Electrochemical Rehabilitation Techniques – An Introduction

Topics Covered

Background

Cathodic Protection

How Does Cathodic Protection Work?

Differences between Cathodic Protection Systems

Design Factors

Installation Procedure

Monitoring Cathodic Protection Systems

The Benefits of Cathodic Protection

Realkalisation and Chloride Removal

Design Factors

Installation Procedure

Temporary Electrode Systems

How a Realkalisation System Works

Chloride Removal or Desalination

Monitoring the System

Benefits of Temporary Cathodic Protection

Summary

Background

In the presence of significant levels of chloride ions or carbonation of the cover concrete, traditional repair techniques at the existing corrosion sites will not provide a long term solution to the problem.

The corrosion sites act in a similar way to sacrificial anodes and so once repaired other corrosion sites will be initiated in adjacent areas. This is commonly know as the incipient anode effect. For a durable conventional repair all contaminated or carbonated concrete at the reinforcement should be replaced. This solution is not only prohibitive due to cost but is also environmentally unacceptable to the building occupiers.

An alternative to the removal of large areas of defective yet sound concrete is to use an electrochemical technique such as cathodic protection, realkalisation or chloride removal.

Cathodic Protection

The field of application of cathodic protection has been considerably extended since its first use as a corrosion prevention strategy in 1824. At the time it was used by Sir Humphrey Davy to protect the copper sheathing on warships. It can in theory be applied to all metal surfaces in contact with an electrolyte and it is now widely used to prevent the corrosion of steel structures in sea water and pipelines in soil. More recently, applications of this technique have included steel in concrete in 1973 and steel and iron in stone structures in 1990. In these cases the pore water in the concrete and stone acts as the electrolyte.

How Does Cathodic Protection Work?

Cathodic protection relies on the passage of a DC current from the environment into the protected metal surface to reverse the direction of the electric currents associated with the corrosion process. It may be viewed as a limited form of electroplating where for example, iron which has dissolved to form rust is encouraged to plate back onto the steelwork. In practice however, the energy expended will only be sufficient to prevent further metal dissolution thus minimising the risk of any deleterious side defects. Iron usually dissolves to form positive ions and the damage caused by corrosion occurs at sites (termed the anodes of the corrosion cell) where a positive current leaves the surface. In reversing the direction of this current flow the cathodic protection system shifts the metal potential in the negative direction. As unlike charges attract, more negative steel potentials provide a barrier to iron dissolution. Thus the performance of a cathodic protection system may be monitored by determining the metal potential shift using an independent reference electrode.

Differences between Cathodic Protection Systems

The above description applies to all cathodic protection systems. The main difference between systems comes from the components used to distribute the current to the protected steelwork. It may be noted that cathodic protection requires the installation of an electrical system with all its associated components. The most important of these is termed the anode. There are primarily four anode systems available and these are:

•        Conductive paint

•        Titanium mesh with a cementitious overlay

•        Conductive cementitious overlay

•        Discrete anodes

For buildings it would be appropriate to use either the conductive paint or discrete anode systems. These are quick and easy to install and cost effective.

A small direct current in the region of 5 to 10 milliamps per square metre of surface concrete is passed between the anode and the steel reinforcement. This applied current causes the steel to become cathodic with respect to the anode system and prevents further corrosion. In addition, polarising the steel causes protective hydroxyl ions to be generated at its surface and aggressive chloride ions to be repelled - essentially localized realkalisation and chloride removal.

Design Factors

Before an electrochemical system is installed the following design factors need to be considered:

•        Potential for alkali aggregate reaction

•        The extent of concrete delamination and the nature and extent of previous repairs

•        Survey results: carbonation depths and chloride ion profiles

•        Concrete cover to the reinforcement

•        Resistivity of concrete and existing repairs

•        Steel density (current requirements)

•        Electrical continuity of the reinforcement

•        Metal fixtures and fittings (isolate or connect to reinforcement)

•        Possible location for electrical cabling system

•        Additional structural load bearing capacity

Installation Procedure

The general installation procedure for a cathodic protection system is as follows:

•        Grit blast/bush hammer the concrete - surface anodes only

•        Identify and repair all delaminated areas of concrete

•        Identify and replace previous repairs where necessary

•        Check the electrical continuity of the reinforcement in each anode zone

•        Check the concrete cover between the anode and the reinforcement

•        Identify isolated metal items in the zone of influence

•        Install the embedded permanent monitoring system

•        Connect the reinforcement to the system negative

•        Install the anode system

•        Complete the electrical cabling and install the transformer rectifier

•        Check the system

•        Energise, commission and monitor the system

Monitoring Cathodic Protection Systems

Cathodic protection systems require monitoring at six monthly intervals. The use of reference electrodes embedded in the concrete enable the monitoring and control to be undertaken remotely either at the transformer rectifier unit in the building or via a modem link in an office off site. The cost of monitoring would be in the region of £750 per annum and the power supply cost would be nominal.

