cathodic protection and the effect of the applied

8/3/2019 Cathodic Protection and the Effect of the Applied

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CATHODIC PROTECTION AND THE

EFFECT OF THE APPLIED CURRENT ON

THE CONCRETE

CATHODIC PROTECTION AND THE

EFFECT OF THE APPLIED CURRENT ON

THE CONCRETE


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BACKGROUNDBACKGROUND

Corrosion

rebar dueto

Cl- and

CO2

Is it the effect on

the concrete

paste

Applied Cathodic

Protection (CP)

Literature

study and

hypothesisElectrochemical

Test and

material

visualization

Theoretical

opinion and

decision

??

Concrete test

simulation in

the Laboratory

Sea Water

Environment


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INTRODUCTIONINTRODUCTION

This Study deals with the cathodic protection of steel in

concrete and the effect of the applied current on the

concrete.

The experiment involves the measurement of the

potential difference along the concrete which is

exposed in two different solutions in both the anodic

and the cathodic compartment. The solution used in

the anodic compartment is sodium chloride 3.5% and

in the cathodic compartment is sodium hydroxide (pH

between 12 and 13)


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Evans diagram of influence of increasing chloride content

on corrosion rate of steel in concrete

Characteristic of Concrete concrete provides chemical protection to

the steel with the formation of highly

alkaline pore solution.

The highly alkaline in the concrete

promotes the formation of an adhering

passive film in the surface of the steel,

which protects it from corrosion.

Corrosion of steel reinforcement

Penetration of unfavourable chemical

species can destroy the protectiveenvironment and eventually cause

corrosion of the steel.

Significant chemical species which can

break down the passive film on the steel

are carbon dioxide and chlorides.

CHARACTERISTIC OF CONCRETECHARACTERISTIC OF CONCRETE


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CORROSION OF STEEL REINFORCEMENTCORROSION OF STEEL REINFORCEMENT

The chloride ion attacks the passive layer of the steel

reinforcement and allow the corrosion process to proceed

quickly. When steel in concrete corrodes, the iron ion

dissolves in the pore water and gives up electrons.

Carbon dioxide has a slow ingress to the concrete and reacts

with the alkaline components in the concrete, neutralising

the alkalinity.

The corrosion product that will be obtained from these

electrochemical reaction is hydrated ferric oxide or rust.

Eventually, the most serious cases of concrete failure occur

through the concrete cracking and spalling as a result of the

volume increase at the steel/concrete interface.


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ELECTROCHEMICAL CORROSION IN THEELECTROCHEMICAL CORROSION IN THEPRESENCE OF CLORIDEPRESENCE OF CLORIDE

.

Cl-


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DETEREORATION OF CONCRETEDETEREORATION OF CONCRETE

There are two consequences of corrosion of steel in

concrete :

a) First, the products of corrosion occupy a volume several

times larger than the original steel so that their

formation results in cracking and spalling of the

concrete cover. This makes it easy for aggressive agents

to ingress towards the steel.

b) Second, the progress of corrosion at the anode reduces

the cross-sectional area of the steel, thus reducing its

load-carrying capacity. In this connection, it should be

pointed out that chloride-induced corrosion is highly

localised at a small anode, pitting of the steel taking

place the alkalinity.


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CRACKING AND SPALLING OF THE CONCRETECRACKING AND SPALLING OF THE CONCRETE

COVERCOVER

.


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Impressed Current Cathodic Protection of Steel

Reinforced Concrete

Impressed Current CP System

a) Impressed current cathodic protection

works by passing a small direct current

(DC) from a permanent anode, fixed on to

the surface or into the concrete, to the

reinforcement.

b) The power supply passes sufficient current

from the anode to the reinforcing steel to

force the anode reaction (metal

dissolution) to stop and make the cathodic

reaction (oxygen reduction) the only one to

occur on the steel surface.

Cathodic reaction after protection

a) O2 reduction and O2 evaolution

O2 + 2H2O + 4e-p 4OH-

2OH- m 1/2O2 + H2O + 2e-

H2O m 1/2O2 + 2H+ + 2e-

a) Chlorination : 2 Cl-m Cl2(gas) + 2e-

CHATODIC PROTECTION OF STEELCHATODIC PROTECTION OF STEELREINFORECED CONCRETEREINFORECED CONCRETE


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Schematic of Sacrificial Anode Cathodic

Protection

Sacrificial anode CP System

a) The sacrificial anode technique

based on potential difference

between the structure and an anode

in the same environment to provide

the driving voltage.

b) Sacrificial anode such as magnesium,

aluminium or zinc is connected to

the steel without using a power

supply.

c) The cathodic protection makes all

the steel negative by dissolution of

anode metal to generated electrons.

Hence, the resistance of the concrete

becomes significant to ensure that

enough current can pass.

CHATODIC PROTECTION OF STEELCHATODIC PROTECTION OF STEELREINFORECED CONCRETEREINFORECED CONCRETE


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LABORATORY TESTLABORATORY TEST

Concrete test

simulation in the

Laboratory

a) Sample

preparation

b) Concrete sample

modification

c) Wiring

arrangement of

testing equipment

d) ElectrochemicalTest

d) Microstructure

analysis using

SEM

Provide the Concrete blocks

Curing the concrete block

Schematic diagram of

Testing sample

Schematic diagram of

Testing equipment

to measure of potential of the

concrete and potentialdistribution within the

concrete

to measure the composition

of chemical found in the pores

of the concrete


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SCHEMATIC DIAGRAM OF TESTINGSCHEMATIC DIAGRAM OF TESTINGSAMPLE AND TESTING EQUIPMENTSAMPLE AND TESTING EQUIPMENT

Testing Equipment

Testing Sample


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Chemical Changes in Concrete under the Influence of Applied Current

DISCUSSIONDISCUSSION


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CONCLUSIONCONCLUSION

a) passage of electricity in concrete from the anodic side to the cathodic

side due to the application of an impressed current cathodic protection

brings about chemical changes in concrete.

b) Under the influence of voltage gradient, the positively charged calcium

ions move towards the cathodic side and the negatively chargedhydroxyl and chloride ions move away from the cathodic side towards

the anodic side.

c) The measured potential will be effected by the physical condition of the

concrete. In this experiment, when the concrete pore is still dry from

either water or other electrolytes, the resistivity of the concrete is very

high. However, when the concrete pore is uniformly filled with water.

The resistivity of the concrete will be reduced significantly, and with

time the resistivity of the concrete under cathodic protection will

increase due to re-precipitation of calcium hydroxide at the highest pH

of the concrete environment (at the cathodic side of the concrete).


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CONCLUSIONCONCLUSION

d) The deposition of calcium hydroxide in the cathodic side will cause

a narrowing of the pores, hence reduce the conductivity of the

concrete.

e) A rise in the water cement ratio of the concrete leads to an

increase in water permeability. Consequently, more current is

passed in the early stages. Then normalised potential distribution

in a high water cement ratio will be better than that in a low water

cement ratio.

f) The error in a fixed voltage cathodic protection will probably occur,

if the electrical field induces the movement of water from the

anode surface, then eventually increases in the anode resistance.

This phenomenon is called osmosis.

g) When the zinc anode is used in the NaCl 3.5% (pH 6-7), the

corrosion of zinc tends to form zinc hydroxide precipitation which

will cover the anode and protect from further corrosion.


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cathodic protection and the effect of the applied

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