Article

Evaluation of passive film behaviour of super austenitic stainless steels at different potential regions using dynamic electrochemical impedance spectroscopy

Journal of Solid State Electrochemistry (Impact Factor: 2.45). 07/2010; 14(7):1197-1204. DOI: 10.1007/s10008-009-0948-5

ABSTRACT

Potentiodynamic anodic polarisation and dynamic electrochemical impedance spectroscopic (DEIS) measurements were carried out
on 316L stainless steel and alloys 926 and 31 in natural seawater in order to assess the crevice corrosion resistance. DEIS
measurements were performed over a wide range of potentials covering the corrosion potential, passive region, breakdown region
and dissolution region. The impedance measurements in potentiodynamic conditions clearly reveal the changes that occur in
the passive layer with change in potential. The impedance spectra at different potential regions were also discussed elaborately.
The surface morphology of the alloy after crevice corrosion was studied using optical microscope and atomic force microscopy.

KeywordsStainless steel-Crevice corrosion-Dynamic electrochemical impedance spectroscopy-Atomic force microscopy

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Available from: Srinivasan Nagarajan, Jul 02, 2015
ORIGINAL PAPER
Evaluation of passive film behaviour of super austenitic
stainless steels at different potential regions using dynamic
electrochemical impedance spectroscopy
S. Nagarajan & V. Raman & N. Rajendran
Received: 3 August 2009 / Revised: 8 September 2009 / Accepted: 5 October 2009 / Published online: 27 October 2009
#
Springer-Verlag 2009
Abstract Potentiodynamic anodic polarisation and dynam-
ic electrochemical impedance spectroscopic (DEIS) meas-
urements were carried out on 316L stainless steel and alloys
926 and 31 in natural seawater in order to assess the crevice
corrosion resistance. DEIS measurements were performed
over a wide range of potentials covering the corrosion
potential, passive region, breakdown region and dissolution
region. The impedance measurements in potentiodynamic
conditions clearly reveal the changes that occur in the
passive layer with change in potential. The impedance
spectra at different potential regions were also discussed
elaborately. The surface morphology of the alloy after
crevice corrosion was studied using optical microscope and
atomic force microscopy.
Keywords Stainless steel
.
Crevice corrosion
.
Dynamic electrochemical impedance spect roscopy
.
Atomic force microscopy
Introduction
Crevice corrosion is one of t he most serious problems
when stainless steel (SS) is used in chloride environments,
which is often very difficult to control. This form of
corrosiononSSisconsideredtoinitiateviaagradual
build up of acidity within the crevice. The various stages
in which this occurs are as follows: depletion of oxygen
in the crevice makes it a net anode; metal ions produced
by passive dissolution within the crevice are only trans-
ported away slowly due to the hydrolysis of these ions
which leads to an increase in the acidity within the crevice
area. To ensure electroneutrality, the anions migrate into
the crevice , an d ev ent ua lly, the cr evi ce s olu ti on b eco me s
sufficiently aggressive to depassivate the metal surface,
i.e. initiate crevice corrosion [14].
In order to avoid such type of corrosion, counter
measures such as structural modifications, reducing the
aggressiveness of the envir onment and materials selection,
have been tried earlier. The selection of materials with
sufficient resistance to crevice corrosion i n the given
environments has also been developed. Many researchers
have reported that SS containing high alloying elements
such as Cr, Mo and N are used to improve the crevice
corrosion resistance [5, 6]. Bond and Dundas [7] conducted
the crevice corrosion test for commercial SS and found that
high Cr and Mo containing ferritic SS possess superior
corrosion resistance than austenitic SS. A strong correlation
between pitting corrosion and crevice corrosion mechanism
was reported by Wilde and William for some metal-
environment systems, particularly in NaCl solutions and
in seawater [8]. Modelling of pit and crevice corrosion
propagation was reviewed and addressed in considerable
depth both empirically and also through mechanical
modelling [9].
Brigham [10] has proposed the mechanism leading to the
initiation of crevice corrosion based on the fundamental
thermodynamic and electrochemical principles. Various
other crevice corrosion mechanisms have been proposed
over the years. Electrochemical impedance measurement
performed on the creviced 316L SS leads to the conclusion
that the initiation stage of crevice corrosion in chloride
environments is similar to that for the initiation of pitting
corrosion [11].
