Influence of Dispersoids on Corrosion Behavior of Oxide Dispersion-Strengthened 18Cr Steels made by High-Energy Milling

Abstract

Corrosion behavior of 18Cr ferritic steel with and without yttria produced by high-energy milling followed by hot extrusion was studied in 3.5% NaCl solution using electrochemical and immersion techniques. The cyclic polarization study showed that pitting corrosion is predominant in all the materials. The pitting rate is higher in yttria dispersed steels and also increases with milling time. Impedance analysis revealed the formation of better corrosion resistance film on the surface of the steel without yttria. Potentiodynamic polarization studies indicated that the corrosion rate decreased up to 48 h of exposure time and increased beyond 48 h. The increase in corrosion rate beyond 48 h is due to the porous passive film. The corrosion behavior of all the materials in immersion studies followed the same trend as observed in electrochemical studies. Even though the corrosion rate of yttria dispersed 18Cr ferritic steel is less than that of the base material, the difference is marginal. The presence of dispersoids appears to promote nucleation of pits when compared to the steel without the dispersoids.

Full text

Influence of Dispersoids on Corrosion Behavior
of Oxide Dispersion-Strengthened 18Cr Steels
made by High-Energy Milling
M. Nagini, A. Jyothirmayi, R. Vijay, Tata N. Rao, A.V. Reddy, Koteswararao V. Rajulapati, and G. Sundararajan
(Submitted August 18, 2015; in revised form November 11, 2015; published online December 21, 2015)
Corrosion behavior of 18Cr ferritic steel with and without yttria produced by high-energy milling followed
by hot extrusion was studied in 3.5% NaCl solution using electrochemical and immersion techniques. The
cyclic polarization study showed that pitting corrosion is predominant in all the materials. The pitting rate
is higher in yttria dispersed steels and also increases with milling time. Impedance analysis revealed the
formation of better corrosion resistance film on the surface of the steel without yttria. Potentiodynamic
polarization studies indicated that the corrosion rate decreased up to 48 h of exposure time and increased
beyond 48 h. The increase in corrosion rate beyond 48 h is due to the porous passive film. The corrosion
behavior of all the materials in immersion studies followed the same trend as observed in electrochemical
studies. Even though the corrosion rate of yttria dispersed 18Cr ferritic steel is less than that of the base
material, the difference is marginal. The presence of dispersoids appears to promote nucleation of pits when
compared to the steel without the dispersoids.
Keywords corrosion rate, cyclic polarization, dispersoids, elec-
trochemical impedance spectroscopy, milling, oxide
dispersion-strengthened ferritic steel, pitting corrosion
1. Introduction
The advanced energy systems being developed to achieve
economy, safety, reliability, and sustainability require high-
performance materials that can withstand extremely hostile
service conditions (Ref 1-6). Nanostructured oxide dispersion-
strengthened (ODS) steels are emerging as promising high-
temperature and high-performance structural materials for fuel
cladding in Gen IV fission reactors, blanket materials for fusion
reactors, and blades for ultra-supercritical stream turbines
because of their high-temperature strength, creep resistance,
and resistance to corrosion, oxidation, and neutron irradiation
damage (Ref 4-12). These steels usually contain a high density of
fine complex (Y-Ti-O) oxide particles along with fine-grained
structure (Ref 13,14). Uniformly distributed fine stable oxide
particles with high number density improve creep strength at
elevated temperatures by restricting the mobility of dislocations
and grain boundaries and also act as trapping sites for both point
defects and helium atoms generated during irradiation to
maintain the superior resistance to irradiation damage (Ref 4).
