Cathodic Protection

Cathodic Protection
Base metal: Metal to be protected- will act as cathode

Active Metal: metal which is made anode
Noble metal: which will always act as anode
Sacrificial Anode
Immersed current cathodic protection
CORROSION
CONTROL
METHODS
CATHODIC
PROTECTIO
N
SACRIFICIAL IMPRESSED
ANODE CURRENT
IMPRESSED CURRENT CATHODIC
PROTECTION
 For larger structures, galvanic anodes can’t 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 source.
 This current is given to insoluble anode like graphite, stainless steel

or scrap iron buried in soil.
 The negative terminal of DC is connected to pipeline to be

protected. The anode is kept in back fill to increase the
electrical contact with the surrounding soil.
Impressed Cathodic
Cathodic Protection Protection
Example of Impressed
Current Installation
Applications of Impressed
Current
 Applied on steel in seawater or soil
 Oil Platforms in steel and concrete
 Subsea Pipelines
 Hull
 Quay structures and sheet pile
curtains
 Concrete bridges placed in seawater
 Pipelines buried in soil
 Vessels/tanks buried in soil
SHIP
S
OFFSHORE
PLATEFORMS
Factors affecting
ICCP
1.Temperature of Sea water
2.Total wetted surface area
3.Salinity
4.Speed of the ship
IMPRESSED
CURRENT
Advantages:
 High driving voltage (30 V)
(potential difference limited only by the size of
power supply).
 Economically feasible when installed on
existing structures.
 Few anodes – reduced resistance
 Large current available, even in very high
resistivity electrolytes
Can provide sufficient current to protect very large, poorly
coated or uncoated structures
 Economically feasible to replace anode system when
Disadvantages:
 Significant operation and maintenance requirements.

 Relatively large chance of premature failure or breakdown.
 Possibility of stray current causing interference damage to
other metallic (foreign) structure
 Vulnerable components
 Need for regulation/control system
 Risk of overprotection of highly charged materials
 Coating damages
 Need for/recommended protection shield around the anodes
STRAY CURRENT
CORROSION
• Stray current corrosion refers to corrosion resulting from
stray current
• the current flowing through paths other than the intended
circuit. ALSO CALLED stray current electrolysis and electrolytic
corrosion.
Stray current is a dynamic and transient phenomenon
Corrosion ONLY occurs at the point of discharge (POD).
At the point of entry (POE) and along the entire system that carries the
stray current, there is NO corrosion.
At the point of entry, stray current provides certain degrees of cathodic
protection to the metallic structure.
 different from other (non-stray current) types of corrosion
 stray current corrosion is an electrolysis process.
 In the electrolysis process, the external current (stray
current) alone drives metal atoms into electrolyte as water-
soluble ions.
 The environmental factors such as oxygen concentration,
chloride, and pH that are so critical to natural corrosion
processes are no longer relevant.
 The extent of damage or loss of metal is directly
proportional to the magnitude of stray current leaving the
structure at the point of discharge.
 Stray current corrosion will be concentrated at certain location
that leads to the lowest electrical resistance in the current
circuit.
Structures affected by stray current corrosion
All metallic structures buried or immersed are susceptible to
stray current corrosion.
DC railways, tunnels, underground pipelines, and storage

tanks without CP systems are particularly susceptible to stray
current corrosion.
Stray current corrosion tends to be localized at locations

where the stray current leaves the metal structure and enters
the electrolyte (soil/concrete/water).
Prevention of stray current corrosion
•Identify the source of stray current
•Stop the leakage from the intended circuit by maintaining good

electrical connections and insulation.
•Install impressed cathodic protection system to offset the effect

of stray current.
ANODIC PROTECTION
Anodic protection (AP) is a technique to control the corrosion of a

metal surface by making it the anode of an electrochemical cell and
controlling the electrode potential in a zone where the metal is
passive.
PROCESS
1.Simply anodic protection is based on the formation of protective film on
metals by externally applied anodic currents.
2.It appears that the application of anodic current to a structure should tend to
increase the dissolution rate of a metal and decrease the rate of hydrogen
evolution.
3.This usually does occur except for metal with active-passive transition such as
Ni,Fe,Cr,Ti and their alloys.
4.If carefully controlled the anodic currents are applied to these materials they
are passivated and the rate of metal dissolution is decreased.
• controls the voltage
between difference
WorkingElectrode
and
a Reference Electrode.
• Both
a electrodes are contained in an
electrochemical cell.
• The potentiostat implements this
control by injecting current into the
cell through an Auxiliary, or Counter,
electrode.
• measures the current flow between
the Working and Counter electrodes.
• The controlled variable in a
potentiostat is the cell potential and
the measured variable is the cell
current.
To anodically protect a structure a device called a potentiostat is
required.
Potentiostat: a potentiostat is an electronic device that maintain a metal at
a constant potential with respect to a reference electrode.
1. The potentiostat has three terminals one connected to the tank, another
to an auxiliary cathode (a platinum clad electrode ) and the third to a
reference electrode.
2. In operation, the potentiostat maintain a constant potential between the

tank and the reference electrode.
3. The optimum potential for protection is determined by

electrochemical measurements.
If an active-passive alloy such as
stainless steel is maintained in
the passive region through an
applied potential (or current)
from a potentiostat, its initial
corrosion rate (icorr) can be
shifted to a low value at ipass as
shown in this Figure.
Applied anodic current density =

oxidation current density –
reduction current density.

Cathodic Protection

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Cathodic Protection

Base metal: Metal to be protected- will act as cathode

 In this method, an impressed current is applied in opposite

 ICCP systems use anodes connected to a DC source.

 This current is given to insoluble anode like graphite, stainless steel

 The negative terminal of DC is connected to pipeline to be

 Significant operation and maintenance requirements.

DC railways, tunnels, underground pipelines, and storage

Stray current corrosion tends to be localized at locations

•Stop the leakage from the intended circuit by maintaining good

•Install impressed cathodic protection system to offset the effect

Anodic protection (AP) is a technique to control the corrosion of a

2. In operation, the potentiostat maintain a constant potential between the

3. The optimum potential for protection is determined by

Applied anodic current density =

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