysis (9mm diameter and 1mm thick disks) were

methods including scanning electron microscopy

extracted from the bulk samples using a spark(SEM) , Scanning Auger Microscopy (SAM) anderoder machine. Once mounted in suitable holders,secondary ion mass spectroscopy (SIMS)[16,16-19]. To

they were polished with 1200 SiC paper, 3 micron

date none has been able to demonstrate the required

diamond paste and finished with silica suspension

high spatial resolution and high elemental sensitivity(with 0.04 μ m average grain size) . This was theexampletraditionalsurface213

Table 1. Chemical content of examined alloys (wt%)
Alloy
20% CW
SUS304
20% CW
15Cr
20% CW
12Cr

Fe
Bal.

C
0.04

Si
0.31

Mn
1.6

P
0.003

S
0.001

Ni
9.2

Cr
18.3

Mo
＜ 0.01

Bal.

0.06

0.5

1.5

0.002

0.001

10

15.0

＜ 0.01

Bal.

0.06

0.5

1.6

0.002

0.001

9.6

12.0

＜ 0.01

only sample preparation required for effective

Ion induced secondary electron images were ac-

analysis of crack and surface chemistry.

quired concurrently. The octopoles controlling thesymmetry of the probe were tuned before eachIn this work, both primary and secondary cracks

acquisition to ensure a stigmatism-free image. In 8

were examined. A primary or dominant crack isdays of operation, with morning and afternoon

defined as a crack which is growing throughout thesessions, more than 100 regions were analyzed.

test as a result of the applied stress and the corrosive
environment. A secondary crack might have been

In order to assess the value of these observations

dominant at some point during propagation but itin the NanoSIMS, a comparison has been made to

was overrun by another crack, being left behind andTEM examination of specimens obtained from theexposed to the environment for a longer period ofsame samples, and also with TEM specimens oftime, mostly under a lower stress than thatequivalent materials examined in previous studies.

experienced by the primary crack. These secondary
cracks are normally located near triple points and4.

Results

are found filled by magnetite.
For each of the alloys shown in Table 1, several
Analyses were performed using the Cameca

cracks and their associated features were analyzed

NanoSIMS 50 in the Department of Materials,

using the NanoSIMS. Both primary and secondary+

University of Oxford. A Cs primary beam, focussed

cracks have been analysed. A cross-sectional view ofto less than 50 nm, was used to sputter negative

the 20％ CW SUS304 sample after testing, cutting

secondary ions from the sample surface. Secondaryand polishing can be seen in Fig. 2. The fatigue pre-ions were mapped on five detectors simultaneously.

crack is at the bottom of the image, oriented

12

−

16

−

11

16

−

The ion species analysed were C , O , B O2 ,
−

58

−

52

16

−

56

16

32

−

vertically. The strain direction was applied perpen-

S , Ni , Cr O , and Fe O , on masses 12, 16, 32,

dicular to this crack, and it is indicated by the arrow.

43, 58, 68, and 72, respectively. The mass spectro-

Once the crack region is located using the in-situ

meter was calibrated using pure metallic Ni, Cr, andCCD camera in the NanoSIMS, the positions of the

Fe standards, and tuned to high mass resolution to

relevant features (crack tips, triple points, etc) areeliminate any isobaric interference.

stored and a series of analysis performed. Up to 15
regions were analyzed per day, enabling the

The areas containing the features of interest werecleaned prior to examination by scanning a defo-

characterization of the most interesting cracks aswell as the surface oxides.

cused primary beam over the surface for up to
1hour, depending on the initial surface condition.
Images were acquired by scanning a 0.5pA primary

4.1

Information from composition mapping

beam over areas ranging from 5 to 35 mm square.
Image resolutions of 512x512 or 256x256 pixels, with

A typical example of the information obtainable incount times of up to 40ms per pixel, were obtained.

the NanoSIMS is shown in Fig. 3. In this figure, major

214

Inner
Oxide

Outer
Oxide

Grain
boundary

SolutionTop
surface

Crack
Flank

Crack
Tip

Fig. 1: Schematic representation of a Stress Corrosion crack and its representative features.

oxide crystals. From electron diffraction data it has
been established that the inner layer is Cr rich spinel
and that the outer oxide is magnetite. The Ni levelsin the oxides have also been shown to be lower thanthe original metallic bulk level. The linear featuresintersecting the crack are discussed below.
Fig. 5 shows the NanoSIMS mapping of major
elements in the oxide formed at crack tip region in a
primary crack in the 20％ CW 12Cr sample. In thiscase a Cr-rich oxide can be seen on the right hand

