electrochemical corrosion testing of solid state sintered ... · immersion method, where their...

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Introduction

Ceramic materials are often used in veryaggressive and corrosive environmentsbecause of their high degree of chemicalinertness and corrosion resistance. However,even though it is slow, corrosion does occur.The standard procedure for measuringcorrosion resistance of ceramic materials is theimmersion method, where their behaviour isinvestigated by immersing samples in acorrosive media and measuring weight loss,corrosion layer depth, residual strength andsometimes the concentration of the corrodedions in the media.1–5 Recent investigations hadshown that the corrosion of SiC can bestrongly influenced by electrical effects.1,5,6

Electrochemical corrosion of sinteredsilicon carbide ceramics has been studied atroom temperature in various dilute acids (HCl,HNO3, HF, H2SO4 and H3PO4).1,5 The principalcorrosion reaction of SiC was assumed to bethe dissolution of Si atoms. For theinvestigated SiSiC materials it is valid, but inSSiC the corrosion is connected with the SiC.Therefore in leaching and the description ofthe electrocorrosion, one has to consider thereaction of both Si and C. In these investi-

gations1,5 there is no indication of Si reactionsin alkaline environments.

The investigations by Cook et al.1 deducedthermodynamical considerations based on Sireactions and from the different behaviour inHF and HCl observed that SSiC is passive inacidic environments because of the formationof a surface silica film. The reduction of O2dissolved in water was proposed as cathodicreaction. A number of anodic reactions forcarbon and silicon in acidic and alkalineenvironments, with their correspondingstandard potentials (E°), are found inliterature.7–10 Based on these reactions, thefollowing corrosion reactions for SSiCinvolving carbon were proposed. The standard(SHE) potentials of the corrosion reactionswere calculated based on thermodynamicconsiderations.11

In acidic environments:

[1]

[2]

[3]

SiC s H O l

SiO s H aq CO g + 8e

E – V

( ) + ( )→

( ) + ( ) + ( )=

+ −

4

0.433

2

2 8 2

o

SiC s H O l

SiO s C H aq e

E – V

( ) + ( )→

( ) + + ( ) +

=

+ −

2

4 4

1.07

2

2

o

C residual H O l

CO g H aq e

E V

( ) + ( )→

( ) + ( ) +

= +

+ −

2

4 4

0.206

2

2

o

Electrochemical corrosion testing ofsolid state sintered silicon carbide inacidic and alkaline environmentsby A. Andrews*, M. Herrmann†, and M. Sephton*

Paper written on project work carried out in partial fulfilment of MSc (Eng) degree

Synopsis

Solid state sintered silicon carbide ceramics are used in aggressiveenvironments as sealings. There exist data concerning corrosion,but there are only few data concerning their electrocorrosionbehaviour. Therefore the electrochemical corrosion of solid statesintered silicon carbide (SSiC) ceramics has been studied at roomtemperature in acidic and alkaline environments by using potentio-dynamic polarization measurements. The investigation showed apronounced electrochemical corrosion in bases and acids. In HCl andHNO3 pseudo-passivity features due to the formation of a thin layerof SiO2 on the surface were observed, whereas in NaOH solublesilicate ions were observed, resulting in more pronounced corrosion.The microstructural observations of initial and corroded samplesreveal that the residual carbon found in the microstructure do notdissolve preferentially.

Keywords: Corrosion; SEM; SSiC; polarization test.

* School of Proc. & Mat. Eng., University of theWitwatersrand, Wits, Johannesburg, South Africa.

† Fraunhofer-Institut für keramische Technology undSinterwerkstoffe, Dresden, Germany.

© The South African Institute of Mining andMetallurgy, 2006. SA ISSN 0038–223X/3.00 +0.00. Paper received Nov. 2005; revised paperreceived Mar. 2006.

269The Journal of The South African Institute of Mining and Metallurgy VOLUME 106 NON-REFEREED PAPER APRIL 2006 s

Electrochemical corrosion testing of solid state sintered silicon carbide

[4]

In alkaline environments:

[5]

[6]

[7]

[8]

In this paper, the corrosion mechanism of SSiC in acidicand alkaline environments is investigated by electrochemicalmethods. The changes of the microstructures duringcorrosion experiments are investigated by SEM andcorrelated with the mechanisms.

