Cathodic Protection of Steel in Concrete

Cathodic protection of steel in concrete

Guided by: prof. R.j.shah prepared by: Bhrugesh patel

SD1210

Introduction

First cathodic protection system installed by sir humphery davy and micheal faraday in 1824.

The first time use of cathodic protection to control corrosion of steel in concrete was made sometime in the period 1945-55 on buried concrete pipeline.

After 1970s variety of cathodic protection system technique is involve. Like disc anodes with overlayers, platinumwire etc.

Corrosion of steel

Corrosion is an electro-chemical process that involves the passage of electrical currents on a micro or macro scale. The change from the metallic to the combined form occurs by an “anodic” reaction:

M → M+ + e-

(metal) (soluble salt) (electron) A common example is:

Fe → Fe++ + 2e-

This reaction produces free electrons, which pass within the metal to another site on the metal surface (the cathode), where it is consumed by the cathodic reaction. In acid solutions the cathodic reaction is:

2H+ + 2e- → H2

(hydrogen ions in solution) (gas)

In neutral solutions the cathodic reaction involves the consumption of oxygen dissolved in the solution:

O2 + 2H2O + 4e- → 4OH-

(alkali) Corrosion thus occurs at the anode but not at the

cathode (unless the metal of the cathode is attacked by alkali).

The anode and cathode in a corrosion process may be on two different metals connected together forming a bimetallic couple, or, as with rusting of steel, they may be close together on the same metal surface.

For steel reinforcement in concrete the ionic flow running through the concrete and attachment between the steel cannot be disconnected as the corrosion circuit is buried in the structure.

Instead it is possible by using an “artificial” anode to add a new and higher current to the original corrosion circuit which runs in opposite direction of the corrosion current .

this makes all the previous + poles (anodes) into current receivers.

Thus all the steel reinforcement is made into a negative pole, i.e. cathod.

It is the principal of cathodic protection.

Principal of Cathodic protection

the corrosion current flows out of the steel at anode, the part corroding , through the concrete and into another part of the steel where there is no corrosion ocuuring , i.e the cathod. This flow called a corrosion circuit.

Steel is dissolved at the

anode And eventually

forms iron oxide

at this location.

Effects on RCC

Cathodic protection is applied to reinforced concrete structures to either prevent the problem of corrosion of the reinforcement.

The water and oxygen in the presence of the chlorides, react with the iron to create corrosion products on the surface of the reinforcing steel.

Rust take up a larger volume than steel, and pressure as high as 5000 psi build up within concrete which adequate to cause cracking and spalling.

When corrosion is started PH at the rust is 3 or less while the PH of surrounding concrete is 12.

The corrosion reaction of reinforcement is catalyzed by the chloride ions although these take no part in the reaction itself. The potential differences between different parts of the reinforcement cage cause electrons to be released from the most negative areas.

This in turn creates positive areas, i.e. ferrous ions, which dissolve into the pore solution. The electrons that were released are consumed by electrolyzing water and oxygen to make hydroxide ions.

The areas where ferrous ions are created are known as anodes or corrosion sites. The areas where electrons are released to form hydroxyl ions are called cathodes, i.e. non-corroding sites.

In cathodic protection a new anode material is introduced into the concrete, which is artificially made more negative than any part of the reinforcement.

This highly negative material releases it’s excess electrons. These released electrons are spread to all areas of reinforcement where

they electrolyze water and oxygen to make hydroxide ions.

The anodic reaction on the

new anode reverses this

process to convert

hydroxideions back into

water and oxygen.

Cathodic protection systems can be grouped into two basic types. impressed current systems galvanic, or sacrificial anode, systems.

The main difference between the two is that impressed current system uses an external power source with inert anodes and

Sacrificial anode system uses the naturally occurring electrochemical potential difference between different metallic elements to provide protection.

Methods of Cathodic Protection

Sacrificial anode cathodic protection

The galvanic cathode protection system is called a sacrificial anode cathodic protection system because the anode corrodes sacrificially to protect the structure.

No external power source is needed with this type of system.

Galvanic anodes are usually made of either magnesium or zinc because of these metals’ higher potential compared to steel structures.

Sacrificial Anode Structure

Sacrificial Anode

(Galvanic)

much less inert object (that is, with much more negative potential, such as a magnesium anode) is placed adjacent to the structure to be protected, and a metallic connection (insulated wire) is installed between the object and the structure, the object will become the anode and the entire structure will become the cathode.

