cathodic protection of reinforced concrete structures
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CATHODIC PROTECTION OF REINFORCED CONCRETE
STRUCTURES
PREPARED BY
Ravi Kumar
Ankan Bairagi
Introduction
Concrete is a porous (microstructure) material, which readily absorbs contaminants from the surrounding environment.
The water and oxygen in presence of the chlorides reacts with iron to create corrosion product on surface of the reinforcing steel.
The growth in volume of these corrosion products exerts tensile stress that eventually causes the concrete to crack.
Contd.
Corrosion of the Reinforcement steel is main cause for the deterioration of steel which is usually initiated through chloride attack, or carbonation. Carbonation: it refers to the penetration of
atmospheric carbon dioxide into concrete matrix. To prevent the corrosion in steel Cathodic
protection method is most widely used worldwide in the developed countries.
Fig:Chloride attack And SpallingRef: Report on Cathodic Protection of reinforced concrete structure at www.cathodic.co.uk
CORROSION
Metal that has been extracted from its primary ore (metal oxide or other free radicals like sulphides, chlorides) has a natural tendency to revert to a native form under the action of oxygen and water.
The process by which this transformation occurs is known as corrosion. It is an electrochemical process. The change from the metallic to the combined forms occurs by an “anodic reaction”.
Contd..
The Anode Reaction Metal Metal ions (dissolved in solution) + electrons Common example:
Fe Fe++ + 2e-
This reaction produces free electrons , which pass within the metal to another site on the metal surface (Cathode), where it consumed by cathodic reaction. The Cathodic Reaction
O2 + 2H2O + 4e- 4OH-
Recombination of these ions at active surface produce following reaction.
2Fe + O2 + 2H2O = 2Fe (OH)2
Contd.
Methods to Prevent Corrosion
Barrier Separation
Paints
Polymer coating
Galvanizing Process
Polymer Injection
Cladding and Chemical Additives
Electrochemical Method (Cathodic Protection)
Sacrificial Anodes
Impressed Current
What is Cathodic Protection (CP) ?
Cathodic protection is a technique used to control the corrosion of a metal surface by making it the cathode of an electrochemical cell.
Principle- The Principle of cathodic protection is in connecting an external anode to metal to be protected and the passing of an electric DC current so that all areas of metal surface become cathodic and do not corrode.
REF: http://www.cedd.gov.hk/eng/about/organisation/images/pm/image007.jpgAccessed on 19-09-2013.
Contd.
Elecrochemical Method
Cathodic Protection
Sacrificial Anodes
Impressed Current
Sacrificial Anode
• In usual application a Galvanic anode, a piece of
more electrochemically active metal is attached
to vulnerable metal surface where it is exposed
to the corrosive liquid. The anode metals
gradually dissipate over a period when
connected to and in a common electrolyte, and
are termed as “Sacrificial Anodes”
Contd.In the case of aluminum anodes, the reaction at the aluminum surface is: (four aluminum ions plus twelve free electrons)
4Al = 4AL+++ + 12 e-
And at the steel surface: (oxygen gas converted to oxygen ions which combine with water to form hydroxyl ions).
3O2 + 12e- + 6H20 = 12OH-
As long as the current (free electrons) arrives at the cathode (steel) faster than oxygen is arriving, no corrosion will occur.
Contd.
Galvanic anodes are selected because they have a more "active" voltage (more negative electrochemical potential) than the metal of the target structure (typically steel)
The galvanic anode continues to corrode, consuming the anode material until eventually it must be replaced.
Anode Material: Alloys of Zinc, Magnesium and Aluminum
Pourbix Diagram
Sacrificial Anode Facts
Alloy BaseWeight Loss per annum (kg/amp)
Practical Life
(Years)Efficiency Use
Magnesium 8 5 50% RC Pipelines
Aluminum 4 10 90% Sea Water
Zinc 12 20 95% Coating
Contd.
Partially corroded sacrificial anode on the hull of a ship. Ref:http://chemwiki.ucdavis.edu/@api/deki/files/6987/800px-Sacrificial_anode.jpg?size=bestfit&width=462&height=346&revision=1
Impressed Current Method
In this method a potential difference is artificially created by applying an external positive voltage to an anode with the negative current return path through the reinforcement cage and cabling.
ICCP system use a nodes connected to DC power source. Usually this will be a cathodic protection rectifier, which converts an AC power supply to DC output.
Contd.
Anode Material can be inert or at least capable of offering up to a nominal a 20 years life
This current given to insoluble anode like graphite stainless steel or scrap iron buried in soil.
The negative terminal of D.C is connected to a pipeline to b protected. The anode is kept in back-fill.
Impressed Current System
Ref: http://www.wv4c.org
Comparison of CP System
Impressed Current System
Sacrificial Anode
Longer Anode Life, Monitoring Required
Simple, No Monitoring & Control
Current can be controlled No requirement for electrical isolation
Can be used in any resistivity environment
Usually Used in lower resistivity electrolyte
Can Be Used for any type structure
Used for very small or well coated structure
Higher Cost Lower Cost
Application
Pipelines are routinely protected by a coating supplemented with cathodic Protection.
Ref: https://upload.wikimedia.org/wikipedia/commons/f/f1/Anodes-on-jacket.jpg
Contd. Placement of Anode on reinforcement Bar
Cage.
Ref: http://faculty.kfupm.edu.sa/CE
Contd.
• The White Patches visible on the ship’s hull are Zinc Block sacrificial anodes.
Ref:www.usmma.edu/life/sea/images/seayear1.jpg
References
K G C Berkeley, S Pathmanaban, “Cathodic Protection of Reinforcement Steel in Concrete”, Butterworth & Co. (Publishers) Ltd, 1990,
Alan R Bird, “Cathodic Protection of reinforced concrete Structures-A Practical Methods of Arresting Rebar Corrosion” Report published at WWW.marineandcivil.com.au, pp 1-5
Baeckman, Schwenck & Prinz, Handbook of cathodic corrosion protection” 3rd edition 1997 ISBN 0-88415-056-9
Contd.
• Dr. S.R Karade & Team, Corrosion Mitigation in RC structures Through Cathodic Protection, CSIR-CBRI News Letter,Vol-32 No-1 Jan-March 2012,pp. 1-2
Denny A.Jones, “Principles and Prevention of Corrosion” 2nd Edition, Prentice Hall, ISBN 0-13-359993-0
V Ashworth, “Principle of Cathodic Protection” Elsevier B.V, 3rd Edition, Volume-2, pp 10:3-10:28
Ref: Cathodic Protection of Reinforced Concrete Structures’, in NACE Technical Report No. 36, 1989, The Concrete Society, London.
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