Materials Transactions, Vol. 53, No. 6 (2012) pp. 1048 to 1055
© 2012 The Japan Institute of Metals
Effect of Copper Addition on the Active Corrosion Behavior
of Hyper Duplex Stainless Steels in Sulfuric Acid
Jun-Seob Lee1, Soon-Tae Kim1, In-Sung Lee1, Gwang-Tae Kim2, Ji-Soo Kim2 and Yong-Soo Park1,+
1
Department of Materials Science and Engineering, Yonsei University, 134 Shinchon-dong, Seodaemun-gu, Seoul 120-749, Korea
Stainless Steel Research Group, POSCO Technical Research Laboratories, Goedong-dong, Nam-Gu, Pohang, Gyeongbuk 790-785, Korea
2
The effect of copper (Cu) addition on the active corrosion behavior of hyper duplex stainless steels in sulfuric acid was investigated. The
addition of Cu in the base alloy enhanced the resistance to general corrosion by decreasing the critical and corrosion current densities, and
increasing the polarization resistance. There are two primary reasons for the considerable enhancement of the corrosion resistance of the
experimental alloys containing Cu. First, the protective surface film was enriched with the noble metallic copper (Cu) due to the selective
dissolution of the active metallic Cr, Fe, and Ni, and the electrochemical dissolution of the corrosion products such as iron-sulfide (FeS2), iron
sulfate (FeSO4), ferrous oxide (FeO) and hydrous iron sulfate (FeSO4·7H2O). Second, chromium oxide (Cr2O3), chromium trioxide (CrO3),
nickel oxide (NiO), molybdenum dioxide (MoO2), molybdenum trioxide (MoO3), and tungsten trioxide (WO3) in an oxide state, molybdenum
oxy-hydroxide (MoO[OH]2) and chromium hydroxide (Cr[OH]3) in a hydro-oxide state, molybdate (MoO42¹) and tungstate (WO42¹) as
corrosion inhibitors in an ion state, and ammonium (NH4+) elevating the pH in an ion state were increased and assisted in improving the
corrosion resistance. [doi:10.2320/matertrans.M2012008]
(Received January 6, 2012; Accepted March 15, 2012; Published May 9, 2012)
Keywords: stainless steel, copper, scanning auger multi-probe (SAM), X-ray photoelectron spectroscopy (XPS), general corrosion
1.
Introduction
Duplex stainless steels (DSSs) with nearly equal fractions
of ferrite (¡) and austenite (£) phases are being increasingly
used for various applications such as fuel gas desulphurization (FGD) facilities in fossil power plants, desalination
facilities, off-shore petroleum facilities, and chemical plants
due to their high resistance to stress corrosion cracking,
pitting corrosion, crevice corrosion, good weldability,
excellent mechanical properties and relatively low cost due
to the addition of low Ni, as compared with austenite
stainless steels.1­3)
In general, super duplex stainless steels (SDSSs), such
as UNS S32750, UNS S32760 and UNS S32550, are
defined as DSSs with a pitting resistance equivalent
(PRE = mass% Cr + (3.3 mass% Mo + 0.5 mass% W) + 16
mass% N)4,5) of 40 to 45. Hyper duplex stainless steels
(HDSSs) such as UNS S32707 are defined as highly alloyed
DSSs with a PRE in excess of 45.4)
It is well known that the addition of copper (Cu) to ferritic,
austenitic or duplex stainless steels improves the resistance to
general corrosion in sulfuric acid.6­10) It has been reported in
previous studies that the mechanism of the beneficial effect of
the Cu addition on the steels is based on the suppression of
the anodic dissolution by the noble metallic Cu enriched in
the surface film of austenitic stainless steels in sulfuric
acid.11,12) Furthermore, it has been also reported that the
enhancement mechanism of the corrosion resistance by Cu
addition is explained by protective, insoluble salt films such
as cuprous chloride (CuCl) or cupric chloride (CuCl2) formed
on the surface of stainless steels in chloride (Cl¹) environments.13)
However, because it is difficult to locate studies that have
quantitatively elucidated which elements of the HDSS, with
high concentrations of not only Cu but also N, Mo, W, Cr
+
Corresponding author, E-mail: yongsoop@yonsei.ac.kr
and Ni, have contributed to the corrosion resistance by being
a specific chemical species on the surface film in an elevated
temperature, concentrated H2SO4 environments, further
in-depth research analysis of surface film is required.
