corrosion of reinforcement...side effects hydrogen embrittlement. ... thus high-strength...

Structure Repair – Electrochemical Technique

MAB 1033 Electrochemical Repair 1

Lecturer:

Prof. Dr. Mohammad Ismail Faculty of Civil Engineering, UTM-Skudai,

Johor Darul Ta’zim, MALAYSIA

Corrosion of Reinforcement

Structure Repair – Electrochemical Technique


Prof. Dr. Mohammad Bin Ismail

Faculty of Civil Engineering

UTM

Introduction

Electrochemical techniques applied for controlling corrosion of reinforcement:

o Cathodic Protection (CP) o Cathodic Prevention (CPre) o Electrochemical Chloride Extraction (ECE) o Electrochemical Realkalization (ECR)


CP:

o Applied to structures already corroded mainly induced by chloride.

CPre:

o Applied to new structures that will presumably be contaminated by chloride.

ECE:

o Can be applied to structures in which corrosion has not or already initiated (chloride).

ECR: Realkalisation

o Can be applied to structures in which corrosion has not or already initiated (carbonation).


For ECE and ECR, there is no need to remove carbonated or chloride contaminated but mechanically sound concrete.

Direct current forced to circulate between an

anode, place on an external surface of the structure, and the reinforcement.

Current density:

o CPre 1-2 mA/m2

o CP 5-20 mA/m2

o ECE & ECR 1000 –2000 mA/m2


Schematic Representation of Application of Electrochemical Technique.


Development CP

o Applied for first time for bridge decks contaminated by de-icing salts by R.F. Stratfull, 1973.

o 1980s anodes based on titanium meshes activated with special oxides or conductive paints were developed

o 1990s, sacrificial cathodic protection of concrete reinforcement introduced.

o CP also effective in repassivating steel in carbonated concrete


CPre o Applied on new reinforced and pre-stressed

structures exposed to the atmosphere in Italy, Pedeferri,1989.

o The technique is based on that, the chloride threshold increases as the potential of steel decreases. In practice, application of low current densities (< 2mA/m2)

can bring the potential to values in which steel operates in conditions of “imperfect passivity” so that initiation of pitting is suppressed.

o This technique also proposed in conjunction with conventional patch repair.


ECE : o First studied in USA, 1970.

o Patented in Europe by Norwegian company Noteby in 1986. Using water retaining substance (paper fibre pulp) or

retarded shotcrete wetted with calcium hydroxide or tap water and moderate voltage (< 40V).

o Original concrete surface is left unchanged after treatment


ECR :

o This technique was introduced by Noteby in the late 1980.

Original system used surface mounted steel mesh anode and sprayed paper pulp wetted by calcium carbonate solution as electrolyte.

Later, titanium mesh anodes and liquid electrolyte were introduced


Effect of the Circulation Current

Beneficial Effect

o Reactions on the steel surface: On the surface of reinforcement - Oxygen reduction :

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

If very –ve potentials are reached, hydrogen evolution also occurs: 2H2O + 2e

- 2OH- + H2

oMigration: Circulation of current in concrete is produced by migration of

ions present in the pore solution

+ve ions (Na+, K+) move towards the steel

-ve ions (OH-, Cl-) in the opposite direction


Profile of Ionic Concentration Measured Between Anode And Cathode


Ionic Concentration Profile in Pore Solution After Treatment


Chloride Concentrations Profile


Side Effects

Hydrogen embrittlement.

If the imposed current is such that the steel potential becomes more negative than –1000mV SCE, hydrogen evolution take place at the surface of the steel and thus high-strength prestressing steel may be subject to hydrogen embrittlement


Alkali-aggregate reaction.

The increased in alkalinity produced at the cathode can cause damage if the concrete contains aggregates potentially susceptible to ASR

May happen only for current densities well over 20 mA/m2


Loss of bond strength

Cannot be excluded if the potentials falls below –1100mV SCE


Anodic Acidification

At anode surface, anodic process of oxygen evolution takes

place:

2H2O O2 + 4H+ + 4e-

In the presence of chlorides, even chlorine develops

2Cl- Cl2 + 2e-

Such process produce acidity and lead to destruction of the cement paste in contact with the anode


Experience shows that such deterioration is negligible for activated titanium mesh anodes if the anodic current does not exceed 100mA/m2


Electrochemical Techniques

Electrochemical techniques are applied in order to avoid corrosion

o By stopping

o By containing/ preventing.