Uses and applications

Generally used for protection of well coated areas where protective requirements and soil or water resistivities are low.

Where the surface area of a protected structure is relatively small due to economic restriction.

No independent source of electric power required.

Limited affects on neighboring structures.

Extremely simple to install. May be directly fixed to the structure.

Simple additions can be made until the desired effect is achieved.

Anode connections are also protected.

Self adjusting but the output generally cannot be controlled

Correct material selection ensures no over protection, thus avoiding metal embrittlement and coating damage.

No possibility of plant damage due to incorrect connections ie. reversed polarity.

Straight forward to install, operate and maintain

Impressed Current Cathodic Protection

Impressed current cathodic protection is achieved by driving a low voltage direct current from a relatively inert anode material, through the concrete to the reinforcing steel.

Typical anodes are titanium

coated with mixed metal oxide

or platinum, silicon iron,

graphite and magnetite.

ICCP Anode

Structure

Uses and application

for structures where protective current requirements and life requirement are high.

Can be used over a wider range of soil and water resistivities. For protection of larger uncoated areas, where relatively few

anodes are required. Requires external power source Can be applied to a wide range of structures in various states

of coating condition.

May be remotely adjusted, monitored and connected to Plant Alarm System.

Anodes are very compact, thus drag and water flow restriction are negligible.

Requires a small number of anodes compared to a galvanic system.

Needs careful design and operation to ensure ongoing protection.

Can affect other structures if not properly monitored. Installation needs to ensure all connections have a high

integrity of insulation and that damage does not occur due to reversed polarity.

Present day use system

Non overlay slotted system Strip system with conventional overlay. Conductive polymer concrete overlay. Conductive coating system. New cathodic protection method for marine concrete.

Non overlay slotted system

this system employs a platinum wire 0.031 in. dia or carbon fibre anode installed in a small slot cut in concrete surface.

This slot is usually ¾ in. deep by ½ or ¼ in. wide. This slots are cut parallel on 10 or 12 in. centers. After the anode is placed it is backfilled with special

conductive polymer grout.

Strip system with conventional overlay

Same as non overlay slotted system but in this method parallel anodes are placed directly on the cleaned concrete surface, no sloted cut provided and covered with a conductive polymer grout.

Conductive polymer concrete overlay

Same as above except the anod wire or fibre filaments are spaced farther apart and the concrete overlay is made of a conductive polymer

Conductive coating system

Conductive coating , which act as contineous distributive anode, require the instalation of a platinum wire to introduce the protective current to the coating (anode).

The platinum wire is embedded in the base coating, and the embedded wire is than covered with wide strip of fine glass fiber screening.

New cathodic protection method for marine concrete structure

this method is highly reliable protection method for concrete structure exposed to salt attacked conditions.

Such as splash zone and structure situated in tidal zone.

The new method was developed using anode panels together with an injection material applied to gaps between the panels and concrete surface.

FRP (glass fiber reinforced resin) and GRC (glass fiber reinforced cement mortar) used as anode.

Design of cathodic protection

Initial considerations

A survey to be made before design. This survey is often combined with a study to establish economic justification for the recommended anti-corrosion proposal while the principal data necessary for design (chemical and physical) are also collected.

If the structure already exists, measurement of existing structure-to-soil potentials is essential to give valuable information as to which areas are anodic and which are cathodic.

Design of a cathodic-protection system for a new structure should include the calculation of

System design Current density requirement. Current distribution. Zones. Anode selection. Cabling Test and monitoring facilities.

System design

Two most important factors for the design of a CP system is Current density requirement and current distribution path.

Current density Unfortunately there is very little or no specific written

information in national and international standards.

Concrete society technical report no.37 says take current density between 10 and 20 mA/m2.

Environment surrounding steel Reinforcement

• Alkaline, no corrosion occurring , low oxygen supply•Alkaline, no corrosion occurring , exposed structure•Alkaline, chloride present, dry, good quality concrete, high cover, light corrosion observed on rebar•Chloride present, wet, poor quality concrete •High chloride level, high oxygen level, hot severe corrosion on steel, low cover

Current densitymA per M2

0.11-33-7

8-2030-50

Current distribution :Highest level of current should be injected at the location of the active corrosion.

Basic Requirements for Cathodic Protection

A galvanic system requires:

i. Sacrificial anodes

ii. Direct welding to the structure or a conductor connecting the anode to the structure

iii. Secure and minimum resistance connections between conductor and structure, and between conductor and anode.