Moreover, it is necessary to quantitatively verify the effects
of the Cu addition on the difference of the resistance to
general corrosion between the £-phase and ¡-phase using a
formula for the sulfuric-acid resistance equivalent (SRE =
mass% Cr + 1.5 mass% Ni + 0.5 mass% Cu + 2 mass% Mo +
2 mass% W + 20 mass% N),14) and to clarify its related stage
of corrosion initiation and propagation.
Thus, in this work, the effect of Cu addition on the active
corrosion behavior of hyper duplex stainless steels (HDSSs)
in highly concentrated sulfuric acid was investigated using
immersion tests, electrochemical measurements, a scanning
Auger multi-probes (SAM) analysis and an X-ray photoelectron spectroscopy (XPS) analysis of surface film.
2.
Experimental Procedures
2.1
Calculation of phase diagram and equilibrium
fractions of each phase
The effects of the Cu addition on the phase diagram and
equilibrium fractions of each phase were calculated against
the temperature for the HDSS alloy using a commercial
Thermo-Calc software package.
2.2 Material and heat treatment
The experimental alloys were manufactured using a high
frequency vacuum induction furnace and then hot rolled into
plates of 6 mm thickness. The experimental alloys were cut
and solution heat-treated for 5 min per 1 mm of thickness at
1363 K and then quenched in water. The chemical composition of the experimental alloys is shown in Table 1.
2.3 Microstructure characterization
In order to observe the optical microstructures of the ¡-
Effect of Copper Addition on the Active Corrosion Behavior of Hyper Duplex Stainless Steels in Sulfuric Acid
Table 1
Alloys
The chemical compositions of the experimental alloys (mass%).
C
Cr
Ni
BASE 0.020 27.36 7.11
1.5Cu 0.017 26.91 6.59
3Cu
0.018 26.84 6.42
Mo
W
N
Mn
Si
S
Results and Discussion
Cu
2.59 3.38 0.31 1.45 0.31 0.004 0.19
2.50 3.30 0.38 0.94 0.34 0.005 1.45
2.51 3.30 0.36 0.96 0.33 0.005 3.06
phases and £-phases in the HDSS, they were ground to 2000
grit using SiC abrasive papers, polished with a 1 µm diamond
paste, and then electrochemically etched using 10 mass%
KOH. The ¡-phase volume fractions were calculated using
the method of manual point count according to
ASTM E562.15) The chemical compositions of the ¡-phase
and £-phase were analyzed using a scanning electron
microscope (SEM)­energy dispersive spectroscope (EDS).
The nitrogen content was analyzed using a SAM.
2.4 Corrosion tests
In order to analyze the effect of the Cu addition on the
resistance to general corrosion of the experimental alloys,
both electrochemical measurements and immersion test were
made. Measurements of potentiodynamic anodic polarization
curves were performed in a deaerated 6.34 N H2SO4 solution
at 353 K according to the ASTM G 5.16) These electrochemical characteristics: the critical current density (Ic),
corrosion current density (Icorr), and polarization resistance
(Rp) were measured from the potentiodynamic anodic
polarization curves. The test was performed at a potential
range of ¹0.4 VSCE to +1.1 VSCE at a scanning rate of
1 mV/min using a saturated calomel electrode (SCE) as a
reference electrode. A potentiostatic polarization test were
performed to measure current transients in a deaerated 6.34 N
H2SO4 solution at 353 K with an applied potential of
¹0.2 VSCE in the active region of the potentiodynamic anodic
polarization curves. The current transients were recorded for
3600 s. The SEM was used to observe the corrosion sites on
the specimen after the potentiostatic test. After immersion of
the same specimens at 20 K intervals from 313 to 353 K for
6 h in 18.4 N H2SO4 solutions, the weight loss was measured
in order to obtain the corrosion rates of the experimental
alloys from the following formula (1) for comparison with
the polarization resistance:
mpy ðmils per yearÞ ¼ 534 W=A µ H
3.
1049
ð1Þ
where ¦W is the weight loss (mg), A is the surface area (in2),
µ is the density (g/cm2), and H is the immersion time (h).
2.5 Surface analysis
The specimens used for a surface analysis were polished
with SiC paper to 2000 grit, then polished with a 1 µm
diamond paste, and washed with acetone (CH3COCH3). The
chemical compositions of each phase were analyzed using
a SAM after the potentiostatic polarization test. The Ar
sputtering rate was approximately 5 nm/min.