The corrosion/oxidation resistance of steels can be enhanced by
increasing Cr content beyond 12%. Even though the corrosion
behavior of ODS steels at high temperatures is well studied in
supercritical pressurized water (SCPW) (Ref 15-17), liquid lead
(Ref 18,19), lead-bismuth eutectic (LBE) (Ref 20,21), sodium
(Ref 22,23), and molten fluoride salts (Ref 24), the general
corrosion behavior of these steels at room temperature is not fully
evaluated. The corrosion behavior of ferritic-martensitic and
ferritic ODS steels with 9-15% Cr was observed in different
chloride and acidic electrolytic solutions (Ref 25-29). The effect
of Al addition to 16Cr ferritic ODS steel on corrosion was
examined by Isselin et al. (Ref 30). Terada et al. (Ref 31) reported
that the corrosion resistance of Eurofer 97 steel decreased with
the dispersion of oxides. Literature on the corrosion behavior of
ODS 18Cr ferritic steels is limited to the studies in SCPW by Hu
et al. (Ref 32) and as a function of Cr content in nitric acid
environment with temperature by Gwinner et al. (Ref 33) and
Dubuisson et al. (Ref 34). Corrosion behavior of ODS steels in
marine environment is of importance because the components
made from ODS steels in systems like nuclear reactors, turbines,
heat exchangers, etc. are likely to be exposed to aqueous NaCl
environment at room temperature during transportation, storage,
and maintenance. As yet, electrochemical corrosion studies are
not carried out on ODS 18Cr ferritic steels. The present study,
therefore, is an attempt to systematically evaluate the influence of
milling time on electrochemical corrosion behavior of 18Cr
ferritic steels with and without dispersoids in 3.5% NaCl solution
at room temperature.
2. Experimental Procedure
The pre-alloyed 18Cr steel powder (Fe-18Cr-2.3W-0.3Ti)
was milled with and without nano yttria in a high-energy
horizontal attritor mill (Simoloyer CM-20, ZOZ GmbH,
M. Nagini, International Advanced Research Centre for Powder
Metallurgy and New Materials (ARCI), Balapur, Hyderabad 500005,
India; and School of Engineering Sciences and Technology (SEST),
University of Hyderabad, Gachibowli, Hyderabad 500046, India;
A. Jyothirmayi, R. Vijay, Tata N. Rao, A.V. Reddy, and
G. Sundararajan, International Advanced Research Centre for
Powder Metallurgy and New Materials (ARCI), Balapur, Hyderabad
500005, India; and Koteswararao V. Rajulapati, School of
Engineering Sciences and Technology (SEST), University of
Hyderabad, Gachibowli, Hyderabad 500046, India. Contact e-mail:
vijay@arci.res.in.
JMEPEG (2016) 25:577–586 ASM International
DOI: 10.1007/s11665-015-1859-5 1059-9495/$19.00
Journal of Materials Engineering and Performance Volume 25(2) February 2016—577

Germany) under argon atmosphere for durations ranging from 1
to 6 h. The milled powders were filled in mild steel cans,
degassed at 450 C under a vacuum of 5.3 910
6
kPa, and
sealed. The sealed powder cans were upset forged at 1050 C
and the upset billets were subsequently extruded at 1150 C
with an extrusion ratio of 19. The extruded rods were annealed
at 900 C for 1 h and then water quenched. Transverse sections
of the 12 mm /rod of 5 mm thickness were mounted and
polished by standard polishing techniques, ultrasonically
cleaned, and dried. The exposed area of the sample in mount
for corrosion studies is 1.327 cm
2
.
For ease of presentation, the annealed ODS steel rods made
from powders milled for 1, 3, and 6 h are denoted as ODS1,
ODS3, and ODS6, respectively. The chemical composition of
all ODS steel samples is same. The base pre-alloyed 18Cr steel
powder milled for 6 h and subsequently extruded and annealed
rod is referenced as NODS. The chemical compositions of the
NODS and ODS steels were determined using ICP-AES
(JOBIN-YUON FRANCE, Model: Ultima-2CHR). The O, N,
and C analysis was carried out using oxygen/nitrogen (LECO,
Model: TC436) and carbon (LECO, Model: CS444) analyzers.
