Electrochemical corrosion testing of
solid state sintered silicon carbide in
acidic and alkaline environments
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by A. Andrews*, M. Herrmann†, and M. Sephton*
Paper written on project work carried out in partial fulfilment of MSc (Eng) degree
Solid state sintered silicon carbide ceramics are used in aggressive
environments as sealings. There exist data concerning corrosion,
but there are only few data concerning their electrocorrosion
behaviour. Therefore the electrochemical corrosion of solid state
sintered silicon carbide (SSiC) ceramics has been studied at room
temperature in acidic and alkaline environments by using potentiodynamic polarization measurements. The investigation showed a
pronounced electrochemical corrosion in bases and acids. In HCl and
HNO3 pseudo-passivity features due to the formation of a thin layer
of SiO2 on the surface were observed, whereas in NaOH soluble
silicate ions were observed, resulting in more pronounced corrosion.
The microstructural observations of initial and corroded samples
reveal that the residual carbon found in the microstructure do not
dissolve preferentially.
Keywords: Corrosion; SEM; SSiC; polarization test.
Introduction
C(residual ) + 2 H2O(l ) →
Ceramic materials are often used in very
aggressive and corrosive environments
because of their high degree of chemical
inertness and corrosion resistance. However,
even though it is slow, corrosion does occur.
The standard procedure for measuring
corrosion resistance of ceramic materials is the
immersion method, where their behaviour is
investigated by immersing samples in a
corrosive media and measuring weight loss,
corrosion layer depth, residual strength and
sometimes the concentration of the corroded
ions in the media.1–5 Recent investigations had
shown that the corrosion of SiC can be
strongly influenced by electrical effects.1,5,6
Electrochemical corrosion of sintered
silicon carbide ceramics has been studied at
room temperature in various dilute acids (HCl,
HNO3, HF, H2SO4 and H3PO4).1,5 The principal
corrosion reaction of SiC was assumed to be
the dissolution of Si atoms. For the
investigated SiSiC materials it is valid, but in
SSiC the corrosion is connected with the SiC.
Therefore in leaching and the description of
the electrocorrosion, one has to consider the
reaction of both Si and C. In these investi-
CO2 ( g) + 4 H + ( aq ) + 4e −
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VOLUME 106
[1]
E o = +0.206 V
SiC(s) + 2 H2O(l ) →
[2]
SiO2 (s) + C + 4 H + ( aq ) + 4e −
E o = –1.07 V
SiC(s) + 4 H2O(l ) →
SiO2 (s) + 8 H + ( aq ) + CO2 ( g) + 8e − [3]
E o = –0.433 V
* School of Proc. & Mat. Eng., University of the
Witwatersrand, Wits, Johannesburg, South Africa.
† Fraunhofer-Institut für keramische Technology und
Sinterwerkstoffe, Dresden, Germany.
© The South African Institute of Mining and
Metallurgy, 2006. SA ISSN 0038–223X/3.00 +
0.00. Paper received Nov. 2005; revised paper
received Mar. 2006.
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Synopsis
gations1,5 there is no indication of Si reactions
in alkaline environments.
The investigations by Cook et al.1 deduced
thermodynamical considerations based on Si
reactions and from the different behaviour in
HF and HCl observed that SSiC is passive in
acidic environments because of the formation
of a surface silica film. The reduction of O2
dissolved in water was proposed as cathodic
reaction. A number of anodic reactions for
carbon and silicon in acidic and alkaline
environments, with their corresponding
standard potentials (E°), are found in
literature.7–10 Based on these reactions, the
following corrosion reactions for SSiC
involving carbon were proposed. The standard
(SHE) potentials of the corrosion reactions
were calculated based on thermodynamic
considerations.11
In acidic environments:
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Electrochemical corrosion testing of solid state sintered silicon carbide
[4]
SiC(s) + 3 H2O(l ) →
SiO2 (s) + 6 H + ( aq ) + CO( g) + 6e −
E o = –0.542 V
In alkaline environments:
C(residual ) + 6OH - ( aq ) →
[5]
CO32– ( aq ) + 3 H2O(l ) + 4e −
E oB = +1.72 V
SiC(s) + 6OH - ( aq ) →
[6]
SiO32– ( aq ) + C + 3 H2O(l ) + 4e −
E oB = –1.874 V
SiC(s) + 12OH - ( aq ) →
SiO32– ( aq ) + 5 H2O(l ) + CO32– (aq ) + 8e −
[7]
E oB = –0.076 V
SiC(s) + 8OH - ( aq ) →
SiO32– ( aq ) + 4 H2O(l ) + CO(g) + 6e −
[8]
E oB = –0.8 V
In this paper, the corrosion mechanism of SSiC in acidic
and alkaline environments is investigated by electrochemical
methods. The changes of the microstructures during
corrosion experiments are investigated by SEM and
correlated with the mechanisms.
