CORROSION CONTROL

Introduction

Steel corrosion costs the UK millions of pounds every year in all sectors of industry. The science of corrosion control has rapidly developed over the last twenty years. Concrete Repairs Limited (CRL) specialises in the control of corrosion for reinforced and steel encased concrete structures in all sectors of industry. It has 50 years experience and is the leading UK contractor for the design and installation of corrosion control systems.

Why does the steel corrode?

Steel in the alkaline environment provided by concrete is in a stable condition because a protective oxide layer forms on the steel surface, which stops corrosion.

There are two situations where the passivating environment at the steel surface can be disrupted:

(1): Carbonation -
This is when atmospheric carbon dioxide dissolves in water to form carbonic acid which neutralises the concrete alkalinity. This carbonation of the concrete proceeds into the cover concrete and eventually reaches the steel. At this point the passive layer is no longer sustained and corrosion will occur.

(2): Chloride attack –
Chlorides in sufficient concentration at the steel (usually >0.4% b.w.c.) will disrupt the passive film and cause rapid corrosion to occur. These chlorides may have been cast in the original mix or from an external source such as de-icing salts or a marine environment.

To support the corrosion activity there must be oxygen and water available and this is normally the case in atmospherically exposed concrete.

Is there a Corrosion Risk?

It is important to establish whether there is a future risk of steel corrosion for both new and existing structures. In both cases it clearly does not make commercial sense to install a corrosion prevention system if the probability of occurrence is low. The first procedure must be to undertake a corrosion risk assessment.

New Structures

With a new structure a desktop study should review items such as:

Environmental conditions
Material qualities
Construction qualities
Service life requirements

If the risk of corrosion is considered to be significant then a corrosion prevention system can be installed during construction.

If the risk of corrosion is moderate the structure can be prepared for a system to be installed in the future. This would include a permanent corrosion monitoring system to identify exactly when to install a prevention system during the service life.

Based on 50 years experience CRL is able to advise on the most appropriate action to take to minimise the likelihood of steel corrosion occuring during the service life of the new structure.

Existing Structures

These structures require a corrosion risk assessment based on the current condition and an extrapolation of the information to the end of the service life.

The initial condition survey should include the following:

Visual survey
Hammer test
Depth of carbonation
Chloride profiles
Half cell potential surveys
Covermeter

This survey will identify possible corrosion risk in certain elements of the structure and a more detailed survey may be used to focus on these areas. The more detailed survey should include corrosion rate monitoring to assess the rate of reinforcement corrosion. This will provide an assessment of the corrosion risk for the structure.

If the risk is moderate and a corrosion prevention system is not installed then a permanent corrosion monitoring system can be used to accurately monitor the structure in the future.

The survey division, CRL Surveys, is able to provide a comprehensive inspection and report service for concrete deterioration with repair recommendations.

Corrosion Prevention

If a risk of corrosion is identified then it is appropriate to take preventative action to stop it occurring or reduce the risk.

Corrosion prevention for existing structures is not an exact science. Some of the corrosion already present may be due to poor maintenance procedures such as leaking deck joints and blocked drainage gullies in the car parks. So standard, simple maintenance procedures may prevent corrosion.

There are four generic corrosion prevention systems, which have been developed for existing reinforced concrete structures:

Corrosion inhibitors
Cathodic protection – sacrificial and impressed current systems
Chloride extraction
Realkalisation

The selection of which system to use, requires expert knowledge from a Corrosion Engineer and there may be more than one option or combination of options for different elements of the structure. CRL have their own qualified Corrosion Engineers who are able to advise on the most appropriate system.

Corrosion Inhibitors

These have been introduced into the market over the last ten years.

These materials are applied to the surface of the concrete and diffuse through the concrete cover to the reinforcement. A thin chemical layer forms on the steel and inhibits corrosion activity.

These materials can be used to prevent corrosion due to carbonation and chlorides. However, they are consumed during the process and they are only effective in areas with low levels of chlorides (less than 2% b.w.c.) Future repeat treatments may be required and if further chloride contamination is expected they are not recommended.

Cathodic Protection

Corrosion is an electrochemical process where the corrosion sites are anodic and passive sites are cathodic. The passive sites are cathodically protected by the anodic corrosion sites. When the anodic sites are repaired corrosion is initiated adjacent to these repair areas and the process is repeated – this is called the incipient anode effect.

Cathodic protection solves this problem by introducing an anode to the concrete and making all the steel cathodic. There are two forms of cathodic protection sacrificial and impressed current.

Impressed current cathodic protection systems have been used to control reinforcement corrosion in structures since 1973. A small direct current is passed between the anode and the reinforcement to cathodically protect the reinforcement. The anode is a permanent addition to the structure and the system is computer controlled to minimise future monitoring costs.

Sacrificial anode systems are based on the use of a galvanic anodes such as zinc or aluminium, which are directly connected to the reinforcement and corrode in favour of the steel. There are zinc sacrificial anode systems now available which compare favourably with impressed current systems.

Chloride Extraction

This is an electrochemical method of extracting the chloride from the concrete. A secondary effect in impressed current cathodic protection systems can be created by using much higher currents, and a temporary anode system in an electrolyte on the concrete surface. The negative chloride ions are repelled from the steel reinforcement and attracted to the positive anode on the concrete surface.

The process takes between 6 and 8 weeks to complete and it is impossible to remove all the chlorides. The immediate area around the reinforcement is left almost chloride free and the passive layer is reformed. However chlorides behind and between the reinforcement will remain and may diffuse back to the reinforcement given time.

Realkalisation

This is again a secondary effect of the cathodic protection process where water is reduced during the cathodic reaction at the reinforcement to produce hydroxyl ions. Using a temporary anode and high current densities the alkalinity in the carbonated concrete around the reinforcement can be restored. The pH of the concrete increases and the passive film reforms on the reinforcement.

The treatment process takes 3-5 days to complete and afterwards it is advisable to apply a protective anti-carbonation coating, which should then be maintained.

The process is most effective in treating carbonated concrete between the reinforcement and external anode. Areas remote from the reinforcement will not be treated such as between the reinforcement and on the back face of pre-cast panel units where the carbonation is likely to be higher.

Summary

Timely intervention is the key to an effective and economical solution.

When?
Where?
Why?

Careful monitoring of the structure with expert interpretation of the results can provide the answers to these questions. Prompt action can then prevent corrosion and avoid expensive concrete repairs. CRL is able to provide a professional service to ensure that durability issues do not reduce the service life of structures.