The chemical species in the outermost surface film
formed on the alloy were analyzed using an XPS after the
potentiostatic polarization test. The energy source was an
Al-K¡ (1486.6 eV) X-ray, and the acquired spectra were
calibrated with a binding energy of C 1 s (284.5 eV).
3.1
Calculation of the phase diagram and equilibrium
fractions of each phase
Figure 1 shows the effects of the Cu addition on the phase
diagram and equilibrium fractions of each phase for the
HDSSs calculated using the Thermo-Calc software package.
The sectional view at the 27 mass% Cr illustrates that the
alloys solidify primarily as an ¡-phase and some of the
this ¡-phase, transforms of £-phase with a decrease in the
temperature (Figs. 1(a) and 1(c)), irrespective of the Cu
addition in the alloy. As the temperature decreases further, the
¡-phase decomposes into a sigma phase (·) and a secondary
austenite (£2) according to the eutectoid reaction.
L!Lþ¡!Lþ¡þ£
! ¡ þ £ ! ¡ þ £2 þ · ! £2 þ ·
ð2Þ
It is well known that Cu as a substitution element stabilizes
the £-phase and provides a solid solution strengthening.17) As
the temperature of the solution heat-treatment decreases in
the region with the dual £/¡-phases, the volume fraction of
the ¡-phase decreases and that of the £-phase increases. It is
predicted that the optimum temperature of the solution heattreatment to obtain the desired microstructure of approximately 50 vol% £-phase and 50 vol% ¡-phase is approximately 1353 to 1363 K (Figs. 1(b) and 1(d)).
3.2
Effects of copper addition and solution temperature
on corrosion rate through immersion test
After the base, 1.5 Cu, and 3 Cu experimental alloys were
immersed in a 18.4 N H2SO4 solution at 313, 333 and 353 K
for 6 h, the corrosion rate was measured (Fig. 2). As the
addition of copper to the base alloy increased, the corrosion
rate decreased. Hence, it is concluded that the Cu addition
has a positive effect on the resistance to general corrosion of
HDSS.
Figure 3 shows the optical microstructure of the experimental alloys observed after 10 min and 6 h immersion in
18.4 N H2SO4 at 353 K. After the immersion test for 10 min,
the base alloy without Cu addition exhibited severe general
corrosion in the ¡-phases and £-phases. However, the alloy
containing 1.5 mass% Cu exhibited severe general corrosion
in all ¡-phases whereas the alloy containing 3 mass% Cu
exhibited general corrosion in partial ¡-phases. In particular,
as the immersion time was increased from 10 min to 6 h, the
general corrosions of the Cu added alloys were propagated
from the ¡-phases to the £-phases.
In order to clarify the difference in the resistance to general
corrosion between the ¡-phases and £-phases, the content of
the alloying elements in the ¡-phases and £-phases annealed
at 1363 K was quantitatively measured using a SEM­EDS
and the N content was measured using a SAM. Then, the
sulfuric-acid resistance equivalent (SRE) values of the ¡phase and £-phase were calculated.
Table 2 shows the effects of the Cu addition on the SRE
values of the ¡-phase and £-phase in the experimental alloys.
As the Cu content to the BASE alloy increased, the SRE
values of the £-phases increased. On the other hand, although
the SRE values of the ¡-phases increased slightly, the Cu
content in the ¡-phases increased with an increase in the
1050
J.-S. Lee et al.
(a)
(b)
27mass%
1363 K
1873
1673
Mole Fraction
Temperature, T / K
1773
1573
1473
1373
1273
1173
1073
773
873
973 1073 1173 1273 1373 1473
Temperature, T / K
Cr, mass%
(c)
(d)
1363 K
27mass%
1873
Temperature, T / K
1773
L
α
L+γ
1673
L+γ+α
L+α
1573
1473
γ+α
γ
1373
σ
γ
γ+α+χ
X
γ+α+σ
1273
γ+χ
1173
γ+σ
Cr2N
1073
10
15
20
25
30
35
40
773
Cr, mass%
873
973 1073 1173 1273 1373 1473
Temperature, T / K
Fig. 1 Effects of the Cu addition on the phase diagram and equilibrium fractions of each phase for the HDSSs calculated using the
Thermo-Calc software package: (a) the phase diagram for the alloy-BASE, (b) the equilibrium fractions of each phase for the alloyBASE, (c) the phase diagram for the alloy-3 Cu, and (d) the equilibrium fractions of each phase for the alloy-3 Cu.