Experimental procedure
Material
Solid state sintered silicon carbide (SSiC) ceramic materials
were gas pressure sintered in Ar at a temperature of 2100°C
with carbon (C) and boron (B) as sintering additives. The
weight per cent of the residual carbon, determined by
Rietveld analysis was 2 ± 0.4 %. The measured density was
3.11 g/cm3. Samples were prepared for image analysis by
grinding on Struers MD-Piano 1200 to achieve a flat surface
and polishing on a Pan cloth to 1 µm finish using diamond
spray and diamond extender (lubricant). The samples were
cleaned afterwards with ethanol and dried. The microstructures and compositions of the material before and after
corrosion were analysed by means of scanning electron
microscopy (LEO 1525), X-ray diffraction (Phillips PW1710
and XRD7 Seifert/GE-Inspection) and Raman spectrometry
(JOBIN-YVON T64000).
Corrosion measurements
The electrochemical behaviour of these materials was
investigated by using potentiodynamic polarization
measurements according to ASTM G 5-9412 and ASTM G 38913. Samples were prepared by attaching an insulated
copper wire to one face of the sample with aluminium
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conductive tape and cold mounting in resin. Samples were
then polished on a Pan cloth to 1 µm surface finish and
coated at the sample-resin interface with Bostik quickset to
prevent the possibility of crevice corrosion. This was followed
by cleaning in an ultrasonic bath with ethanol before
immersion in the electrolyte. All tests were conducted at
room temperature (~25°C). Solutions were replaced after each
polarization scan. A Schlumberger (SI 1286) electrochemical
interface was connected to a personal computer with electrochemical application software for control and data-logging. A
platinum counter electrode (CE) was used. A saturated
calomel reference electrode (SCE) was connected to the cell
via a Luggin capillary. The probe tip was suspended approximately 1 cm above the sample surface. All potentials reported
are versus SCE.
Electrolytes used included 1 M solutions of HCl, HNO3
and NaOH. A mixture of 40.0g sodium carbonate (Na2CO3)
and 0.4 litres of 1 M NaOH was also used as electrolyte; the
details are explained later. Before immersion of the samples,
the corrosive solutions were purged with ultra-high purity
(99.999%) nitrogen gas (except where CO2 was used, as
explained later) for approximately 45 minutes to reduce the
dissolved-oxygen content. Purging was continued throughout the tests. The open-circuit potential (Ecorr) was taken 5
minutes after immersion of samples. A potential scan was
then initiated with a scan rate of 1 mV/s. The width of the
scan was from -200 mV to about +1000 mV relative to Ecorr.
At least three full scans were conducted per electrolyte. The
corrosion current density (icorr), which is directly proportional
to corrosion rate, was determined by the Tafel extrapolation
method.14
An inductively coupled plasma optical emission
spectrometer (SPECTRO CIRUS CCD) was used to determine
the concentrations of Si concentration released into the
corrosive solutions.
Results
Corrosion behaviour in acidic environments
The determined corrosion currents and the open circuit
potentials measured under the different conditions are given
in Table I. Fig. 1 shows typical polarization scans of SSiC in
HCl. Similar behaviour was obtained in HNO3. Reproducibility was good (Figure 1a). The curves show pseudopassivity features at about +160 mV in 1 M HCl. In both
acids different open circuit potentials were observed,
indicating the influence of the acidic anion on the corrosion
(Table I). In comparison to the corrosion of pure silicon, the
open circuit potentials are shifted to much more positive
values and the corrosion current density was increased
(Figure 1b).
