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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.
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:
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
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
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
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
(solid line) and Y
(dashed line) in ODS1 and (b) Y
Table 1 Chemical composition (wt.%) of NODS and ODS6 samples
Sample/composition (wt.%) Fe Cr W Ti C Total O N Y
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
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
) 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
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 (Vilellas
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
), mV
), mV
current density
), lA/cm
rate, MPY
Area of
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
(10 nm) and a few Y
(£5 nm)
particles in ODS1 and a large number of relatively finer
(£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
) is calculated using the following equation (Ref 40):
MPYpit 20 Ipit
eðÞ;ðEq 1Þ
where I
is the pitting current density [A/cm
], Kis the com-
bination of several conversion terms which is 1.2866 910
[equivalents. sec. mils/Coulombs. cm. years], qis the metal
density [g/cc], and eis the equivalent weight [grams/equiva-
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
) is lower than the
corrosion potential (E
Important parameters like corrosion potential (E
), corro-
sion current density (I
), primary passive potential (E
critical current density (I
), passive current density (I
breakdown/pitting potential (E
), potential range of passive
region E
(DV), repassivation potential (E
), pitting
current density (I
), 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
time, h
Sample name
, F/cm
Cdl, F/cm
Rct, Xcm
rate from
data, MPY
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
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
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
) are almost similar for all the
samples studied. Corrosion current density (I
) 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
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
) is more negative in NODS which
shows the early formation of passive layer compared to ODS
samples. The critical current density (I
) and passive current
density (I
) 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 DV1200 mV. The more ex-
tended the passive region (difference between E
and E
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
) and
pitting corrosion starts beyond E
. 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
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
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
) 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
is more negative than the E
, susceptibility to the
pitting corrosion is high. At the potentials between the pitting and
repassivation potential (E
), new pits will not form,
but existing pits will develop into bigger pits (Ref 43). In the
present study, E
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
) 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
, 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
value less than 1is considered as a good fit, and
the obtained v
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
). 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
) formed at
the interface of test sample and electrolyte and charge transfer
resistance (R
) 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
) 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
) 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
) also increases. ODS samples
have high C
values compared to NODS. The charge transfer
resistance (R
) 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
values compared to ODS6 samples
due to the presence of the less number of dispersoids. R
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 nis an empirical constant, which lies
between 0 and 1 and can be related to surface roughness of the
electrode (Ref 44). The n
value for passive layer is closer to 1,
which shows that the nature of film is like pure capacitor. The
decrease of n
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
, 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
. 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
, lower I
values, higher
, broader passive region (DV), and nobler E
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
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
) 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.
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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... However, these properties could be compared with only low yttria (0.35%) containing extruded alloys. Extrusion of high yttria alloys has not been very successful, because these alloys exhibited poor ductility.There is limited literature available on 18Cr ferritic ODS steels (Ref 34,39,47). Most of the research has been done on ODS steels in which the yttria content was limited to 0.35% (Ref 34,47). ...
... Extrusion of high yttria alloys has not been very successful, because these alloys exhibited poor ductility.There is limited literature available on 18Cr ferritic ODS steels (Ref 34,39,47). Most of the research has been done on ODS steels in which the yttria content was limited to 0.35% (Ref 34,47). Increasing the yttria content beyond 0.35% leads to a significant decrease in ductility. ...
Full-text available
Oxide dispersion strengthened (ODS) ferritic steels are candidate materials for clad tubes in the upcoming Generation IV nuclear reactors. In the present work, a powder forging consolidation technique has been used for fabrication of ODS steels. Two alloys having nominal compositions (in weight %) of Fe-18Cr-2W-0.285Ti-0.5Y 2 O 3 and Fe-18Cr-2W-0.571Ti-1Y 2 O 3 , respectively, have been studied in this work. The alloys were prepared by mechanical alloying of elemental powders with yttria in a Simoloyer high energy horizontal attritor. The milled powders were consolidated at 1473 K by powder forging in a flowing hydrogen gas atmosphere. Yttria to titanium ratio was kept constant at $ 1.75 for both the alloys. TEM micrographs of the forged alloys showed fine recrystallized grains with a dispersion of nano-size Y-Ti-O oxide particles. High-resolution transmission electron microscope fringes and the corresponding fast Fourier transformation confirmed the presence of orthorhombic Y 2 TiO 5 oxide particles in a ferrite matrix. These were the predominant oxide particles in the forged alloys. The Y 2 TiO 5 particles were incoherent with the matrix and exhibited a cuboidal morphology. Despite their high yttria content, both the alloys showed high tensile strength and ductility at room temperature and 973 K. Reasons for this are discussed.