The mechanism of working is different form different technique


CP – of Steel in Soil or Seawater

Stop

Of corrosion


10-200 mA/m2 Potential

CP – Steel in Chloride Containing Concrete


8-20 mA/m2 Chloride

removal

Chemical reactions

(OH- production)

Lowering of

potential

Decrease in

Cl-/OH- at the

Steel surface

Lowering of

Driving voltage

For corrosion

Increase in the

Kinetic resistance

to corrosion

Reduction or

stop of corrosion

rate

CP – Steel in Carbonated Concrete


4 - 8 mA/m2

Chemical

Reactions

(OH- production)

Repassivation

Of steel

Stop of

corrosion

CPre – of Steel in Concrete in Contact With Chloride Environment


1- 2 mA/m2 Barrier to

Chloride ingress

Lowering of

potential

pH control

Increase

Epit

Maintaining of

E < Epit

Maintaining of

Passive

conditions

CPre (1-Evolution paths of potential and chloride content on the rebar surface of an aerial construction during its service

life for 2-3)

CP restoring passivity (4-5), CP reducing rate (4-6)


CP

Initial current densities 5 – 15mA/m2. Much lower densities for oxygen deficient area. Under water 0.2 – 2 mA/m2

Also current can decrease with time due to passivity established on steel surface to value (2 – 5mA/m2).

Development of passivity also favoured by the decrease of the [Cl-]/[OH-]


CPre Applied to non corroding structures for prevention purposes.

Application of CP in carbonated concrete even though lower only slightly steel potential,

However can produce enough alkalinity to restore the pH to values > 12 on the RC surface. Thus promote passivation


Anode System

Normally: Titanium activated with oxides of different metals (ruthenium or iridium) used in a form of mesh, wire or strip

o Good mechanical properties.

o Coated with an overlay of mortar but can also be embedded directly into the concrete

o Current densities up to 100 mA/m2 with short term maximum levels 300-400 mA/m2

o Service life range from 20-100 years


Design

Consider the type and location of anodes in order to achieve sufficient and durable protection

Power is delivered by transformers/ rectifiers

If main power not available, solar or wind powered system can be used


Examples of Anode Layouts with Respect to a Concrete Cross-Section

Monitoring System

System based mainly on potential measurement of the reinforcing steel with respect to embedded electrodes

o True reference electrodes for permanent embedment are: silver-silver chloride (Ag/AgCl/KCl-0.5M) and manganese dioxide (Mn/MnO2/KOH-0.5M)

o To avoid overprotection, the potential should not be more –ve than –1100mV for plain RC or –900mV for prestressing steel (VS Ag/AgCl).


Execution

Complete concrete surface is checked for cracking, delamination, cover depth, steel continuity and the presence of metal objects that might caused short circuits

Reference electrodes and other monitoring probes are embedded

Anode is applied, with overlay or top coat

Power source is installed MAB 1033 Electrochemical Repair 34

Operation and Maintenance

CP is to work properly by checking voltage, current and depolarization regularly (2-4 times a year).

Once a year the installation is visually inspected for cracks, rust spots, loss of adhesion, cable defects, etc.



ECE-Principle Reactions Involved.


Electrochemical Realkalization or Electrochemical Chloride Extraction

1 – 2 A/m2

Chloride

removal

(OH- production)

Protective

concrete

Restoring of

Passive

conditions

• A direct current is applied between the reinforcement and the anode that is placed temporarily on the outer surface of the concrete.

• The anode: Activated titanium wire mesh or a reinforcing steel mesh surrounded by tap water or saturated Ca(OH)2 solution.

• Current density (1-2 A/m2) applied for 6-10 weeks

MAB 1033

38 Electrochemical Repair

ECE/ Desalination

• Strong polarization brings the potential of the steel below –1000 mV Vs SCE and the electrolysis of water produces a significant amount of hydrogen gas evolution.

• Hydroxide produced increase the pH, chloride reduced - hence steel repassivated.