An impressed-current system requires:

i. Inert anodes (clusters of which, connected together often in a backfill, are called the “ground bed”).

ii. A DC power source.

iii. Electrically well insulated, minimum resistance and secure conductors between anodes and power source.

iv. Secure and minimum resistance connections between power source and structure.

Requirements

Electrical Continuity The resistance of the conductor and structure should be such as to

minimize the potential drop of the return protective currents through the structure.

CoatingsThe provision of a protective/insulating coating to the structure will

greatly reduce the current demanded for cathodic protection of the metallic surface.

Increases the effective spread of cathodic protection current. A combination of applying both a coating and cathodic protection

will normally result in the most practical and economic overall protection system.

Ideal coatings are those that have a high electrical resistance, are continuous and will adhere strongly to the surface to be protected.

Other desirable coating characteristics include; stability in the environment, abrasion resistance, and compatibility with the alkaline environment created or enhanced by cathodic protection.

Test facilitiesIt is important to consider the location of test facilities,

test stations, corrosion monitoring coupons, permanent half cells (reference electrodes), and the manner that data can be routinely collected or viewed

Types of equipment

Various galvanic anode alloys of magnesium, aluminium or zinc are available in a variety of block, rod or wire forms.

These alloys are cast around steel inserts to enable fixing of the anode and to maintain electrical continuity and mechanical strength towards the end of the anode life.

The insert may be directly welded or bolted to the structure to be protected, or anodes may be connected to the structure by means of an insulated lead, usually of copper

Impressed-current ground beds in soils have traditionally consisted of high-silicon cast iron. However, mixed metal oxide (MMO) anodes are becoming increasingly popular for all environments because characteristics and compact size of their good mechanical and electrical

There are automatic control units available that will adjust current output in accordance with potential changes for current supply.

Various Anode Systems

Surface monitored anode system without overlaysThese anode systems are mounted on the surface of the

concrete and do not require a cementitious overlay. Generally, these anode systems are not used in high wear applications.

Conductive mastic systemsThese consist of a conductive coating with embedded

anodes on the surface of concrete. They are used on vertical surfaces, ceilings, and columns.

Plate type systemsTheses consist of manufactured anode plates that are glued

to the concrete surface.

Surface mounted anode systems with overlaysThese anode systems are generally used

on horizontal surfaces and require a cementitious overlay of 0.5’’ (13mm) minimum thickness.

Mesh type noble metal anodesA mesh of a noble metal anode is fixed

to the concrete with a multiplicity of pins and then covered with a cementitious material.

Saw slot anode systemsA series of small depth and width saw

slots is made in the concrete surface. The slots are filled with a noble metal anode and a conductive polymer concrete

Embedded anode systems The anode system is embedded in the

surface of the concrete or at the level of the reinforcing in new construction.

Conductive polymer concrete strips A series of conductive polymer

concrete strips containing a noble metal anode is fixed to the concrete surface and covered with a cementitious overlay.

New construction Anodes can be placed at the level of

the reinforcing during new construction.

However, care must be taken to prevent contact between the anode and the reinforcing steel.

Repair Methodology - Embedded galvanic anodes

Purpose of repairEmbedded galvanic anodes reduce the corrosion activity of the

reinforcing steel in the vicinity of the installed anode.Anodes are installed in areas of the concrete where there is a high

likelihood of corrosion occurring or recurring

When to use this methodFor repairs in either chloride-contaminated or carbonated concrete,

embedded galvanic anodes can be incorporated in the repair to minimize corrosion of the reinforcing steel adjacent to the repair.

Embedded galvanic anodes can also be attached to reinforcement at the interface of new and existing chloride-contaminated concrete

During concrete condition inspections, areas of potentially active corrosion of the reinforcing steel are often discovered in mechanically sound concrete.

Embedded galvanic anodes can be installed in these areas to delay corrosion damage to the concrete.

These anodes can be installed on a grid pattern over a large area to provide protection for reinforcing steel in concrete that is found to be or is suspected to be contaminated

Repair procedure

Anode spacing in either repair type is often determined by the

engineer, and differs for each situation. Spacing of the anodes is mainly a function of steel density and the corrosiveness of the environment. Structures with heavy reinforcement or structures in highly corrosive environments often require closer spacing for the anodes to function effectively.