Experimental procedure

Material

Solid state sintered silicon carbide (SSiC) ceramic materialswere gas pressure sintered in Ar at a temperature of 2100°Cwith carbon (C) and boron (B) as sintering additives. Theweight per cent of the residual carbon, determined byRietveld analysis was 2 ± 0.4 %. The measured density was3.11 g/cm3. Samples were prepared for image analysis bygrinding on Struers MD-Piano 1200 to achieve a flat surfaceand polishing on a Pan cloth to 1 µm finish using diamondspray and diamond extender (lubricant). The samples werecleaned afterwards with ethanol and dried. The micro-structures and compositions of the material before and aftercorrosion were analysed by means of scanning electronmicroscopy (LEO 1525), X-ray diffraction (Phillips PW1710and XRD7 Seifert/GE-Inspection) and Raman spectrometry(JOBIN-YVON T64000).

Corrosion measurements

The electrochemical behaviour of these materials wasinvestigated by using potentiodynamic polarizationmeasurements according to ASTM G 5-9412 and ASTM G 3-8913. Samples were prepared by attaching an insulatedcopper wire to one face of the sample with aluminium

conductive tape and cold mounting in resin. Samples werethen polished on a Pan cloth to 1 µm surface finish andcoated at the sample-resin interface with Bostik quickset toprevent the possibility of crevice corrosion. This was followedby cleaning in an ultrasonic bath with ethanol beforeimmersion in the electrolyte. All tests were conducted atroom temperature (~25°C). Solutions were replaced after eachpolarization scan. A Schlumberger (SI 1286) electrochemicalinterface was connected to a personal computer with electro-chemical application software for control and data-logging. Aplatinum counter electrode (CE) was used. A saturatedcalomel reference electrode (SCE) was connected to the cellvia a Luggin capillary. The probe tip was suspended approxi-mately 1 cm above the sample surface. All potentials reportedare 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; thedetails 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, asexplained later) for approximately 45 minutes to reduce thedissolved-oxygen content. Purging was continued through-out the tests. The open-circuit potential (Ecorr) was taken 5minutes after immersion of samples. A potential scan wasthen initiated with a scan rate of 1 mV/s. The width of thescan was from -200 mV to about +1000 mV relative to Ecorr.At least three full scans were conducted per electrolyte. Thecorrosion current density (icorr), which is directly proportionalto corrosion rate, was determined by the Tafel extrapolationmethod.14

An inductively coupled plasma optical emissionspectrometer (SPECTRO CIRUS CCD) was used to determinethe concentrations of Si concentration released into thecorrosive solutions.

Results

Corrosion behaviour in acidic environments

The determined corrosion currents and the open circuitpotentials measured under the different conditions are givenin Table I. Fig. 1 shows typical polarization scans of SSiC inHCl. Similar behaviour was obtained in HNO3. Repro-ducibility was good (Figure 1a). The curves show pseudo-passivity features at about +160 mV in 1 M HCl. In bothacids 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, theopen circuit potentials are shifted to much more positivevalues and the corrosion current density was increased(Figure 1b).

Additionally, the corrosion was investigated by holdingthe sample at a more positive than corrosion potential (+500mV vs. SCE) in 1 M HNO3 for 10, 20, 30 and 60 minutes,

SiC s OH aq

SiO aq H O l CO e

E – V

-

B

( ) + ( )→

( ) + ( ) + ( )=

−

8

432–

2 g + 6

0.8o

SiC s OH aq

SiO aq H O l CO e

E – V

-

B

( ) + ( )→

( ) + ( ) + ( )=

−

1

aq + 8

0.076

32–

2

2

5 32–

o

SiC s OH aq

SiO aq C H O l e

E – V

-

B

( ) + ( )→

( ) + + ( ) +

=

−

6

3 4

1.874

32–

2

o

C residual OH aq

CO aq H O l e

E + V

-

B

( ) + ( )→

( ) + ( ) +

=

−

6

3 4

1.72

32–

2

o

SiC s H O l

SiO s H aq CO g +6e

E – V

( ) + ( )→

( ) + ( ) + ( )=

+ −

3

0.542

2

2 6o

s

270 APRIL 2006 VOLUME 106 NON-REFEREED PAPER The Journal of The South African Institute of Mining and Metallurgy

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 60minutes holding times, respectively, were recorded. Ecorr

values shifted to more positive values as the prior holdingtimes increased. These results indicate the formation of apassivating surface layer.

