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Journal of The Electrochemical Society, 147 (7) 2525-2531 (2000) 2525
S0013-4651(99)12-033-0 CCC: $7.00 © The Electrochemical Society, Inc.
Characterization of materials in situ offers a new window of
opportunity for several areas of science, including electrochemistry,
biology, chemistry, physics, and medicine. At the present time, few
research groups are working toward building more versatile micro-
scopes that extend modern microscopy to studies in situ in liquids.
Smyrl’s group, for example, demonstrated that by using a modified
near-field scanning optical microscope (NSOM), with a home-built
tuning fork, it is possible to obtain in situ topography in liquids
together with functional imaging (FI) on some model systems.1
William’s group2modified an atomic force microscope (AFM) to
study the onset of pits on austenitic stainless steels, by using it as a
scanning electrochemical microscope (SECM). They also modified
a confocal microscope to obtain a photoelectrochemical microscopy
(PEM) and reported images with resolution of the order of 0.5 mm.3
Smyrl’s group has also used PEM to study precursor sites for pitting
in polycrystalline Ti with a resolution in the order of 100 nm.4In the
present work, we show that by using a modified near-field scanning
optical microscope (NSOM), with a home-built tuning fork, one can
obtain high resolution optical images concurrently with the topogra-
phy of different systems while the substrate (sample) and the probe
are immersed in the liquid. This technique has been used to show, for
the first time, metastable pitting experiments in a 25% Cr duplex
stainless steel (DSS) UNS S32550.
It is well established that MnS inclusions are the main cause of
pitting in commercial stainless steels.2,3,5,6 However, in highly al-
loyed steels (as in the case of duplex stainless steels), the cause for
pitting is a more complex phenomenon, mainly because of the pres-
ence of different precursors (not only MnS inclusions) and because
of the development of metastable pits preceding stable pitting.7In
the present work we briefly describe the method for obtaining con-
current topography and optical imaging by using an optical fiber
with an apex smaller than 100 nm. The optical image is obtained by
using a three-way connector that allows the focusing of a laser light
through the fiber to illuminate the sample, and at the same time, that
allows the collection of the reflected light. After finding, analyzing,
and isolating the particles in some areas of a DSS sample, prior to
the corrosion tests, they were imaged in situ in both 3.5% NaCl and
1 M HCl solutions. Then, the metastable pitting process was initiat-
ed with further imaging of the selected areas.
Experimental
Tip preparation.—All NSOM probes were prepared from type F-
AS optical fiber (3.7 mm in core diameter with 125 62 mm of
cladding and 245 615 mm of polymer coating), which were obtained
from Newport Corporation. The optical fibers were pulled with a
commercial pulling machine (from Sutter). After pulling the fibers
they were observed under a 63 times magnification microscope to
check the shape of the fiber before it could be mounted in the fork.
The apex of the fibers were 100 nm or less.8Those fibers with either
a large diameter or that had crashed during the mounting were dis-
carded. More details about the preparation of the nanoprobes used to
perform these tests will be reported in future publications.8
Materials.—All samples used in the present research were cut
from commercially available 25Cr DSS of the type UNS S32550,
which were obtained in wrought form (bars of 10 mm diam). Table I
shows the melt composition of the alloy. Two main annealing proce-
dures were selected, involving solutions treated at 1020 or 11408C.
Annealing at 10208C gives a critical pitting temperature (CPT) 208C
higher than annealing at 11408C.9In both cases, the samples were
annealed for 2 h and then water quenched. The resulting grain size
was about 15 mm in both cases. However, in order to obtain a sam-
ple with coarser grain size, one piece of the bar, already annealed
once at 10208C, was reannealed at 13008C for 2 h (in the ferrite re-
gion), followed by 1 h at 10208C (to grow a controlled amount of
austenite phase in the ferrite matrix), and finally water quenched.
The structure is similar to the other two heat-treatments, but the
grain size was about 100 mm. This alloy is referred to as “rean-
nealed” throughout this work. The chemical compositions of both
ferrite and austenite phases for all three heat-treatments were deter-
mined by electron probe microanalysis (EPM), as described before7
and are presented in Table II.
