Materials Transactions, Vol. 50, No. 5 (2009) pp. 1214 to 1218
#2009 The Japan Institute of Metals
EXPRESS REGULAR ARTICLE
Selective Dissolution Characteristics of 26Cr-7Ni-2.5Mo-3W Duplex Stainless Steel
in H2 SO4 /HCl Mixed Solution
Heejoon Hwang, Gwanyong Lee, Soonhyeok Jeon and Yongsoo Park
Department of Metallurgical Engineering, Yonsei University, 134 Shinchon-dong, Seodaemun-gu, Seoul 120-749, Korea
Selective dissolution of hyper duplex stainless steel was studied by potentiodynamic and potentiostatic test in various concentrations of
H2 SO4 /HCl solutions at various temperatures. There were two peaks in the active-to-passive transition region in potentiodynamic test in 2 M
H2 SO4 + 0.5 M HCl solution at 60 C. In potentiostatic tests, the curve at 340 mV showed stable current density. As the potential increased,
the current density increased and at above 310 mV potential, there was a much longer initial period of nonsteady current value. As the potential
reached at 280 mV, the current density started to be stabilized and the current density was completely stabilized at 250 mV. It was found that
a preferential dissolution of ferrite phase occurred at 330 mV and with the increase of potential, austenite phase was corroded at a high rate. On
the other hand, both two phases were passivated at the potential above 270 mV, so that selective dissolution was absent.
[doi:10.2320/matertrans.MER2008182]
(Received June 5, 2008; Accepted March 3, 2009; Published April 15, 2009)
Keywords: hyper duplex stainless steel, selective dissolution, potentiostatic test, active-to-passive transition
1.
Introduction
Duplex stainless steel (DSS) is the stainless steel (SS) that
has microstructure where both ferrite and austenite phases are
present in approximately equal volume fraction. DSS has a
high mechanical strength, excellent corrosion resistance, and
a better cost performance than austenitic SS because of its
lower Ni content. Highly alloyed DSS with Cr, Mo and N has
excellent resistance to localized corrosion and SCC, so that it
is suitable for chemical industries or an marine equipment
that requires excellent corrosion resistance.1,2) The corrosion
behavior of DSS is greatly affected by the difference in
chemical composition between ferrite and austenite phases.
In general, Cr, Mo and W contents are higher in ferrite while
Ni and N are much higher in austenite. For this reason, there
is a difference in dissolution rate between ferrite and
austenite phases of DSS.3–6) Selective dissolution of the
two phases of DSS in acid solutions has been widely
studied.7–14) If lower potential than passivation potential is
applied to the specimen, dissolution of ferrite phase takes
place preferentially, whereas if higher potential is done,
usually austenite phase is dissolved and ferrite phase
dissolves very slowly or is passivated. It is well known that
if DSS is dipped in acidic solution, galvanic coupling
between the two phases affects preferential dissolution.
Symniotis measured weight loss as a function of potential for
the two phases of SAF 2205 in 2 M H2 SO4 + 0.1 M HCl
solution.9) According to Symniotis, the ferrite and austenite
phases had similar dissolution rates at different potentials,
and the potential of the maximum dissolution rate of
austenite was 100 mV more noble than ferrite. These results
mean that austenite phase facilitates the dissolution of ferrite
phase by galvanic coupling. By Tsai et al.,15) it is revealed
that two separate peaks exist in anodic active-to-passive
transition region of polarization curve of SAF2205 in
H2 SO4 /HCl mixed solution. At higher anodic peak, the
preferential dissolution of austenite phase occured, while
the lower peak corresponded to the ferrite phase.16,17) The
existing researches were progressed for DSS and super DSS
of PREW 40 grades. However, to date, few studies have
been focused on hyper DSS of PREW 50 grades, having
higher Cr, Mo and W than DSS and super DSS, to substitute the 6% Mo austenite stainless steels in more severe
environment.
The purposes of this study are to observe the active-topassive transition of 26.2Cr-6.99Ni-2.37Mo-2.88W-0.36N
hyper DSS having PREW 49.5 in H2 SO4 /HCl mixed solution
through potentiodynamic polarization test, and to verify the
selective dissolution of ferrite and austenite phases through
potentiostatic polarization test at different potentials close to
passivation potential.
2.
