the influence of chloride on electrochemistry and...
TRANSCRIPT
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The Influence of Chloride on Electrochemistry and Corrosion of
Copper in Aqueous Sulphide Solutions
J. M. Smitha,b, Z. Qina, F. Kingc and D. W. Shoesmitha
a Department of Chemistry, The University of Western Ontario, London, ON, N6A 5B7b Kinectrics, 800 Kipling Avenue, Toronto, ON, M8Z 6C4c Integrity Corrosion Consulting Ltd, Nanaimo, BC, V9T 1K2
This research was funded by the Swedish Nuclear Fuel and Waste Management Company (SKB), Stockholm, Sweden
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Copper nuclear waste containers
Copper is the canister material of choice, since it is thermodynamically stable under the anoxic aqueous conditions anticipated
in a 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
•
Sulphide
remotely produced by sulfate-reducing bacteria (SRB) could be transported slowly to the Cu surface.
•
The formation of copper sulphide films would lower the potential
for Cu oxidation to a value sufficiently negative to make corrosion of Cu thermodynamically viable via the reduction of water.
Cu is not stable in sulphide solutions
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Influences of chloride
•
In aerated chloride dominated groundwater, Cu dissolution as Cu+
is stabilized as various copper chloride species (CuCl2
-, CuCl32-) and supported by the cathodic
reduction of oxygen to water. Thermodynamically, this would not be expected under anoxic conditions.
•
However, local pore chemistry within a Cux
S
film could possess a combination of low [SH-] and high [Cl-], especially with a high chloride groundwater content when the Cu/SH-
reaction has become transport controlled.
•
This could lead to a significant influence of Cl-
on reactions in sulphide
solutions.
The effect of increasing [Cl-] The effect of decreasing [SH-]
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Experimental procedures
EIS EIS
E
Time
SEM
EDX2 mV/s
EvertexE
Time
•
To avoid O2
contamination, experiments were performed in Ar-purged solutions in an anaerobic chamber.
•
Electrochemical impedance spectra (EIS) were periodically recorded under natural corrosion conditions. At the end of experiments, surfaces of corroded samples were examined by scanning electron microscopy (SEM) and energy dispersive X-ray spectroscopy (EDX).
•
A series of cyclic voltammograms
(CV) was performed to various anodic limits.
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Film formed: 0.1 mol/L NaCl
1hr 5hrs 30hrs
1.50 μm 1.50 μm 1.50 μm
1hr 5hrs 30hrs
Cou
nts
Energy (keV)
•
SEM shows dual layer film formation: a slightly porous coherent base layer and a compact, crystalline outer deposit.
•
EDX analysis of the electrode after 30 hrs of exposure yields Cu:S
≅
2:1.
•
XRD confirms the base layer is digenite
(Cu1.8
S) and the deposit is chalcocite
(Cu2
S).
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Film formed: 1.0 mol/L NaCl
1hr 5hrs 30hrs
1.50 μm 1.50 μm 1.50 μm
30hrs1hr 5hrs •
With time, an initially formed base layer becomes porous, and a scattered, only partially protective, outer layer forms.
•
EDX sulphur signal is barely distinguishable after 1 hour of exposure indicating the initially formed base layer is very thin.
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Film formed: 5.0 mol/L NaCl
1hr 30hrs5hrs
(a) (b) (c)
3.00 μm 3.00 μm 3.00 μm
1hr 5hrs 30hrs
3.00µm 3.00µm 3.00µm
•
Film morphology very different to that formed at lower chloride concentrations.
•
As the [Cl-]
increases, the base layer becomes less coherent, and the outer deposit becomes less well-formed and more scattered.
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EIS spectra: 0.1 mol/L NaCl
•
EIS response contains two time constants. The shorter time constant (at ~10 Hz) is attributed to charge transfer at the Cu metal surface and
the longer time constant (at ~10-2
Hz) to the properties of the Cu2
S surface film.
•
Total impedance at the low frequency limit increases with time.
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Rct
Rpore
Cdl
CporeRs
Cu base layer solution
•
Based on the SEM and other evidences, a two time constant equivalent circuit
was proposed to represent the sulphide
film structure.