Sample preparation.—The samples were prepared by successive
polishing with 6, 1, and 0.25 mm diamond paste. The polishing pro-
cedure on these samples was similar to the polishing procedure
described before.4,10 That is, all the samples were polished and test-
ed a few times without being in contact with any particulate other
than diamond polishing pastes (to avoid any contamination from Al
or Si contained in a normal polishing procedure). At the end of the
polishing, the samples were lightly electrolytically etched for 3 s in
40% KOH at an applied voltage of 2.5 V, to allow the ferrite and
In Situ High-Resolution Microscopy on Duplex Stainless Steels
L. F. Garfias* and D. J. Siconolfi
Lucent Technologies, Bell Laboratories, Materials Reliability and Component Processing Research Department, Murray Hill,
New Jersey 07974-0636, USA
The aim of the present work is to focus on the characterization of materials in situ by using a modified near-field scanning optical
microscope (NSOM) with a home-built tuning fork head that allowed us to obtain optical imaging concurrently with topography
of some systems. The first example is related to finding precursor sites for pitting corrosion on duplex stainless steels (DSS). Pre-
cursor sites for pitting on DSS were found to be inclusions that are complex in structure and where metastable pits develop at tem-
peratures below the critical pitting temperature (CPT), but where stable pits, with large corrosion current, occur at the CPT. These
inclusions were analyzed and found to be inhomogeneous in nature and consisting of a mixture of various elements (Si, Al, Mg,
Ca, Ti, Mn, and S). After analysis of the particles, in situ observation of the particles in 3.5% NaCl and 1 M HCl solutions showed
that they developed metastable pits. The pits and corrosion products developed in both particles present in the austenite grains and
in particles contained within the ferrite matrix.
© 2000 The Electrochemical Society. S0013-4651(99)12-033-0. All rights reserved.
Manuscript submitted December 7, 1999; revised manuscript received March 22, 2000. This was Paper 572 presented at the Hon-
olulu, Hawaii, Meeting of the Society, October 17-22, 1999.
* Electrochemical Society Active Member.
Table I. UNS S32550 alloy used in the present work.
Composition (wt %)
Cr Ni Mo Cu N Mn Si C S P
25.92 5.9 3.19 1.62 0.2 0.96 0.47 0.03 0.007 0.024
2526 Journal of The Electrochemical Society, 147 (7) 2525-2531 (2000)
S0013-4651(99)12-033-0 CCC: $7.00 © The Electrochemical Society, Inc.
austenite to be distinguished in all the microscopes. The etching was
done in order to remove the air-formed oxide (native oxide), to pre-
pare a “fresh” surface, and to find the grains and inclusions for each
test. After etching, the whole sample was covered with an insulating
lacquer except on the area of interest (normally square regions with
an area smaller than 200 3200 mm). No crevices were ever ob-
served at the edges of the lacquer, and this procedure allowed us to
isolate a small part of the surface and consequently to keep the back-
ground passive current very low.4,10
Optical microscopy and atomic force microscopy (AFM).—Opti-
cal microscopy with a Zeiss microscope with different magnifica-
tions up to 1000 times was done in order to select the area or areas
in the sample where the inclusions are located. After location of the
particles of interest, they were transferred to an AFM (Topometrix,
Explorer) to identify the topography of the selected areas and to con-
firm the size of the inclusions and adjacent grains (see Fig. 1).
Particle analysis.—Following optical and AFM imaging, the
grains and matrix in the samples together with the inclusions were
mapped by using a scanning electron microscope (SEM) Cambridge
250, equipped with an energy dispersive spectrometer (EDS), which
provides X-ray mapping and particle analysis (see Fig. 2 and 3).