The chemical compositions of NODS and ODS6 used in the
present study are given in Table 1. The morphology and
microstructures of the annealed samples before and after
corrosion tests were observed using scanning electron micro-
scope (SEM) (Hitachi, Model: S-3400N). Transmission elec-
tron microscopic (TEM) investigations of ODS1 and ODS6
were carried out using an FEI Tecnai G2 200 kV (LaB
6
)
microscope equipped with Gatan image filter.
To determine the corrosion behavior of NODS and ODS
samples, electrochemical experiments like cyclic polarization
(CP), electrochemical impedance spectroscopy (EIS), and
potentiodynamic polarization and immersion tests were carried
out at room temperature in 3.5% NaCl solution prepared using
Fig. 1 SEM microstructures of annealed samples: (a) ODS1, (b) ODS6, and (c) NODS
Fig. 2 TEM images of annealed samples showing dispersoids: (a) Y
2
O
3
(solid line) and Y
2
Ti
2
O
7
(dashed line) in ODS1 and (b) Y
2
Ti
2
O
7
in
ODS6
Table 1 Chemical composition (wt.%) of NODS and ODS6 samples
Sample/composition (wt.%) Fe Cr W Ti C Total O N Y
2
O
3
Excess O
NODS Balance 17.6 2.3 0.34 <0.03 0.05 0.012 ÆÆÆ ÆÆÆ
ODS6 Balance 17.4 2.2 0.31 <0.03 0.14 0.012 0.36 0.06
578—Volume 25(2) February 2016 Journal of Materials Engineering and Performance

analytical grade reagent and distilled water. The as-prepared
3.5% NaCl solution was purged with N
2
gas in order to remove
the dissolved oxygen. A standard three-electrode cell assembly
was connected to the computer-controlled Solartron Electro-
chemical Interface (Model SI 1287) with Corrware and
Corrview-2 softwares and Solartron (Model SI 1260) Impe-
dance/Gain Phase analyzer with Z-plot, Z-view softwares to
obtain and analyze the corrosion data. The corrosion cell
consisting of three electrodes, in which the sample is taken as a
working electrode along with a saturated calomel electrode
(SCE) as a reference electrode and a platinum electrode as a
counter electrode. CP experiments were performed to evaluate
the pitting resistance of the alloys from a potential of 0.8 V
relative to the reference electrode potential with a scan rate of
1 mV/s up to 1 V, and the scan was reversed back to the
potential of 0.85 V.
EIS and potentiodynamic polarization experiments were
carried out on samples exposed to 3.5% NaCl for different
exposure times of 1, 24, 48, and 96 h. EIS spectra were
recorded prior to polarization scans by applying an AC signal
with 10 mV amplitude and a frequency range of 25 kHz-
0.02 Hz at open circuit potential (OCP). After EIS scans,
potentiodynamic scan was performed from cathodic to anodic
region starting with a potential of 0.8 up to 1.2 V relative to
the reference electrode potential at a scan rate of 1 mV/s. A set
of three samples were tested in each test condition, and all the
results were reproducible. Prior to the start of the scans, OCP of
the samples was stabilized in the test solution. Since the
passivation started immediately on immersion, polarization
resistance (R
p
) fit method (linear polarization technique) is used
to analyze the data obtained in the present work. In this method,
applied potential (E) is plotted with measured current (i) which
is linear within ±20 mV of corrosion potential and R
p
is
obtained from the slope of the plot (DE/Di) (Ref 35). Long-term
corrosion behavior was tested on samples immersed in 3.5%
NaCl solution for 365 days.
