2007d_1 corrosion basics.pdf

Concrete Materials, Mechanics & Engineering Lab., Yonsei Univ.

Corrosion of Reinforced Concrete

Ha-Won Song

Professor School of Civil and Environmental Engineering

Yonsei Univ., Seoul 120-749, KOREA


Introduction

When a metal is inserted in an aqueous environment, a potential difference develops at the metal-aqueous solution interface.

The rate at which the reactions and the movement of charges across the interface occurs is determined by the magnitude of the potential difference.

In aqueous solution, the distribution of particles close to the metal surface is no longer homogeneous, nor are the forces isotropic.


Double LayerWater molecules are polar and therefore are attracted to the charged surface and orient themselves along the interface. Charged ions can also form hydrated units. In analogy with a parallel-plate condenser, the system of the two oppositely charged planes is referred to as a double layer


Double Layer

Small ions generate large Coloumb forces, and therefore have greater chance to become hydrated.

This means that most cations (陽이온) are solvated and most anions ( 陰이온), being large, are not solvated. This also explains why some anions cause more corrosion damage.

The large anions are unhydrated and can get closer to the metal surface, even though they may not participate directly in the corrosion reactions.


Double layer

The ion can be adsorbed on the metal surface forming an inner-sphere complex when no water molecule is between the surface function group and the ion, or an outer-sphere complex when at least one water molecule exists between the surface and the ion. Ions can also be adsorbed in the diffuse swarm of the double layer in order to neutralize the surface charge


Corrosion of reinforced concreteCorrosion of reinforcing bars in the electrolytic concrete pore solution involves electron or charge transfer through the chemical reactions at the interface. Electrode potential difference between the reinforcing bars and electrolyte is the driving force for the charge transfer to occur.

Corrosion damage


Electrochemical process of steel corrosion in concrete


Volumetric change


Carbonation of Concrete

Carbonated concrete

Painting with Phenolphthalein

Concrete exposed to CO2

(accelerated test)


Corrosion due to Carbonation

CO2

CO2CO2

CO2CO2


Corrosion due to chlorides

Cl-Cl- Cl-

Cl- Cl-


Factors needed for steel corrosion

O2 H2O

+ humidity

O2

H2O H2OO2

H2O

pHcracks

Cover concrete quality, W/C


Corrosion Potential The corrosion potential of the steel in reinforced concrete can be measured as the voltage difference between the steel and a reference electrode in contact with the surface of the concrete. Half-cell measurements may be made relatively easily, using only a high impedance voltmeter and a standard reference electrode, such as a copper-copper sulfate electrode.


The potential recorded in the half-cell measurement can be used to indicate the probability of corrosion of the steel reinforcement.

Measured potential (mV vs. CSE)

Corrosion probability

>-200 Low, less than10% probability of corrosion

-200 ~ -350 Uncertain <-350 High, greater than 90% probability

of corrosion

System for measuring the half-cell potential


Results affected byDegree of humidity in concrete. The measurement is very sensitive to the humidity existing in the concrete. More negative potentials result for concrete with higher degree of saturation.

Stray currents. The presence of stray currents will significantly affect the measurements of the half-cell potential.

Oxygen content near the reinforcement. The lack of oxygen near the reinforcement results in more negative potentials as compared to more aerated zones.

Microcracks. Localized corrosion can be generated by microcracks, which also modify the concrete resistivity, consequently affecting the corrosion potential measurement.


Polarization curveElectrode potential difference between the reinforcing bars and electrolyte is the driving force for the charge transfer to occur. Their electrode potentials will change with the corrosion reaction rate until a stable or equilibrium state (Ecorr) is achieved. At this potential the anodic (ia) and cathodic (ic) current densities are opposite and equal and to icorr.

Deviation from the steady-state condition can be expressed by the electrode polarization potential, also known, as overpotential (ηa or ηc) where

corra EE −=η

EEcorrc −=η


Polarization curve

corra EE −=η

EEcorrc −=η


Polarization Resistance

The slope at the origin of the polarization curve is defined as the polarization resistance, Rp:

( )cacorr

ca

0c,appp i3.2id

dRβ+β

ββ=⎟

⎟⎠

⎞⎜⎜⎝

⎛ η=

→η


)(3.2 caP

cacorr R

Iββ

ββ+

=

polarization resistance=RP

cathodic curve slop=βc

anodic curve slop=βa

corrosion rate=Icorr

Tafel’s extrapolation technique

-600

-500

-400

-300

-200

-100

0

0.01 0.1 1 10 100 1000Current(㎂)

Pote

ntia

l(mV

)

Cathodic curve

Anodic curve

Corrosion potential

Corrosion rate

βa

βc

Tafel’s extrapolation technique corrosion rate


Measurement


Polarization resistance curve

Typical polarization resistance for steel in concrete

Rate of corrosion Polarization resistance,

Rp (kΩ.cm2) Corrosion penetration,

p (μm/year) Very high 0.25 < Rp < 2.5 100 < p < 1000

High 2.5 < Rp < 25 10 < p < 100 Low/moderate 25 < Rp < 250 1 < p < 10

Passive 250 < Rp p < 1


Experimental set-up


Electrochemical Impedance

Electrochemical impedance spectroscopy, or AC impedance, is an informative method because not Rp only is measured, but also the physical processes in concrete and steel/ concrete interface are assessed. Impedance measurement employs small-amplitude alternating (AC) signals in a wide range of frequency as a perturbation.

In electrochemical impedance spectroscopy measurements, a sine or cosine wave of AC current with magnitude I0 and frequency f is commonly used as the input. The output is recorded as a voltage response with the magnitude V(f) and a phase angle φ(f) with respect to the current


Voltage response, V, to sinusoidal current signal i.

time

V (t)

i (t)

V or i


Equivalent circuit

Equivalent circuits have been used to model the impedance of complex systems.

Pure resistance and pure capacitance represent two types of impedance to the charge transfer.

Energy is dissipated through electrons or ions flowing through aresistance element, which constitutes a non-frequency-dependent impedance that has only a real part.

A capacitance element represents an energy storage process or charge separation under an electrical field. It creates an alternating electric current under an alternating electrical field, and the impedance will decrease with frequency.


Model of equivalent circuit


Nyquist plot for the impedance of the electric circuit


Representation of the cracking-corrosion-cracking cycles

2007d_1 corrosion basics.pdf

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