• Not suitable applied to prestressed concrete

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• If no new chloride would penetrate, a safe upper limit for accepting the remaining chloride would be 0.4% by mass of cement

MAB 1033


Durability after ECE

Effectiveness depends on characteristics of individual structures, (concrete composition, actual chloride penetration profile and cover depth)

Carry trial on 1-10 m2 at least 4-8 weeks

Monitoring progress o by monitoring chloride profile from cores

o Monitoring chloride in electrolyte MAB 1033


Trials and Monitoring

• Carried out half-cell potential measurement. It should be realized that due to strong polarization, immediately after chloride extraction steel potentials are very negative

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After Treatment

1. ASR

2. Loss of Bond

3. Softening of cement paste around steel

– One month at 4 A/m2 or 4 months at 1A/m2 is equivalent to 2880 A.h/m2 found to cause 7-15% loss of bond strength

MAB 1033


Side Effects

ECR - Realkalisation • Similar to ECE. But anode is surrounded by a sodium (or other

alkali metal) carbonate solution of about 1M/l

• Only part of hydroxyl ions migrates to the anode, the remaining part being balanced by sodium ions migrating in

• Carbonate ions penetrate from the electrolyte into the concrete by electro-osmotic flow, diffusion and capillary absorption

• pH increase to 14 or above

• Sodium of sufficient concentration stabilizes pH between 10.5 - 11


ECR- Principle Reactions Involved.

MAB 1033 45 Electrochemical Repair

Development of Realkalisation


Untreated After a short treatment

After longer treatment

Treatment Effectiveness

When treatment can stop Total charge passed 200-450 A.h/m2 (current density

1A/m2 for 8-18 days)

Measure sodium content from cores taken during and after treatment

Using pH indicator liquids (phenolphthalein or Thymolphthalein) on cores taken during or after treatment

Carried out half cell measurement


Durability

Amount of charged needed may depend on cement type.

Potential measurement after 6 months and 1 year was homogeneous with values around –100mV SCE.

Total charged 200 A.h/m2 sufficient but for for blended cement.

Reduction of pH can be prevented by using CO2-resistant coating on concrete surface


Side Effects

ASR, very minimal

– Use lithium-based electrolyte


Practical Aspects

Achieve sufficient protection for at least 10 years.

Monitoring the process (where control cores to be taken).

Preparation includes cleaning of surface, filling wide cracks and preventing short circuits and points excessively low surface surface-to-steel resistance.

Calcium hydroxide or sodium carbonate solution concentration should be maintained.


CORROSION PREVENTION FOR CONCRETE STRUCTURES (1)


Use of sufficient cover

Use of impermeable good quality concrete o Lower water binder ratio

o Use of mineral admixtures

o Use of optimum cement content

o Optimum compaction

o Early and comprehensive curing

o Apply surface treatments

o Use of durability related tests for compliance (gas & water permeability, chloride permeability, chloride diffusion)



Isolation of reinforcement from the chemical effect of corrosion by means of physical barrier or chemical inhibition o Use of epoxy coated reinforcement

o Use of galvanized reinforcement

o Use of stainless steel reinforcement

o Use of bar primer

o Use of zinc rich paint



Reversing the effect of corrosion by cathodic protection (CP)

It works based on the principles of eliminating the anodic sites (corrosion sites) by progressing the steel to a cathodic state o Sacrificial anodes CP

o Impressed current (CP)



Preserving or restoring passivity (reserving the effect carbonation and chloride attack by electrochemical processes) o Realkalization : Technique to introduce alkaline solution into concrete to

arrest and prevent further deterioration due to carbonation. Produce hydroxyl ions & restoring pH levels

o Chloride extraction (Desalination) : Technique to remove ingressed or cast in chlorides in order to arrest deterioration due to carbonation

Say: “ If the ocean were ink (wherewith to write out) the word of my lord, sooner would the ocean be exhausted than would the word of my lord, even if we added another ocean like it, for its aid” Say : “I am but a man like yourselves, (but) the inspiration has come to me that your God is one God: whoever expects to meet his lord, let him work righteousness, and, in the worship of his lord, admit no one as partner.” Al-kahfi: 109-110

Say: “ If the ocean were ink (wherewith to write out) the word of my lord, sooner would the ocean be exhausted than would the word of my lord, even if we added another ocean like it, for its aid” Say : “I am but a man like yourselves, (but) the inspiration has come to me that your God is one God: whoever expects to meet his lord, let him work righteousness, and, in the worship of his lord, admit no one as partner.” Al-kahfi: 109-110

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MAB 1033 61

corrosion of reinforcement...side effects hydrogen embrittlement. ... thus high-strength...

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