In situ near-field scanning optical microscopy (NSOM) with con-
current topography.—The topography and optical images in situ in
the liquid were performed on a Topometrix Aurora NSOM with a
custom modified head incorporating a tuning fork transducer, which
has been described in earlier publications.1,4 The experimental setup
is shown in Fig. 4. An optical fiber nanoprobe (described above) was
used to monitor the surface topography and to focus the 480 nm laser
beam onto the DSS surface. The nanoprobe was glued onto one of
the arms of the tuning fork following the feedback procedure devel-
oped by Karrai and Grober.11,12 The fork is rigidly attached to a
piezoelectric vibrator (the driving piezo or dither), which vibrates at
a frequency of 33 kHz with an amplitude of nearly 0.01 nm.11,12 The
shear-force feedback method (a detailed description of our setup can
be found in Ref. 4) was then used to obtain the topography of the
sample concurrently with the optical image in the near-field. We
used the shear-force feedback to maintain a constant distance be-
tween the nanoprobe and the surface.13 The distance is normally reg-
ulated by monitoring the damping of the oscillations of the nano-
probe, while it is in resonance, as it encounters viscous damping near
the sample surface. The method is basically noncontact with a typi-
cal interaction distance between 10 to 30 nm in air.14 After the cell
containing the sample was fixed to the scanner, the other two elec-
trodes (reference and counter) were placed near the surface of the
cell avoiding any contact with either the sample or the cell (see
Fig. 4). All electrodes were connected in order to avoid stirring the
liquid while the scanning was performed. In some cases a “fast”
scan in air was necessary to find the area of interest. Then, the solu-
tion (either 3.5% NaCl or 1 M HCl) was added into the cell, and the
images were obtained.
Electrochemical experiments.—After the first in situ images
were obtained, an EG&G potentiostat/galvanostat model 273 was
used in all our electrochemical experiments (see Fig. 5). We used a
three-electrode electrochemical cell using a saturated calomel elec-
trode (SCE) as the reference and Pt wire as the counter electrode. All
potentials mentioned in this work are vs. the SCE. Both the sample
potential and current were collected through a general purpose inter-
face bus (GPIB) interface into a personal computer. After the meta-
stable pitting experiments, a second set of images was obtained with
the NSOM setup described above.
Results and Discussion
Brief description of metastable pitting in DSS.—The first observa-
tion since 1995 of metastable pitting in highly alloyed steels was made
by one of the authors.15-17 In several studies with DSS containing 22,
25, and 28% Cr content, a number of metastable pits were observed in
large electrodes. (An extensive description on the metastable pitting of
DSS can be found in Ref. 7.) Until now all metastable pitting studies
have been made only on microelectrodes.18-23 While testing the DSS
Table II. Average composition of ferrite and austenite for each
heat treated DSS sample.
Annealing Composition (wt %)
temp. (8C) Phase % Cr Ni Mo NaCu P
1020 Ferrite 47 26.36 4.63 3.69 0.05 1.27 0.043
Austenite 53 23.14 7.36 2.35 0.33 1.95 0.025
1140 Ferrite 64 26.37 5.05 3.36 0.05 1.36 0.038
Austenite 36 24.04 7.15 2.33 0.47 1.83 0.026
Reannealed Ferrite 50 27.27 4.58 3.89 0.05 1.33 0.052
Austenite 50 23.44 7.58 2.36 0.35 1.98 0.023
aNitrogen in ferrite is taken as fixed at the saturation value of ,0.05%,
the remaining portions are attributed to austenite.
Figure 1. Optical micro-
graph (center) from the DSS
sample showing the austenite
grains (dark) and the ferrite
matrix (light color). Notice
that this area contained three
different particles, two in the
ferrite and one in the austen-
ite. The respectvie AFM
images from the areas are
shown in the boxes next to
them (the austenite grains are
also darker than the lighter
ferrite matrix).
Journal of The Electrochemical Society, 147 (7) 2525-2531 (2000) 2527
S0013-4651(99)12-033-0 CCC: $7.00 © The Electrochemical Society, Inc.
below the CPT in neutral NaCl solutions, a region of very large peaks
associated with metastable pits was observed at low potentials
(between 200 and 350 mVSCE), followed by a decrease of activity at
higher potentials.7Above the CPT, stable pits were initiated at the
same low potentials where metastability occurs but preceded by only
a few metastable events. We found observable differences between the
growth of metastable pits and its transition to stable pitting in high-
chromium DSS from that seen in single-phase austenitic steels.
Inclusions as precursors sites for pitting in DSS.—Figure 3
shows typical EDX spectrum for the particles found in the ferrite
matrix and in the austenite grains in 25% Cr DSS. Table III shows a
Figure 2. EDX spectra from
the DSS sample showing the
characteristic peaks for the
austenite grains and those for
the ferrite matrix. Notice that
the ferrite matrix contains a
larger Mo peak and smaller
Ni peak than the austenite
spectrum.
Figure 3. EDX spectra from
two inclusions in the DSS.