Experimental Procedure
The DSS was vacuum induction melted and then hot rolled
to 6 mm-thick plate. Chemical composition of the DSS used
in this study is shown in Table 1. The specimen was cut and
solution heat treated at 1090 C for 30 minutes. This was
ground with SiC papers, followed by polishing with 1 mm
diamond paste, and then electrochemically etched at 2.5 V in
10 N KOH solution at room temperature. Microstructure of
DSS was observed with optical microscope.
In order to investigate the electrochemical behavior of
DSS, the specimen was mounted in epoxy-resin and ground
with SiC paper down to #2000, the specimen was conducted
to potentiodynamic anodic polarization test (APT) in 2 M
H2 SO4 , 2 M H2 SO4 + 0.5 M HCl and 2 M H2 SO4 + 1 M
HCl mixed solutions at 40 and 60 C. After potentiostatic test
was conducted at potential close to passivation potential for
2 h, the surface of the specimen was examined with SEM.
Besides, to the specimen after the potentiostatic test, XRD
was conducted to identify the phases existing on the surface.
3.
Results and Discussion
Figure 1 shows the microstructure of DSS which was
solution heat treated at 1090 C for 30 minutes. The austenite
phase can be found as island phase on the background of
Selective Dissolution Characteristics of 26Cr-7Ni-2.5Mo-3W Duplex Stainless Steel in H2 SO4 /HCl Mixed Solution
Table 1
1215
Chemical composition of duplex stainless steel (DSS) in the present study.
Element
Cr
Ni
Mo
W
Mn
N
P
S
Ce
Ba
Fe
PREW
mass%
26.2
6.99
2.37
2.88
2.23
0.3567
0.007
0.008
0.01
0.003
Bal.
49.5
PRE (Pitting Corrosion Equivalent) ¼ %Cr þ 3:3 ð%Mo þ 1=2 %WÞ þ 30 %N
Fig. 1 Optical micrograph of DSS after solution heat treatment at 1090 C
for 30 min.
ferrite phase which looks relatively dark. Besides, affected
by hot rolling, the specimen has elongated texture parallel to
the rolling direction. Potentiodynamic anodic polarization
test was conducted in 2 M H2 SO4 , 2 M H2 SO4 + 0.5 M HCl
and 2 M H2 SO4 + 1 M HCl mixed solutions at different
temperatures to see anodic polarization behavior. Results of
the APT in H2 SO4 /HCl mixed solution at 60 C are shown in
Fig. 2. Among the solutions of different concentrations, two
peaks were detected in the active-passive transition region at
Fig. 3 Potentiodynamic polarization curves of 2205 DSS and the
respective constituent phases in 2 M H2SO4 + 0.5 M HCl solution by
Tsai et al.15)
from 400 mV to 200 mV on the polarization curve only
in 2 M H2 SO4 + 0.5 M HCl solution at 60 C. This result is
very similar to the polarization behavior of SAF 2205 (DSS
of PRE 38) in 2 M H2 SO4 + 0.5 M HCl solution at room
temperature reported by Tsai et al.17) (Fig. 3). However,
Fig. 2 Potentiodynamic anodic polarization curves of DSS in (a) 2 M H2 SO4 at 60 C, (b) 2 M H2 SO4 + 0.5 M HCl at 60 C, (c) 2 M
H2 SO4 + 1 M HCl at 60 C and (d) 2 M H2 SO4 + 0.5 M HCl solution at 40 C.
1216
H. Hwang, G. Lee, S. Jeon and Y. Park
Fig. 4
The enlarged transition region of Fig. 2(b) of DSS in deaerated 2 M H2 SO4 + 0.5 M HCl solution at 60 C.
(a)
(b)
(c)
(d)
(e)
(f)
(g)
(h)
(i)
(j)
Fig. 5 Potentiostatic measurements in 2 M H2 SO4 + 0.5 M HCl solution at 60 C for 2 h at different anodic potential.
Selective Dissolution Characteristics of 26Cr-7Ni-2.5Mo-3W Duplex Stainless Steel in H2 SO4 /HCl Mixed Solution
(a)
(b)
1217
(c)
Fig. 6 SEM micrographs of DSS after potentiostatic test at (a) 330 mV, (b) 300 mV and (c) 270 mV for 2 h in 2 M H2 SO4 + 0.5 M
HCl solution at 60 C.