•
The spectra can be accurately fitted by the equivalent circuit providing constant phase elements are used to account for the non-ideality
in the capacitances.
Equivalent circuit
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•
Charge transfer resistance (Rct
) for corrosion at the base of pores increases as the pore resistance (Rpore
) increases; i.e., corrosion is being stifled by a pore closure process.
•
Double layer capacitance (Cdl
) at the sulphide/solution interface almost constant.
•
Pore capacitance (Cpore
) decreases as the pores close since the number of polarizable
species in the pores decreases.
EIS results: 0.1 mol/L NaCl
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EIS spectra: 1.0 mol/L NaCl
•
Little change in the spectra after the shortest exposure time.
•
The inability to fit the spectra at low frequencies indicates additional processes (e.g. diffusion in pores) may also be important.
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EIS results: 1.0 mol/L NaCl
•
Charge transfer resistance for corrosion at the base of pores is
almost constant (0.6 ~ 0.7 kΩ.cm2).
•
Pore resistance decreases indicating pore opening.
•
The small increase in pore capacitance would be consistent with pore opening, since more polarizable
(diffusing) species would be present .
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Comparison of the two solutions
•
The most obvious difference between the impedance behaviours in the two chloride concentrations is in the pore characteristics.
•
EIS results are consistent with the SEM observations. 0.1 mol/L NaCl 1.0 mol/L NaCl
0.1 mol/L NaCl 1.0 mol/L NaCl
Pore closurePore opening
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•
Spectra cannot be fitted to the two time constant circuit used for lower chloride concentrations.
•
The impedance at the low frequency limit is an order of magnitude lower than at the two lower concentrations, suggesting a much lower degree of protection of the surface by the deposited sulphide layer, consistent with SEM
observations.
•
This is consistent with the assignment of the time constant at high frequency to rapid charge transfer at the base of open pores.
EIS spectra: 5.0 mol/L NaCl
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•
An anodic peak with the shape typical of a diffusion-controlled process.
•
The single cathodic
reduction peak indicates the formation of a dense, compact sulphide
layer, consistent with SEM observations and closed pores.
CV: 0.1 mol/L NaCl
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•
The anodic behaviour similar to that at lower concentrations.
•
Two distinct cathodic
reduction peaks indicate the reduction of two layers, one (base layer) in good electrical contact with the Cu, and a second (scattered deposit) not in good electrical contact with the Cu.
CV: 5.0 mol/L NaCl
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A possible mechanism
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When SH-
is present, the formation of adsorbed species (CuSH)ads
occurs rapidly on the Cu surface.
•
(CuSH)ads
adsorbed on the Cu surface may be complexed
at high [Cl–] to produce soluble CuCl2
– which allows CuI
species to be transported out of pores.
•
CuCl2–
could be formed at the Cu surface (where [SH-]:[Cl–] is low), transported out of the defect site, and deposited as Cu2
S on the outer surface (where [SH-]:[Cl-] is higher).
SH-, Cl-
Cu
(CuSH)ads CuCl2-Cu1.8 S
Cu2 S
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Summary
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Chloride could influence Cu corrosion in sulphide
solutions by
accelerating Cu dissolution and/or interfering with sulphide film formation.
•
Under conditions of low [SH–] and high [Cl–], the complexation
and dissolution of CuI
as CuCl2–
would compete with the film formation.•
Sulphide
films are comprised of a Cu1.8
S (digenite)
base layer and an outer Cu2
S (chalcocite)
deposited layer.•
At 0.1 mol/L NaCl, the deposited layer is dense and eventually closes the pores in the base layer.
•
At 1.0 mol/L NaCl, the deposited layer is less coherent allowing the pores in the base layer to open slightly with time.
•
At 5.0 mol/L NaCl, the pores are unprotected by the deposited layer and corrosion rates could be high.
•
When the chloride to sulphide concentration ratio is high, CuI
species, originally formed as surface adsorbed CuSH
species can be complexed
by chloride to produce soluble complexes (CuCln
(n-1)-). Transport of these species away from the metal surface would maintain porosity in the base layer and limit the deposition of the outer protective deposit of Cu2
S.
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JMS DWS ZQ
2006
Electrochemical and Corrosion Studies at Western
Group website: http://sun.chem.uwo.ca/
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