CP

1. Cathodic Protection
Chapter 14
Instructor:
Dr. Md. Easir Arafat Khan
Associate Professor
Department of Chemical Engineering, BUET, Dhaka-
1000
July 2022
ChE 409

2. How to prevent or minimize corrosion?
• changing the metallic material to another one
• altering the environment making it less aggressive
• separating the metallic material from the environment
• changing the electrode potential of the metal/alloy
• appropriate design at the design stage

3. Methods of corrosion protections
Design Change
of Metal
Change of
Environment
Change of
Electrode
potential
Use of
Coating
Cathodic
Protection
Anodic
Protection

4. Principle of Cathodic Protection

5. Historical background
• 1824: Sir Humphry Davy proposed cathodic protection to British ships
• 1829: Edmund Davy successfully protected iron buoys in the sea by attaching
zinc blocks
• Lately Cathodic Protection (CP) has been used to protect thousands of miles
of buried pipelines and cables.
• Other CP applications are to submarines, water tanks, canal gates, marine
piling, offshore oil/gas drilling structures, etc.

6. CP cannot be used
• Above the waterline and in vapour
• In non-conducting liquids, such as oil
• In electrically screened areas
• In extremely corrosive environments. Although theoretically possible,
power costs would be too high and hence CP would be impractical

7. CP with sacrificial anodes

8. Sacrificial anode – domestic hot water
tank

9. CP with sacrificial anodes
• Advantages
• no external power is needed. Hence, this method can be used in remote
rural areas with no power supply lines.
• low cost of installation
• low maintenance cost
• Disadvantages
• limited current output
• limitations of soil resistivity

10. CP by impressed current

11. Use of insulating coating
• to improve current distribution
• total current needed is lower
• one anode can protect longer pipeline

12. Use of backfill
• Backfill is used to reduce the resistivity of the nearby soil
• A backfill may consist of approximately 20% bentonite, 75% gypsum
and 5% Na2SO4. Bentonite is an inorganic colloid used for moisture
retention.

13. Economics of CP
• For underground and underwater pipelines, CP is the least expensive
method offering equal protection
• CP has made it possible to transport oil and natural gas (at high
pressure) across thousands of miles of land or Sea
• With CP system working, enables the pipeline design engineer to
specify thinner pipe wall as long as that can withstand the pressures.
Corrosion allowance need not be provided. This saves lot of money.

14. Examples of CP use
• Gates of canals (e.g. Panama canal gates)
• All natural gas pipelines in Bangladesh and elsewhere
• All modern ships (sacrificial anode). Some ships of Canadian Navy
have been protected by impressed current CP.

15. Anodic protection

16. Revisiting anodic polarization curve

17. Protection of acid storage tank

18. Some facts
• Anodic protection can be applied only to metals/alloys which show active-
passive behaviour and are readily passivated when anodically polarized
• Contrary to cathodic protection, corrosion rates are never reduced to zero
• In anodic protection, throwing power is rather high.
• Anodic protection cannot be applied in environments containing Cl- ions
• If protection breaks down due to failure of electrical system severe pitting may
take place

19. Applications
• Carbon steel equipment vs. oleum, sulfuric acid, super phosphoric
acid.
• Stainless steel equipment vs. sulfuric acid, phosphoric acid, mixed
sulfuric and nitric acid.

20. Cathodic vs anodic protection