Fig. 7 XRD of DSS after potentiostatic test in 2 M H2 SO4 + 0.5 M HCl
solution at 60 C for 2 h.
when the Potentiodynamic test was conducted in 2 M
H2 SO4 + 0.5 M HCl solution at 40 C, only one peak was
detected. Femenia et al.18) reported that the higher alloyed
duplex steels exhibited a more equal corrosion resistance
between the two phases. High contents of the alloying
elements Cr, Mo, W and N in the DSS improved dissolution
resistance of the ferrite and austenite phases as well as of
phase boundaries, and subsequently this type of DSS showed
more homogeneous dissolution behavior. The hyper DSS of
PRE 49.5 used in this study contained aforementioned
elements highly, so that the dissolution tendency of ferrite
and austenite phases in 2 M H2 SO4 + 0.5 M HCl solution at
40 C was balanced, but it was thought that the balance was
broken as the temperature increased. So, the two peaks of
hyper DSS in 2 M H2 SO4 + 0.5 M HCl solution were
detected at 60 C although the two peaks of SAF 2205, 1st
generated DSS were found at room temperature.
Figure 4 is the enlarged transition region of Fig. 2(b). The
lower peak is located at around 330 mV and the upper at
around 270 mV. According to previous reports the peak at
lower potential corresponded to the dissolution of ferrite
phase, whereas the peak at upper potential to that of austenite
phase.
Figure 5 showed tendencies of anodic current as a function
of time through potentiostatic test in 2 M H2 SO4 + 0.5 M
HCl at 60 C for 2 h at different potentials. Figure 5(a)
representing the test at 340 mV for 2 h showed stable
current density. As the potential increased, the current
density increased and at above 310 mV potential, the curve
denoted that at the beginning the current density decreased
but it increased later as time passed. As the potential reached
at 280 mV, the current density started to be stabilized, at the
same time, time required for the stabilization of the current
density was shortened gradually and current density was
completely stabilized at 250 mV. Namely, the current
density increased as the selective dissolution of ferrite phase
started with increasing potential at 340 mV and at the
potential of the transition range from 330 mV to 280 mV,
ferrite phase was passivated gradually whereas austenite
phase was activated. And both two phases were passivated at
above 280 mV, so that they could have a constant current
density. Figure 6 is microstructures of the specimens
conducted to potentiostatic test by SEM. It was found that
a selective dissolution between ferrite and austenite phases of
the specimen occurred. It could be ascertained that ferrite
phase is dissoluted preferentially when potentiostatic tested
at 330 mV for 2 h. in SEM image of Fig. 6(a), but in
Fig. 6(c), austenite phase was dissolved selectively with
ferrite phase remained after tested at 270 mV.
Figure 7 shows the results of X-ray diffraction (XRD) of
the specimen before and after the potentiostatic test. In XRD
result (a) before the potentiostatic test, both peaks of ferrite
and austenite phases were present. But after the potentiostatic
test at 330 mV (b), some of the peaks of ferrite phase didn’t
show up and after the test at 300 mV (c) the peak of
austenite phase disappeared. It was found that a preferential
dissolution of ferrite phase occurred at 330 mV and with the
increase of potential, austenite phase was corroded at a fast
rate. On the other hand, both two phases were passivated at
the potential above 270 mV, so that normal XRD peaks
could be detected.
4.
Conclusions
(1) When the potentiodynamic tests were conducted in
H2 SO4 /HCl mixed solutions, two separate peaks
related to ferrite and austenite phases were detected
in active-to-passive transition region of polarization
curve only in 2 M H2 SO4 + 0.5 M HCl solution at
60 C.
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H. Hwang, G. Lee, S. Jeon and Y. Park
(2) In the potentiostatic test at from 340 mV to 250 mV
in 2 M H2 SO4 + 0.5 M HCl solution at 60 C for 2 h, the
curve showed stable current density at first, but as the
potential increased, the current density increased and at
above 310 mV potential, the curve denoted that at the
beginning the current density decreased but it increased
later as time passed. The current density started to
be stabilized from 280 mV, at the same time, time
required for the stabilization of the current density
reduced gradually and the current density was completely stabilized at 250 mV.
(3) In XRD result on the surface of the specimen after
potentiostatic test, both peaks of ferrite and austenite
phases were present before the potentiostatic test. But
after the potentiostatic test at 330 mV, some of the
peaks of ferrite phase didn’t show up and after the test
at 300 mV the peak of austenite phase disappeared.
It was found that a preferential dissolution of ferrite
phase occurred at 330 mV and with the increase of
potential, austenite phase was corroded at a fast rate.
On the other hand, both two phases were passivated
at the potential above 270 mV, so that selective
dissolution was stopped.
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