... The yttria (Y 2 O 3 ) which has a high melting point and a stable structure at high temperature is the main choice for oxide dispersoid in ODS steel synthesis [9][10]. However, several research showed that zirconia can replace the Yttria as a dispersoid and increased the high-temperature corrosion resistance [11][12][13]. ...
Full-text available
An Austenitic ODS steel was developed for reactor structural material by dispersed 0.5 wt % of nano powder zirconia (ZrO 2 ) to the AISI 316L steel. The synthesis was carried out by the powder metallurgy process with high energy milling and cooled compacting process. A new apparatus of APS (Arc Plasma Sintering) was used for consolidation the sample in the sintering process. Characterizations of the microstructure and elemental composition distribution were performed using the Scanning Electron Microscope (SEM) with X-ray Diffraction Spectroscopy (EDX) and area mapping. Identification for the change of phasesand hardness were analyzed using the XRD-test and Vickers Hardness measurement. Austenitic phase with relatively equiaxed grain and homogeny distribution of the ZrO 2 dispersoid were identified after the sintering process followed by the improvement of hardness due to the pinning effect of the grain boundaries.
... The increasing second phase precipitates may induce an increment of corrosion current. The dispersion particles act as the initiation site of corrosion process, which induces the further dissolution of dispersion particles and matrix in nitric acid medium and results in the formation of pitting corrosion, as shown in Figure 10(d) [38,40]. ...
In this paper, 15Cr-ODS steels containing 0, 1 wt%, 2 wt% and 3 wt% Al element were fabricated by combining wet-milling and spark plasma sintering (SPS) methods. The microstructure and mechanical properties of ODS steel were investigated by XRD, SEM, TEM, EBSD and tensile tests. The results demonstrate that the Al addition significantly refines the particle precipitates in the Fe−Cr matrix, leading to the obvious refinement in grain size of matrix and the improvement of mechanical properties. The dispersion particles in ODS steels with Al addition are identified as Al2O3 and Y2Ti2O7 nanoparticles, which has a heterogeneous size distribution in the range of 5 nm to 300 nm. Increasing Al addition causes an obvious increase in tensile strength and a decline in elongation. The tensile strength and elongation of 15Cr-ODS steel containing 3 wt% Al are 775.3 MPa and 15.1%, respectively. The existence of Al element improves the corrosion resistance of materials. The ODS steel containing 2 wt% Al shows corrosion potential of 0.39 V and passivation current density of 2.61×10−3 A/cm2(1.37 V). This work shows that Al-doped ODS steels prepared by wet-milling and SPS methods have a potential application in structural parts for nuclear system.
... The new Fe-Cr-Al-based OPH steel has received significant attention because of its good corrosion resistance and high mechanical properties [3]. Aluminum and chromium are considered the most effective alloying elements for decreasing corrosion at high temperatures as well as at room temperature [4]. Forming a passive oxide layer can protect the surface against electrochemical corrosion and act as a barrier film. ...
Full-text available
The microstructure, mechanical, tribological, and corrosion properties of Fe–Cr–Al–Y-based oxide-precipitation-hardened (OPH) alloy at room temperature are presented. Two OPH alloys with a composition of 0.72Fe–0.15Cr–0.06Al–0.03Mo–0.01Ta–0.02Y2O3 and 0.03Y2O3 (wt.%) were prepared by mechanical alloying with different milling times. After consolidation by hot rolling, the alloys presented a very fine microstructure with a grain size of approximately 180 nm. Such a structure is relatively brittle, and its mechanical properties are enhanced by heat treatment. Annealing was performed at three temperatures (1000 °C, 1100 °C, and 1200 °C), with a holding time from 1 to 20 h. Tensile testing, wear testing, and corrosion testing were performed to evaluate the effect of heat treatment on the behavior and microstructural properties. The grain size increased almost 10 times by heat treatment, which influenced the mechanical properties. The ultimate tensile strength increased up to 300% more compared to the initial state. On the other hand, heat treatment has a negative effect on corrosion and wear resistance.