S. Nagarajan
:
V. Raman
:
N. Rajendran (*)
Department of Chemistry, Anna University Chennai,
Chennai, India
e-mail: nrajendran@annauniv.edu
J Solid State Electrochem (2010) 14:11971204
DOI 10.1007/s10008-009-0948-5
Page 1
During the crevice corrosion process, the nature of the
passive film on the metal surface may be varied with
respect to its environment. In such a case, the stationary
condition for the investigated system is not fulfilled, and
the classical EIS fails. This disadvantage in using the
classical EIS technique initiate us to carry out dynamic
electrochemical impedance spectroscopy (DEIS), which can
possibly follow the passive layer changes on the metal
surface during the crevice corrosion process. Darowicki et
al. [1215] developed the dynamic electrochemical imped-
ance techni que and investigated the pitting corrosion, pit
pre-initiation state and trans ports of o rganic coatings.
Hence, the objective of this study is to understand the
passive film behaviour of 316L SS, alloy 926 and alloy 31
in natural seawater environment under the influence of
changes in the electrode potential.
Experimental
The chemical compositions of the 316L SS and alloys 926
and 31 are given in Table 1 . Bars of dimensions 50×50×
3 mm with 10 mm hole in the centre were cut out and wet-
polished with emery paper up to 600. The specimens were
then washed with distilled water, degreased with acetone
followed by alcohol and dried. Multi-crevice corrosion test
assembly was prepared based on the standard guide for
crevice corrosion test, ASTM G78. The crevice former was
a polyacetal resin rings, which had 20 teeth of dimensions
2×2 mm; two of these ri ngs were pressed onto the
specimen at a torques of 8.5 Nm with titanium bolt and a
nut, so that 20 small crevice sites were formed on each side
of the specimen. The bolt was electrically insulated from
the specimens using a polytetrafluoroethylene tape. The
multi-crevice assembly thus obtained were immersed in the
electrolyte. A conventional three-electrode cell was used for
all the electrochemical measurements. A saturated calomel
electrode (SCE) was used as a reference electrode, platinum
foil as a counter electrode and the test material as the
working electrode. Natural seawater collected from the
coastal area of Che nnai, India, served as the electrolyte.
Potentiodynamic cyclic polarisation studies were carried
out for the test specimens in natural seawater. The
potentiostat (model PGSTAT 12, AUTOLAB, The Nether-
lands B.V) was controlled by a personal computer. A
dedicated software (GPES version 4.5) was used for
conducting the p olarisation experiments. The potential
was applied on the working electrode at a scan rate of
0.167 mV s
1
. In order to test the reproducibility, the
experiments were performed in triplicate.
DEIS measurements were carried out using a frequency
response analyser (FRA), which included a potentiostat
model PGSTAT 12. Impedance spectra were acquired
from the corrosion potential to the dissolution region with
a step potential of 20 mV in the frequency range of
55 kHz0.1 Hz with a 10-mV amplitude sine wave
generatedbytheFRA.
The surface morphology was recorded by atomic force
microscopy (AFM) using a Nanosurf easy scan 2 (Nanosurf
AG, Grammetstrasse 14, 4410 Liestal, Switzerland). The
images were acquired by contact mode, using silicon nitride
cantilevers with a spring constant of 0.15 N /m at a
resonance frequency of 13 kHz. All images were recorded
under air atmosphere at room temperature.
Results and discussion
Potentiodynamic polarisation studies
The anodic polarisation curves of the three specimens
viz type 316L SS, alloy 926 and alloy 31 is given in
Fig. 1. The corrosion potentials for the alloys were
observed at 300, 238 an d 197 mV vs SCE, respec-
tively, in natural seawater. Compared to the 316L SS, the
corrosion potential values of alloy 926 and alloy 31 were
shifted to nobler region, due to the presence of higher
amounts of nickel, molybdenum, chromium and nitrogen.
Table 1 Chemical composition of investigated alloys (wt.%)
Alloy Main alloying elements (wt.%)
Cr Ni Mo N C Mn
316L SS 17.2 12.6 2.4 0.02 0.030 1.95
Alloy 926 21.0 25.0 6.5 0.20 0.003 1.95
Alloy 31 27.0 31.0 6.5 0.20 0.003 0.68
-400
0
400
800
1200
1x10
-3
1x10
-5
1x10
-6
1x10
-4
1x10
-2
316L SS
Alloy 926
Alloy 31
I (A cm
-2
)
E (mv vs SCE)
Fig. 1 Potentiodynamic polarisation curves for 316L SS, alloy 926
and alloy 31 in natural seawater
1198 J Solid State Electrochem (2010) 14:11971204
Page 2
Crevice potential of the materials is the criterion for
evaluating resistance to crevice attack, whi ch is directly
influenced by the amount of passivating elements present in
the alloy. The critical crevice potential for the type 316L SS
was 133 mV, whereas for alloy 926 and alloy 31, the
critical c revice potentials were observed at 936 and
989 mV, respectively. Thus, alloy 926 and alloy 31 exhibits
better crevice corrosion resistance in the natural seawater
compared to reference material 316L SS.
