Cathodic protection of reinforced concrete structures


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Cathodic protection of reinforced concrete structures

  2. 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. 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. 4. Fig:Chloride attack And Spalling Ref: Report on Cathodic Protection of reinforced concrete structure at
  5. 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. 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. 7. Contd.
  8. 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. 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. 10. REF: Accessed on 19-09-2013.
  11. 11. Contd. Elecrochemical Method Cathodic Protection Sacrificial Anodes Impressed Current
  12. 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. 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.
  14. 14. 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
  15. 15. Pourbix Diagram
  16. 16. 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
  17. 17. Contd. Partially corroded sacrificial anode on the hull of a ship. Ref: Sacrificial_anode.jpg?size=bestfit&width=462&height=346&revision=1
  18. 18. Cathodic Protection-Galvanic System Ref:
  19. 19. 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.
  20. 20. 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.
  21. 21. Impressed Current System Ref:
  22. 22. 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
  23. 23. Application  Pipelines are routinely protected by a coating supplemented with cathodic Protection. Ref:
  24. 24. Contd.  Placement of Anode on reinforcement Bar Cage. Ref:
  25. 25. Contd. • The White Patches visible on the ship’s hull are Zinc Block sacrificial anodes.
  26. 26. 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, pp 1-5  Baeckman, Schwenck & Prinz, Handbook of cathodic corrosion protection” 3rd edition 1997 ISBN 0-88415-056-9
  27. 27. 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.