Log Corrosoin Rate (mpy)
6
5
The nitrogen solubility in the ¡-phase in HDSSs has a
maximum value of 0.05 mass% whereas that in the £-phase
has a value of 0.5 to 0.57 mass% (Table 2). Hence, based on
the calculated SRE£ and SRE¡ values, it is reasonable to
explain that general corrosion must occur selectively in the
¡-phases in a highly concentrated H2SO4 environment
because the SRE value of the £-phase is larger than that of the
¡-phase, irrespective of the Cu addition to the alloys (Fig. 3).
BASE
1.5 Cu
3 Cu
4
3
2
1
0
300
320
340
360
Temperature, T / K
Fig. 2 Effect of copper addition on the corrosion rate of the experimental
alloys in 18.4 N H2SO4 at elevated temperature for 6 h.
added Cu, thereby enhancing the resistance to general
corrosion of the ¡-phases in the HDSS. Furthermore, as
verified in previous studies, nitrogen (N) has the most
powerful influence on the balance of corrosion resistance
between the ¡-phase and £-phase, compared with Cr, Mo and
W, because N, which is a £ stabilizer, is nearly completely
solutionized in the £-phase whereas it is rarely solutionized in
the ¡-phase in HDSSs.18­20)
3.3
Effects of Cu addition on potentiodynamic and
potentiostatic polarization behaviors
The potentiodynamic anodic polarization curves in the
deaerated 6.34 N H2SO4 at 353 K are presented in Fig. 4.
Table 3 shows the effects of the copper addition on the
critical current density (Ic), corrosion current density (Icorr),
and polarization resistance (RP) obtained from Fig. 4. As the
addition of copper to the base alloy increased, Ic and Icorr
decreased whereas RP increased. This indicates that the
general corrosion resistance of the Cu added alloys is
superior to that of the base alloy due to lower Ic, lower Icorr
and higher RP values.
Figure 5 shows the effect of the Cu content on the change
of anodic current density with time at an applied potential of
Effect of Copper Addition on the Active Corrosion Behavior of Hyper Duplex Stainless Steels in Sulfuric Acid
1000
Immersion for 6 h
Potential, E / mV vs SCE
Immersion for 10 min
γ
(a)
α
100 μ m
100 μ m
800
BASE
1.5Cu
3Cu
600
3Cu
400
1.5Cu
200
BASE
0
-200
γ
(b)
1051
10-5
10-4
10-3
10-2
Current density, I / Acm-2
α
Fig. 4 Effect of copper addition on potentiodynamic polarization behavior
in the deaerated 6.34 N H2SO4 at 353 K.
(c)
γ
Table 3 Effects of Cu addition on the critical current density (Ic), corrosion
current density (Icorr), and polarization resistance (RP) measured from the
potentiodynamic polarization curves of Fig. 4.
α
Alloy
Fig. 3 Optical micrographs of the experimental alloys after immersion test
for 10 min and 6 h in 18.4 N H2SO4 at 353 K: (a) the alloy-BASE, (b) the
alloy-1.5 Cu and (c) the alloy-3 Cu.
¢a
¢c
Rp*2
Icorr
Ic
B*1
(µA/cm2) (mV/decade) (mV/decade)
(µA/cm2) (³-cm2)
BASE
600
33
67
9.6
515
18
1.5Cu
258
31
60
8.9
185
48
3Cu
174
50
40
9.7
111
87
*1
Table 2 Effects of Cu addition on the SRE values of the ¡-phase and
£-phase in the experimental alloys.