The observed increase in crevice corrosion resistance of
the super austenitic SS can be explained based on the
following postulates. During active dissolution, nickel and
chromium were generally dissolved, whereas non-active
elements such as nitrogen were enrich at the surface. Such
an enrichment of the passive film inhibits the anodic
dissolution of the materials by two orders of magnitude,
presumably through the formation of iron nitride. The
presence of iron nitride inhibits the autocatalytic process of
pit formation and also it helps in the healing of the already
formed pit. Thus, addition of small amounts of nitrogen can
enhance the crevice resistance and passivation character-
istics [1619]. It is believed that the nitrogen in the steel
dissolves; it consumes protons in the pit to form ammonia,
thus preventing the lowering of the pH within the pit, which
contributes to the suppression of acidification inside the pit.
It is also speculated that surface films are stabilised through
passivation or nitrogen enrichment at the film/m etal
interface to prevent the attack of the chloride anions [20].
A synergistic influence of nitrogen and molybdenum
on pitting and crevice corrosion resistance at the surface
has been reported earlier [21]. Newman et al. [22]
reported that enrichment of nitrogen and molybdenum at
the interface is the predominant factor for preventing
further dissolution of the substrate consequent to the
destruction of the passive film.
Potentiodynamic impedance spectroscopic studies
The impedance spectra were obtained for every 20-mV
increase in the potent ial, beginning from the open-circuit
potential (OCP) to the dissolution potential. Figure 2ac
shows that three-dimensional representations of all the
impedance spectra of 316L SS, alloys 926 and 31 were
obtained from the potentiodynamic impedance studies. A
linear increase in the magnitude of impedance was
observed from OCP irrespective of the alloy. In the case
of 316L SS, the impedance values increases with potent ial
and attains a maximum around 110 mV. Further increase in
potential causes a sudden decrease in the impedance values.
However, in the case of alloys 9 26 and 31, the magnitude
of impedance attained a maximum value approximately at
180 and 140 mV, respectively. Further increase in the
potential results in the decrease of the impedance values up
to a certain extent and then the magnitude of impedance
increases. The results obtained from DEIS, when correlated
with polarisation curves of 316L SS, alloys 926 and 31,
0
525
1050
0
450
900
0
450
900
b
Z' (ohmcm
2
)
E
(mV vs SCE)
-
Z'' (ohmcm
2
)
-200
0
200
0
Z'
(ohmcm
2
)
700
350
700
350
-Z'' (ohmcm
2
)
E (mV vs SCE)
a
0
0
550
1100
0
500
1000
0
500
100
0
c
E
(mV vs SCE)
- Z'' (ohmcm
2
)
Z' (ohmcm
2
)
Fig. 2 The impedance versus
potential diagram of (a)316L
SS, (b) alloy 926 and (c)alloy31
J Solid State Electrochem (2010) 14:11971204 1199
Page 3
revealed a precise information on the passive film behav-
iour from OCP till dissolution potential. For instance,
firstly, all the alloys exhibited a maximum resistance before
breakdown. Tho ugh polarisation curves also depicted the
same, the result obtained from DEIS was more apparent.
Secondly, in the case of alloys 926 and 31, the decline in
the value before the secondary maxima, essentially due to
the thinning of the passive film, was not clearly visible
from the polarisation c urves. Thirdly, the breakdown
potentials can also be determined precisely using DEIS
compared to the polar isation studies.
In order to have a better unders tanding of the crevice
corrosion process, elect rochemical impedance spectra was
obtained at selec ted regions: OCP, passive, breakdown and
dissolution. Figure 3ac shows the Nyquist plots at OCP,
and Fig. 4ac shows the Nyquist plots in the passive region
for 316L SS, alloys 926 and 31, respectively. All the
spectra exhibited a similar behaviour containing a straight
line with a high magnitude of impedance, which indicates a
highly resistant passive film. This is in good agreement
with the earlier report [23].