The inclusion in the ferrite
matrix contains large Al and
Mn peaks and smaller Si, Ca,
and Ti peaks; the Fe and Mo
peaks are smaller than in the
matrix. In the austenite inclu-
sion the large peaks are from
Al, Si, Ca, and Mn with small
peaks corresponding to Mg
and Ti. The peaks corre-
sponding to Fe, Ni, and Cu
are smaller than in the nor-
mal austenite grains.
2528 Journal of The Electrochemical Society, 147 (7) 2525-2531 (2000)
S0013-4651(99)12-033-0 CCC: $7.00 © The Electrochemical Society, Inc.
more detailed distribution of the peaks in some of the DSS samples.
The inclusions were inhomogeneous in nature and consisted of a
mixture of various elements (Si, Al, Mg, Ca, Ti, Mn, and S); each
inclusion was normally different from the others. However, in most
of the inclusions a large Al peak was always present and very often
the Mo and Fe peaks were recessed (compared to the ferrite or
austenite spectrum).
Most of the inclusions found in the ferrite matrix, showed rela-
tively large Al and Mn peaks, denoting the presence of both elements
in the particle. In comparison the Mo and Fe peaks were recessed. In
a previous investigation,16 with a different microprobe that could
resolve and differentiate the Mo and S peaks, we observed that in
this kind of steel a high Mn peak was not always associated with a
high S content in the inclusion. Those inclusions with Mn and S may
appear similar to the ones observed in conventional austenitic stain-
less steels.2,3,5,6 However, the presence of other, different elements
shows the complexity of these inclusions. Indeed, in the present
research, only in one inclusion did we observe the Mo peak to be
larger than in the normal austenite or ferrite spectrum (see Table III).
This can only happen if Mo is enriched in the inclusion or if S is
enriched in the inclusion, we believe that (in this case) sulfur enrich-
ment in the particle is the correct explanation.
The number of inclusions found in the austenite grains was nor-
mally less than in the case of the ferrite matrix. We estimated that
about 30% of the inclusions found were located in the austenite (see
Table III). The austenite particles showed a more complex and dis-
tinct composition than the inclusions found in the ferrite (different
relevant peaks). However, they also can trigger metastable activity
(see below).
We propose that the initiation sites on DSS can be attributed to
four distinct causes: inclusions on the ferrite matrix, inclusions in the
austenite grains, inclusions in the boundary between austenite grains
and matrix and (of less relevance) defective oxide growth over active
grain boundaries or other surface features. In the case of the inclu-
sions, they appear to be complex particles that in some cases can be
found near the grain boundaries, most probably because of the
migration of these contaminants during heat-treatment (see, for ex-
ample, Fig. 6). Although most of them contain Al, Si, Ca, Ti, and
Mn, they have other impurities in lower concentrations, which may
also contribute to the developing of the metastable pits.
After location and analysis of the inclusions, in situ imaging in
the area with the inclusion or inclusions desired was done first in air
and then after the solution was introduced in the cell. We did not find
any difference between the dry images and the wet images (topogra-
phy and optical imaging) before the electrochemical experiments.
Metastable pitting in DSS.—Most of the electrochemical tests
done have already been reported in the same type of DSS.4,15-17 In
most of the experiments we try to isolate a small working area sam-
ple, with the least number of inclusions, typically on the order of
0.1 mm2. In some cases, depending of the location of the inclusions,
we used rectangles with approximate lengths between 300 to
400 mm. By using a small area we ensure that the passive current will
be relatively low and that we will be measuring the current directly
related to the metastable events. Figure 5 shows a typical anodic
polarization in 3.5% NaCl of a DSS sample (scan rate 1 mV/s and test
temperature around 218C). The current transient show metastable
Figure 4. Optical imaging with concurrent topography setup.
Figure 5. Anodic polarization in 3.5% NaCl (room temperature, scan rate 1
mV/s) of DSS (sample 3) showing metastable pitting. The largest event
strarted at 150 mVSCE followed by two events, one starting at 300 mVSCE and
the other at 450 mVSCE.
Table III. Detailed distribution of peaks obtained by EDX in the inclusions on DSS samples used in the present work.