Cr
Ni
Cu
Mo
W
N
Substrate 27.36 7.11 0.19 2.59 3.38 0.31
Sulfuric-acid
Resistance
Equivalent (SRE*)
®
¡
28.87 5.37 0.12 3.05 4.06 0.05
BASE (51 vol%)
52.1
£
25.78 8.76 0.22 2.01 2.59 0.49
(49 vol%)
58.0
Substrate 26.91 6.59 1.45 2.50 3.30 0.379
¡
28.57 5.22 1.25 3.06 4.01 0.05
1.5Cu (49 vol%)
800
Current density, I / Acm-2
Chemical Compositions (mass%)
B = ¢a © ¢c/2.303(¢a + ¢c)
Rp = B/Icorr
*2
®
52.2
BASE
1.5Cu
3Cu
BASE
700
600
500
400
1.5Cu
300
3Cu
200
100
£
25.41 8.05 1.70 1.96 2.55 0.567
(51 vol%)
58.7
Substrate 26.84 6.42 3.06 2.51 3.30 0.361
®
¡
28.34 4.96 2.52 3.06 4.06 0.05
3Cu (50 vol%)
52.3
£
25.32 7.88 3.60 1.96 2.54 0.55
(50 vol%)
58.9
*SRE = [mass% Cr] + 1.5 [mass% Ni] + 0.5 [mass% Cu] + 2 [mass%
Mo] + 2 [mass% W] + 20 [mass% N]
¹0.2 VSCE in the active region of the potentiodynamic
polarization curves of Fig. 4 in the deaerated 6.34 N H2SO4
at 353 K. The test was performed in order to observe the
current transients at a primary passivation potential (Epp) that
correspond to the critical current density at the potentiodynamic anodic polrarization curves. As the addition of copper
to the base alloy increased, the critical current density
decreased.
0
0
600
1200
1800
2400
3000
3600
Time, t / s
Fig. 5 Effect of copper addition on the change of electrochemical current
density with time behavior at an applied potential of ¹0.2 VSCE in the
deaerated 6.34 N H2SO4 at 353 K.
In summary, based on these electrochemical parameters (Ic,
Icorr and RP) and corrosion rate measured from the immersion
test, it was elucidated that Cu considerably enhanced the
resistance to general corrosion at an active region in a highly
concentrated sulfuric acid environment.
3.4
Effects of Cu in alloy and pure metallic Cu and on
the active corrosion behavior
Figure 6 shows the corrosion rate of the pure metals and
experimental alloys measured after the immersion test (IT)
and potentiodynamic anodic polarization test (PAPT) in the
1052
J.-S. Lee et al.
Atomic concentration (at.%)
Icorr, I / μAcm-2
IT, mpy
1000
100
10
1
Alloy Alloy Alloy Fe
BASE 1.5Cu 3Cu
Cr
Ni
Cu
Mo
W
Potential, E / mV vs SCE
Fig. 6 The corrosion rate of the pure metals and experimental alloys for the
immersion test (IT) and corrosion current density (Icorr) obtained from
polarization resistance by the potentiodynamic polarization curves in
6.34 N H2SO4 solution at 353 K.
1200
Alloy-3Cu
1000
Cr
Fe
800
Ni
Cu
600
Mo
400
W
200
0 W
Cu
-200
Mo Ni
-400
Fe
-600
Cr
-800
-1000
10-7 10-6 10-5 10-4 10-3 10-2 10-1
70
S
O
Ni
(a)
60
Mo(x2)
N(x3)
Fe
Cr
Cu
W
50
40
30
20
10
0
0
3
6
9
12
15
Sputter time, t / min
Atomic concentration (at.%)
Corrosion Rate, mpy or I / μAcm -2
10000
70
S
Mo(x2)
N(x3)
O
Cr
Fe
Ni(x2)
Cu
W
(b)
60
50
40
30
20
10
0
0
3
6
9
12
15
Sputter time, t / min
100
Fig. 8 The Auger depth profile of the £-phase and ¡-phase in the alloy3 Cu after the potentiostatic polarization test at an applied potential
of ¹0.2 VSCE in the deaerated 6.34 N H2SO4 solution at 353 K: (a) the
£-phase and (b) the ¡-phase.
Current density, I / Acm-2
Fig. 7 Potentiodynamic polarization behavior of the pure metals and the
alloy-3 Cu in the deaerated 6.34 N H2SO4 at 353 K.
6.34 N H2SO4 solution at 353 K. The corrosion rate (mpy)
after the IT was measured using the formula (1) and that
(µA/cm2) after the PAPT was measured using a cathodic
Tafel extrapolation method. The corrosion rates of the pure
metallic Cu were very low (8.2 mpy or 17.4 µA/cm2)
compared with that of the pure metallic Cr (3152 mpy or
9513 µA/cm2), pure metallic Fe (1650 mpy or 2135 µA/
cm2), and pure metallic Ni (1021 mpy or 1055 µA/cm2).
According to the results of the potentiodynamic anodic
polarization test given in Fig. 6, the pure metals showed
various corrosion current density in this order: Cr >
Fe > Ni > base alloy > 1.5 Cu alloy > 3 Cu alloy > Cu µ
Mo µ W.
