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Electrochemical and Microscopic Investigation of thePassivation and Depassivation of Iron and Steel in
Simulated Concrete Pore Solutions
H. Burak Gunay
Supervisor: O. Burkan Isgor
Carleton University
Department of Civil and Environmental Engineering
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Introduction
Corrosion of reinforcement in concrete:
Mainly due to salts used in de-icing and anti-icing activities
Also exist in marine environments
Affects the safety of existing infrastructure
Costs over $20 billion annually in North America.
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Introduction
Protective nature of concrete:
Low permeability
Ability to bind and immobilise aggressive agents
High electrical resistance of concrete
High alkalinity (pH > 12.5), from calcium/sodium/potassium
hydroxides, makes corrosion of reinforcement less
favourable. A passive iron oxide/hydroxide layer (passive film) protects
steel from corrosion.
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Introduction
Ghods et al. (2005-11), supported by NSERC, CANMET-MTL PERD/RIEM program
Used a nano-scale and multi-technique (FIB/TEM, EELS, Diffraction, XPS) approachto study passivity and depasivation of carbon steel.
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Introduction
Ghods et al. (2005-11)
Before chlorides - passive After chlorides - depassivated
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Introduction
Ghods (2010) advanced the understanding about: Passivity and chloride-induced depassivation of carbon
steel in concrete
It was shown that passive film was around 5-13 nm and
consisted of two layers with an indistict border.
The inner layer (above steel) was a thin Fe(II)-rich
protective oxide/hydroxide.
The outer layer was a thicker Fe-(III)-rich unprotective
(porous) oxide/hydroxide. Chlorides penetrates through the porous layer and
converts Fe(II)-rich oxides/hydroxides to Fe(III)-rich
oxides/hydroxides, making the film unprotective.
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Introduction
Three points left for further investigation:1. Static nature of his data did not explain kinetics of
passivation and depassivation processes
2. Although oxidation states of iron in the passive film were
identified, chemical compositions were not predicted
3. Fe2+ → Fe3+ mechanism in presence of Cl- was not
explained.
OBJECTIVETo address the first two items for future work left by
Ghods et al.
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Experimental: Electrochemical Studies
Electrochemical studies involve the use of: Electrochemical Quartz Crystal Nanobalance (EQCN)
Electrochemical Impedance Spectroscopy (EIS)
Open Circuit Potential (OCP)
EQCN is a technique to detect small mass changes
in nanogram scale due to the electrochemical
processes that occur on an electrode that isdeposited on a quartz crystal (QC).
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Experimental: Electrochemical Studies
Quartz has piezoelectricproperties such that
electrical impulse can
create shear wave
oscillations in the firstmode of vibration.
To apply potential across
the quartz crystal metalplates are deposited on
either side of the QC.
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Experimental: Electrochemical Studies
As the mass of the electrode that is deposited on the quartzcrystal change due to electrochemical processes (e.g. film
formation or dissolution), the resonance frequency of the
oscillations also change.
Sauerbrey (1959) developed a relationship that correlatesthe frequency change of the quartz crystal to the mass
change.
2 2
2 2 0.867A A
o o
q q q q
f f m f
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Experimental: Electrochemical Studies
EIS is a technique todetermine the impedance
characteristics of an
interface
Impedance in AC circuitsis analogous to resistance
in DC circuits
Data can be collected in
terms of phase angle,modulus of impedance,
and real/imaginary part of
impedances
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Experimental: Electrochemical Studies
Low frequency response isrelated to film properties
High frequency response isrelated to solution properties
A passive film becomes moreprotective as: The phase angle converges to -
90⁰ (i.e. acts similar to acapacitor)
The modulus of impedance atlow frequencies increases
The Zimg decreases more than theincrease of Zreal
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Experimental: Electrochemical Studies
OCP is technique to measure the free potential of an electrode with respect to a reference electrode
(e.g. Saturated Calomel Electrode).
When the passive film becomes more protective,
OCP increases.
When a metal corrodes, its OCP is lower than that
of its passive state.
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Experimental: Electrochemical Studies
Experimental set-up consists of
A quartz oscillator circuit
A waveform generator
An oscilloscope
A typical three electrode set-up
A data logger
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Experimental: Electrochemical Studies
Two simulated concrete pore solutions are used: CH (Saturated Ca(OH)2)
CP (Concrete pore solution consists of 0.1 M Ca(OH)2, 0.1 MNa(OH), 0.2 M K(OH), and 0.03 M Ca(SO4))
A typical three electrode set-up is used: A saturated calomel reference electrode A platinum counter electrode
A pure iron electrode that is electron sputtered on a quartz crystal.