Figure 5ac shows the impedance spectra of the alloys at
the breakdown potential. The reference material 316L SS
exhibited a two-time constant. However, for alloys 926 and
2.1 Hz
0.38 Hz
0.19 Hz
0.1 Hz
(a)
(b)
(c)
-Z'' (ohmcm
2
)
300
60
0
300
600
0
0
0 450 900
0
450
900
0.27 Hz
10.3 Hz
1.4 Hz
0.1 Hz
-Z'' (ohmcm
2
)
0 500 1000
0
500
1000
0.27 Hz
2.7 Hz
1.1 Hz
0.1 Hz
-Z'' (ohmcm
2
)
Z' (ohmcm
2
)
Z'
(ohmcm
2
)
Z'
(ohmcm
2
)
Fig. 4 Nyquist plot at passive region for (a) 316L SS, (b) alloy 926
and (c) alloy 31
(a)
-Z'' (ohmcm
2
)
Z' (ohmcm
2
)
300150
300
150
0
0
0.19 Hz
0.1 Hz
0.38 Hz
2.8 Hz
0 300 600
0
300
600
0.37 Hz
0.1 Hz
(b)
-Z'' (ohmcm
2
)
Z' (ohmcm
2
)
14.3 Hz
1.4 Hz
0 300 600
0
300
600
0.37 Hz
27.1 Hz
1.4 Hz
0.1 Hz
(c)
-Z'' (ohmcm
2
)
Z' (ohmcm
2
)
Fig. 3 Nyquist plot at open-circuit potential for (a) 316L SS, (b) alloy
926 and (c) alloy 31
1200 J Solid State Electrochem (2010) 14:11971204
Page 4
31, a two-time constant and also a low-magnitude inductive
loop at lower frequency was observed.
The appearance of a capacitive loop in the form of a
semicircle for alloys 926 and 31 can be attributed to a
charge transfer at the film/solution interface. A fast charge
transfer process, which takes place at the interface for all
the alloys, was revealed from a very small and depressed
high frequency capacitive loop. The inductive loop ob-
served at lower frequency is most probably due to the
relaxation process of an intermediate species of the
dissolution reaction. Substantial reports are available for
the presence of inductive loops in the low frequency range
of the impedance spectra obtained for high alloys [24, 25].
Figure 6ac shows the Nyquist plot of 316L SS, alloys 926
and 31 at the dissolution region. The 316L SS exhibited a
semicircle, having lower impedance value with a more
pronounced Warburg tail. The Warburg impedance ob-
served may be attribut ed to an ionic diffusion through the
solid corrosion products that precipitated near the pit mouth
as reported by Dawson and Ferreira [11]. The Nyquist plot
correspond ing to alloy 926 at the dissolution region
exhibits two capacitive loops at higher frequency and an
inductive loop at intermediate frequency, and this was
0.1 Hz
1.0 Hz
44.6 Hz
10
5
44.6 Hz
5 10
(a)
-Z'' (ohmcm
2
)
Z' (ohmcm
2
)
0
0
20
10
40 kHz
1.0 Hz
14.3 Hz
0.1 Hz
0
0
10
(b)
-Z'' (ohmcm
2
)
Z' (ohmcm
2
)
20
30
14.0 Hz
40 kHz
1.0 Hz
0.1 Hz
15
15
(c)
-Z'' (ohmcm
2
)
Z' (ohmcm
2
)
30
Fig. 5 Nyquist plot at breakdown potential for (a) 316L SS, (b) alloy
926 and (c) alloy 31
5
2.5
0.1Hz
31.8 Hz
1.3 kHz
2.5 5
-Z'' (ohmcm
2
) -Z'' (ohmcm
2
) -Z'' (ohmcm
2
)
(a)
(b)
(c)
7.2 kHz
510
0
5
10
40 kHz
74 Hz
1.9 Hz
0.1 Hz
Z' (ohmcm
2
)
Z'
(ohmcm
2
)
Z'
(ohmcm
2
)
3.5 7.0
0.0
3.5
7.0
7.6 kHz
74.7 Hz
1.1 Hz
0.1 Hz
Fig. 6 Nyquist plot at dissolution region for (a) 316L SS, (b) alloy
926 and (c) alloy 31
J Solid State Electrochem (2010) 14:11971204 1201
Page 5
presumably due to the metal charge transfer and relaxation
process of an intermediate species at the film/solution
interface, while the low frequency capacitive loop can be
assigned to the secondary passivation. The lowest freque n-
cy inductive loop obtained was not considered in the
subsequent analysis because of the possibility that it was
due to the slow process of the system. Whereas in case of
alloy 31, two capacitive loops at higher and intermediate
frequency regions, another small capacitive loop followed
by an inductive loop at lower frequency was observed. The
small semicircle at the intermediate frequency can be due to
the adsorption effects, and it has been attributed to the
increase in the surface coverage of intermediate species
formed during metal dissolution. The formation of such
intermediate species can be associated with the enhanced
crevice corrosion resistance of alloy 31. Further, the very
low frequency inductive loop could also be related to an
