Available online at www.sciencedirect.com
Scripta Materialia 62 (2010) 45–48
www.elsevier.com/locate/scriptamat
In situ surface investigation of austenitic stainless steel in nitric acid
medium using electrochemical atomic force microscopy
N. Padhy, S. Ningshen, U. Kamachi Mudali* and Baldev Raj
Corrosion Science and Technology Division, Indira Gandhi Center for Atomic Research, Kalpakkam 603102, Tamil Nadu, India
Received 7 August 2009; revised 19 September 2009; accepted 19 September 2009
Available online 24 September 2009
Using potentiodynamic polarization and electrochemical atomic force microscope, the surface morphology of austenitic stainless
steel has been investigated with the aim of understanding the passive film property in nitric acid medium. It has been found that
passive film consists of platelet-like structures with a moiré pattern at lower concentrations (0.1, 0.5 M); at higher concentrations
(0.6, 1 M) the platelet-like structures disappear, revealing the grain boundary structure.
Ó 2009 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.
Keywords: Stainless steels; Atomic force microscope; Corrosion; Grain boundaries; Surface morphology
Austenitic stainless steels (SS) of type 304L SS are
used at different concentrations and temperatures of
HNO3 in nuclear fuel reprocessing plants due to their
good passivation property, high corrosion resistance
and good mechanical property [1–3]. However, certain
corrosion problems have been observed, depending on
the concentration of HNO3 acid, the presence of redox
electrochemical species and the temperature, leading to
degradation in the corrosion resistance of components
and pipings of type 304L SS [1,2]. 304L SS is indispensable for successful fuel reprocessing application due its
affordable cost, fabricability and availability, thus a detailed fundamental study to understand the corrosion
aspects in nitric acid medium is demanded.
A number (albeit small) of significant attempts have
been made to comprehend the phenomenon of localized
corrosion in stainless steel in various electrochemical
environments using an in situ scanning probe technique.
Kamachi Mudali and Katada [4] investigated the nanomechanical properties of passive film of 304LN SS and
predicted a decrease in stiffness value and increase in
height of passivated surface in 0.5 M NaCl medium under different surface conditions. Williford et al. [5] studied pitting and intergranular corrosion of 304 SS in
NaCl and oxalic acid medium, and demonstrated the
growth of pit and initiation of intergranular corrosion.
Zhang et al. [6] analyzed the role of corrosion products
* Corresponding author. Tel.: +91 44 27480121; fax: +91 44
27480301; e-mail: [email protected]
on pitting corrosion of SUS 304 SS and reported that
the current density for the pit growth is higher than that
of the applied current. However, to our knowledge, no
in situ study has been carried out to date to understand
the process of corrosion in austenitic stainless steel in
HNO3 medium which is a major concern in spent nuclear fuel reprocessing plants. The aim of the present work
is to understand the in situ surface morphology of 304L
SS in HNO3 medium using an electrochemical atomic
force microscope (EC-AFM).
Specimens of dimensions 10 10 1.6 mm were cut
from a sheet of 304L SS and solution annealed at
1323 K for 30 min. The specimens were mechanically
polished in SiC emery sheet up to 1400 grade and then
polished in diamond paste (0.25 lm) to get a mirror-like
finish. Finally, all the polished specimens were ultrasonically cleaned in acetone and double-distilled water.
Potentiodynamic anodic polarization study was carried out to understand the polarization behavior of
304L SS in 0.1, 0.5, 0.6 and 1 M HNO3. Polarization
tests were conducted at room temperature using a Solartron 1287 electrochemical interface associated with a
standard three-electrode cell comprising Pt as the counter electrode, Ag/AgCl as the reference electrode and the
sample as the working electrode. The test solution was
not deaerated as the experiments were carried out to
simulate the plant condition. Five sets of experiments
were carried out for each concentration to check the
reproducibility. Corrosion potential (Ecorr) and corrosion current density (Icorr) were calculated by the Tafel
extrapolation method by fitting Tafel lines in the close
1359-6462/$ - see front matter Ó 2009 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.
doi:10.1016/j.scriptamat.2009.09.026
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N. Padhy et al. / Scripta Materialia 62 (2010) 45–48
vicinity (100 mV) of the corrosion potential. Passive current density (Ipass) is the current density in the passive
range of type 304L SS at a particular concentration,
and the transpassive potential (Etranspass) was determined at the potential where the anodic current increased monotonically to exceed the mean passive
current density. The range of anodic current for determination of the transpassive potential was 65–150 lA.
