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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



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.



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.



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.



Introduction

Ghods et al. (2005-11)

Before chlorides - passive After chlorides - depassivated



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.



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.



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).




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.




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




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




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




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.




Experimental set-up consists of

A quartz oscillator circuit

A waveform generator

An oscilloscope

A typical three electrode set-up

A data logger




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



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.




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.




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.




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.



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



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



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.



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



Results: TEM/EELS studies

Samples passivatedin chloride-free CH

solution:




Samples passivatedin chloride-free CP

solution:




Samples passivated inpresence of chloride

in CH solution:




Samples passivated inpresence of chloride

in CP solution:



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



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



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



Acknowledgements

NSERCCANMET-MTL

Dr. Pouria Ghods, UBCDr. Graham Carpenter, CANMET-MTL

Dr. Sankara Papavinasam, CANMET-MTL



Thank you…

Any Questions



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