To determine the type of passivating layer that is formed,the working electrode was held at a more positive thancorrosion potential (+500 mV vs. SCE) in 1 M HNO3 for 30minutes to allow the layer to form and this resulted in theshift of the open circuit potential to positive potentials

(Figure 2b). This sample was then immersed in 1 M HF for10 minutes (without applying potential). The polarizationscan of this sample in 1 M HNO3 is nearly identical with theobserved one for the as-prepared samples (Figure 2b). Thisindicates that the formed oxide layer is dissolved by the HF.Therefore it is expected that the layer formed is SiO2 x H2Obecause HF will dissolve SiO2.9–11

SEM images of the surfaces prior to and after corrosionare shown in Figure 3. The micrographs showed that evenafter corrosion at +500 mV vs. SCE in 1 M HNO3 for 60minutes, residual carbon seems not to have been attackedand corrosion occurred mainly at the grain boundaries. XRD

Electrochemical corrosion testing of solid state sintered silicon carbideJournal

Paper

271The Journal of The South African Institute of Mining and Metallurgy VOLUME 106 NON-REFEREED PAPER APRIL 2006 s

Table I

Summary of corrosion current densities and corrosion potentials at different conditions

Materials Condition (s) Corrosion Potential (mV vs. SCE) Corrosion Potential (mV vs. SHE) Current Density, icorr (µA/cm2)

SSiC 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

Si 1 M HCl -380 -139 1

1 M NaOH -846 -605 0.9

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

-500 -300 -100 100 300 500

Potential (mV vs. SCE)

icorr~35 µA/cm2

icorr~50 µA/cm2

icorr~55 µA/cm2

Cor

rosi

on c

urre

nt d

ensi

ty (

µΑ

/cm

2 ) (a)

(b)

1.00E+03

1.00E+02

1.00E+01

1.00E+00

1.00E–01

1.00E–02

-500 -300 -100 100 300 500


SSiC

Si

Cor

rosi

on c

urre

nt d

ensi

ty (

µΑ

/cm

2 ) 1.00E+03

1.00E+02

1.00E+01

1.00E+00

1.00E–01

1.00E–0-2

1.00E–03


s


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

(b)

1 µm SSiC WD = 6 mm Mag = 5.00 KX EHT = 10.00kV Detector = SE2H

1 µm SSiC-1 Mag = 5.00 KX EHT = 10.00kV WD = 6 mm Detector = SE2 23 Feb. 2006

(a)

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 heldat +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 minutesimmersion in 1 M HF before performing a full scan

-200 0 200 400 600 800 1000Potential (mV vs. SCE)

No hold10 mins.20 mins.30 mins.

Cur

rent

den

sity

(µΑ

/cm

2 )

(a)

(b)

1.00E+03

1.00E+02

1.00E+01

1.00E+00

1.00E–01

1.00E–02

1.00E–03

1.00E-04

-200 0 200 400 600 800 1000Potential (mV vs. SCE)

30 mins

No hold

30 mins/HFimmersion

Cur

rent

den

sity

(µΑ

/cm

2 )

1.0E+04

1.0E+03

1.0E+02

1.0E+01

1.0E+00

1.0E–01

1.0E–02

1.0E–03

analysis performed on a corroded surface (20 minutes hold at+500 mV before scan) showed no decrease in the intensity ofresidual carbon peaks (Figure 4). Raman spectrometry on asample held at +500 mV for 60 minutes in HNO3 showedsome evidence of a thin layer of carbon on the surface(Figure 5).

To test the oxidation of C to CO2, additional polarizationscans were carried out in HCl saturated with CO2. De-aerating1 M HCl with CO2 gas (99.0 % purity) for approximately 45

minutes was done to reduce the dissolved oxygen contentand increase the CO2. Purging was continued throughout thetest. A comparison of the behaviour in 1 M HCl, de-aeratedwith N2 gas and 1 M HCl, de-aerated with CO2 is shown inFigure 6. The icorr and Ecorr remained approximately thesame. However, the current density drop in CO2 environmentat the pseudo-passive region was less (about 20 µA/cm2

compared to 75 µA/cm2 in the N2 environment and 46µA/cm2 without purging with a gas).