The quantitative values given for the corrosion related
parameters are the mean of all the tests carried out in
a particular solution, with the standard deviation given
to indicate the accuracy.
An in situ surface morphological investigation in the
aforementioned HNO3 concentrations at a fixed location
of the specimen (without changing the specimen position)
was carried out using an EC-AFM (Solver Pro EC, NTMDT, Moscow) in ambient conditions with potentiostatic control to monitor the sequential morphological
changes occurring over the surface. Like the polarization
study, the solution was not deaerated. The electrochemical cell is made up of Teflon and has a volume capacity of
5 ml. It consists of Pt as the counter electrode, Ag/AgCl as
the reference electrode and the sample as the working
electrode. Prior to measurement, the bipotentiostat was
calibrated for the applied and measured potential ranges.
Similarly, the scanner used was also calibrated using standard grating. All the morphological measurements were
carried out in semi-contact mode using a conical silicon
tip having a force constant of 5 nN m1 with a frequency
range of 50–150 Hz and a cone angle of less then 22°. The
potential for the in situ study was varied from the cathodic to the anodic region, based on the polarization curve
obtained, at a step of 50 mV vs. Ag/AgCl, and by holding
at that potential for 10 min.
The results for anodic polarization study are as shown
in Figure 1. The polarization plots did not reveal any active–passive transition behavior, as after the cathodic region it quickly entered into the passive region over a wide
potential range, followed by the transpassive regime. The
quantitative values of all the corrosion related parameters are given in Table 1. The maximum standard deviations for Ecorr, Icorr, Ipass and Etranspass for all the
concentrations of HNO3 given in Table 2 were within
±6 mV vs. Ag/AgCl, ±5 103 lA cm2, ±3 lA cm2
Potential (V) Vs Ag/AgCl
2.0
1.5
0.1 M HNO3
0.5 M HNO3
0.6 M HNO3
1 M HNO3
1.0
0.5
0.0
-0.5
-1.0
-2
10
-1
10
0
10
1
10
2
3
10
10
4
10
2
Log Current density (µ A/cm )
Figure 1. Potentiodynamic polarization behavior of 304L SS in 0.1,
0.5, 0.6 and 1 M HNO3.
and ±27 mV vs. Ag/AgCl, respectively. By going from
the lowest to the highest concentration, the results revealed that Ecorr increased by 50 mV vs. Ag/AgCl, Icorr
increased by two orders of magnitude, Ipass increased
by one order of magnitude and Etranspass decreased by
around 150 mV vs. Ag/AgCl. It is well known that the
reduction of nitric acid is autocatalytic in nature, with
the generation of aqueous HNO2 and gaseous species
such as NO and NO2 depending on the concentration
used [3,7]. With increasing nitric acid concentration,
the reduction rate, and thus the oxidizing power, also increases as the ratio of nitrous acid to nitric acid increases.
Thus, from a kinetics point of view, the generation of nitrous acid, which maintains the autocatalytic nature,
accelerates the corrosion rate due to the oxidation of
alloying elements such as Fe and Cr. Consequently,
chromium, which is key to passive film stability, depletes
from the surface, leading to an increase in the corrosion
potential, and increases in the passive current density
and corrosion current density. Similarly, owing
to the higher catalytic activity at higher concentrations,
the transpassive dissolution of passive film also
becomes faster, leading to a decrease in the tranpassive
potential.
The results for the in situ surface morphology study
of type 304L SS in 0.1, 0.5, 0.6 and 1 M HNO3 at
1100 mV vs. Ag/AgCl are presented in Figure 2(a–d).
The justification for showing the morphology at
1100 mV is that this is the best passive condition at all
the concentrations, and contrasting surface features
were observed clearly at this potential in all the concentrations. Figure 2a represents the surface morphology
inside the solution at 1100 mV in 0.1 M HNO3, Figure
2b represents the surface morphology at 1100 mV in
0.5 M HNO3, Figure 2c represents the surface morphology at 1100 mV in 0.6 M HNO3 and Figure 2d represents the surface morphology at 1100 mV in 1 M
HNO3. As can be seen from Figure 2a, in 0.1 M
HNO3 the surface is covered with platelet-like structures
[8] in the form of moiré pattern, i.e. there is a successive
formation of layer structures. This type of growth mode
of the platelets has been correlated with the growth of
the oxide layer by Vignal et al. [8] using a limited height
method by plotting the platelets’ mean area vs. Z-height.
