1

Zack Qin, J.M. Smith, and D.W. Shoesmith

The University of Western OntarioLondon, Ontario, Canada

Electrochemical Impedance Studies of The Growth of Sulfide Films on Copper

17th

International Corrosion Congress, Las Vegas, Nevada, Oct. 6-10, 2008.

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Deep geologic repository

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Cu Microbially Depleted Zone

Unaffected Vault Environment

SRBBlocked

SO42-

SH-

SRB

Cu

Cu2 S

e-

H2 O

H2

Remotely produced sulfide by sulfate-reducing bacteria (SRB) could be transported slowly to the Cu surface.

The formation of copper sulfide films would lower the potential for Cu dissolution to a value sufficiently negative to make corrosion of Cu thermodynamically viable via the reduction of water.

Sulfide attack on Cu surface

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EISEIS

EISEIS

10 mVE

time

Ecorr

Solutions: 0.1 mol/L NaCl with various total sulfide concentration ([S]T = 10-4 ~ 10-3 mol/L), Ar-degassed.All experiments were performed within an anaerobic chamber to maintain the anoxic conditions anticipated in a repository.Sulfide films grown anodically by stepping the potential in +10 mV increments from the corrosion potential (Ecorr).EIS were recorded at each potential after polarization for 2 hours to ensure steady-state.

Experimental procedures

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For the lowest [S]T, the current density increases slowly with potential, whereas, at the higher [S]T, an active to passive transition is observed.It is possible, but not demonstrated, that this transition is due to the formation of a partially passivating CuS layer on the inner layer.The current density increases and the active to passive transition becomes more defined as [S]T is increased.

-0.92 -0.90 -0.88 -0.86 -0.84 -0.82 -0.80 -0.78 -0.76

0

2

4

6

8

10

12

14

16

18

20

22 [S]T=10-4M

5x10-4M 10-3M

Stea

dy-S

tate

Cur

rent

Den

sity

(μA

/cm

2 )

Applied Potential (V/SCE)

Growth of sulfide films

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The peak at ca. –1.1V can be attributed to the reduction of the Cu2S base layer, while the peak at a more negative potential can be attributed to the reduction of the second deposited layer.

Two distinct morphologies were observed after 2 hours treatment; a nodular film similar to that formed at short times and faceted crystals indicative of a deposit via solution deposition.

Dual layer film5 min

2 hrs

2 hrs

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CellVDC (IDC )

VACin VAC

in

IACout

EIS is an in-situ, nondestructive technique which can be used to probe the electrical properties of, reaction kinetics, charge transfer and mass transfer processes occurring at an interface or film.

The technique involves the application of a small sinusoidal potential perturbation and measurement of the resultant current (which occurs at the same frequency but with a different amplitude and phase).

Impedance, Z, is a transfer function between the excitation and response defined in the frequency domain allowing the separation of coincident processes on a frequency scale.

V(jω) = Z(jω) x I(jω) (j2 = -1)

Electrochemical Impedance Spectroscopy (EIS)

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EIS spectra -

[S]T

= 10-4

mol/L

Impedance data are presented in Bode (left) and Nyquist (right) plots.EIS response contains at least two time constants. Diffusion contribution is indicated by ‘45° line’ in the Nyquist plot.

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EIS spectra -

[S]T

= 5x10-4

mol/L

Spectra evolve into two well defined time constants after active to passive transition.

Increased total impedance (i.e. |Z| at 10-3 Hz) in the passive region.

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EIS spectra -

[S]T

= 10-3

mol/L

EIS response undergoes various behaviours before and after active to passive transition.

How can these spectra be interpreted ?

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Equivalent circuitsEIS spectrum may be represented by electrical analogues, called equivalent circuits (EC). Quantitative data is extracted by modeling the system with the EC that fits the spectrum.EC should be established on the basis of a physical model of the system.There is evidence that the sulfide film forms as two distinct layers; an inner layer (IL) which is thin and coherent, and an outer layer (OL) formed of deposited crystals.

Rct – charge transfer resistanceROL – outer layer resistanceRs – solution resistanceZdl – double layer CPEZOL – outer layer CPEWIL – inner layer Warburg element

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0.5

2 2

Infinite diffusion

Warburg coefficient

2

(1 )W

s

RT

n F c D

Z j

σ

σ ω −

=

= −

Diffusion impedance (Warburg)

( )

2

Finite transmissive boundary

2 tanh

Diffusion time constant

WS D

D

Z jj

D

σ ωτω

δτ

=

≡

The origins of the diffusion impedance lie in the mathematical isomorphism between mass transfer and charge transfer processes.A pseudo diffusion impedance is observed whenever a reaction is under partial or complete mass transport control.

