Volume 6 Preprint 33
Chromium Implantation in AISI 304L Stainless Steel: Electrochemical Study of the Passive Film
C. M. Abreu, M. J. CristÃƒÂ³bal, X. R. NÃƒÂ³voa, G. Pena, M. C. PÃƒÂ©rez
Keywords: Ion Implantation, AISI 304L, Cyclic Voltammetry, EIS, XPS.Introduction
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Volume 6 Paper C078
STAINLESS STEEL: ELECTROCHEMICAL STUDY
OF THE PASSIVE FILM
C. M. Abreu, M. J. Cristóbal, X. R. Nóvoa, G. Pena, M. C. Pérez
E. T. S. E. I., University of Vigo, Lagoas-Marcosende, 9, 36310 – Vigo
The present work focuses on the effect of chromium implantation on
the formation and evolution of the passive layers formed on an
austenitic stainless steel (AISI 304L) in an alkaline medium. Cyclic
Voltammetry and electrochemical Impedance Spectroscopy have been
used for this study. The characterization of the developed passive
films has been performed by means of X-Ray Photoelectron
Spectroscopy (XPS) and Scanning Electron Microscopy (SEM).
Electrochemical measurements show an important increase in the
intensity of the peak assigned to the Cr3+/Cr6+ oxidation together
with, leading to an increase on the Cr/Fe and Ni/Fe ratio in the oxide
layer of the implanted steel compared to the unimplanted one. This
effect of Ni enrichment is also reflected on the EIS diagrams of the
implanted steel, indicating a modification in the resistance and
capacitance values. High frequency impedance experiments have also
shown the effect of the Cr implantation in the conductive properties of
the passive film.
Keywords: Ion Implantation, AISI 304L, Cyclic Voltammetry, EIS,
Since the late 1970s, high dose ion implantation of carbon, nitrogen
and metallic ions has been reported as an effective solution for
enhancing the lifetime and improving the performance of many kinds
of tools for which wear, friction fatigue and corrosion problems are
presents. The most abundantly studied materials with regard to
surface modification by ion implantation are steels. Chromium ion
implantation is a technique used to increase corrosion resistance of
low alloy steels [1,2]. For alloyed steels and stainless steels, some
works point out that the combined implantation of Cr and N is a good
alternative for obtaining excellent combinations of wear and friction
properties and corrosion resistance [3,4]. Although previous studies
have demonstrated some of the merits of the co-implantation, still
there are some ambiguities remained, particularly the effect on the
modification of tribology.
In the present study we analyze the effect of chromium implantation
on the formation and evolution of the passive layer formed on an
austenitic stainless steel in an alkaline medium, as first step to study
the effect of nitrogen and chromium co-implantation on the
development of passive layers on different stainless steels.
Coupons of AISI 304L of 1 x 1x 1.5 cm (18.22 wt.% Cr, 8.58 wt.% Ni,
1.79 wt.% Mn, 0.34 wt.% Si, 0.43 wt.% Mo, 0.023 wt.% C, Fe balance)
were cut from cold-rolled plates. Before implantation, the samples
were ground with silicon carbide paper up to 600 grit, and then
mechanically polished with diamond paste to 6 µm finish.
Implantation at a nominal dose of 2x1017 Cr ions/cm2, was undertaken
with an acceleration voltage of 150 KeV. The dose used (saturation
dose) was selected taking in consideration the theoretical implantation
profiles calculated by means of the PROFILE  code, which take into
account the material loss due to sputtering.
Electrochemical experiments were performed at 30°C in pre-dearated
NaOH 0.1M solutions and under continuous N2 bubbling, in an
electrochemical cell with two identical working electrodes (exposed
surface of 0.28 cm2 each). A Pt mesh was used as large area counter
electrode and a Hg/HgO 0.1M KOH as the reference one. An AUTOLAB
30 Potentiostat (from EcoChemie) was used for cyclic voltammetry and
electrochemical impedance measurements.
The oxide films are formed, after a cathodic reduction of the native
films at –1.4 V for 1 minute, scanning the potential from hydrogen to
oxygen evolution reactions. In order to establish the evolution of the
voltammetric curves and to generate films thick enough to be
characterised, each sample was cycled eight times at a scan rate of
1mVs-1, which allows relaxation of the redox processes taking place in
the passive layers. Just after the voltammetric tests the electrochemical
impedance spectra (from 10 kHz down to 1 mHz and 10 mV rms signal
amplitude) were registered between –1.4 V and +0.5V, at 100 mV
Immediately after the electrochemical tests, specimens were
ultrasonically cleaned, rinsed and dried to observe the morphology of
the generated oxide layer, using a JEOL 5410 scanning electron
microscope. The chemical characterisation was performed by X-ray
photoelectron spectroscopy (XPS) using a VG ESCALAB 250iXL
spectrometer. The XPS data were collected using monochromatic Al Kα
radiation at 1486.92 eV at a constant analyser pass energy of 20 eV.