The surface morphology in 0.5 M HNO3 (Fig. 2b) at
1100 mV also shows the formation of the platelet-like
structures; however, the size of the platelets is smaller
compared to in 0.1 M HNO3. The platelets’ projected
area (AS), the inclination (h) between adjacent layers,
the disorientation angle (u) between platelets in the
same layer and the root mean square roughness (Rq) value for 0.1 and 0.5 M were calculated using NOVA software, and are presented in Table 2. The size of the
platelets was determined as rectangular grains, the inclination (h) was determined by taking three platelets (two
in the same layer and the third in an adjacent layer), the
disorientation angle was determined by taking three
platelets in the same layer (top to bottom) and the root
mean square roughness was determined by using Eq. (1):
sffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi
Pn
i¼1 ðZ i ZÞ
Rq ¼
ð1Þ
n
N. Padhy et al. / Scripta Materialia 62 (2010) 45–48
47
Table 1. Polarization parameters for 304L SS in 0.1, 0.5, 0.6 and 1 M HNO3.
Conc. of HNO3 (M)
Ecorr (mV vs. Ag/AgCl)
Icorr (lA cm2)
Ipass (lA cm2)
Etranspass (mV vs. Ag/AgCl)
0.1
0.5
0.6
1
125
100
97
75
1.35 101
7.50 101
1.01 101
1.65 101
6 101
9.5 101
11 101
1.45 102
1400
1345
1330
1250
Table 2. Statistical EC-AFM data for 0.1, 0.5, 0.6 and 1 M HNO3.
Conc. of HNO3 (M)
AS (lm2)
h (deg)
u (deg)
Rq (nm)
0.1
0.5
0.6
1
0.8–1
0.4–0.7
–
–
45–65
10–20
–
–
2
1
–
–
9
6
4
20
where Zi is the height value of each single point, Z is the
mean of all the height values and n is the number of data
points within the image. From Table 2, it is evident that
the size of the platelet, the angle of inclination, the disorientation angle and the root mean square roughness
value decreased with increasing HNO3 concentration,
giving a uniform and smooth morphology at 0.5 M
HNO3. The interesting feature in both 0.1 M and
0.5 M HNO3 is that no visible corrosion process was observed on the surface, with the surface being covered
fully by the platelets. The morphology in 0.6 M HNO3
is quite interesting and different from that in both 0.1
and 0.5 M HNO3. Both the formation of platelet-like
structures and the initiation of the surface dissolution
process were observed. However, the platelets formed
were very few in number, and the surface dissolution occurred at the brighter regions of the surface, which are
the elevated portions. The morphology in 1 M HNO3
is totally different as compared to those in 0.1, 0.5 and
0.6 M HNO3. In 1 M HNO3 the formation of plateletlike structures was not observed at all; instead, grain
boundaries opened up at a potential of 1100 mV. At
higher concentrations of HNO3, the oxidizing power increases, which preferentially attacks oxide boundaries as
these are the high-energy regions.
Overall, the in situ results demonstrate the formation and breakdown of passive film with increasing
oxidizing power of nitric acid. According to the passivation model for austenitic stainless steel in acidic media [9], passive film is a gel-like structure, which starts
decaying in an aggressive electrochemical environment
and eventually breaks down where local inhomogenities expose a small amount of the surface, thereby initiating surface dissolution. In the present investigation,
a decrease in surface roughness value was observed
from the surface morphology study up to 0.6 M
HNO3. The decaying of the passive film is supported
by the decrease in roughness value, as the height of
the passivated surface is correlated with the roughness
of the surface [4]. The decrease in roughness from 0.1
to 0.6 M HNO3 is due to thinning of the passive film,
which symbolizes the start of the decay nature. Supportive evidence for the breakdown of passive film
and the initiation of surface dissolution is apparent
from the morphology at 0.6 M HNO3, where surface
dissolution is observed in the presence of platelet-like
structures. However, towards higher concentrations
(1 M) the increase in roughness (20 nm) is due to the
increase in the surface dissolution process and the
opening up of oxide boundaries.
Figure 2. Surface morphology of 304L SS at 1100 mV in (a) 0.1 M HNO3, (b) 0.5 M HNO3, (c) 0.6 M HNO3 and (d) 1 M HNO3.
48
N. Padhy et al. / Scripta Materialia 62 (2010) 45–48
In summary, this investigative study on the surface of
austenitic stainless steel in nitric acid medium using an
electrochemical atomic force microscope revealed that
platelet-like structures formed in lower concentrations
(0.1, 0.5 M) of HNO3, providing effective protection
for the surface, but cease to form at concentrations beyond 0.6 M HNO3, when grain boundaries are revealed
and selective dissolution starts.
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