1. Definition of Corrosion
Corrion is a destructive attack of a metal by chemical or electrochemical reaction with its environment.

2. Types of Corrosions Uniform Attack Galvanic Corrosion
Crevice Corrosion
Intergranular Corrosion
Stress Corrosion
High Temperature Corrosion

3. Uniform Attack
Uniform attack is a form of electrochemical corrosion that occurs with equal intensity of the entire surface of the metal. Iron rust when exposed to air and water, and silver tarnishes due to exposure to air.

4. Galvanic corrosion
Galvanic corrosion occurs when two metals having different composition are electrically coupled in the presence of an electrolyte. The more reactive metal will experience sever corrosion while the more noble metal will be quite well protected. Perhaps the most infamous examples of this type of combination such as steel and brass or copper and steel.

5. Crevice Corrosion
Crevice corrosion is consequence of concentration difference of ions or dissolved gases in an electrolytic solution. A solution became trapped between a pipe and the flange on the left. The stagnant liquid in the crevice eventually had a lowered dissolved oxygen concentration and crevice corrosion took over and destroyed the flange.

6. Intergranular Corrosion
Occurring along grain boundaries for some alloy, the heating of the some material causes chromium carbide to form from the chromium and carbon in the metal. This leaves a chromium deficient boundary just shy of the where the metal was heated for welding.

7. PREVENTION OF CORROSION
By Material Selection
By Environmental modification
By Corrosion inhibitors
By Paints
By Coating
By Cthodic protection
By Interaction between the above methods

8. Principle of Cathodic Protection
Cathodic Protection is to make the potential of the whole surface of the steel structure sufficiently negative with respect to the surround medium to ensure that no current flows from the metal into the medium.

9. Method of Cathodic Protection
Sacrificial Anode System
Impressed direct current System

10. Sacrificial Anode System
With the Sacrificial anode method, use is made of galvanic action to provide the cathodic protection current. The surface of the structure is made by connecting it electrically to mass of less noble metal buried or immersed in to common electrolyte.

11. Sacrificial Anode Cathodic Protection
Metallic Conductor
Current
PIPE LINE
PIPE LINE
Sacrificial Anode
ZINIC
Protected pipe

12. Impress Direct Current System
Impress direct current method, the structure is connected to the negative of a direct current power supply and the positive of the supply is connected to a number of anodes buried in ground or impressed in water.

13. Impress Current Cathodic Protection System
Direct Current Source
Current
Metallic Conductor
PIPE LINE
Ground Bed Anode
Protected pipe

14. THE ELECTROMOTIVE SERIES
Electrodes Electrode Potential, volt
Base End or Anodic end
Sodium
Magnesium
Aluminum
Zinc
Chromium (Trivalent)
Iron (divalent)
Nickel
Tin
Lead
Hydrogen
Copper (divalent)
Copper (monovalent)
Silver
Platinum
Gold (monovalent)
Noble End Cathodic End

15. THE GALVANIC SERIES Anode - Corroding End Least Noble,
Magnesium
Aluminum
Zinc
Cadmium
Steel
Cast iron
Chromium (Active)
Stain less Steel (Active)
Soft Solder
Tin
Lead
Nickel
Brass
Bronze
Copper
Silver solder
Chromium (Passive)
Silver
Graphite
Gold
Platinum
Anode - Corroding End
Least Noble,
Electro - negative
Cathode - Protected End
Most Noble,
Electro - Positive

16. Types of Anodes Graphite Anode High Silicon cast iron Anode
Mixed- metal oxide Anode
Platinum coated Anode
Lead alloy Anode

17. Types of ground Bed Deep Ground Bed Ground bed more then 50feet deep.
Shallow Ground Bed
Ground bed up to 50 feet depth

18. Types of backfill Standard metallurgical coke breeze
Petroleum coke backfill
Natural or manufactured graphite

19. Corrosion cell due to dissimilarity of metals
Protective current to old pipe-Large Cathode
Old pipe
Old pipe
New pipe
Corrosion current to new pipe- Small Anode

20. RESISTIVITY
A resistor is an element that resist the movement of electrical charge. The unit of resistance is the ohm. A resistance of one ohm will allow one ampere to pass when one volt of potential difference is applied. Electrical resistivity (ρ) has the ohm-cm and is physical property of the material R= ρ L/A
R = resistanenc (ohm)
L = length (cm) of the current flow path
A = area (cm2) perpendicular to current flow path
ρ = Resistivity (ohm-cm)

21. SOIL RESISTIVITY
The resistivity of the soil( electrolyte) largely determine the magnitude of corrosion current in any corrosion cell. Thus lower the resistivity the greater would be the intensity of the corrosive attack. And if cathodic protection is required, the soil resistivity will determine the magnitude of current generated by sacrificial anode or the resistance to ground of impressed current anode.

22. Cathodic Protection
A technique to prevent corrosion of a metal surface by making it the cathode of an electrochemical cell