Figure 7 shows the potentiodynamic polarization behavior
of the pure metals and the alloy-3 Cu in deaerated 6.34 N
H2SO4 at 353 K. The figure indicates that the anodic current
densities of the pure metallic Cu, Mo and W were much
lower than those of the pure metallic Cr, Fe and Ni whereas
the corrosion potentials of the pure metallic Cu, Mo and
W were much higher than those of the pure metallic Cr,
Fe and Ni. They also showed their corrosion potentials
in the following order: Cr < Fe < Ni < Mo < 3 Cu alloy <
Cu < W.
Accordingly, the corrosion current density decreasing and
the corrosion potential increasing when Cu was added to the
alloy can be attributed to the noble property of Cu.
3.5
Mechanism of Cu addition for enhancement of
corrosion resistance by Cu addition
Figure 8 shows the Auger depth profile of the surface film
formed on the £-phases and ¡-phases in the alloy-3 Cu after
the potentiostatic polarization test at an applied potential of
¹0.2 VSCE in the deaerated 6.34 N H2SO4 solution at 353 K.
Figure 9 shows the distribution of the alloying elements of
the surface film formed on in the austenite phase and the
ferrite phase in the alloy-3 Cu obtained from the Auger depth
profile presented in Fig. 8 after the poteniostatic polarization
test at an applied potential of ¹0.2 VSCE in the deaerated
6.34 N H2SO4 solution at 353 K. The Cr, Mo and W, which
act as ¡-stabilizers, are enriched in the ¡-phase and are
diluted in the £-phase. In contrast, the Cu, Ni and N, which
act as £-stabilizers, are enriched in the £-phase and are diluted
in the ¡-phase. While the atomic concentration of the Fe, Cr
and Ni in the outermost surface film of both the ¡-phases and
Fe-α
Fe-γ
(a)
3
15
Atomic concentration (%)
(c)
Ni-α
12
Ni-γ
9
6
3
0
10
3
6
9
12
Sputter time, t / min
(e)
4
2
0
7
3
6
9
12
Sputter time, t / min
(g)
6
5
3
2
1
0
3
10
0
6
9
12
Sputter time, t / min
3
(d)
25
15
Cu-α
Cu-γ
20
15
10
5
0
10
3
6
9
12
Sputter time, t / min
15
W-α
(f)
8
W-γ
6
4
2
0
80
3
6
9
12
Sputter time, t / min
(h)
15
O-α
60
O-γ
40
20
0
0
15
6
9
12
Sputter time, t / min
30
0
4
0
Cr-γ
20
15
N-α
N-γ
Cr-α
30
0
6
0
(b)
40
15
Mo-α
α
Mo-γ
8
50
0
15
0
Atomic concentration (%)
6
9
12
Sputter time, t / min
Atomic concentration (%)
Atomic concentration (%)
0
Atomic concentration (%)
Atomic concentration (%)
80
70
60
50
40
30
20
10
0
Atomic concentration (%)
Atomic concentration (%)
Effect of Copper Addition on the Active Corrosion Behavior of Hyper Duplex Stainless Steels in Sulfuric Acid
3
6
9
12
Sputter time, t / min
15
Fig. 9 Comparison of the alloying elements in the ¡-phase with those in
the £-phase in the alloy-3 Cu measured by the scanning Auger multiprobes (SAM) after the potentiostatic polarization test at an applied
potential of ¹0.2 VSCE in the deaerated 6.34 N H2SO4 solution at 353 K:
(a) Fe, (b) Cr, (c) Ni, (d) Cu, (e) Mo, (f ) W, (g) N, and (h) O.
£-phases decreased significantly, that of the Cu, Mo and N
in the outmost surface film increased significantly compared
with that of the substrate. This indicates that the novel metals
such as Cu, N and Mo can be enriched on the alloy surface
due to the selective dissolution of the active metals such as
Fe, Cr, and Ni in the alloy. However, there was no significant
difference in the W concentrations in the ¡-phase and £-phase
1053
compared with the substrate. Consequently, Cr, Fe and Ni
cannot contribute to the corrosion resistance in highly
concentrated sulfuric acid environments due to their
corrosion potential being lower than the other alloying
elements. However, Cu, N and Mo can contribute to the
corrosion resistance. In particular, the Cu on the outermost
surface film of the £-phase was enriched by approximately
26 at%, which is much larger than that of the substrate and
that of the ¡-phase was enriched by approximately 7 at%,
which is also larger than that of the substrate.