The Benefits of Cathodic Protection

To summarise the beneficial effects of cathodic protection are as follows:

•        Primary

•        Potential of reinforcement is made more negative

•        All locally generated corrosion cells are overcome

•        Secondary

•        Aggressive chloride ion removal via ionic migration

•        Rise in the concentration of hydroxyl ions at the steel reinforcement

•        Available oxygen for reinforcement corrosion reduced

Realkalisation and Chloride Removal

These electrochemical systems have been developed over the last 10 years. They are essentially temporary cathodic protection systems which exploit the secondary effects of a cathodic protection system by using much higher current densities and voltages over a finite period.

Design Factors

The design factors are similar to those for a cathodic protection system. The higher current densities and voltages focus the need to ensure that AAR aggregates are not present and there is no risk of stray current corrosion.

Installation Procedure

The installation procedure is similar to that for a cathodic protection system but the anode used is only temporarily fixed to the concrete surface in contact with an electrolyte to form an electrode and a protective coating system must be applied and maintained on completion.

Temporary Electrode Systems

There are three temporary electrode systems which have been used in the UK:

•        Sprayed cellulose fibre soaked with electrolyte encapsulating a titanium or mild steel mesh anode.

•        Electrolyte filled shuttering system encapsulating a titanium or mild steel mesh anode.

•        Gel filled sponge with a titanium or conductive paint anode surface mounted to the sponge.

For realkalisation the electrolytes used are normally a combination of the following cations and anions:

Table 1. Typical cations and anions in realkalisation.

Possible Cations

Possible Anions

Hydrogen

Hydroxide

Sodium

Carbonate

Calcium

Bicarbonate

Lithium

 

Sodium carbonate is the most common electrolyte used for realkalisation.

How a Realkalisation System Works

During realkalisation a current of 1 Amp per square metre of surface concrete is applied at 10-20 volts. Hydroxyl ions are generated at the reinforcement by the reduction of water and the positive sodium cation in the electrolyte moves towards the negative reinforcement which “fixes” the hydroxyl ions to produce sodium hydroxide in the pore solution of the cover concrete. This process normally takes 4 days to complete.

Chloride Removal or Desalination

Chloride removal or desalination as it is sometimes known can be used to remove free chlorides in the pore solution in the cover concrete. It is not possible to remove the chlorides in the concrete outside the field of current influence. So chlorides added as calcium chloride in the original mix cannot be removed throughout the concrete to sufficiently low levels to prevent further reinforcement corrosion.

This process is most likely to be successful for buildings in a marine environment where chloride contamination of the cover concrete has occurred.

The process is similar to that for realkalisation except that the electrolyte can be tap water and the process usually takes 6-8 weeks to complete.

Monitoring the System

To monitor the success of these systems the following tests should be undertaken during and after the process:

•        Common

•        Total charge passed (Amp-Hours)

•        Petrographic examination for AAR

•        Realkalisation

•        Sodium ion profile

•        pH test of concrete samples (Phenolphthalein)

•        Desalination

•        Chloride ion profile in concrete samples

•        Concentration of chloride ions in electrolyte

Benefits of Temporary Cathodic Protection

The beneficial effects of these temporary cathodic protection systems are as follows:

•        Primary

•        Rise in the concentration of hydroxyl ions at the steel reinforcement

•        Aggressive chloride ions removal via ionic migration

•        Short Term Benefits

•        All locally generated corrosion cells are overcome

•        Potential of reinforcement is made more negative

•        Available oxygen for reinforcement corrosion is reduced

Summary

•        Electrochemical repair is cost effective

•        Cathodic protection is a proven method to control corrosion

•        Realkalisation and desalination have more limited uses

•        Choice of method is structure specific

•        Appoint electrochemical repair specialists

Primary author: John Drewett

Source: Corrosion Protection Association Monograph no. 2

 

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