Sample Inclusion
no. no. Phase Large peaks Small peaks Changed
1 2 Austenite Al, Si, Mn, Ca Mg, Ti None
2 Ferrite Al, Mn Si, Ca, Ti FefMof
4 2 Ferrite Al, Mn, Ti(Med) Si FefMof
(A2)
2 Ferrite Al, Mn Ti Mof
3 Ferrite Al, Mn Ti Mof
5 1 Ferrite Al, Si, Mg, Ca Ti CrfFefMof
2 Ferrite Al, Si, Mg, Ca Ti CrfFefMof
3A Austenite Si, Ca Ti, Al Crf
3B Between Al, Mn, SaTi MoaF
4 Between Al, Si, Ca Ti CrfFefMof
aThe Mo peak increased because of the significant contribution from sulfur.
Journal of The Electrochemical Society, 147 (7) 2525-2531 (2000) 2529
S0013-4651(99)12-033-0 CCC: $7.00 © The Electrochemical Society, Inc.
activity at three different potentials. The first metastable event started
at 150 mVSCE and ended at 250 mVSCE, the maximum current
achieved was nearly 70 nA. After the test, the transient for that event
was isolated and its passive current subtracted (in that range of poten-
tial, between 150 and 250 mVSCE, was around 20 nA). By using the
same kind of analysis described by Pistorius and Burstein,22 and
assuming hemispherical pit geometry, it was demonstrated that
metastable activity in DSS shows distinct transients from that of con-
ventional austenitic steels.7In this case, the transient was analyzed
and a final pit radius of about 10 mm was calculated. The relevance
of this result is discussed below. The other two transients in Fig. 5
showed relatively lower currents, one of them (most probably the one
started at 450 mVSCE) could be related to a different metastable pit,
whereas the one started at 300 mVSCE could correspond to a smaller
pit (not found), or as a part of any of the other two events.
In situ NSOM and topography after metastable pitting.—In the
present investigation, we have confirmed that in most cases the
inclusions found showed metastable activity and growth of the pits
under a cover, mainly formed of corrosion products (see Fig. 7 and
9). Figure 6 shows a side-by-side concurrent topography and NSOM
images of the same area in a DSS sample while immersed in 1 M
HCl solution (before the anodic polarization in that solution). Both
of the images on the top show a square area of 60 360 mm. From
the topography images, one can measure the size of the inclusion,
which is approximately 8 mm across and 135 nm in height (see scale
bar in the left of the topography image). The NSOM image shows
the light reflection signal at the same area, with the inclusion having
very little reflection of the light. In this case, higher resolution
images (30 330 mm) are shown on the bottom of each image. The
resolution of the images in both cases was probably better 100 nm.
One important feature in those images corresponds to dark areas of
nearly 1 mm, near the inclusion in the grain boundary. The size of
these dark areas cannot be fully estimated from the topography, but
only from the optical images. Furthermore, they appear to be more
similar to the inclusions than to any of the two adjacent phases. Fig-
ure 7 shows images of the same inclusions before (top) and after
(bottom) the metastable test in 1 M 1 HCl. The bottom images (after
the test) show two important features. First, the inclusions have de-
veloped into a pit of larger size that contains corrosion products,
most probably redeposited outside the pit where the inclusion was
originally located. Some small holes can be seen together with a rel-
atively large aperture of the cavity. It is also important to notice the
change in the Z scale, which turned from 135 nm before the test to
1154 nm after the test. The main contribution in the Zdirection is
from the corrosion products found outside of the pit. The second rel-
evant piece of information is related to the partial “etching” of the
austenite grains (at 2200 mVSCE). The austenite grains, which be-
fore the test were higher in the Zdirection, now appear to be re-
cessed after the (selective) anodic dissolution at lower potentials.
Nevertheless, we have observed in the past that metastable pits de-
veloped with more open cavities in HCl solutions than in the NaCl
solution.7,9,15-17 After the tests, the samples were slightly polished,
and etched. Figure 8 shows the topographical images of two inclu-
sions before (top) and after corrosion tests and repolishing (bottom).
In the case of particle 3 (left side images), the pit was not sufficient-
ly deep and can be seen only as a small hole in the ferrite matrix.