3. Results and Discussion
3.1 Microstructural Studies of Annealed Samples
Typical SEM micrographs taken on etched (Vilellas
reagent) longitudinal sections of ODS1, ODS6, and NODS
Fig. 3 CP curves of ODS 18Cr samples along with NODS sample
Table 2 Data obtained from analysis of CP hysteresis curves of NODS and ODS samples
Sample
name
Corrosion
potential
(E
corr
), mV
Corrosion
current
density
(I
corr
),
lA/cm
2
Primary
passive
potential
(E
PP
), mV
Critical
current
density
(I
crit
),
lA/cm
2
Passive
current
density
(I
pass
),
lA/cm
2
Breakdown
potential
(E
BP
),
mV
E
B
2E
corr
DV,
mV
Repassivation
Potential
[E
RP
],
mV
Pitting
current density
(I
pit
), lA/cm
2
Pitting
rate, MPY
Area of
hysteresis
loop (I 3V)
NODS 583 1.717 526 3.110 5.425 607 1190 607 2.961 25.00 40
ODS1 577 1.777 444 4.025 6.127 600 1177 610 3.152 26.89 48
ODS3 565 2.201 509 3.154 6.593 615 1180 607 3.847 32.48 47
ODS6 604 2.694 450 4.435 8.688 608 1212 651 3.951 33.35 54
Journal of Materials Engineering and Performance Volume 25(2) February 2016—579

are shown in Fig. 1(a-c), respectively. Microstructures revealed
that the ODS samples consist of fibrous grain structure, the
grain fibers being thinner in ODS6 (0.48 lm) than ODS1
(1.17 lm). In contrast, NODS showed coarse (10 lm) equi-
axed grain structure. In general, the fibrous structure tends to
become finer in rods made from powders with increasing
milling time. TEM examination of ODS steels indicated the
presence of Y
2
O
3
(‡10 nm) and a few Y
2
Ti
2
O
7
(£5 nm)
particles in ODS1 and a large number of relatively finer
Y
2
Ti
2
O
7
(£5 nm) dispersoids in ODS6 (Fig. 2). The number
density of dispersoids in ODS6 is an order of magnitude higher
than that in ODS1. The increased fibrous nature of the grain
structure is due to the retarded recrystallization as a result of the
effective pinning of the grain boundaries by dispersoids during
hot working and subsequent annealing. The progressive
refinement of the grain size is caused by the decreased size
and increased number density of the dispersoids at higher
milling times.
3.2 Electrochemical Studies
3.2.1 Cyclic Polarization. Cyclic polarization (CP) stud-
ies are carried out on ODS1 and ODS6 along with NODS
samples in 3.5% NaCl solution after pre-immersion (0.5 h) to
stabilize OCP. Typical CP curves of the experimental samples
described with relevant parameters are shown in Fig. 3.
CP measurements are carried out to determine the pitting
tendencies (passivation breakdown and repassivation behavior)
of the active-passive alloys which undergo localized (pitting)
corrosion in the metal-solution system. In the CP curve,
potential is increased from cathodic to anodic direction until the
current density reached the given magnitude, then the potential
is decreased in the reverse direction. Here the anodic branch
extends to a wider potential range reaching up to 1 V with
OCP, then the potential decreased toward OCP. Pitting
phenomenon can be explained with the nature of hysteresis
loop formed with forward and reverse scans. Positive hysteresis
indicates (loop is on the right) the localized corrosion
susceptibility and the amount of the localized corrosion
incurred by the material and is represented by the area under
the hysteresis loop (Ref 36,37). The presence of huge positive
hysteresis loop is an indication of the nucleation and growth of
pits. Negative hysteresis (loop is on the left) indicates the
formation of passive film which is protective and self-healing.
In other words, it indicates that the damaged passive film
repairs itself (Ref 38,39). No hysteresis also indicates high
resistance to localized corrosion (Ref 40).
Since the localized (pitting) corrosion is observed to be the
dominant mode of damage in the steels studied, pitting rate
(MPY
pit
) is calculated using the following equation (Ref 40):
MPYpit ffi20 Ipit

KðÞ1
q
eðÞ;ðEq 1Þ
where I
pit
is the pitting current density [A/cm
2
], Kis the com-
bination of several conversion terms which is 1.2866 910
5
[equivalents. sec. mils/Coulombs. cm. years], qis the metal
density [g/cc], and eis the equivalent weight [grams/equiva-
lent].