Depth profile experiments were performed using an EX05 Ar+ Ion Gun
at 3 kV. To compensate for sample charging during analyses all the
binding energies were referred to the C 1s signal at 285 eV.
Quantification of the species in the passive film was performed via a
commercial curve fitting routine in which the XPS peaks were
deconvoluted into single species peaks. Sensitivity factors for Fe, Cr,
and Ni have been calculated using the bulk material as standard; the
same values have been used for the ions of these elements, while for
oxygen sensitivity factor has been extracted from the Wagner Library
. Sputtering rate values were fixed using perfilometry
3.1. Cyclic Voltammetry
The voltammograms obtained on the AISI 304L and Cr-implanted steel
are depicted in figure 1 and 2 (detailed descriptions have been
reported in a previous work ). As it can be seen in the figure, three
different regions of potential are considered, were similar processes
are evolving in both steels. But comparing the evolution of the shown
curves, some differences can be pointed out.
Current Density / µA cm
Potential / V vs. Hg/HgO
Figure 1: Cyclic voltammograms obtained in NaOH 0.1M solution for
AISI 304L. Potential range: –1.4 V to 0.5 V (vs. Hg/HgO).
(dE/dt=1 mVs-1 ).
Current Density / µA cm
Potential / V vs. Hg/HgO
Figure 2: Cyclic voltammograms obtained in NaOH 0.1M solution for
Cr implanted AISI 304L . Potential range: –1.4 V to 0.5 V (vs. Hg/HgO).
(dE/dt = 1 mVs-1 ).
In the iron activity region, implantation reduced the current density of
the main redox processes (Fe+2/Fe3O4 and Fe3O4/ Fe+2), moreover, it is
interested to note that also modifies the peak shape broadening its
base. As a consequence of this, the implanted steel shows a less
extended passive region than unimplanted steel. In the Cr and Ni
activity region, both the morphology and the intensity of the peak
Cr3+/Cr6+ change with the implantation, showing the Cr-implanted
steel the maximum peak intensity. A high current level in the Ni2+/Ni3+
process and a displacement towards more cathodic potentials is also
registered in the implanted steel.
3.2. Electrochemical Impedance Spectroscopy
The experimental impedance data have been modelled using an
equivalent circuit already proposed for magnetite based passive films
. A good agreement exists between experimental and fitted data. All
over the considered potential range, three time constants are well
differentiated in the obtained impedance spectra for both tested
materials. The time constant at higher frequencies is attributed to the
double layer process, while R2C2 and R3C3 have been assigned to
different redox process taking place in the oxide layers.
3.3. Surface study of the passive films formed by cyclic polarisation.
After the electrochemical tests, the surface morphology of all samples
was observed by SEM. High magnification micrographs (figure 3)
reveals that the oxide surface morphology, which has been growth on
both implanted and unimplanted steel, could be described as
homogeneous layers of small acicular crystal; however, the film
formed on implanted steel is severely cracked.
Figure 3: SEM microstructures on a) Unimplanted AISI 304L and b) CrImplanted AISI 304L
3.4. XPS analysis of the passive films on implanted and unimplanted
AISI 304L steel.
The XPS analysis of the passive film generated on AISI 304L after
electrochemical measurements shows that the elemental composition
is similar to that observed for the native layer, i.e.: OH−, O2−, Fe2+, Fe3+,
Cr3+ and Ni2+. The curve fitting of the spectra gives the following
results. The O 1s peak could be fitted with two peaks: one at 530.1 eV
corresponding to O2- in chromium and/or iron oxide, and one at 531.3
eV corresponding to OH− in hydroxide species. A metallic peak at
574.2 eV and a trivalent oxide peak at 576.4 eV could fit the
chromium spectra (Cr 2p3/2). Unfortunately, the chromium
implantation has been contaminated with carbon C (binding energy
283.0 eV) along the implanted region. Figure 4 depicts the highresolution Cr 2p3/2 peak on implanted AISI 304L, indicating that
chromium has been implanted with a binding energy of 574.2 eV, very
similar (BE: 574.5 eV) to that reported for metallic chromium and Crcarbide mixture .