... The effect of process parameters on the microstructure, Received 1 October 2019; Received in revised form 27 March 2020; Accepted 29 March 2020 mechanical and corrosion properties of ODS-18Cr steel has already been reported by us [18][19][20]. The present work is therefore aimed at detailing the near atomic scale microstructure of the dispersoids in ODS-18Cr steel using multiple techniques in order to understand their influence on strength related properties. ...
Dispersion of nano Y2O3 (0.35 wt%) in 18Cr ferritic steel (Fe–18Cr–2.33 W–0.34Ti) was achieved by high energy ball milling of pre–alloyed powders after 6 h. The severe deformation induced nano-structuring during ball milling led to metastable solid solution formation, which gets stabilized during consolidation by upset forging and hot extrusion. Transmission electron microscopy, atom probe tomography and small angle X–ray scattering were combined to comprehensively characterize the crystal structure, morphology and the chemical composition of the dispersoids. Accordingly, the dispersoids of the type Y2Ti2O7 with cuboidal shape and Fd3¯m diamond cubic crystal structure having a lattice parameter of 1.01 nm were observed. The plastic deformation behavior of ODS steels at different operating temperatures was studied using the tensile test and the results were correlated with the size and morphology of the dispersoids.
Electrochemical corrosion and passivation characteristics of the oxide dispersion strengthened (ODS) Fe3Al–Ti and ODS Fe3Al–Ti–Zr alloys immersed for different exposure times of 1, 72, 120, 166 and 240 h in 3.5 wt. % NaCl solution were studied using open-circuit potential, cyclic polarization and electrochemical impedance spectroscopy techniques. Both ODS Fe3Al–Ti and ODS Fe3Al–Ti–Zr alloys exhibit pitting corrosion under chloride ions exposure. In both alloys, corrosion rate decreased as immersion time increased and decreased beyond 72 h and similarly up to 240 h due to the formation of a passive layer that prevented further corrosion. ODS Fe3Al–Ti–Zr alloy exhibited more resistance to pitting corrosion for all exposure times than ODS Fe3Al–Ti alloy. Electrochemical impedance spectroscopy results indicated that the presence of Zr in ODS Fe3Al–Ti–Zr alloy improved the corrosion resistance, charge transfer resistance and its passive layer stability which can be attributed to the formation of a better protective passive layer as compared to ODS Fe3Al–Ti alloy. Even though the ODS Fe3Al–Ti–Zr alloy exhibited better passivation characteristics and corrosion resistance, the difference in electrochemical properties of the two alloys was marginal.
The effect of Zr and ZrO2 on corrosion behaviour and passive film characteristics of the oxide dispersion strengthened (ODS) steels with 9%Cr (ODS9Cr-Zr and ODS9Cr-ZrO2) and those without the addition of oxide dispersions (NODS) has been studied in 0.6 M NaCl solution using cyclic polarization and electrochemical impedance spectroscopy measurements. Electrochemical corrosion studies indicated that Zr and ZrO2 addition increases the corrosion resistance of ODS9Cr steels, with the best results obtained for ODS9Cr-ZrO2 steel. The better corrosion behaviour of ODS9Cr steels is due to its surface morphology and Cr2O3 enriched passive film. Dispersion of Zr and ZrO2 along with Y2O3 in 9Cr steel considerably improved the corrosion resistance further.
Full-text available
The structural materials are essential for the present important advancements in economics, for reliability, safety, sustainability, etc., over presently operating reactor technologies of Generation IV reactors, advanced fast reactor core and fusion reactors. In order to avoid the release of radioactive fission products in the environment, the consistency of the cladding tube is necessary. Thus, increasing the operating temperature of these steels by oxide dispersion strengthening (ODS) makes them promising candidate materials. For the present study, the samples were prepared from the 430 L pre-alloyed powder using mechanical alloying through vacuum hot pressing. The hot-pressed alloys A, B, and C structural changes were examined through transmission electron microscope-energy dispersive spectroscopy (TEM-EDS) analysis and the mechanical properties, such as sintered density(g/cc), Vickers hardness (HV), compressive strength (650°C), and elongation (%) were measured. The corrosion resistance was measured through an electrochemical corrosion test of the alloys A, B, and C. From the TEMEDS analysis, it was observed that the nano oxide particles and complex oxides particles were uniformly distributed. Alloy A has a higher compressive strength of 1307 MPa when compared to alloy Band alloy C. Moreover, alloy Chas higher corrosion resistance was observed than alloys A& B.