increase in the ionic conductivity of the oxide film with
potential. It has been reported that the chloride activity
within their crevices increases only very slightly compared
to the bulk solution, and it has been proposed that much of
the chloride in the crevice must have formed complexes
(a)
(b)
Fig. 7 Surface morphology of
316L SS after crevice corrosion
test (a) photographs and (b)
optical micrographs
(a)
(b)
0612
0
400
800
ZAxis, nm
X Axis, µm
685.07 nm
755.14 nm
(c)
Fig. 8 Surface morphology of
alloy 926 after crevice corrosion
test (a) photographs (b) optical
micrographs and (c) atomic
force microscopy images
1202 J Solid State Electrochem (2010) 14:11971204
Page 6
like (MOMOHCl)
ads
and (MOMCl)
ads
with the metal ions,
which accelerates metal dissolution [2628]. The lower
frequency inductive loop may be due to the relaxation
process of an intermediate species, which takes place at a
slower phase [29]. Generally, such slower relaxation
processes does not play a significant role in the passivation
behaviour. However, alloy 31 and alloy 926 have better
crevice corrosion resistance than 316L SS.
Surface characterisation
Figure 7a shows the visual image of 316L SS after crevice
corrosion test. The image clearly revealed that a severe
damage on the surface of the specimen has taken place due
to the crevice corrosion process. In order to get further
insight on the crevice corrosion process, optical micro-
graphs were taken at the crevice site and are shown in
Fig. 7b. The micrographs exhibited a surface with a few
millimetre depth of crevice-affected area. Similar surface
characterisation studies were also carried out for alloy 926
and31andareshowninFig.8a, b and 9a, b, respectively.
The optical microscopic image of the alloys 926 and 31
does not give any clear evidence for the crevice corrosion
process. In order to probe in further detail, AFM micro-
graphs were taken at the crevice site for these specimens,
and the images are given in Figs. 8c an d 9c, res pe ct ive ly,
for the alloys 926 a nd 31. Th is pit app earance is
presumably due to a particle-enhanced local dissolution
of the matrix at the boundary. The depths of the pit were
calculated for these two all oys; alloy 926 exhibited a pit
depth of around 720 nm, and alloy 31 exhibited a pit depth
around 600 nm. From the pit depth values, it was
envisaged that the alloy 31 exhibits better crevice
corrosion resistance than the other two alloys viz 926
and 316L SS.
Conclusion
The crevice corrosion resistance observed for alloys 926
and 31 in natural seawater shows higher crevice corrosion
resistance compared to that of 316L S S, and this is
attributed to the presence of both nitrogen and molybdenum
in the super austenitic alloys. The DEIS technique has been
used to evaluate the crevice corrosion resistance of alloys
926 and 31 in natural seawater. The Nyquist plot at
different potential regions, namely, OCP, passive region,
breakdown region and dissolution region, reveals that the
impedance changes as a function of potential. The critical
crevice potential measured from the polarisation data is in
good agreement with the DEIS data. An atomic force
microscopic study reveals nano-crevice attack on these
alloys even though alloys 926 and 31 exhibits higher
crevice corrosion resistance than 316L SS.
(a)
(b)
0.00 3.75 7.50
0
325
650
X Axis, µm
ZAxis, nm
378.76 nm
618.81 nm
(c)
Fig. 9 Surface morphology of
alloy 31 after crevice corrosion
test (a) photographs (b) optical
micrographs and (c) atomic
force microscopy images
J Solid State Electrochem (2010) 14:11971204 1203
Page 7
Acknowledgements TEQIP is gratefully acknowledged for provid-
ing instrumental facilities.
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1204 J Solid State Electrochem (2010) 14:11971204
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  • Source
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