100µmside of the crack tip and an Fe-rich oxide is seen on

Fig. 2. Optical micrograph showing a bifurcated stresscorrosion crack in a 20％ CW SUS304 sample
after the test. The wider crack at the bottom of
the image is the fatigue pre-crack lying perpendicular to the strain direction (indicated by thearrow).

can be observed where the Cr-rich oxide has formed

the left hand side of the crack. Regions depleted in Ni
and rejected nickel accumulates around it. Again
linear features intersecting the crack flank arevisible.
The NanoSIMS can also be a powerful tool for theelement distribution in the oxide formed on the

study of trace element distributions around cracks

flanks of a secondary crack in the 20％ CW 12Cr

because of the very high sensitivity of SIMS analysis

alloy are shown. The oxides found always consist offor many impurities in steel samples. A NanoSIMS

an inner Cr-rich layer and an outer Fe rich oxideline profile across the Cr-rich oxide layer of the flankwhich tends to fill the crack as it opens (see Fig 1 forof the fatigue pre-crack in the 20％ CW SUS304alloy

definition of inner and outer). This observation is in

is presented in Fig. 6 to illustrate the detection ofagreement with previous work, showing a double

minor constituents of the oxide. Line profiles wereoxide layer in similarly tested 304 alloys[15,20]. In Fig.

measured directly from the ion images, which were4, results from a STEM observations of a similaracquired using 10ms/pixel and a current of 7.9nA.

sample are shown. The Dark-Field (DF) STEM

All profiles have been normalized in order to accountimage shows a portion of a secondary crack, which

for the different yields. Boron is enriched at thewas found to be filled by oxides. Interpretation of theoriginal sample interface, while S seems to stay at thestructure is easier with the aid of EDX elemental

outside, probably as an impurity from the sample

maps, which are also shown in Fig. 4. It can be seen

preparation process. No traces of B could be found bythat the inner oxide layer is Cr-rich (depleted in Fe

EDX or EELS analysis in the TEM samples

but not in Cr), and the outer is formed by Fe-rich

extracted from the crack tip region of a secondary

215

16

O-

56

Fe16O-

5µm

5µm

52

Cr 16O-

5µm
Fig. 3. NanoSIMS maps from a secondary crack in the 20％ CW 12Cr sample.

crack in the 20％ CW 15Cr alloy. In addition, inbe obtained in a systematic manner on the influence

regions of the crack flank, where B was observed inof pre-existing crystallographic features in the metal

the NanoSIMS (with a SNR of 〜 25) , no B was

matrix on subsequent oxide formation.

detected in the Scanning Auger Microscope.
In Fig. 7 deformation along slip bands in the grain

4.2

Oxide interaction with the crystallographic features in the matrix.

interiors can be seen as a result of the slight etchingcaused by the silica polishing in the 20％ CW 15 Cr
alloy. NanoSIMS images of similar regions in the

The NanoSIMS cannot provide data (other than

20％ CW 12Cr alloy are shown in Fig. 8. It is clear

composition) that allows the structure of the oxides

that the enhanced contrast next to the crack flank is

to be identified, although this can be provided byactually due to formation of a Cr-rich oxide along

conventional TEM techniques. However, it has beenthese deformation bands. It can also be seen thatfound in this study that the ability to characterise

there the oxide contains Fe but that, as expected, Ni

extensive regions of crack flanks and a number oflevels are reduced (since it is rejected by thecrack tips in several samples enables information toformation of Cr-rich oxide) . It is also obvious that

216

Cr

O

Ni

Fe

Fig. 4. DF STEM image showing a secondary crack flank in a SUS304 sample (without CW). On the sides, STEM EDX
elemental maps are shown for O, Cr, Ni and Fe.

each grain oxidises differently.

5.

Discussion and conclusions

Observations from the NanoSIMS have alsoenabled preliminary interpretations on how the

The high lateral resolution of the NanoSIMS

oxide at a crack tip forms in relation to the grain

together with the high chemical sensitivity to certain

boundary chemistry. In Fig. 9, a dominant crack tip

elements means that it can be an important new tool

has been analyzed in the 20％ CW SUS304 alloy andin establishing a multi-scale approach to the study ofsome line profiles have been measured from thestress corrosion cracks in steels. The benefits ofcrack tip region along the continuing grain bound-applying the NanoSIMS are:

