Vibhanshu Ppt

8/3/2019 Vibhanshu Ppt

1/21

CATHODIC PROTECTION TECHNIQUES

SEMINAR ON

PREPARED BY :

VIBHANSHU KOHLI

ROLL NO. : 909

CLASS : B.E

IV, METALLURGY


2/21

CORROSION:

Destruction or deterioration of a material because of reaction with its environment.

Reaction can be chemical or electrochemical in nature.

It involves loss of electrons i.e. oxidation and gain of electrons i.e. reduction.


3/21

It reduces metal thickness leading to loss of mechanical strength and structuralfailure or breakdown.

It can cause hazards or injuries to people arising from structural failure or

breakdown (e.g. bridges, cars, aircraft).

It reduces value of goods due to deterioration of appearance. It contaminates fluids in vessels and pipes.

Perforation of vessels and pipes allowing escape of their contents and possible

harm to the surroundings.

Hence, losses due to corrosion are very high. So, corrosion and its prevention is

a problem of great importance.

WHY CORROSION PREVENTION IS ESSENTIAL ?


4/21

DIFFERENT TECHNIQUES OF CORROSION CONTROL :

1. Proper design

2. Use of high purity metals

3. Use of alloy additions

4. Use of special heat treatments5. Use of inhibitors

6. Cathodic protection

7. Use of protective surface coatings

8. Environment control


5/21

CATHODIC PROTECTION TECHNIQUES :

Very old and effective method to prevent corrosion. It is capable of completelypreventing corrosion.

PRINCIPLE :

It is accomplished by placing a metal that is higher in electro potential series

with the metal which is to be protected.

Hence , the metal which acts as an anode protects the metal which acts as

cathode and itself gets corroded. Hence, it is called sacrificial anode.

Example: Galvanized iron


6/21

GALVANIC SERIES

Magnesium

Zinc

Aluminum

Cadmium

Iron or Steel

Lead

Tin

Nickel

Brass

Copper

Bronze

Stainless Steels

Silver

Titanium

Graphite

GoldPlatinum

Corroded End,

Anodic or less noble or

more active

Protected end,

Cathodic or most noble or

less active


7/21

The first reported practical use of

Cathodic protection is generally

credited to Sir Humphrey Davy in the

1820s.

He investigated the corrosion of

copper sheeting used for cladding the

hulls of naval vessels and found that he

could preserve copper in seawater

by the attachment of small quantities ofiron, zinc or tin.

The copper became cathodically

protected.

HISTORY :


8/21

TECHNIQUES OF APPLYING CATHODIC PROTECTION :

Cathodic protection mat be achieved in the following ways :

1) By the use of a sacrificial anode.

Example : Application of galvanic zinc

a) Zinc metalizingb) Zinc rich paints

c) Hot dip galvanizing

2) By the use of impressed current from an external source.


9/21

1. USE OF A SACRIFICIAL ANODE :

Sacrificial anodic protection occurs when a metal

is coupled to a more reactive(anode) metal.

This connection is referred to as galvanic

coupling.

A metal which is more anodic is connected to a

metal which is to be protected.

The metal to be protected is less anodic i.e. acts as

cathode.

The anode in this case is called sacrificial anode

since it is consumed during the protection of the

structure.Electrolyte


10/21

ANODIC MATERIALS:

Commercial sacrificial anodes include zinc, magnesium and aluminum . Theyshould have sufficient potential difference so that they can supply electrons when

connected.

Zinc is used for Cathodic protection in fresh water and marine water. It can also

be used in low resistivity soils. It is used to protect mechanical components of the

ship, coastal power plants, heat exchangers, etc.

Magnesium anodes are most widely used for underground soil applications.

They are used to protect underground pipelines.

Aluminum anodes are less active than Zn and Mg anodes. They are used for

protection of offshore structures, interiors of tanks.


11/21

CATHODIC PROTECTION OF STEEL PIPES :

Zn, Mg, Al are more electronegative than steel. Hence they supply electrons to

the more electropositive steel.


12/21

PROTECTION OF UNDERGROUND PIPES :

Mg anodes are most widely used for

protection of underground pipes.

Mg acts as anode and protects the

underground pipes.