Additionally, the corrosion was investigated by holding
the sample at a more positive than corrosion potential (+500
mV vs. SCE) in 1 M HNO3 for 10, 20, 30 and 60 minutes,
The Journal of The South African Institute of Mining and Metallurgy
Electrochemical corrosion testing of solid state sintered silicon carbide
Table I
Summary of corrosion current densities and corrosion potentials at different conditions
Materials
SSiC
Si
Condition (s)
Corrosion Potential (mV vs. SCE)
Corrosion Potential (mV vs. SHE)
Current Density, icorr (µA/cm2)
1 M HCl
-125
+116
46
1 M HNO3
+77
+318
58
1 M HNO3 (30 mins. hold)
+183
+424
2
1 M HNO3 (60 mins. hold)
+147
+38
0.04
1 M HNO3 + 1 M HF
+78
+319
40
1 M NaOH
+53
+294
33
1 M NaOH (30 mins. hold)
+73
+314
35
1 M HCl
-380
-139
1
1 M NaOH
-846
-605
0.9
P
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Corrosion current density (µΑ/cm2)
1.00E+03
(a)
1.00E+02
1.00E+01
icorr~35 µA/cm2
icorr~50 µA/cm2
1.00E+00
icorr~55 µA/cm2
1.00E–01
1.00E–02
-500
-300
-100
100
300
500
Corrosion current density (µΑ/cm2)
Potential (mV vs. SCE)
1.00E+03
(b)
1.00E+02
1.00E+01
1.00E+00
1.00E–01
SSiC
Si
1.00E–0-2
1.00E–03
-500
-300
-100
100
300
500
Potential (mV vs. SCE)
Figure 1—Polarization scans of SSiC and Si in different environments. (a) 1 M HCI showing good reproducibility (b) SSiS and pure Si in 1 M HCI
The Journal of The South African Institute of Mining and Metallurgy
(Figure 2b). This sample was then immersed in 1 M HF for
10 minutes (without applying potential). The polarization
scan of this sample in 1 M HNO3 is nearly identical with the
observed one for the as-prepared samples (Figure 2b). This
indicates that the formed oxide layer is dissolved by the HF.
Therefore it is expected that the layer formed is SiO2 x H2O
because HF will dissolve SiO2.9–11
SEM images of the surfaces prior to and after corrosion
are shown in Figure 3. The micrographs showed that even
after corrosion at +500 mV vs. SCE in 1 M HNO3 for 60
minutes, residual carbon seems not to have been attacked
and corrosion occurred mainly at the grain boundaries. XRD
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respectively, before performing polarization tests (Figure 2).
The icorr decreased with increasing prior holding time.
Decreases of 1 to 3 orders of magnitude after 30 to 60
minutes holding times, respectively, were recorded. Ecorr
values shifted to more positive values as the prior holding
times increased. These results indicate the formation of a
passivating surface layer.
To determine the type of passivating layer that is formed,
the working electrode was held at a more positive than
corrosion potential (+500 mV vs. SCE) in 1 M HNO3 for 30
minutes to allow the layer to form and this resulted in the
shift of the open circuit potential to positive potentials
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Electrochemical corrosion testing of solid state sintered silicon carbide
1.00E+03
(a)
Current density (µΑ/cm2)
1.00E+02
1.00E+01
1.00E+00
1.00E–01
No hold
10 mins.
20 mins.
30 mins.
1.00E–02
1.00E–03
1.00E-04
-200
0
200
400
600
800
1000
800
1000
Potential (mV vs. SCE)
Current density (µΑ/cm2)
1.0E+04
(b)
1.0E+03
1.0E+02
1.0E+01
1.0E+00
No hold
1.0E–01
30 mins/HF
immersion
1.0E–02
1.0E–03
-200
0
30 mins
200
400
600
Potential (mV vs. SCE)
Figure 2—Polarization scans of SSiC in 1 M HNO3. (a) Samples were held at +500 mV for 10, 20, 30 and 60 minutes before scanning. (b) Samples were held
at +500 mV for 30 minutes and compared to a scan in 1 M HNO3. Sample was also held at +500 mV for 30 minutes in 1 M HNO3 followed by 10 minutes
immersion in 1 M HF before performing a full scan
(a)
1 µm
H
SSiC
WD = 6 mm
Mag = 5.00 KX
EHT = 10.00kV
Detector = SE2
(b)
1 µm
SSiC-1 Mag = 5.00 KX EHT = 10.00kV WD = 6 mm Detector = SE2 23 Feb. 2006
Figure 3—SEM (secondary) micrograph of SSiC. The following features are marked: (A) residual carbon (B) porosity. (a) As-sintered before corrosion (b)
after full polarization scan in 1 M HNO3. Sample was held at +500 mV for 60 minutes
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Electrochemical corrosion testing of solid state sintered silicon carbide
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2500
100
After Corrosion
SiC
A
A
C
Before Corrosion
10
1500
1
20 21 22 23 24 25 26 27 28 29 30
SiC
500
SiC
1000
SiC
Intensity (counts)
2000
C
A
0
20
P
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25
30
35
40
45
50
Angle (°2Theta)
Figure 4—XRD pattern of SSIC before and after holding the sample in 1 M HNO3 at +500 mV for 20 minutes before performing full polirization scan. The
enlargement of peak at 26° is shown in the insert. There was no change in the intensity (height) of the residual carbon
14000
Itensity (arbitray units)
12000
(a)
10000
SiC
SiC
8000
a
6000
4000
2000
0
250
450
650
850
1050
1250
1450 1650
1850 2050
Wavenumber (cm-1)
14000
(b)
Itensity (arbitray units)
12000
C
SiC
10000
SiC
8000
6000
4000
2000
0
250
450
650
850
1050
1250 1450
1650
1850
2050
Wavenumber (cm-1)
Figure 5—Results of Raman measurements of SSiC (a) Before corrosion (b) After corrosion in 1 M
The Journal of The South African Institute of Mining and Metallurgy
minutes was done to reduce the dissolved oxygen content
and increase the CO2. Purging was continued throughout the
test. A comparison of the behaviour in 1 M HCl, de-aerated
with N2 gas and 1 M HCl, de-aerated with CO2 is shown in
Figure 6. The icorr and Ecorr remained approximately the
same. However, the current density drop in CO2 environment
at the pseudo-passive region was less (about 20 µA/cm2
compared to 75 µA/cm2 in the N2 environment and 46
µA/cm2 without purging with a gas).
