Cathodic Protection

1. CATHODIC
PROTECTION

2. Facts
• 1 Ton of steel rusts every 90 sec
• Energy required to produce 1 ton of steel
can be consumed in a household family for
3 months
• 50% steel produced worldwide is to replace
rusted steel

3. CORROSION
• Loss of metal due to its interaction with
environment.
• Corrosion is an electrochemical process.
• The corrosion is a complex process which
depends on so many factors such as
physical ,chemical, metallurgical,
electrochemical and thermodynamics .

4. FACTORS WHICH INFLUENCE CORROSION
RESISTANCE
CORROSION RESISTANCE
ELECTROCHEMICAL
PHYSICAL
CHEMICAL
METALLURGICAL
THEMODYNAMIC

5. CORROSION
• THE LESS NOBEL METAL BECOME ANODE (POSITIVE ELECTRODE)
WHILE MORE NOBLE METAL ACT A CATHODE( NEGATIVE
ELECTRODE)
• ANODIC REACTION – DISSOLUTION OF METAL M++ + 2e-
• CATHODIC REACTION – DEPOSITION OF IONS N++ + 2e- N
• REACTION AT ANODE WHICH CAUSES DISSOLUTION OF METAL
LEADS TO CORROSION OF METAL M

6. Corrosion Cell
electrical path
electrolyte (soil)
cathode anode
Conventional current

7. e-
e-
e-
cathode
e-
e-
e-
Iron anode
Electron migrations (e-
)
Conventional current flow ( + to –v)
e-
e-
H2
H+
e-
e-
H2
H+
Fe(OH)2 OH-
Fe+
Fe(OH)2 OH-
Fe+
HOW CORROSION TAKES PLACLE

8. • At Anode:
• 2H2O (OH)-
+H +
+ 2e-
FE +
+ 2(OH) -
FE(OH)2
FE(OH)2+ FE +
+ (OH) -
FE2(OH)3
At Cathode
2e-
+ H + H(atom)
H +H plarisation film

9. • Corrosion potential – Natural potential of object
with no protection or interference current present
• Polarization -A non ohmic component that will
apparently exist very close to electrode/ electrolyte
surface of the cathode and anode
• Polarization films are very important factor in
controlling the amount of current flow . In one
sense , the film of hydrogen formed on the surface
of cathode may be thought of as an insulating
layer which introduced ohmic resistance and
reduce the flow of ohmic current

10. CORROSION: degradation of material,
unintended attack from environment
(electrochemical mechanism)
Requirements:
1. Anode (oxidation)
+ve for electrolytic cell, -ve for galvanic cell
2. Cathode (reduction)
-ve for electrolytic cell, +ve for galvanic cell
3. Electrolyte
4. Electrical path between anode and cathode

11. • The amount of metal that will be removed is
directly proportional to the amount of current flow
• Effective resistance in the anode cathode circuit
goverens the amount of current flow in the
corrosion cell which is then the function of
resitivity of soil & size of anodic and cathodic
area
• Polarization film formed is important in
controlling the amount of current flow
• Depolarization effect tend to remove the hydrozen
polarized film either by mechanical effect or by
supply of dissolved oxygen

12. Factors affecting corrosion
1. Activation energy/polarisation
2. Concentration polarisation (concentration
of ions near electrode)
3. Resistance polarisation (resistance to flow
of current, e.g., painting, coating, etc).
Localised corrosion is more critical than uniform
corrosion

13. TYPES OF CORROSION
CELLS ON PIPELINE
• DISSIMILAR METAL CORROSION
• CORROSION DUE TO DISIMILAR SOIL
• DIFFERENTIAL AERATION
CORROSION CELL
• NEW & OLD PIPE

14. Corrosion can also occur due to
• Difference in aeration (crevice corrosion)
• Contact with a foreign body
• Due to stress (stress corrosion), e.g., mild
steel in alkaline environment

15. Control of Corrosion
• Coating
• Cathodic protection
• Corrosion inhibitors

16. Cathodic Protection
A TECHNIQUE TO REDUCE THE
CORROSION OF A METAL SURFACE
BY MAKING THAT SURFACE THE
CATHODE OF AN
ELECTROCHEMICAL CELL
… NACE

17. Corrosion takes place at the
anode only !
• Anode: the electrode from which current
leaves to the electrolyte
Anode gets corroded
• Cathode: the electrode where current is
collected from the electrolyte
No corrosion takes place at the cathode

18. IMPRESSED CURRENT GROUND BEDS
Insulated
header cable
Cable
trench
Insulating joint of
header cable & pigtail
of anode
High silicon cast
iron anode
Carbonaceous
backfill material,
well temped
Angured hole
for anode &
backfill

19. Impressed Current CP
• Requires external power supply
• TRU output can be controlled (automatic /
manual)
• Can be used in high resistivity soil, can
protect uncoated structures
• Wide choice on anode material; anodes
have long life

20. Anode
Protected Pipe Pipe
CP unit
Anode Cable
Cathode Cable
TYPICAL CP UNIT
+
-
Ground bed
Anodic Area

21. Half cell
• Copper / copper sulphate (used for land
PSP)
• Silver / silver chloride (used in marine
environment)
• Hydrogen (used in laboratory)
• Calomel (used in laboratory)