A metallic peak at 706.9 eV, a divalent oxide peak at 708.5 eV, a
trivalent oxide peak at 710.6 eV, and a satellite peak at 715.1 eV could
fit the iron spectra (Fe 2p3/2). In order to simplify the figures, the
contributions of both Fe ions are summed. The two oxidation states of
iron could correspond to a Fe3O4, as indicated by the shape and
position of Fe 2p3/2 spectral line, when compared with the X-ray
photoelectron spectra of some typical iron corrosion products as: FeO,
α-Fe2O3, γ-Fe2O3 and Fe3O4 .
Figure 4: XPS high resolution Cr 2p emission through the passive film
on AISI 304L
The fitting of the Cr 2p3/2 and Fe 2p3/2 do not take into account the
contributions of the hydroxide species. However, it is reasonable to
consider that the component at 576, eV assigned to the Cr3+ (oxide)
and the component 710,6 eV assigned to Fe3+ (oxide) include some
amount of hydroxide spices together with the oxide, as indicate the
fact that the FWHM (full width at half maximum) is more than 3 eV
, as well as the presence of the OH- peak throughout the film,
especially in the outer most part of the layer. The high resolution
spectra of Ni 2p3/2 could be fitted with two peaks: one at 852.9 eV
corresponding to a metallic state, and the second one with a binding
energy of 855,4 eV, which could be assigned to a divalent oxide state
Ni+2 on Ni (OH)2 or NiFe2O4 .
The XPS depth profile obtained on the AISI 304L after polarisation in
NaOH (figure 5a) shows that the film is composed mainly of iron oxide,
with some amount of Cr3+ distributed throughout the film with a
maximum located close to the oxide/metal interface, when the iron
signal is decreasing. So, the oxide film could be described as an
outermost layer of iron oxide (with small amount of Cr3+) and an inner
layer constituted by both chromium and iron oxides. Nickel ions are
detected only as traces at the outer part of the film; but it is
interesting to note that enrichment in the Ni0 signal at the oxide/alloy
interface is observed. A similar behaviour is observed for the native
oxide layer. This fact has been already reported in the literature
It is important to note that, in any case, it is impossible to determine
whether the oxide film is a mixture of two oxides (Cr2O3 + Fe3O4) or
whether it is a mixed iron-chromium spinel in which chromium
replaces some of the iron positions.
The elemental depth profile of figure 5b, illustrate the effects of
chromium implantation in the composition and structure of the oxide
layer generated on the stainless steel. First consequence is a reduction
in the film thickness, about 115nm in the unimplanted AIS304L and 75
nm in the Cr implanted steel.
A second aspect to be considered is the change in the element
distribution along the passive films: in the implanted 304L, an
significant increase on the Cr/Fe ratio is noted, standing out the
higher concentration of Cr3+ and Ni2+ in the outermost part of the
AISI 304L - cycled
Atomic Conc. / %
Depth / nm
Figure 5a : XPS depth profile of AISI 304L cycled in NaOH 0.1M
AISI 304L Cr implanted - Cycled
Atomic Conc. / %
Depth / nm
Figure 5b : XPS depth profile of Cr-implanted AISI 304L cycled in NaOH
The obtained results suggest the formation of a passive film on the
AISI 304L based on the magnetite structure, with the incorporation of
Cr3+ mainly to the internal part of the layer. In the implanted steel, the
evolution in the voltammetric curves and the higher detected levels of
Cr and Ni indicate an increasing deviation from this structure.
The higher Cr/Fe ratio measured by XPS elemental profile in the
implanted steel could be considered as a sign of a best corrosion
resistance . Nevertheless, the film is generated under
potentiodynamic conditions reaching the potential at which the
oxidation Cr3+/Cr6+ takes place. The solubility of Cr6+ species in this
medium enhances the diffusion of Cr3+ ions towards the outermost
part of the film. The ions mobility can generate stresses into the film
high enough to cause the cracking of the oxide layer, observed in
Electrochemical impedance results in the frequency range 10kHz down
to 1 mHz show the presence of three time constants, as it was
previously mentioned. Constants at medium and low frequencies, R2C2
and R3C3, were assigned to the iron redox processes taking place into
the film at the different scanned potentials. The presence of two
environments for iron cations with different energy levels can explain
the presence of two time constants, .