EMBEDDED ANODES INSTALLED IN STANDARD REPAIRS

•All deteriorated concrete should be removed from around and behind the reinforcing steel inside the repair area in accordance with good concrete repair practice

•Sufficient clearance between the anode and the substrate concrete should be provided (minimum of 3/4 in. [19 mm] or 1/4 in. [6 mm] larger than the nominal maximum size of the coarse aggregate used in the repair material, whichever is greater).

Deteriorated concrete removed

The exposed reinforcing bar in the repair area should be thoroughly cleaned and at least the visible surfaces should be cleaned to a bright metal surface to facilitate good electrical connections where the anodes are attached.

Prior to installation, electrical continuity of the reinforcing bar within the repair area should be confirmed with the use of a DC ohm meter

Checking continuity of reinforcing steel

Anode spacing is as specified by the engineer, with the anodes placed along the perimeter of the repair area. Each anode should then be securely connected to the reinforcing steel.

If less than 1 in. (25 mm) of cover exists, the anode should be placed beneath the bar (away from the surface of the concrete)

Tying in anode

•Once installed, the electrical connection between the anode and the reinforcing steel should be confirmed. •The resistance of the electrical connection should be less than 1 ohm.• Finally, the patch cavity is filled with a compatible repair material, using normal patching procedures and taking care to completely encase the anode.

Confirming connection to reinforcing steel

Embedded anodes installed in sound concrete

Reinforcing steel in the area of the desired installation should be located and marked on the concrete surface.

Based on the location of the reinforcing steel, the anode location should be marked, and a hole of appropriate size should be drilled to accommodate the anode

Locating reinforcing steel

Coring hole for anode

A location for connection of the anode to the reinforcing steel should then be marked, drilled if necessary, and a connection made, either within the original hole or in a secondary hole.

Continuity of the reinforcing steel in the location of installation should be verified with a DC ohm meter.Secondary hole with reinforcing steel connection

All holes should be cleaned of debris and dust. The anode should be securely connected to the reinforcing steel, and the contact should be confirmed using the DC ohm meter. Connection resistance should be less than 1 ohm

Confirming connection to reinforcing steel

Anode placed in hole with patching material

Any connections between dissimilar metals (such as copper wires to steel) should be sealed with silicone or a two-part epoxy to prevent localized corrosion.

The drilled hole(s) can then be filled using the appropriate repair material

Advantages

The main advantage of cathodic protection over other forms of anti-corrosion treatment is that it is applied simply by maintaining a DC circuit and its effectiveness may be monitored continuously.

Cathodic protection is commonly applied to a coated structure to provide corrosion control to areas where the coating may be damaged.

It may be applied to existing structures to prolong their life.

It is most suited to remedial works on chlorine-contaminated reinforced concrete structures. The chloride contamination within the concrete tends to reduce its resistivity of the concrete and, therefore, will increase the driving voltage necessary to supply adequate current flow to provide cathodic protection to the reinforcement.

Limitations

Cathodic protection will not replace corroded steel At the present time, cathodic protection is not recommended

for general usage on prestressed concrete structures. Hydrogen evolution at the cathode surface may, on high-strength steels, result in hydrogen embrittlement of the steel, with subsequent loss of strength. On some high strength steels, this may lead to catastrophic failures.

Post tensioned structures may be cathodically protected after an analysis by a corrosion engineer to determine the material of the sheathing and reinforcement, and the electrical continuity of the reinforcement

The reinforcing steel must be electrically continuous for the cathodic protection to function. In structures where the reinforcing is covered with an inorganic coating such as epoxy, a determination of the electrical continuity of the reinforcing must be made before considering the use of cathodic protection.

Cathodic Protection involves the transport of positively charged alkali ions towards the reinforcement, which in theory could result in local concentrations of alkali. This could be detrimental in concretes containing aggregates which are susceptible to ASR.

The chemical reactions occurring at the reinforcement may result in a reduction in the bond strength.

The electrochemical reaction at the anode surface generates acid products. It has been suggested that excessive evolution of acid causing softening of the concrete surface has been responsible for the failure of some anode systems.

Cathodic Protection of Steel in ConcreteEdited By: Paul Chess, Gronvold and Karnov

Cathodic Protection – An Overview By: Cathodic Protection Co. Limited, UK

Sp 102-8 By: j. lehmann

Sp 170-58By: T.fukute, M. Takewaka

Installation of Embedded Galvanic AnodesReported by ACI Committee E 706

References

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