The shapes of CP curves are broadly similar for both ODS
samples with different milling times and NODS sample. From
the obtained CP curves, it is evident that (a) distinct active,
passive, and transpassive regions exist, (b) current density
increases slightly in the passive region with increasing
potential, (c) transition from passive to transpassive state is
sharp, (d) current density increased significantly in transpassive
region with a small increase in potential, (e) potential reversal
resulted in positive hysteresis, (f) the passive region width is
high, and (g) the repassivation potential (E
RP
) is lower than the
corrosion potential (E
corr
).
Important parameters like corrosion potential (E
corr
), corro-
sion current density (I
corr
), primary passive potential (E
pp
),
critical current density (I
crit
), passive current density (I
pass
),
breakdown/pitting potential (E
BP
), potential range of passive
region E
BP
E
corr
(DV), repassivation potential (E
RP
), pitting
current density (I
pit
), pitting rate, and the area under hysteresis
loop are derived from the CP curves and presented in Table 2.
Table 3 Parameters obtained from the impedance circuit and corrosion rate calculated from potentiodynamic polariza-
tion data for ODS and NODS samples
Exposure
time, h
Sample name
R
s
,Xcm
2
C
film
, F/cm
2
310
25
R
film
,Xcm
2
n
1
Cdl, F/cm
2
310
25
Rct, Xcm
2
310
5
n
2
v2
Corrosion
rate from
polarization
data, MPY
NODS
1 1.279 1.36 52.39 0.97 4.179 2.564 0.73 0.0006 0.438 ±0.0.29
24 1.252 5.667 79.41 0.98 3.402 3.249 0.76 0.0057 0.291 ±0.029
48 1.322 1.198 55.16 0.97 3.934 5.347 0.84 0.0021 0.27 ±0.037
96 0.899 4.498 98.91 0.98 2.769 0.951 0.87 0.0038 0.508 ±0.025
ODS1
1 0.807 1.900 40.63 0.97 7.982 2.258 0.76 0.009 0.589 ±0.042
24 0.676 2.695 42.11 0.98 7.431 3.169 0.77 0.0004 0.522 ±0.051
48 0.606 3.428 53.12 0.98 7.097 3.438 0.77 0.0007 0.434 ±0.031
96 3.896 0.973 43.73 0.97 7.644 0.734 0.84 0.0071 0.592 ±0.061
ODS6
1 1.078 2.885 53.69 0.99 6.198 0.583 0.70 0.0003 0.864 ±0.061
24 9.13 1.644 69.92 0.97 5.45 0.781 0.76 0.0017 0.74 ±0.049
48 2.446 6.534 41.85 0.98 7.871 1.73 0.83 0.0014 0.683 ±0.049
96 1.094 0.86 34.13 0.99 7.253 0.551 0.75 0.0057 0.823 ±0.064
580—Volume 25(2) February 2016 Journal of Materials Engineering and Performance

The corrosion potentials (E
corr
) are almost similar for all the
samples studied. Corrosion current density (I
corr
) values are
higher for ODS samples than for the NODS sample and it is
higher for ODS6 than ODS1. In all the samples, passive region
has started immediately after E
corr
showing the formation of a
protective barrier oxide layer on the surface of the sample and
the extent of the passive region is similar. The primary
passivation potential (E
pp
) is more negative in NODS which
shows the early formation of passive layer compared to ODS
samples. The critical current density (I
crit
) and passive current
density (I
pass
) provide information on nucleation and growth of
the passive oxide layer and the nature of the passive oxide layer
formed on the surface of the sample, respectively. There is a
variation in both of these values from NODS to ODS and
higher in ODS samples which is due to the porosity generation
through the passive oxide layer. All the samples exhibited well-
defined passive region with DV1200 mV. The more ex-
tended the passive region (difference between E
BP
and E
corr
),
the better the resistance to pitting corrosion with better
passivation characteristics (Ref 41). For all the samples, this
passive region ends at the breakdown potential (E
BP
) and
pitting corrosion starts beyond E
BP
. The reason for corrosion of
passive materials is the damage of the passive oxide layer on
the metal surface because of the existence of chloride or other
aggressive anions (Ref 36,39,42). The higher the value of E
BP
,
the greater the resistance to initiation of pitting corrosion is.