However, in some cases as in the case of particle 2 (right side
images), the pit is still deep after the mild polishing. This information
was used to associate a transient with its correspondent inclusion
when testing an area with multiple inclusions. Evidently, a larger
transient was always associated with those large pits. Figure 9 shows
the topography (3D) and NSOM images of the inclusion associated
with the large transient described above (in the NaCl solution). In this
case, after the metastable pitting experiment, the inclusion developed
into a pit with the typical corrosion products protruding outside the
pit (after the test). Similarly to the cases described above, the Zdirec-
tion in the image below (after the corrosion test) shows an increase
related to the corrosion products. The inside of the pit before and after
metastable pitting shows a different contrast. Before metastable pit-
ting the cavity basically does not reflect the light (therefore the cavi-
ty appears dark), but after metastable pitting the cavity shows a bet-
ter contrast in and outside the pit (despite the fact that the bottom of
the pit is about 1 mm deep in reference to the top of the corrosion
products). This is most probably due to the reflection of the light by
the corrosion products inside the pit. The size of this inclusion was
Figure 6. Concurrent topography and
optical imaging in 1 M HCl of inclusion
P3 (in the ferrite matrix). Notice that in
some cases some features are easily seen
in the optical image. However, both
images contain useful information that
complement each other.
2530 Journal of The Electrochemical Society, 147 (7) 2525-2531 (2000)
S0013-4651(99)12-033-0 CCC: $7.00 © The Electrochemical Society, Inc.
found to be around 15 mm. As mentioned in the previous section, the
calculated transient corresponded to a hemispherical pit of 10 mm
radii. The main difference in the calculated size lies in the depth of
the pit. While assuming hemispherical pits is a reasonable approxi-
mation, in the case of DSS this may be an overestimation, mainly
because an inclusion may develop into a pit as deep as the inclusion.
We believe that metastable events that develop within an inclusion
will eventually find a more resistant material underneath and near the
walls of the pit that can hinder the further growth. However, some
other circumstances may impede metastable growth.24 We believe
Figure 8. Topography of both particles (2
and 3) before (top) and after (bottom) cor-
rosion tests. In both cases a pit developed
in the inclusion. The bottom images were
obtained after the sample was polished,
cleaned, and etched.
Figure 7. Concurrent topography and opti-
cal imaging (in 1 M HCl) of inclusion P3
(in the ferrite matrix), before (top) and after
(bottom) anodic polarization between
2300 mVSCE and 1500 mV. The topogra-
phy shows that the particle developed into
a pit, whereas the austenite grains are par-
tially etched. The optical image shows
some of the deposits created in the surface
near the particle.
Journal of The Electrochemical Society, 147 (7) 2525-2531 (2000) 2531
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that those conditions were not achieved in our experiments, particu-
larly since we have not found any evidence of salt films in the bottom
of the pits, but rather crystallographic etch pits.25,26 Those pits grow
with a cover that, as the pit developed, enhanced the aggressive envi-
ronment within the pit and most likely attacked the cover from inside,
as described in Ref. 26.
Conclusions
Metastable pitting in DSS was studied in situ in both 1 M HCl
and 3.5% NaCl solutions. Topography and NSOM images of the
selected areas containing inclusions showed that they developed into
pits that repassivate (metastable pits). The inclusions are complex in
structure, where metastable pits develop at temperatures below the
CPT, while stable pits, with large corrosion currents, occur at the
CPT. These inclusions were analyzed and found to be inhomoge-
neous in nature and consisting of a mixture of various elements (Si,
Al, Mg, Ca, Ti, Mn, and S). After analysis of the particles, in situ
observation of the particles in 3.5% NaCl and 1 M HCl solutions
showed that they developed metastable pits. Pits and corrosion prod-
ucts developed in both particles present in the austenite grains and in
particles contained within the ferrite matrix. The resolution and
accuracy of the topographic information in liquid is retained (com-
pared to conventional scanning probe methods in air). At the present
time the lateral resolution in this kind of test is around 100 nm.
Acknowledgments
We acknowledge useful discussions with M. Buechler, W. H.
Smyrl, R. B. Comizzoli, J. D. Sinclair and R. P. Frankenthal.
Lucent Technologies, Bell Laboratories, assisted in meeting the publica-
tion costs of this article.
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Figure 9. Concurrent topography and
optical imaging (in 3.5% NaCl at room
temperature) of an inclusion in the ferrite
matrix (sample 3), before (top) and after
(bottom) anodic polarization from 2300
mVSCE to 1850 mVSCE. The topography
image shows how the partially recessed
particle developed into a pit with sur-
rounding corrosion products, whereas the
optical image shows the deposits created
in the surface near the particle.
