cathodic protection

Presented by
*Abhishek mishra
*Arunn kumar
*Manish kumar
Corrosion control methods

Cathodic protection

Sacrificial Impressed
anode current

Applications

What is Cathodic protection?
 Cathodic protection (CP) is a method of corrosion
control that can be applied to buried and submerged
metallic structures.
 It is normally used in conjunction with coatings and
can be considered as a secondary corrosion control
technique.
 Cathodic protection can, in some cases, prevent stress
corrosion cracking.

Principle involved

 The principle of cathodic protection is to prevent
anodic sites occurring on the structure under
protection by allowing the anodic reactions to occur
on specially designed and installed anodes

HISTORY:
 Cathodic protection was first described by Sir Humphry Davy in
a series of papers presented to the Royal Society in London in
1824.
 Thomas Edison experimented with impressed current cathodic
protection on ships in 1890, but was unsuccessful due to the lack of a
suitable current source and anode materials.
 In the USA by 1945 the use of CP was commonly applied to the
rapidly expanding oil and natural gas industry. In the UK CP was
applied from the 1950s onwards and Cathodic Protection
Company Limited was established in this period, pioneering it’s
use in the UK.
 CP is now well established on a large variety of immersed and
buried metallic structures as well as reinforced concrete
structures, and provides effective corrosion control.

Discription:
 The simplest method to apply CP is by connecting the metal to be
protected with another more easily corroded "sacrificial metal" to
act as the anode of the electrochemical cell.
 The sacrificial metal then corrodes instead of the protected metal.
For structures where passive galvanic CP is not adequate, for
example in long pipelines, an external DC electrical power source is
sometimes used to provide current.
 Cathodic protection systems are used to protect a wide range of
metallic structures in various environments. Common applications
are; steel water or fuel pipelines and storage tanks such as home
water heaters, steel pier piles; ship and boat hulls; offshore oil
platforms and onshore oil well casings and metal reinforcement
bars in concrete buildings and structures. Another common
application is in galvanized steel, in which a sacrificial coating of
zinc on steel parts protects them from rust.
 Another method of protection impresses a small direct current on
a structure.

Why is it important?
 Corrosion costs money.
 Corrosion of metals costs the USA economy almost
$300 billion per year and it is estimated that one third
of this value could be saved with better selection of
corrosion prevention techniques, including cathodic
protection.
o Our country has been losing around Rs 1.52 lakh crore
every year owing to corrosion in various sectors,
including infrastructure, utility services, production &
manufacturing, and defence & nuclear waste.

Why does corrosion occur?
 The corrosion of metals, in particular steel in an aqueous
environment (which can be either soil or water), occurs because
the metal interacts with the local environment.
 In the case of steel, man has mined iron ore and processed it into
steel. However due to certain characteristics of the steel, it is
not ‘stable’ once in contact with an aqueous environment and
interacts with the local environment in an attempt to return to
its naturally occurring state. This process is corrosion.

Place diagrams depicting corrosion

Basic Reactions of corrosion
 The basic process at an anodic site is the release of iron (Fe)
from the steel surface into the environment and can be
expressed as:
Fe  Fe2+ + 2e-
 During the process two electrons (2e-) are generated which
must be consumed by the environment (in aerated
systems) and can be expressed as:
 4H+ + O2 + 4e- 2H2O
 A summary of the above half reactions can be expressed as:
2Fe + 2H2O + O2 2Fe(OH)2
 The term Fe(OH)2 is iron oxide which can be oxidized to
form the red-brown Fe(OH)3 commonly referred to as rust.

How does it work?

 Simply CP works by preventing the anodic reaction of
metal dissolution occurring on the structure

In the usual application, a galvanic anode, a piece of a more electrochemically
"active" metal, is attached to the vulnerable metal surface where it is exposed to the
corrosive liquid. Galvanic anodes are designed and selected to have a more "active"
voltage (more negative electrochemical potential) than the metal of the target
structure (typically steel). For effective CP, the potential of the steel surface is
polarized (pushed) more negative until the surface has a uniform potential.

Metals like (Zn,Al,Mg) are used for making anode because they have very low
electrochemical potential as compared to steel hence more ‘active’.
These metals act as anode and get corroded.

For this purpose of increasing electrical contact, the active metal is placed in back fill
(coal and NaCl).
When it is consumed completely then replaced by a newer one.

Insulated wire
Ground level

backfill

Underground
pipeline Sacrificial anode
( Zn, Al)

Impressed Current (ICCP)
 For larger structures, galvanic anodes cannot economically deliver enough
current to provide complete protection.
 In this method,an impressed current is applied in opposite direction to
nullify the corrosion current and convert the corroding metal from anode
to cathode.
 ICCP systems use anodes connected to a DC power source. Usually this will
be a cathodic protection rectifier, which converts an AC power supply to a
DC output. In the absence of an AC supply, alternative power sources may
be used, such as solar panels, wind power, etc.
 This current is given to insoluble anode like graphite, stainless steel or
scrap iron burried in soil.
 The negative terminal of D.C. is connected to pipeline to be protected.The
anode is kept in back-fill(composed of gypsum or coke breeze) to increase
electrical contact with the surrounding soil.

Applications
1.Piplines:
Pipelines are routinely protected by a coating
supplemented with cathodic protection.

2.Ships:
Cathodic protection on ships is often
implemented by galvanic anodes attached to the
hull, rather than using ICCP.

3. Marine:
Marine CP covers many areas, jetties, harbors,
offshore structures.

4.Internal CP:
Vessels, pipelines and tanks which are used to
store or transport liquids can also be protected
from corrosion on their internal surfaces by the
use of cathodic protection.

5. Galvanized steel:
Galvanizing generally refers to hot-dip
galvanizing which is a way of coating steel with a
layer of metallic zinc.

Problems arised due to cp:
 Production of hydrogen ions
A side effect of improperly applied cathodic protection is the production
of hydrogen ions, leading to its absorption in the protected metal and
subsequent hydrogen embrittlementof welds and materials with high
hardness.
 Cathodic Disbonding
This is a process of disbondment of protective coatings from the
protected structure (cathode) due to the formation of hydrogen ions
over the surface of the protected material (cathode)
o Cathodic Shielding
 Effectiveness of cathodic protection systems on steel pipelines can be
impaired by the use of solid film backed dielectric coatings such as
polyethylene tapes, shrinkable pipeline sleeves, and factory applied
single or multiple solid film coatings.
 Protective electric current from the cathodic protection system is
blocked or shielded from reaching the underlying metal by the highly
resistive film backing.

o Safety
Rectifier safety has recently become an industry
concern for cathodic protection technicians and
personnel who are responsible for rectifier
operation, maintenance and repair. While
rectifiers are manufactured according to national
electrical codes and standards and inspected by
authorities, the basic rectifier designs have not
changed significantly over the past 25 years in
regards to safety.

cathodic protection

Recommandé

Recommandé

Contenu connexe

Tendances

Tendances (20)

Similaire à cathodic protection

Similaire à cathodic protection (20)

Dernier

Dernier (20)

cathodic protection