Fig. 4 Nyquist and Bode plots of ODS and NODS samples for exposure times of 1 and 48 h
Journal of Materials Engineering and Performance Volume 25(2) February 2016—581

The sudden increase in the passive current density (i
pass
)atE
BP
shows the breakdown of the passive oxide film formed and
initiation of the pit (Ref 36).
In the CP curve, the presence of the hysteresis loop also
represents a delay in repassivation of an existing pit, when the
scan is reversed. If the hysteresis loop is larger, then repassivation
of the pit becomes difficult (Ref 42). Repassivation potential
(E
RP
) represents the potential where the reverse scan intersects
the forward scan (loop closes on the reverse scan). At this
potential, pit growth is arrested and current decreases drastically.
If the E
RP
is more negative than the E
BP
, susceptibility to the
pitting corrosion is high. At the potentials between the pitting and
repassivation potential (E
RP
<E<E
BP
), new pits will not form,
but existing pits will develop into bigger pits (Ref 43). In the
present study, E
RP
is more negative for ODS6 compared to ODS1
and NODS steels. The experimental results also indicate that the
area under the loop which represents the amount of pitting
corrosion is higher for ODS than NODS. Pitting current density
(I
pit
) and pitting rate values are higher in ODS than NODS and
also increase with milling time.
3.2.2 Electrochemical Impedance Spectroscopy.
Impedance analysis is carried out in order to study the
properties of electric double layer formed at the interface of
the sample and electrolyte at different exposure times. The
impedance test results of ODS1, ODS6, and NODS samples at
exposure times of 1 and 48 h plotted in the form of Nyquist and
Bode plots are shown in Fig. 4. Similarly, Nyquist and Bode
plots of ODS1, ODS6, and NODS samples as a function of
exposure time are shown in Fig. 5. The Nyquist plot shows real
and imaginary components of impedance which is expressed in
Xcm
2
, and in the Bode plot the frequency dependence of the
phase angle hand modulus are plotted. The frequency
dependence of the phase angle hand modulus of impedance
indicate whether one or more time constants are present in the
system. Examination of different equivalent electric circuit
models has been carried out to fit the plots using the Z-view
software. An equivalent circuit with two time constants was
found to be suitable for fitting the data at OCP and is shown in
Fig. 6. The v
2
value less than 1is considered as a good fit, and
the obtained v
2
values (Table 3) showed a good fit of
experimental data. The Bode plots of (Fig. 4and 5) all the
samples show the existence of two time constants, where the
time constant at higher frequencies attributes to the resistance
and capacitance values of a very thin oxide film formed on the
sample substrate along with the uncompensated solution
resistance (R
s
). The other time constant at lower frequencies
Fig. 5 Nyquist and Bode plots of ODS and NODS samples with different exposure times of 1, 24, 48, and 96 h
Fig. 6 Equivalent circuit used to fit the EIS data
582—Volume 25(2) February 2016 Journal of Materials Engineering and Performance

shows the capacitance of electric double layer (C
dl
) formed at
the interface of test sample and electrolyte and charge transfer
resistance (R
ct
) of the test sample. The numerical values of EIS
parameters obtained from the fitted circuit are summarized in
Table 3.