The objective of the present work is to investigate the microstructural evolution during hot deformation of nano oxide dispersion strengthened (n-ODS) Fe – 18Cr ferritic steel over a wide range of temperatures 1273–1573 K and strain rates 10⁻² – 10 s⁻¹. Electron backscatter diffraction analysis was performed extensively on deformed samples to investigate the restoration mechanisms operating in the material. Small angle X-ray scattering (SAXS) analysis was performed on the deformed samples to investigate the nanoprecipitate characteristics. Flow curves obtained in the current study exhibit initial strain hardening followed by flow softening for all the studied deformation conditions. An inflection point was noticed in the plot between strain hardening rate (θ=(dσdε)T,ε˙) and flow stress for almost all test conditions except the ones corresponding to 1573 K and at 10⁻² and 10⁻¹ s⁻¹, which is an obvious indication for the occurrence of dynamic recrystallization (DRX) in the n-ODS-18Cr steel. Microstructural analysis revealed the occurrence of continuous DRX and it is responsible for the grain refinement in the present steel. Grain growth was dominant in the samples deformed at 1423 K (and above) and 10⁻² s⁻¹. SAXS analysis revealed considerable coarsening of nanoprecipitates with bimodal particle distribution in the sample deformed at 1573 K and 10⁻² s⁻¹. It showed that the average size of fine nanoprecipitates increased to ~6.2 nm from 3.8 nm and coarse precipitates of size 20–30 nm.
Higher chromium containing 17Cr oxide dispersion strengthened (ODS) steel (Fe-16.78Cr-4.46Al-0.5Ti-0.45Y2O3-0.36Y) with the ferritic structure are a prospective applicant fuel cladding materials for the high operating temperatures of future advanced nuclear reactors system. In this study, the microstructure, passive oxide composition and corrosion resistance of the Al-containing 17Cr ODS steel in different concentrations of nitric acid were investigated by means of SEM, TEM, XPS and electrochemical methods. The corrosion result shows that with increasing concentration of 3 M–11.5 M HNO3, open circuit potential revealed nobler potential and the potentiodynamic polarization plots exhibited a shift in corrosion potential toward transpassive region. The boiling nitric acid test after 240 h exposed in 3 M–11.5 M HNO3 showed desirable corrosion rate of ∼0.075–0.25 mm/y. As the nitric acid concentration increased, the corrosion morphology varied from smaller pits to enlarged pits with groove-like features. The improved corrosion resistance of the ODS alloy is attributed to the alloy composition, nature of dispersed oxide and passive film nature, where Al2O3 is prominently enriched. The corrosion mechanisms of ODS steel is further discussed.
Full-text available
The resistance of austenitic stainless steel type 304 to pitting corrosion was studied in solutions of sulphuric acid (2M and 5 M) with and without sodium chloride addition by linear polarization technique. The pitting and passivation potentials, corrosion rate and current density were analyzed with respect to the chloride ion concentration. Under anodic polarization the stainless steels in sulphuric acid solution acquired a passive state, with breakdown at the transpassive region (pitting potential), however this was greatly reduced with the addition of sodium chloride which led to a sharp increase in current at potentials significantly lower than the value that necessitates pitting in the acid media due to rapid breakdown of the passive film and development of local pits. Results obtained establish the dynamic relationship and interaction between the sulphate/chloride ion concentration and electrochemical potentials in the corrosion behavior of the ferrous alloy at ambient temperature.
Powder Metallurgy Stainless Steels: Processing, Microstructures, and Properties covers every step in the production of powder metal stainless steel parts, examining the effect of each processing variable on corrosion resistance as well as strength, ductility, and other properties. It provides a brief history of the technology and an overview of the metallurgy and composition of wrought and powder stainless steels. It explains how powders are made and how they are characterized based on chemical composition, particle size and shape, compressibility, and other factors. It describes compacting and shaping methods, including rigid die compaction, metal injection molding, extrusion, and hot isostatic pressing, and discusses several sintering processes, explaining how furnace atmosphere, heating and cooling rates, and part density affect property development and corrosion resistance. It also covers secondary operations such as machining, welding, sinter bonding, and resin impregnation, describes the effect of thermal and deformation processes on creep, fatigue, and other mechanical properties, and explains how to evaluate and test corrosion performance. The book includes an atlas of microstructures, a glossary of terms, and detailed application examples. For information on the print version, ISBN: 978-0-87170-848-9, follow this link.