After the potentiostatic polarization test of the alloy-3 Cu
at an applied potential of ¹0.2 VSCE in the deaerated 6.34 N
H2SO4 solution at 353 K, the chemical species in the
outermost surface film were analyzed using XPS; the results
are presented in Fig. 10. The binding energy of each
chemical element used for the XPS analysis is given in
Table 4. For the 3 Cu experimental alloy, Fe existed as Fe
(M), ferrous oxide (FeO), ferric sulfate (Fe2[SO4]3) and
hydrous iron sulfate (FeSO4·7H2O); Cr existed as Cr (M),
chromium oxide (Cr2O3), chromium trioxide (CrO3), and
chromium hydroxide (Cr[OH]3); Ni existed as Ni (M), nickel
oxide (NiO), and nickel iron oxide (NiFe2O4); Cu existed as
metallic copper (Cu), and copper oxide (CuO); Mo existed
as Mo (M), molybdenum dioxide (MoO2), molybdenum
trioxide (MoO3), molybdenum oxy-hydroxide (MoO[OH]2),
and molybdate (MoO42¹); W existed as W (M), tungsten
trioxide (WO3), and tungstate (WO42¹); N existed as
ammonium (NH4+) and nitric oxide (NO) and S existed as
iron-sulfide (FeS2), copper mono-sulfide (CuS), nickel sulfide
(NiS), copper sulfate (CuSO4), and iron sulfate (FeSO4).
Based on the results of the XPS analysis, the contribution
of the Cu addition to the enhancement of corrosion resistance
can be explained as the followings. With an increase in the
Cu addition to the base alloy, Fe, Cr and Ni in the active
metal state were dissolved, and FeS2 sulfides, FeSO4 sulfates,
FeO oxides, and FeSO4·7H2O hydrous iron sulfate increased.
The FeS2, FeO, FeSO4 and FeSO4·7H2O are corrosion
products that dissolve electrochemically. This leads to the
understanding that these chemical species were rapidly
dissolved in the highly concentrated H2SO4 solution, and
the noble Cu in the relative metallic state was considerably
enriched, which greatly enhances the corrosion resistance.
Table 4 The binding energies of some chemical species for the XPS analysis.
Binding Energies
(eV)
Species
Fe(M)
Species
Binding Energies
(eV)
Species
707.1
Cu(M)
2p3/2
932.6
Fe2+ 2p3/2
FeO 2p3/2
708.6
709.4
CuO 2p3/2
Cu(M) 2p1/2
933.9
952.2
FeO
710.0
MoO3
711.3
MoO2
2p3/2
2p3/2
FeSO4·7H2O
2p3/2
Fe42(SO4)2 2p3/2
713.1
NiS
2p3/2
161.4
CuS
2p3/2
162.5
Mo(M)
3d5/2
MoO(OH)2 3d5/2
MoO42¹ 3d5/2
3d5/2
3d3/2
MoO42¹ 3d5/2
Binding Energies
(eV)
227.7
Ni(M)
230.7
231.4
NiO
NiO
232.6
853.5
854.2
2p3/2
854.6
855.4
235.1
Ni2O3 2p3/2
855.8
Ni(OH)4 2p3/2
856.0
NiSO4 2p3/2
857.0
NO 1s
NH4+ 1s
399.2
400.2
FeS2 2p3/2
163.1
W(M)
Cr2O3 2p3/2
576.0
FeSO4 2p3/2
168.3
WO3 4f7/2
33.1
Cr(OH)3 2p3/2
CrO3 2p3/2
577.0
578.1
CuSO4 2p3/2
CuSO4·5H2O 2p3/2
169.1
169.5
W(M) 4f7/2
WO3 4f7/2
34
35.5
WO42¹
37.4
4f7/2
852.5
2p3/2
2p3/2
NiFe2O4 2p3/2
574.1
4f7/2
NiO
2p3/2
Binding Energies
(eV)
233.4
2p3/2
Cr(M)
Species
33
NO
1s
401.0
1054
J.-S. Lee et al.