A careful review of electrochemical impedance results
shows the following:
The film resistance (R
film
) is less in ODS steels compared to
that of NODS steel. This is due to the presence of dispersoids,
which act as discontinuities in the oxide layer. The oxide layer
formed on the surface of ODS steels due to corrosion is thus not
uniform and hence is not very effective in offering protection
unlike an effective continuous passive film formed on NODS
steel surface. As the area of the oxide layer on the surface is
less, the capacitance of the passive film (C
film
) is less in ODS
steels when compared to NODS steels. The exposed area of
ODS samples increases with immersion time due to the
presence of discontinuous passive layer on the surface. As the
area of interface of test sample and electrolyte increases, the
double-layer capacitance (C
dl
) also increases. ODS samples
have high C
dl
values compared to NODS. The charge transfer
resistance (R
ct
) is high for NODS for all exposure times than
that for ODS samples. This may be due to the presence of
continuous passive film in NODS and dispersoid-induced
discontinuities in the passive film of ODS samples. ODS1
sample showed better R
ct
values compared to ODS6 samples
due to the presence of the less number of dispersoids. R
ct
values
for all samples increase up to 48 h and then decrease for 96 h
of exposure which shows that the passive film is protective up
to 48 h only. The factor nis an empirical constant, which lies
between 0 and 1 and can be related to surface roughness of the
electrode (Ref 44). The n
1
value for passive layer is closer to 1,
which shows that the nature of film is like pure capacitor. The
decrease of n
2
values shows the increase in surface roughness
of the electrode, and the values are increased with the exposure
time for all the samples.
3.2.3 Potentiodynamic Polarization. To study the cor-
rosion behavior as a function of immersion time, the potentio-
dynamic polarization experiments are carried out on ODS1,
ODS6, and NODS steels in 3.5% NaCl solution after pre-
immersion for different exposure times of 1, 24, 48, and 96 h.
The corrosion rates of NODS and ODS steels with exposure
time calculated from the graphs are given in Table 3.
From the corrosion rate data, it is evident that (a) the
corrosion rate is higher for all ODS steels than that for NODS at
all exposure times, and (b) the corrosion rate increases in ODS
steels with milling time and is maximum in ODS6. The
differences in the quality of oxide film and grain size may be
responsible for the higher corrosion rate in ODS samples, and
(c) in both NODS and ODS steel samples the corrosion rate
decreases up to 48 h and increases thereafter. The passive layer
formed on the substrate is protective up to 48 h, as the
thickness of the passive layer increases; it may spall or become
porous and allows easy access of the electrolyte to oxide-steel
interface thereafter increasing the corrosion rate (Ref 36).
3.3 Immersion Studies
To study the corrosion behavior of the samples for longer
time, immersion tests are carried out by dipping all the samples
in 3.5% NaCl solution for 365 days. The samples were
examined periodically for any indications of corrosion. Initially,
there was no observable change, but after 6 months small pits
were formed and severe corrosion was noticed after 1 year of
immersion. Figure 7shows the SEM images of the samples
immersed for 365 days. The pit densities of ODS1, ODS6, and
NODS are 261, 336, and 195 cm
2
, respectively. Pit density
increased with milling time in ODS steels and is higher than
that of NODS. The observed trend in this study followed well
with electrochemical experiments.
3.4 Microstructural Studies
The surface morphologies of corroded samples of ODS1,
ODS6, and NODS after potentiodynamic polarization tests in
3.5% NaCl solution after an exposure time of 48 h are shown in
Fig. 8. The size, distribution, and morphology of pits formed
are imaged on cross sections of corrosion-tested samples after
48-h immersion of ODS6 and NODS steels and are shown in
Fig. 9.
The microstructures reveal the occurrence of predominantly
localized (pitting) corrosion along with general corrosion in
both ODS and NODS samples. In ODS samples, pits are
smaller, large in number, and partly covered with debris, while
in NODS pits are less, deeper, and open. The relatively smaller
and shallower pits with high number density in ODS6 when
compared to ODS1 are considered to be due to the increased
number density of finer dispersoids. From the cross-sectional
studies (Fig. 9), pit shapes mainly appeared to be hemi-
spherical and globular. The formation of more number of pits in
ODS steels is probably due to the non-uniformity of passive
layer. The pit formation in NODS is considered as a general
galvanic corrosion, occurring as a small active area, which is
being accelerated by the large continuous passivated area. This
relative area difference causes acceleration of the corrosion,
resulting in deeper penetration of the pits (Ref 45). More rapid
propagation of the pits may occur due to their initiation at
Fig. 7 SEM Morphologies of immersed samples: (a) ODS1, (b) ODS6, and (c) NODS
Journal of Materials Engineering and Performance Volume 25(2) February 2016—583

metallurgical features, e.g., inclusions and grain boundaries,
second-phase particles, solute segregated grain boundaries, etc.