Paper

The Journal of The South African Institute of Mining and Metallurgy VOLUME 106 NON-REFEREED PAPER APRIL 2006 273 s

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. Theenlargement of peak at 26° is shown in the insert. There was no change in the intensity (height) of the residual carbon

20 25 30 35 40 45 50

Angle (°2Theta)

20 21 22 23 24 25 26 27 28 29 30

100

10

1

After Corrosion

Before Corrosion

Inte

nsity

(co

unts

)

CAA

A C

SiC

SiC

SiC

SiC

2500

2000

1500

1000

500

0

Figure 5—Results of Raman measurements of SSiC (a) Before corrosion (b) After corrosion in 1 M

250 450 650 850 1050 1250 1450 1650 1850 2050Wavenumber (cm-1)

SiCSiC

a

Iten

sity

(ar

bitr

ay u

nits

)

(a)

(b)

14000

12000

10000

8000

6000

4000

2000

0

250 450 650 850 1050 1250 1450 1650 1850 2050Wavenumber (cm-1)

SiCC

SiC

Iten

sity

(ar

bitr

ay u

nits

)

14000

12000

10000

8000

6000

4000

2000

0


s


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

Figure 7—Polarization scans of SSiC in 1 M NaOH, showing good reproducibility

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)

-400 -200 0 200 400 600 800 1000


N2

CO2

Cor

rosi

oncu

rren

t den

sity

(µ

A/c

m2 )

1.0E+03

1.0E+02

1.0E+01

1.0E+00

1.0E-01

1.0E-02

-200 0 200 400 600 800


icorr~30 µA/cm2

icorr~40 µA/cm2

icorr~30 µA/cm2

Cor

rosi

oncu

rren

t den

sity

(µ

A/c

m2 ) 1.00E+03

1.00E+02

1.00E+01

1.00E+00

1.00E-01

1.00E-02

1.00E-03

-200 0 200 400 600 800


No hold

30 mins. hold

Cur

rent

den

sity

(µ

A/c

m2 )

1.00E+03

1.00E+02

1.00E+01

1.00E+00

1.00E-01

1.00E-02

Corrosion behaviour in alkaline environments

Figure 7 shows polarization scans of SSiC in NaOH. In Table Ithe measured open circuit potentials and the corrosioncurrent densities are given. The results were reproducible andno pseudo-passivity feature (as was seen in the acidicenvironments) could be observed. The average icorr wasapproximately 33 µA/cm2. In comparing the polarization plotof Si to SSiC, the icorr of SSiC increased and Ecorr shifted tomore positive values (Table I).

Additionally, the corrosion was tested by holding theworking electrode at a more positive than corrosion potential(+500 mV vs. SCE) in 1 M NaOH for 30 minutes prior to apolarization scan. The result was compared to a scan without priorholding. Figure 8 shows the results of this investigation. Theicorr values were about the same after 30 minutes prior toholding. The shift of the open circuit potential is also muchlower than observed for the same experiments in acids. Thisseems to suggest that no protective layer was formed inalkaline environments, but rather silicic ions were forming.Analyses of Si concentration in the corrosive solution after30 minutes prior to holding before scanning in 1 M HNO3

and 1 M NaOH reveal a high Si-concentration in NaOH incomparison to the similar experiment in HNO3 (Table II). Toexclude the oxidation of C to CO2, a polarization scan of SSiCin mixture of 40.0g sodium carbonate (Na2CO3) and 0.4 litresof 1 M NaOH was conducted. The results were compared to apolarization scan in 1 M NaOH (Figure 9). The icorr werelower in the NaOH/Na2CO3 mixture than in the NaOH. TheEcorr values shifted to slightly higher positive values.

Discussion

The corrosion current density in HNO3 and HCl strongly

reduces with time. By adding HF to the acid or by treatmentof the samples with HF after a polarization scan this drop ofthe corrosion current can be avoided. Also in NaOH such adrop of corrosion was not observed. These results confirmthe proposal by Cook et al. about the formation of apassivating SiO2 surface layer. The formation of a protectingSiO2 layer on the surface is also proved by the much lowerSi-concentration in the solution observed after corrosion inHCl in comparison to the corrosion in NaOH (the currentdensities in both cases were similar).

