The Influence of Groundwater Anions on the Impedance Behavior of Carbon Steel

Corroding under Anoxic Conditions

Charmaine Lee, Zack Qin, and David Shoesmith

Sponsored byNational Science and Engineering Research Council of Canada (NSERC)Ontario Power Generation (OPG)

Department of ChemistryThe University of Western Ontario

London, Ontario, Canada

6th International Symposium on Electrochemical Impedance Spectroscopy, Cocoa Beach, Florida, May 16-21, 2004

Nuclear energy produces 16% of the electricity in Canada, 45% in OntarioA proposed used fuel disposal scenario: waste stored in containers and buried deep in granitic rock in the Canadian Shield

Copper corrosion barrier with a carbon steel (CS) liner to provide structural rigidityGroundwater entering a failed container will be anoxicCorrosion of CS could lead to scavenging of radiolytic oxidants thereby preventing corrosion of the fuel

Canadian Nuclear Waste Container

WaterRadiolysis

RedoxScavenging

FuelDissolution

SteelCorrosion

UO2Fuel

Aqueous Solution In A Failed Container CarbonSteelLiner

Cu

H2 O H2 O

O2 FeIII(s) FeII(aq) Fe2e-H2 O

H2

H2 O

H2 O

H2 Oα

H2 O2

2e-

UO22+UO2

CuShell

HCO3-/CO3

2- SO42- Cl- pH

A 1 M - - ~9B 0.2 M 0.1 M - ~9C 0.2 M 0.1 M 2 M ~9D 0.2 M 0.1 M 5 M ~9E - - 4 M ~9

Experimental

SEM

Periodic EIS every 24 hours up to 400 hoursRaman spectroscopy performed in-situ in a second experiment

A

B

C

D

E

0 100 200 300 400-0.90

-0.85

-0.80

-0.75

-0.70

Time (Hours)

Eco

rr (V

/SCE

)

-0.90

-0.85

-0.80

-0.75

-0.70

Corrosion Potential (Ecorr )A. 1 M HCO3

-/CO3=

B. 0.2M HCO3-/CO3

=

+ 0.1M SO4=

C. “B” + 2 M Cl-

D. “B” + 5 M Cl-

E. 4 M Cl-

Ee (H+|H2 )

Ee (Fe|Fe2+)

Ee(Fe|Fe2+) < Ecorr < Ee(H+|H2)Iron dissolution is strongly polarized and rate-determining for solution E For all other solutions with carbonate, no clear rate control by either the cathodic or anodic reaction

Thermodynamics of Solution E

No buffer capacityNo significant thermodynamic driving force for corrosionin absence of HCO3

-

/CO3= buffering, local

pH can increase as the consequence of corrosion A slight local pH increase ( ) would shift Ecorr into the stability region for magnetiteA local increase in Fe2+

concentration would expand the stability field for Fe3O4 to a lower pH

M. Pourbaix, Atlas of Electrochemical Equilibria in Aqueous Solutions, (1966), p312.

0 5000 10000 15000 20000

-15000

-10000

-5000

0

5000

Z'

Z''

Solution D

0 2000 4000 6000 8000 10000 12000

-8000

-6000

-4000

-2000

0

2000

4000

Z'

Z''

Solution C

0 2500 5000 7500 10000 12500 15000

-10000

-7500

-5000

-2500

0

2500

5000

Z'

Z''

Solution B

0 2000 4000 6000 8000

-6000

-4000

-2000

0

2000

Z'

Z''

Solution A

TimeFirst 96h

Thereafter

Time

Up to 168h

0 5000 10000 15000 20000 25000 30000 35000

-25000

-20000

-15000

-10000

-5000

0

5000

10000

Z'

Z''

Solution E

Up to 96h

Impedance Changes with Exposure Time

A. 1 M HCO3-/CO3

=

B. 0.2M HCO3-/CO3

=

+ 0.1M SO4=

C. “B” + 2 M Cl-

D. “B” + 5 M Cl-

E. 4 M Cl-

1 t

CT

RT

Rs

Equivalent Circuits

10- 2 10-1 100 101 102 103 104 105100

101

102

103

104

Frequency (Hz)

|Z|

FitResult

10- 2 10-1 100 101 102 103 104 105

-100

-75

-50

-25

0

Frequency (Hz)

thet

a

Rp

Rpore

2 t

Rs

Cdl

Cfilm

10- 2 10-1 100 101 102 103 104 105100

101

102

103

104

Frequency (Hz)

|Z|

FitResult

10- 2 10-1 100 101 102 103 104 105

-75

-50

-25

0

Frequency (Hz)

thet

a

0 100 200 300 400

103

104

105

Pola

rizat

ion

resi

stan

ce (Ω

cm

2 )