(Ref 36). In the present study, the nucleation of pits occurs
predominantly at dispersoids which in this case are Y
2
O
3
and
Y
2
Ti
2
O
7
. Both these oxides are noble when compared to the
matrix and exhibit similar electrochemical behavior. Morphol-
ogy of pits observed under SEM (Fig. 9a) shows lacelike
patterns with a central hole. This lacy metal structure contains
white sponge portion with enrichment of Ti caused by
preferential leaching of Fe from the corrosion debris.
Fig. 8 Surface morphologies of potentiodynamic polarization tested samples of (a) ODS1, (b) ODS6, and (c) NODS after 48-h exposure to the
3.5% NaCl solution
Fig. 9 Morphology of pits (a) on the surface; cross section of (b) and (c) ODS6 and (d) and (e) NODS
584—Volume 25(2) February 2016 Journal of Materials Engineering and Performance

The salient features of the corrosion studies carried out on ODS
and NODS steels using various electrochemical (CP, EIS, and
potentiodynamic) and immersion techniques are summarized below.
In general, the resistance to corrosion, passivation character-
istics, and the anodic protection ability of a material in CP studies
are associated with the negative E
pp
, lower I
crit
,I
pass
values, higher
E
BP
, broader passive region (DV), and nobler E
RP
.NODSsample
exhibited better properties among all. Pitting corrosion is the main
corrosion occurred in all the samples and the order of pitting rate is
NODS <ODS1 <ODS6. The charge transfer resistance (R
ct
)is
inversely proportional to the pitting rate and is lowest for ODS6
which exhibited the highest pitting rate. Impedance analysis
revealed a better corrosion resistance of surface film for NODS
than ODS steels. The corrosion rate calculated from the PD
polarization data compares well with the results of impedance
data. Immersion studies also reflect the same trend for corrosion
resistance as observed by electrochemical techniques. Even
though all the above-mentioned corrosion studies indicate that
ODS samples exhibit lower corrosion resistance than NODS
samples, the difference is not very significant.
4. Conclusions
The corrosion behavior of ODS1 and ODS6 along with
NODS alloy steels in 3.5% NaCl solution was investigated
using CP, EIS, potentiodynamic polarization, and immersion
studies. The conclusions from the study are given below:
•ODS steels are more prone to pitting corrosion when com-
pared to NODS among ODS steels whose pitting tendency
increases with milling time.
•The marginally higher pitting rates in ODS steels are due
to the presence of dispersoids which act as initiating sites
for the formation of pits.
•The charge transfer resistance (R
ct
) values are less for
ODS steels when compared to NODS because of the dif-
ference in the formation of passive film.
•In both ODS and NODS steels, corrosion rate decreases up
to 48 h due to the formation of protective passive layer and
increases beyond due to the increase of porosity in the pas-
sive layer.
•Corrosion behavior of both ODS and NODS steels is found
to be similar in both electrochemical and immersion studies.
Acknowledgments
The authors thank Mr. G. V. R. Reddy and Mr. M. Ramakrishna
for carrying out electron microscopy analysis. They are thankful to
Dr. K. Satya Prasad and Dr. B. V. Sarada for valuable technical
discussions. The authors gratefully acknowledge Indira Gandhi
Centre for Atomic Research (IGCAR), Kalpakkam, for funding
(No. IGC/MMG/MMD/ODS/01/2010) the work and NFC, Hyder-
abad, for carrying out hot extrusion.
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