The following parameters were investigated:
Effect of exposure solution (CH vs. CP) Effect of electrochemical cleaning
Effect of chloride
Effect of passivation time on chloride-induced depasivation
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Results: Electrochemical studies
Effect of solution: Rapid electrochemical and
mass changes in the
beginning slows down in
time, but continues.
Confirms that there is a thinbut protective film on the
steel surface and a thick but
unprotective film above it.
Films in CH are thicker.
The impedance of the films
formed in CP is higher.
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Results: Electrochemical studies
Effect of cleaning: Film formation takes
place faster on cleaned
surface.
Rapid electrochemical
and mass changes in thebeginning slows down in
time, and they become
similar in both cleaned
and as-received samples.
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Results: Electrochemical studies
Effect of chloride: Gradual mass gain is
followed by a relatively
faster mass gain, then
followed by mass loss.
Impedance/phase angleand OCP remains stable
during initial chloride
ingress (induction time).
Depassivation occurs
when a drastic drop inimpedance and OCP, anddivergence from -90⁰ in
phase angle, are observed.
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Results: Electrochemical studies
Effect of passivation time: Similarly, the rapid mass
changes in the beginning
slowed down, then followed
by a final mass gain
succeeded by a mass loss OCP results show that
depassivation takes place
similar chloride
concetrations in both
samples passivated for 36hours and 2 hours.
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Discussion: Electrochemical studies
If a film is protective, growth of it should change the impedancecharacteristics. Therefore, a protective film forms rapidly, followed by
an unprotective film.
Chloride increments:
Mass gain: Chloride ingress
Induction time: Duration of chloride ingress Mass loss: Dissolution of the film
Films formed inside CP solution:
Thinner and more protective
Less likely to encounter defects
Films formed on cleaned samples
Form faster initially thanks to absence of air formed oxide film resistance
Become similar to the films formed on as-received samples
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Discussion: Electrochemical studies
Thickness of the oxide films are estimated based on the assumptions: Uniform film thickness
Inner oxide film FeO (Fe-II oxide); outer oxide film Fe2O3 (Fe-III oxide)
No imperfections
Small mass changes
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Experimental: TEM/EELS studies
Electron energy lossspectroscopy was used to
characterize the chemical form
of the oxide films
Transmission electron
microscopy was used to showthe microscopic images where
the EELS analysis was carried
out.
Characterization was carried
out with the fingerprints foriron oxides (FeO,Fe2O3,Fe3O4)
and oxide hydroxides (FeOOH)
published or standardized.
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Experimental: TEM/EELS studies
Samples are exposed to:
Chloride-free CH
Chloride-free CP
CH with chloride
CP with chloride
TEM Samples
From carbon steel rebar
Using FIB technique
By Ghods (2010)
Summary of
fingerprints used in
this study
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Results: TEM/EELS studies
Samples passivatedin chloride-free CH
solution:
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Results: TEM/EELS studies
Samples passivatedin chloride-free CP
solution:
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Results: TEM/EELS studies
Samples passivated inpresence of chloride
in CH solution:
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Results: TEM/EELS studies
Samples passivated inpresence of chloride
in CP solution:
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Discussion: TEM/EELS studies
Thermodynamic stability of iron oxides is lowest in FeOand highest in Fe2O3.
In presence of chloride inner FeO layer deteriorates most,
therefore:
Outer FeIII
layer does not protect inner FeO, perhaps due to itsporous nature
The inner oxide layer may convert into FeIII and/or dissolve into the
electrolyte solution
Despite Cl- concentration is below the threshold values determined
by Ghods (2010), extreme thinning of the oxide film is observed Films formed in CP may be more protective despite they are thinner
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Concluding Remarks: Electrochemical Studies
Passivity is attained rapidly, gradual oxide film formationcontinuum does not change the film quality
The thickness of the protective films vary between 1-15 nm
Films formed inside CP solution is thinner, but relative tio
CH, shows better protection against corrosion.There may be an inverse relationship between thickness and
film quality.
Chloride induced depassivation was described with three
critical events: Chloride ingress through the porous oxide (causes a mass gain)
Onset of inner film mass dissolution
Breakdown of passivity
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Concluding Remarks: TEM/EELS Study
The inner oxide film resembles FeIIO layerIt is covered by a Fe3+ rich Fe2
IIIO3 (in CP) or FeIIIOOH (in
CH) layer
The transition between Fe+2 to Fe3+ layers is indistinct, that
is probably in the form of Fe3O4 (FeIIO. Fe2IIIO3)Chemical form of the oxide layers depends on: Electrolyte solution
Chloride presence
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Acknowledgements
NSERCCANMET-MTL
Dr. Pouria Ghods, UBCDr. Graham Carpenter, CANMET-MTL
Dr. Sankara Papavinasam, CANMET-MTL
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Thank you…
Any Questions