In the case of AISI 304L, a clear difference between the external part of
the film, that can be considered a nearly pure iron oxide, and the inner
region can be considered as a Fe-Cr spinel oxide (figure 4a). For the
implanted stainless steels, a similar consideration can be made taking
into account the different concentrarion of chromium and nickel
species trough the film (figure 4b).
An example of the Nyquist diagrams obtained in the three potential
regions considered in the voltammetric curves are depicted in figure 6.
Both unimplanted and implanted steels show similar tendencies in
electrical parameters obtained by fitting to the equivalent circuit,
reaching lower capacitance values (close to double layer capacity) and
high resistance values, as expected, in the passive region.
AISI 304L-Cr implanted
- Imaginary part
Real Part/ k Ω. cm
AISI 304L + Cr implanted
- Imaginary part
Real Part/ k Ω. cm
- Imaginary part
AISI 304L + Cr implanted
50 10 mHz
Real Part/ k Ω. cm
Figure 6 : Impedance spectrum obtained for unimplanted and Crimplanted in NaOH 0.1M solution, using as reference electrode (vs.
Hg/HgO) A) -0.657 V, B) -0.257 V, C) 0.343 V
Nevertheless, some differences between them are observed on figure
6. In general, the Cr implanted steel reach lower values of capacitance
and resistance in the three regions, owing to the influence of the
allowing elements reactions.
In the first potential region, the predominance of the iron redox
process above the others reactions, make the registered parameters to
approximate between both materials. As the relative importance of
iron reaction decreases with the anodic displacement of the potential,
the difference increases. In previous works  the electrochemical
behaviour of pure chromium and nickel electrodes was studied in
order to establish their influence in the passive film formation on
stainless steels. Regarding these reported results, it can be concluded
that in the passive region the main influence is Ni redox process and
in the more anodic zone, the Cr3+/Cr6+ reaction. Both mentioned
processes are much more important in the implanted stainless steel as
it can be derived from XPS analysis, that register a significant increase
in Ni2+ and in Cr/Fe ratio.
Another aspect of behaviour of the developed films was considered in
the present work. Previous research of our group suggest that the
conducting properties of passive films formed on stainless steel in
alkaline media can also be detected in the high frequency region (40
MHz down to 100Hz). In order to determine the influence of chromium
implantation this range was examined in more detail, using an
Impedance Analyser (HP4194A) which allows capacitance
measurements in a wide range (from 10-14 to 0.1F) with a resolution of
10-16F. For these experiences, the reference and the counter
electrodes were removed form the cell, so only the two working
electrodes were measured. To minimise noise during the tests the
dearation was stopped.
Figure 7 displays the registered impedance response of both steels in
the high frequency region. As it can be noted, in the unimplanted steel
spectra a small but well-defined capacitive arc is noted at the higher
frequencies. Using the capacity value obtained from this time constant,
a dielectric constant ε = 15 is obtained , which is in accordance
with reported value for magnetite .
In the chromium implanted steel this signal does not exist or it is
overlapped with the medium and lower frequencies processes, which
can be related to the existence of low resistance paths through the
cracks present in the film of this stainless steel. On the other hand,
this change can be attributed to the different conductive properties of
the Cr rich spinel structure formed as consequence of the Cr
However further work will be needed in order to deepen in the high
frequency impedance response of both systems.
- Imaginary part
304L Cr implanted
Real Part/ k Ω. cm
Figure 7: High frequency impedance spectra obtained for implanted
and unimplanted 304L in NaOH 0.1M solution at zero current in a twoelectrode cell.
The performed experiences on chromium implanted AISI 304L revealed
that the passive layer developed under potentiodynamic conditions in
0.1M NaOH is thinner.
The higher disposal of Cr in this steel is the origin of the increase in
the Cr/Fe ratio through the film, as compared to the unimplanted
steel, and the diffusion of chromium species from the metal to the
solution can be the reason of the cracking of the oxide layer. Another
significant aspect about the composition of the film formed on the
implanted steel is the Ni2+ ions enrichment especially in the outermost
EIS experiences performed in the medium and low frequency range
have revealed the influence of the higher amounts of Ni and Cr species
present in the film on the electrochemical behaviour. At higher
frequencies, the obtained results show a change in the conductivity
properties of the film induced by the chromium implantation.
The authors gratefully acknowledge the financial support from the
MCYT (Project Nº ) and materials supplying from ACERINOX, S.A. The
technical assistance of Dr. Carmen Serra in XPS analysis is specially
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