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analysis performed on a corroded surface (20 minutes hold at
+500 mV before scan) showed no decrease in the intensity of
residual carbon peaks (Figure 4). Raman spectrometry on a
sample held at +500 mV for 60 minutes in HNO3 showed
some evidence of a thin layer of carbon on the surface
(Figure 5).
To test the oxidation of C to CO2, additional polarization
scans were carried out in HCl saturated with CO2. De-aerating
1 M HCl with CO2 gas (99.0 % purity) for approximately 45
Electrochemical corrosion testing of solid state sintered silicon carbide
Corrosion current density (µA/cm2)
1.0E+03
1.0E+02
1.0E+01
CO2
N2
1.0E+00
1.0E-01
1.0E-02
-400
-200
0
200
400
600
800
1000
Potential (mV vs. SCE)
Corrosion current density (µA/cm2)
Figure 6—Polarization scans of SSiC in 1 M HCI indicating approximately the same icorr and Ecorr when de-aerated with N2 and CO2 gas
1.00E+03
1.00E+02
1.00E+01
icorr~30 µA/cm2
icorr~40 µA/cm2
1.00E+00
icorr~30 µA/cm2
1.00E-01
1.00E-02
1.00E-03
-200
0
200
400
600
800
600
800
Potential (mV vs. SCE)
Figure 7—Polarization scans of SSiC in 1 M NaOH, showing good reproducibility
Current density (µA/cm2)
1.00E+03
1.00E+02
1.00E+01
1.00E+00
No hold
30 mins. hold
1.00E-01
1.00E-02
-200
0
200
400
Potential (mV vs. SCE)
Figure 8—Polarization scans of SSiC in 1 M NaOH. Sample was held at +500 mV for 30 minutes and compared to a normal scan in HNO3 (without holding)
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Current density (µA/cm2)
1.00E+03
1.00E+02
1.00E+01
1.00E+00
1 M NaOH.Na2CO3
1.00E-02
-200
P
a
p
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1 M NaOH
1.00E-01
0
200
400
600
800
Potential (mV vs. SCE)
Figure 9—Comparison of polarization scans of SSiC in 1 M NaOH and 1 M NaOH/Na2CO3 mixture, indicating a lower icorr and a slightly positive shift in Ecorr
in NaOH/Na2CO3 mixture
Corrosion behaviour in alkaline environments
Figure 7 shows polarization scans of SSiC in NaOH. In Table I
the measured open circuit potentials and the corrosion
current densities are given. The results were reproducible and
no pseudo-passivity feature (as was seen in the acidic
environments) could be observed. The average icorr was
approximately 33 µA/cm2. In comparing the polarization plot
of Si to SSiC, the icorr of SSiC increased and Ecorr shifted to
more positive values (Table I).
Additionally, the corrosion was tested by holding the
working electrode at a more positive than corrosion potential
(+500 mV vs. SCE) in 1 M NaOH for 30 minutes prior to a
polarization
scan. The result was compared to a scan without prior
holding. Figure 8 shows the results of this investigation. The
icorr values were about the same after 30 minutes prior to
holding. The shift of the open circuit potential is also much
lower than observed for the same experiments in acids. This
seems to suggest that no protective layer was formed in
alkaline environments, but rather silicic ions were forming.