Counts
600
400
(a) Fe 2p
Fe2(SO4)3
Fe(M)
FeO
Counts
800
FeSO47H2O
700
(b) Cr 2p
600
Cr(OH)3
Cr2O3
500
Cr(M)
400
CrO3
300
200
200
100
0
0
582 580 578 576 574 572 570
716 714 712 710 708 706 704
Binding Energy, eV
Binding Energy, eV
300
(c) Ni 2p
NiO NiO Ni(M)
NiSO4 NiFe2O4
200
150 Ni(OH)2
100
8000
Cu(M)
2000
0
858 857 856 855 854 853 852 851
0
960 955 950 945 940 935 930 925
Binding Energy (eV)
Binding Energy, eV
200
MoO(OH)2
180 (e) Mo 3d
MoO3 MoO42160
Mo(M)
140
120
MoO2
100
80 MoO4260
40
20
0
240 237 234 231 228 225 222
700
400
300
WO4
WO3
WO3
100
0
40
38
36
34
32
Binding Energy, eV
200
+
Counts
150
NO
398
2-
200
NH4
400
W(M)
500
Binding Energy, eV
1600 (g) N 1s
1400
NO
1200
1000
800
600
400
200
0
404 402
(f) W 4f
600
Counts
Counts
CuO
6000
4000
50
Counts
Cu(M)
(d) Cu 2p
10000
Counts
Counts
250
12000
FeS2
(h) S 2p
CuS
CuSO4
FeSO4
NiS
100
50
396
Binding Energy, eV
0
174 171 168 165 162 159 156
Binding Energy (eV)
Fig. 10 Deconvolution of chemical species by the XPS in the outer surface film formed on the alloy-3 Cu after the potentiostatic
polarization test at an applied potential of ¹0.2 VSCE in the deaerated 6.34 N H2SO4 solution at 353 K: (a) Fe, (b) Cr, (c) Ni, (d) Cu,
(e) Mo, (f ) W, (g) N and (h) S.
In addition, Cr2O3, CrO3, NiO, MoO2, MoO3 and
WO321,22) in the oxide state, MoO(OH)2 and Cr(OH)323,24)
in the hydro-oxide state, MoO42¹ and WO42¹ 24,25) as a
corrosion inhibitor in the ion state, and NH4+,26) which
increases the pH in the ion state, were increased. Therefore,
it is believed that these chemical species also assist in
improving the corrosion resistance.
4.
Conclusions
In this work, the effect of Cu addition on the active
corrosion behavior in hyper duplex stainless steels (HDSSs)
in highly concentrated sulfuric acid environments was
investigated using an immersion test, a potentiodynamic
polarization test, a potentiostaic polarization test, a scanning
Auger multi-probes (SAM) analysis and an X-ray photoelectron spectroscopy (XPS) analysis of the surface film
formed on the steels. From the results of these tests, the
following conclusions have been drawn.
(1) The addition of Cu in the base alloy greatly enhanced
the general corrosion resistance by decreasing the
critical current density (Ic) and corrosion current density
(Icorr), and increasing the polarization resistance (Rp)
This effect is a result of the noble metal Cu. In addition,
the corrosion rates of the elements composing the alloys
were similar to those of pure metals.
Effect of Copper Addition on the Active Corrosion Behavior of Hyper Duplex Stainless Steels in Sulfuric Acid
(2) Based upon the SRE£ and SRE¡ values, the general
corrosion of the solution heat-treated HDSSs was
selectively initiated in the ¡-phases because the SRE
value of the £-phase is larger than that of the ¡-phase,
irrespective of the Cu addition to the HDSSs. The
general corrosion was finally propagated from the
¡-phase to the £-phase.
(3) There are two primary reasons for the considerable
enhancement of the corrosion resistance of the
experimental alloys containing Cu. First, the protective
surface film was enriched with the noble metallic
copper (Cu) due to the selective dissolution of the active
metallic Cr, Fe and Ni, and the electrochemical
dissolution of corrosion products such as iron-sulfide
(FeS2), iron sulfate (FeSO4), ferrous oxide (FeO) and
hydrous iron sulfate (FeSO4·7H2O). Second, chromium
oxide (Cr2O3), chromium trioxide (CrO3), nickel oxide
(NiO), molybdenum dioxide (MoO2), molybdenum
trioxide (MoO3), and tungsten trioxide (WO3) in an
oxide state, molybdenum oxy-hydroxide (MoO[OH]2)
and chromic hydroxide (Cr[OH]3) in a hydro-oxide
state, molybdate (MoO42¹) and tungstate (WO42¹) as
corrosion inhibitors in an ion state, and ammonium
(NH4+) elevating the pH in an ion state were increased
and assisted in improving the corrosion resistance.
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