Time (hours)

A: 1M HCO3-/CO3

=

B: 0.2M HCO3-/CO3

=

+ 0.1M SO4=

C: 'B' + 2M Cl-

D: 'B' + 5M Cl-

E: 4M Cl-

Polarization Resistance (Rp )

Rp ∝ 1/icorrCorrosion is fast in the concentrated carbonate solution (category I)Corrosion rates are intermediate and similar for mixed anion solutions (category II) The high Rp for solution E (category III) indicates that corrosion is suppressed in a solution with no buffer capacity

III

II

IRT (1t)

RP (2t)

Pore Resistance (Rpore )

Rpore = ρ lpore/Apore

0 100 200 300 400100

101

102

103

104

Pore

resi

stan

ce (Ω

cm

2 )

Time (hours)

A: 1M HCO3-/CO3

=

B: 0.2M HCO3-/CO3

=

+ 0.1M SO4=

C: 'B' + 2M Cl-

D: 'B' + 5M Cl-

E: 4M Cl-

Rpore Cfilm

r

↑ ↑ —lpore ↑ ↑ ↓

Apore ↑ ↓ ↓

CPE Related to Film Capacitance (Cfilm )

Constant phase element (CPE):ZCPE = 1/[Y(jω)a]Y ~ C, if a ~ 1 Cfilm = εε0Af /dfThe drop in Cfilmand Rpore at the same time suggests an increase in porosity at long exposure times in solution E

0 100 200 300 4000.00000

0.00005

0.00010

0.00015

0.00020

0.00025

0.00030

0.0

0.2

0.4

0.6

0.8

1.0

1.2

CPE

-T (

)

Time (hours)

B: 0.2M HCO3-/CO3

=

+ 0.1M SO4=

C: 'B' + 2M Cl-

D: 'B' + 5M Cl-

E: 4M Cl-

CPE

-P (

)

-1050 -950 -850 -750 -650 -550 -450 -350

-0.4

-0.2

0.0

0.2

0.4

0.6

Cur

rent

Den

sity

(mA/

cm2 )

Potential (mV vs SCE)

1M HCO-3/CO=

3

0.2M HCO-3/CO=

3

+ 0.1M SO=4

Cyclic Voltammetry on a Rotating Disc Electrode (RDE)

Carbonate accelerates both anodic and cathodic processes:

a) iron dissolution is enhanced by ferrous ion complexation

Fe2+ + HCO3- Ø Fe(HCO3 )+

b) proton reduction is maintained by bicarbonate dissociation

2HCO3- + 2e- Ø H2 + 2CO3

=

(a)

(b)

5.5 h

FeCO3 FeCO3 ·H2 O

Fe3 C

Concentrated Carbonate Solution

HCO3-

FeCO3 ⋅H2 O

Fe(HCO3 )+

Fe α-Fe α-Feα-FeFe3 C Fe3 CFe3 C

H2 O H2

98 h

Fe(HCO3 )+

A

e-

Fe2+

Mixed-Anion Solution

Fe

GRFe3 O4

Fe2+

FeCO3 ⋅H2 O

24 h

42 h

79 h

FeCO3 ·H2 OSO4=

Fe3 O4

Fe2+-OH Fe3+-OH

GRGreen rust (GR): 4Fe(OH)2•2Fe(OH)3CO3

FeCO3

HCO3-

GR

B

H2 O

Pure Concentrated Chloride Solution

Fe3 O4 & a-Fe2 O3

FePassive oxide

Ex-situE

Hematite (a-Fe2O3) is not formed in solution, but in air by conversion from magnetite (Fe3O4)

ConclusionsConcentrated carbonate solution

Corrosion is ‘fast’ since carbonate accelerates both anodic (by Fe2+ complexation) and cathodic (by HCO3

- acting as a proton reservoir) reactionsCorrosion is partially blocked by siderite formation but maintained within poresVisibility of carbide residue from the underlying steel confirms the presence of porosity

Mixed-anion solutionCorrosion is suppressed at lower carbonate concentrations which appears to be the primary corrosion controlling factorThe surface is covered by compact green rust deposits, and no carbide residue is exposed

ConclusionsPure concentrated chloride solution

Corrosion is extremely slow since no significant thermodynamic driving force existsDue to the absence of buffer capacity, a slight increase in local pH could substantially reduce the thermodynamic driving force for corrosion and enforce the formation of a passive oxide filmThe increase of porosity after long exposure could indicate breakdown of the oxide film by chloride

Electrochemistry and Corrosion Studies at Western

Group leader: Professor D. W. Shoesmith (NSERC/OPG industry chair in nuclear fuel disposal chemistry)

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