Cathodic protection is used to prevent corrosion of reinforcing steel in concrete structures. There are two main cathodic protection methods: sacrificial anodes and impressed current. Sacrificial anodes made of metals like zinc or aluminum corrode instead of the steel. Impressed current uses an external DC power source connected to anodes to supply current and protect larger structures. Both methods make the steel cathode and prevent corrosion by driving electrochemical reactions. Cathodic protection is widely used to extend the life of reinforced concrete infrastructure.

1. CATHODIC PROTECTION OF
REINFORCED CONCRETE
STRUCTURES
PREPARED BY
Ravi Kumar
Ankan Bairagi

2. 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.

3. 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.

4. Fig:Chloride attack And Spalling
Ref: Report on Cathodic Protection of reinforced concrete structure at
www.cathodic.co.uk

5. 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”.

6. 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

7. Contd.

8. 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

9. 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.

10. REF: http://www.cedd.gov.hk/eng/about/organisation/images/pm/image007.jpg
Accessed on 19-09-2013.

11. Contd.
Elecrochemical
Method
Cathodic
Protection
Sacrificial
Anodes
Impressed
Current

12. 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”

13. 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.

15. 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

16. Pourbix Diagram

17. Sacrificial Anode Facts
Alloy Base
Weight 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

18. 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

19. Cathodic Protection-Galvanic System
Ref: http://www.wv4c.org

20. 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.

21. 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.

22. Impressed Current System
Ref: http://www.wv4c.org

23. 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

24. 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

25. Contd.
 Placement of Anode on reinforcement Bar
Cage.
Ref: http://faculty.kfupm.edu.sa/CE

26. Contd.
• The White Patches visible on the ship’s hull are Zinc
Block sacrificial anodes.
Ref:www.usmma.edu/life/sea/images/seayear1.jpg

27. 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

28. 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.