PROTECTION OF SHIPS :

Galvanic anodes are mounted on the

steel fixtures.

Aluminum or zinc hulls are placed on

the steel fixtures.


13/21

GALVANIC ZINC APPLICATION :

1) Zinc Metalizing (plating)

Feeding zinc into a heated gun, where it is melted and

sprayed on a structure or part using combustion gases or

auxiliary compressed air.

2) Zinc-rich Paints

Zinc-rich paints contain various amounts of metallic zinc

dust and are applied by brush or spray.

3) Hot-dip Galvanizing

Complete immersion of steel into a bath of molten zinc.


14/21

2. USE OF IMPRESSED CURRENT FROM AN EXTERNAL SOURCE :

External source of direct current power is connected (or impressed)

between the structure to be protected and the metal which acts anode.

The anode used is surrounded by backfill consisting of coke breeze,

gypsum or bentonite.

A rectifier is used for the purpose of providing direct current.

Half-wave rectifier Full-wave rectifier


15/21

PRINCIPLE:

The metal connected directly to the positive terminal of the power

supply behaves as an anode while the metal to be protected is connected

to the negative terminal of the power supply.

The anode gets corroded and the cathode is protected.

The metal acting as anode is replaced once it gets corroded completely.

Impressed current anodes must be corrosion resistant and durable in

environment. Commercially used impressed current anodes are

platinum, graphite, lead alloy, high silicon cast iron, etc.


16/21

CATHODIC PROTECTION OF AN UNDERGROUND TANK/PIPE USING IMPRESSED CURRENTS :

The backfill helps to improve electric contact between the electrode and the surrounding

medium(soil).


17/21

COMPARISON BETWEEN SACRIFICIAL ANODE AND IMPRESSED

CURRENT CATHODIC SYSTEMS :

SR. NO.

1.

2.

3.

4.

5.

6.

7.

SACRIFICIAL ANODE SYSTEM

Simple setup

Low maintenance

Works best in conductive or

low resistive electrolytes

Lower installation costs

Fixed driving voltage andcurrent

No external power is required

It does not cause stray current

corrosion

IMPRESSED CURRENT SYSTEM

Complex setup

High maintenance

It can work in low conductive

electrolytes

High installation costs

Adjustable driving voltage andcurrent

External power is required

It can cause stray current

corrosion .


18/21

The main advantage of cathodic protection over other forms of anti-corrosion

treatment is that it is applied simply by maintaining a dc circuit and its effectiveness

may be monitored continuously.

Economically feasible to replace anode system when required.

It is applied to existing structures to prolong their life.

Its main use is to protect steel structures buried in soil or immersed in water.

Structures that are commonly protected by cathodic protection

are the exterior surfaces of:

Pipelines, Ships hulls, Storage tank bases, harbour structures, Steel sheet, etc

Cathodic protection is also used to protect the internal surfaces of:

Large diameter pipelines, Ships tanks, Storage tanks , Water-circulating systems

ADVANTAGES AND LIMITATIONS OF CATHODIC PROTECTION :


19/21

LIMITATIONS:

Significant operation and maintenance requirements.

Relatively large chance of premature failure or breakdown.

Possibility of stray current causing interference damage to other metallic

structures

.

It cannot be used to completely prevent atmospheric corrosion on

metals.

There is a possibility of stray current corrosion.

Anode metals are required to be replaced frequently.


20/21

REFERENCES :

1. Fontana, M., and N. Greene, Corrosion Engineering, McGraw-Hill Book

Company (1967).

2. Garverick, L., Corrosion in the Petrochemical Industry, ASM International

(1994).

3. Davies, K., and R. Kean, Cathodic Protection, DTI publication (1981).

4. Scannell, W., and A. Sohanghpurwala,Cathodic Protection as a corrosion

control alternative, CONCORR Inc. (1993).

5. Langill, T.,Corrosion Protection: Basic corrosion theory and protection

methods,American Galvanizers Association (2006).


21/21

THANK YOU

Date post:	07-Apr-2018
Category:	Documents
Upload:	devashish-joshi
View:	219 times
Download:	0 times

Vibhanshu Ppt

Documents