The comparison of the microstructures before and aftercorrosion reveals that the corrosion attack is not homo-geneous but mostly takes place on the interfaces. Themicrographs as well as the XRD results and the Ramanmeasurements also indicate that C inclusions in the samplewill not be attacked. Furthermore, the standard potentialsindicate that the oxidation of carbon can take place only atmuch higher applied voltage in comparison to the SiCcorrosion (Reactions 1–9). However, according to Ramanmeasurements, a C layer is formed on the surface during thecorrosion. This indicates that the dissolution of the SiC canbe described by Reaction [9]


Paper

The Journal of The South African Institute of Mining and Metallurgy VOLUME 106 NON-REFEREED PAPER APRIL 2006 275 s

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 Ecorrin NaOH/Na2CO3 mixture

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

-200 0 200 400 600 800


1 M NaOH.Na2CO3

1 M NaOH

Cur

rent

den

sity

(µ

A/c

m2 )

1.00E+03

1.00E+02

1.00E+01

1.00E+00

1.00E-01

1.00E-02


[9]

and

[7]

This is in agreement with the very low influence of CO2 orCO3

2- in the solution on the current density and open circuitpotential.

The corrosion current densities of the SSiC materialsobserved here in acids are higher than that observed by Cooket al. In 1 M HCl in our experiments, values of 46 µA/cm2

were observed, whereas Cook et al. observed values less than0.07 A/m2= 7 µA/cm2. Our experiments with differentholding times indicate that the current density can slow downby several orders of magnitude, so the differences areproperly connected with some differences in the oxide layerthickness due to different preparation methods. Despite thelower corrosion stability of SiC in NaOH in comparison to thecorrosion in acids, the initial corrosion current densitiesmeasured for both conditions were similar. The differentlong-term stabilities arise from the fast formation of SiO2

layers in HCl and HNO3 resulting in a strong reduction of thecorrosion current density.

The corrosion of SSiC investigated here and SiSiCinvestigated1,5 previously could be described by electro-chemical methods and therefore the same methods forcorrosion protection as used for metals can be applied toprevent the corrosion. Nevertheless, one should keep in mindthat the corrosion current density is several orders ofmagnitude lower than that for metals. First investigations ofliquid phase sintered materials, which contain an oxide grainboundary, indicate that the corrosion of these materials couldnot be described by electrochemical methods because thedegradation of these materials is controlled by the dissolutionof the grain boundary, which is a simple hydrolysis or oxideacid reaction without change of the state of oxidation.11

Conclusion

The corrosion behaviour of SSiC ceramic materials has beeninvestigated using electrochemical techniques and micro-structural examinations. The electrochemical corrosionreaction of SiC was established to be the formation of C andSiO2 protective layers in HCl and HNO3 and carbon andsoluble silicate ions in the NaOH environment. The formedSiO2 layer results in a strong reduction of the corrosioncurrent densities and in a shift of the open circuit potential tomore positive values. The residual carbon does not dissolveunder the test conditions in both acidic and alkalineenvironments.

Acknowledgements

This work was partially supported by DAAD under an ANSTIfellowship award.

References

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pp. E27–E34.

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for Corrosion Rate Determination in Ceramics, J. Am. Ceram. Soc., 1989,

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Electrochemistry, A.J. Bard (ed.), New York, Marcel Dekker, 1985,

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9. POURBAIX, M., MUYLDER, J.V., BESSON, J., and KUNZ, W. Atlas of

Electrochemical Equilibria in Aqueous Solution, Houston. TX. NACE.

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10. SCIENTIFIC GROUP THERMODATA EUROPE, Pure Substance Database, 2001

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Measurements, ASTM G5-94, Annual Books of ASTM Standards, 2000,

vol. 03, no. 02, pp. 57–67.

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vol. 03, no. 02, pp. 38–42.

14. JONES, D.A. Principles and Prevention of Corrosion, 2nd edn., 1996,

Prentice-Hall, Inc. u

SiC s OH aq

SiO aq C H O l e

E – V

-

B

( ) + ( )→

( ) + + ( ) +

=

−

6

3 4

1.874

32–

2

o

SiC s H O l

SiO s C H aq e

E – V

+

( ) + ( )→

( ) + + ( ) +

=

−

2

4 4

1.07

2

2

o

s


electrochemical corrosion testing of solid state sintered ... · immersion method, where their...

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