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10[ ( ) ]CPEZ Y j αω −= ⋅

α 1 0 -1 0.5Y0 C 1/R 1/L 1/√2σ

CPE is often related to frequency dispersion attributed to surface inhomogeneities and distributed time constants. The dimension of Y0 is secα/Ω, and the physical justification is not obvious except for the following circumstances:

CPE is often used to describe non-ideal capacitive behaviour, and can be converted to the capacitance C by the equation,

10 (0.8 1)chaC Y α αω − < <=

=(1-a)p/2

Constant phase element (CPE)

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Fitting by equivalent circuit

Prior to analysis, EIS data were checked for compliance with the constraints by linear Kramers-Kronig transforms.Reasonable fits of the EIS to the EC obtained.In most cases, the value of the exponent of ZOL, αOL, was fixed at either 1 (capacitor) or 0.5 (infinite Warburg element).

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Parameters derived

-0.92 -0.90 -0.88 -0.86 -0.84 -0.82 -0.80 -0.78 -0.76101

102

103

104

[S]T=10-4M

5x10-4M 10-3M

Rct (Ω

cm2 )

Eapp (V/SCE)

The charge transfer resistance Rct and double layer capacitance Cdl are similar for all concentrations.This similarity suggests that conditions at the Cu/IL interface are not particularly dependent on [S]T, suggesting that the electrochemical behaviour is primarily controlled by the properties of the sulfide films.Values of Cdl are in the range one might expect for a rough surface, and the decrease in Cdl might indicate a decrease in roughness.

-0.92 -0.90 -0.88 -0.86 -0.84 -0.82 -0.80 -0.78 -0.7610-6

10-5

10-4

10-3

[S]T=10-4M

5x10-4M 10-3M

Cdl (F

/cm

2 )

Eapp (V/SCE)

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Inner layer parameters

-0.92 -0.90 -0.88 -0.86 -0.84 -0.82 -0.80 -0.78 -0.76101

102

103

104

[S]T=10-4M

5x10-4M 10-3M

War

burg

coe

ffici

ent σ

IL (Ω

cm2 /s

0.5 )

Eapp (V/SCE)

-0.92 -0.90 -0.88 -0.86 -0.84 -0.82 -0.80 -0.78 -0.7610-1

100

101

102

[S=]T=10-4M

5x10-4M 10-3M

Diff

usio

n tim

e co

nsta

nt τ

IL (s

ec)

Eapp (V/SCE)

The IL is electronically conductive, but would act as a diffusion barrier to the ionic species involved in film growth.A comparison of σIL suggests the transport properties of the IL are similar at all concentrations.For the two higher [S]T, the large increase in τIL in the transition region would reflect a significant decrease in the diffusion coefficient as a result of a partial conversion of Cu2S to CuS.

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Outer layer parameters

-0.92 -0.90 -0.88 -0.86 -0.84 -0.82 -0.80 -0.7810-4

10-3

10-2

10-1

100

[S]T=10-3M

5x10-4M

CO

L (F/

cm2 )

Eapp (V/SCE)-0.92 -0.90 -0.88 -0.86 -0.84 -0.82 -0.80 -0.78 -0.76

102

103

104

105

106

[S]T=10-4M

5x10-4M 10-3M

RO

L (Ω

cm2 )

Eapp (V/SCE)

The behaviour of the OL is much more defined for the two higher concentrations, and is modeled by an RC parallel circuit. The ROL increases with potential and the active to passive transition leads to very large changes in COL.The extremely large COL values in the active region may suggest that a significant diffusional response is disguised by the CPE in this range.The COL values after the transition are consistent with that when a surface adsorption is captured by EIS as a pseudo-capacitance.

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Outer layer transport

-0.90 -0.88 -0.86 -0.84 -0.82 -0.80 -0.78 -0.76 -0.74102

103

104

War

burg

coe

ffici

ent σ

OL (Ω

cm2 /s

0.5 )

Eapp (V/SCE)

σOL

0.5

0.6

0.7

0.8

0.9

CPE

exp

onen

t αO

L

αOL

([S]T=10-4M)

The diffusive transport of SH- in the outer layer is most obvious in the solution containing the lowest [S]T, suggesting, at this [S]T, the film formation is under at least partial diffusion control. The value of the diffusion coefficient calculated from σOL is ~4x10-6 cm2/s, consistent with diffusive transport of SH- in solution.In the passive region, αOL increases to much higher values, suggesting that an RC parallel combination would become more appropriate.

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Conclutions

Anodically grown copper sulfide films on Cu appear to occur in two stages, involving a compact, but flawed inner layer, and a thicker, deposited outer layer.As the concentration of sulfide is increased, an active to passive transition develops.Properties of sulfide films are important since they affect both charge transfer and mass transfer processes. EIS spectra were fitted to the equivalent circuits containing two time constants and one or two diffusion elements, depending on the sulfide concentration and the film growth potential.From the fits, the values of Rct, Cdl, σIL, τIL, ROL, COL, and σOLwere determined.Considerable difficulty was experienced in separating capacitative and diffusional impedances.

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Electrochemical and Corrosion Studies at Western

Thank You

http://publish.uwo.ca/~ecsweb/

AcknowledgementsFunded by Svensk

Kärnbränslehantering

AB, Swedish Nuclear Fuel & Waste Management Co. (SKB), Stockholm, Sweden.