56

16

−

11

16

−

ary. Both Fe O and B O2 maps are shown to aid
11

16

−

the interpretation. The B O2 map is very useful as

1. Ease of sample preparation. It has to be

it shows B segregation to the grain boundary, thusemphasised that the elaborate preparation

enabling its location in the matrix to be determined

techniques necessary for TEM or Atom Probeat the same time as studying the crack and grain

analysis are not required for NanoSIMS, making

boundary chemistry. It can also be seen thatthe analysis quick and easy to perform

oxidation occurs preferentially on the bottom grain,

2. A lateral spatial resolution of less than 50nm forpossibly due to its more favourable crystallographic

the major elements found in the oxides associ-orientation.

ated with cracks
3. Ability to detect both major elements (Cr, Fe
and Ni) and also minor elements such as S, P and
B. It is to be noted that S and B segregation inthese samples could not be detected using TEM

217

16

O-

12

2 µm

2 µm

Ni-

56

2 µm

2 µm

58

52

C-

Fe16O-

Cr16 O-

2 µm
Fig. 5. NanoSIMS maps showing the crack tip region in the 20％ CW 12Cr sample.

or SAM.

a steel matrix have not been performed in the
NanoSIMS, preventing the acquisition of truly

The major disadvantage is that the necessary

quantitative data at this stage. All the data shown incalibrations for the yield of the elements of interest in

this work is thus qualitative. Several random grain

218

SE image

a）

2µm
1.2

b）
CrO
32S
16O
BO2
FeO

1

Normalized intensity

0.8
0.6
0.4
0.2
0

0

0.5

1

1.5

2

2.5

3

-0.2
d（microns）

Fig. 6 Detail of surface oxidation in the 20％ CW SUS304 sample. (a) SE image showing position of line profile (b)NanoSIMS line profile results (normalized to unity).

boundaries were analyzed in the bulk (a few mm far
from the cracked region) in order to confirm theobserved B segregation to grain boundaries. A
similar B enrichment was observed. Due to the highyield of B, NanoSIMS is able concentrations of theorder of ppm. As mentioned before, no B was
observed at boundaries with any of the othertechniques, so its enrichment is expected to be well

20µm
Fig. 7. Optical micrograph showing a portion of a SC
crack in the 20％ CW 15Cr alloy. Deformation
bands along slip directions can be seen as aresult of the slight etching caused by the silica
polishing. The arrow represents the strain
direction.

below 1 at％ .
Critically, in a typical experimental campaign on
the NanoSIMS a significant number of cracks can be
examined. This has two benefits; firstly the oxide
structures in different cracks can be compared andʻtypical behaviourʼestablished, and secondly, re-

219

16

O-

2µm

58

Ni-

2µm

56

Fe16O-

2µm

52

Cr 16O-2µm

Fig 8. NanoSIMS maps showing the dominant crack in the 20％ CW 12Cr sample.

gions for subsequent examination in the TEM or

grains microstructure is being performed. These

3DAP can be identified. The latter enables theseobservations might have important implications onhigher resolution techniques, which involve time-the mechanisms of crack propagation and are

consuming specimen preparation, to be focussed on

currently being investigated into more detail.

the critical regions.
In conclusion, the high lateral and mass resolution
A detailed analysis of the crack flank and crack tip

together with the high sensitivity of the NanoSIMS

region has provided very important information.

provide a unique tool for the extensive investigation

Firstly, the results agree with previously reportedof stress corrosion cracks in stainless steels. Informa-

observations of a dual oxide layer consisting of antion with fine detail complementing that obtained byinner Cr-rich oxide layer and an outer Fe-rich oxide

TEM, but with better chemical sensitivity, was easily

layer (see Fig. 4)[15,16,20]. Second, it has been possible

obtained on polished bulk samples. No special sample

to observe oxide penetrating along crystallographic

preparation was necessary. It has been shown that

deformation bands into the adjacent grain. This

the NanoSIMS can provide information on the oxide

observation has not been found in the literature andcomposition with a resolution good enough tomore detailed analysis of the relation between oxide

separate the different oxide layers. It also provides

formation along deformation bands and surrounding

detailed compositional profiles across/along features

220

56

a)

Fe16O-

11b)

B16O2-

2µm

2µm
1.2

c）

Normalized intensity

1
32S
16O
BO2

0.8
0.6
0.4
0.2
0

3

3.5

-0.2

4

4.5

5

5.5

d
（microns）
56

16

-

Fig 9. Dominant crack tip region
in- the20％ CW SUS304 sample (a) NanoSIMS Fe O map showing position of line
11
16
profile (b) NanoSIMS B O2 map from same region and (c) NanoSIMS line profiles (normalized to unity).

of interest and very valuable information on the
distribution of minor segregants (S, B).

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