Analyses of Si concentration in the corrosive solution after
30 minutes prior to holding before scanning in 1 M HNO3
and 1 M NaOH reveal a high Si-concentration in NaOH in
comparison to the similar experiment in HNO3 (Table II). To
exclude the oxidation of C to CO2, a polarization scan of SSiC
in mixture of 40.0g sodium carbonate (Na2CO3) and 0.4 litres
of 1 M NaOH was conducted. The results were compared to a
polarization scan in 1 M NaOH (Figure 9). The icorr were
lower in the NaOH/Na2CO3 mixture than in the NaOH. The
Ecorr values shifted to slightly higher positive values.
Discussion
reduces with time. By adding HF to the acid or by treatment
of the samples with HF after a polarization scan this drop of
the corrosion current can be avoided. Also in NaOH such a
drop of corrosion was not observed. These results confirm
the proposal by Cook et al. about the formation of a
passivating SiO2 surface layer. The formation of a protecting
SiO2 layer on the surface is also proved by the much lower
Si-concentration in the solution observed after corrosion in
HCl in comparison to the corrosion in NaOH (the current
densities in both cases were similar).
The comparison of the microstructures before and after
corrosion reveals that the corrosion attack is not homogeneous but mostly takes place on the interfaces. The
micrographs as well as the XRD results and the Raman
measurements also indicate that C inclusions in the sample
will not be attacked. Furthermore, the standard potentials
indicate that the oxidation of carbon can take place only at
much higher applied voltage in comparison to the SiC
corrosion (Reactions 1–9). However, according to Raman
measurements, a C layer is formed on the surface during the
corrosion. This indicates that the dissolution of the SiC can
be described by Reaction [9]
Table II
Dissolved Si ion concentration after holding at
+500 mV prior to polarization scan
Si concentration (ppm)
30 mins. hold prior to scanning in 1 M HNO3
0.366
30 mins. hold prior to scanning in 1 M NaOH
20.91
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The corrosion current density in HNO3 and HCl strongly
Electrochemical corrosion testing of solid state sintered silicon carbide
Acknowledgements
SiC(s) + 2 H2O(l ) →
SiO2 (s) + C + 4 H + ( aq ) + 4e −
[9]
E o = –1.07 V
and
References
SiC(s) + 6OH - ( aq ) →
SiO32– ( aq ) + C + 3 H2O(l ) + 4e −
This work was partially supported by DAAD under an ANSTI
fellowship award.
[7]
E oB = –1.874 V
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This is in agreement with the very low influence of CO2 or
CO32- in the solution on the current density and open circuit
potential.
The corrosion current densities of the SSiC materials
observed here in acids are higher than that observed by Cook
et al. In 1 M HCl in our experiments, values of 46 µA/cm2
were observed, whereas Cook et al. observed values less than
0.07 A/m2= 7 µA/cm2. Our experiments with different
holding times indicate that the current density can slow down
by several orders of magnitude, so the differences are
properly connected with some differences in the oxide layer
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corrosion in acids, the initial corrosion current densities
measured for both conditions were similar. The different
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layers in HCl and HNO3 resulting in a strong reduction of the
corrosion current density.
The corrosion of SSiC investigated here and SiSiC
investigated1,5 previously could be described by electrochemical methods and therefore the same methods for
corrosion protection as used for metals can be applied to
prevent the corrosion. Nevertheless, one should keep in mind
that the corrosion current density is several orders of
magnitude lower than that for metals. First investigations of
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Conclusion
Carbide and Liquid Phase Sintered Silicon Carbide Ceramics, MSc disser-
The corrosion behaviour of SSiC ceramic materials has been
investigated using electrochemical techniques and microstructural examinations. The electrochemical corrosion
reaction of SiC was established to be the formation of C and
SiO2 protective layers in HCl and HNO3 and carbon and
soluble silicate ions in the NaOH environment. The formed
SiO2 layer results in a strong reduction of the corrosion
current densities and in a shift of the open circuit potential to
more positive values. The residual carbon does not dissolve
under the test conditions in both acidic and alkaline
environments.
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Measurements, ASTM G5-94, Annual Books of ASTM Standards, 2000,
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13. Conventions Applicable to Electrochemical Measurements in Corrosion
Testing, ASTM G3-89, Annual Books of ASTM Standards, 2000,
vol. 03, no. 02, pp. 38–42.
14. JONES, D.A. Principles and Prevention of Corrosion, 2nd edn., 1996,
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