9Cr- and 12Cr-ODS (oxide dispersion strengthened) steel claddings are candidates for the advanced sodium-cooled fast reactor fuels, due to controlling nano-size oxide particles and micron-size grain morphology. Al-added 16Cr-ODS steel cladding offers the potential under use of lead-bismuth eutectic and supercritical pressurized water environment. Their development, mechanical properties, corrosion, and irradiation performance were reviewed. The development of MA957 and European DT2203Y05 was also presented from a historical viewpoint.
Cladding materials development is crucial to realize highly efficient and high-burnup operation over 100 GWd/t of so called Generation IV nuclear energy systems, such as supercritical-water-cooled reactor (SCWR) and lead-cooled fast reactor (LFR). Oxide dispersion strengthening (ODS) ferritic/martensitic steels, which contain 9-12%Cr, show rather high resistance to neutron irradiation embrittlement and high strength at elevated temperatures. However, their corrosion resistance is not good enough in SCW and in lead at high temperatures. In order to improve corrosion resistance of the ODS steels in such environment, high-Cr ODS steels have been developed at Kyoto University. An increase in Cr content resulted in a drastic improvement of corrosion resistance in SCW and in lead, while it was expected to cause an enhancement of aging embrittlement as well as irradiation embrittlement. Anisotropy in tensile properties is another issue. In order to overwhelm these issues, surveillance tests of the material performance have been performed for high Cr-ODS steels produced by new processing technologies. It is demonstrated that high-Cr ODS steels have a high potential as fuel cladding materials for SCWR and LFR with high efficiency and high burnup.
Pitting corrosion is localized accelerated dissolution of metal that occurs as a result of a breakdown of the otherwise protective passive film on the metal surface. This paper provides an overview of the critical factors influencing the pitting corrosion of metals. The phenomenology of pitting corrosion is discussed, including the effects of alloy composition, environment, potential, and temperature. A summary is then given of studies that have focused on various stages of the pitting process, including breakdown of the passive film, metastable pitting, and pit growth.
The corrosion resistance and passive film compositions of 9% Cr oxide dispersion-strengthened steel in different acidified and chloride media was evaluated. The results presented below show that with increasing concentrations of 1 M to 11.5 M nitric acid (HNO3), the open-circuit potential reveals a more noble potential, while in chloride-containing media, a less noble potential was observed. The potentiodynamic polarization plots exhibited higher breakdown/transpassive potentials in acidic media with 0.5 M sulfuric acid (H2SO4) and 1 M to 11.5 M HNO3, and there was a shift in the corrosion potential toward the transpassive region as the HNO3 concentration increased from 1 M to 11.5 M. However, no intergranular corrosion attack was observed at the HNO3 concentrations studied. In acidic-chloride media (0.5 M H2SO4 with 0.1 M and 0.5 M sodium chloride [NaCl]) and in chloride only media (0.1 M NaCl and 0.5 M NaCl) poor pitting corrosion resistance was attributable to the microstructural inhomogeneity and inclusions in the steel. The x-ray photoelectron spectroscopy analysis indicated that the passive film has iron(III) oxide (Fe2O3) and chromium(III) oxide (Cr2O3), along with yttrium oxide (Y2O3). The scanning electron microscopy (SEM) examinations showed that the pits on the specimen were hemispherical and formed lace-like patterns. The corrosion properties affected by the dispersed oxide are also discussed in the paper.
Several steels were exposed to either static or flowing liquid lead-bismuth eutectic under various exposure conditions, Steels T91, EP-823, S2439 and S2440 were exposed to oxygen-rich static LBE at 490 degrees C for similar to 5016 h. The experiments in flowing LBE were carried out in the CORRIDA loop at 550 degrees C and around 10(-6) mass% dissolved oxygen. The steels tested in the CORRIDA loop included the reduced activation steel EUROFER 97 and two heats of an oxide dispersion strengthened steel produced by mixing EUROFER 97 and yttria powders. The exposure time varied between 1007 and 7511 h for the EUROFER 97 steel and between 5012 and 20039 h for the two ODS heats. The exposed steels were characterized by means of scanning electron microscopy and energy-dispersive X-ray spectrometry.