22. On Potential
on potential
PSP
corrosion potential (550 mV)
days 0 2 4 6

23. CURRENT
MEASUREMENTS
-V+
-V+ ++ +
+
B
AA A
Buried Pipeline- Use of two electrode method

24. Foreign Pipelines close to Cathodic Protection Ground Bed and crossing
Protected line
Rectifier
Remote GB
Influence of ground bed
surrounding foreign line
Current flow from foreign structure
to protected line in crossing area
Foreign line

25. Foreign Pipelines close to Cathodic Protection
Ground Bed but not crossing
Protected line
Rectifier
Remote GB
Foreign line
Current discharge from
foreign line
+
-

26. Pipe-to-soil potential
measurement
PSP
pipe earth
copper
electrode
CuSO4
solution
porous plug

27. FOREIGN LINES CROSSING BARE
CATHODICALLY PROTECTED LINE
Foreign line
Influence of GB
P/Line
Foreign line tends to
become positive to soil.
Current picked up by foreign
line in electrically remote
sections and discharge to the
protected line in the crossing
area
Most intense damage to foreign line

28. PEARSON TECHNIQUE
 
NB:
1Receiver indicates maximum as operator A passes directly over defect.
2Receiver indicates full reading when both operators are equidistant from defect.
3Receiver indicates maximum as operator & passes directly over defect.
Electrically connected
Coating defect
soil
transmitter
Operator A Operator B
Receiver
EarthOperators use cleat shoes

29. CLOSE INTERVAL PIPE TO SOIL POTENTIAL SURVEYS (CIPS

Pipeline under investigation
Trailing wire
Long Half Cell
Mounting
Board
Back pack

30. Protection criteria
NACE RP-0169-96
• A negative 850 mV PSP w.r.t. Cu/CuSO4
reference electrode with CP current applied
• A negative 850 mV polarised PSP w.r.t.
Cu/CuSO4 reference electrode
• A minimum 100 mV cathodic polarisation
• A net protective current
• 300 mV shift (not per NACE)

31. IR Drop in On Potential
power supply
PSP
half cell
anode
pipe
IR drop

32. Off Potential
IR drop
PSP 100 mV min
corrosion potential
time 0

33. On-Off PSP Survey
• Current interrupters are used
• on:off time = 4:1
• Off potential should be measured at least
after 0.5 sec.
• Off period should be less than 3 sec to
avoid depolarisation.

34. Current Interrupters and
Synchronisation
• GPS
- Costly
• SCADA
- Not available everywhere
• Quartz clock
- Can loose synchronisation by 0.5 sec/day

35. Sacrificial System CP
• No external power requirement
• Suitable in low resistivity soil / marine
environment only
• Choice on anode material is limited, e.g.,
magnesium, zinc, aluminium, etc.;anodes
get corroded fast;
• Can protect well coated structures only,
protected area is small

36. CP Design
• Impressed current system
• Sacrificial anode system
• Hybrid system

37. Designing CP Installation-
impressed current
• Establish soil resistivity
• Estimate total current requirement
• Select suitable anode material
• Calculate anode bed size, shape and
configuration / calculate total mass of anode
for design life
• Select location of CP / test stations
• Consider facilities required for control
monitoring

38. Span of protection for CP
Stations
CP stn 1 CP stn 2 CP stn
3
l l l l
pipeline

39. Required Protective Current
• 2I0 = 2πDJL
I0 = current to one side of CP station(A)
D = diameter of pipeline( m)
J = protective current density(A/m2)
L = spread of protection on one side of CP
station

40. SPREAD PROTECTION
• 2L= 8 U L/ 3.14 D J R’
I0 = current to one side of CP station(A)
D = diameter of pipeline( m)
J = protective current density(A/m2)
L = spread of protection on one side of CP
station
R’= Resistance of the pipe (ohm/m)
^UL= 0.30 Volts(max . Potential diff
between drain point and end protection)

41. Types of Anode Bed
• Horizontal anode bed
Lower resistance
R = (ρ/2πL)ln(L2
/tD)
• Vertical anode bed
R = (ρ/2πL)ln(2l/d×(4t+3l)½
/(4t+l) ½
)

42. Sacrificial Anode Materials
• Zinc Alloy (C-Sentry)
Sea water, low resistivity soil
• Aluminium Alloy (Galvalum I, Galvalam
II, Galvalum III, Alanode)
Sea water
• Magnesium Alloy (Galvomag)
soil

43. Anode Life
L = Ecm/I
L = anode life
E = efficiency
c = capacity(Ah/Kg)
m = anode weight (kg)
I = anode output current(A)

44. Anode Life
L = Ecm/I (HOURS)
L = anode life
E = 50% say for Mg anode, 90% for Zn
C = 2200Ah/Kg for Mg & 820 Ah/ Kg Zn
m = anode weight (kg)
I = anode output current(A)

45. Impressed Current Anode
Materials
• Platinum and platinised metals
• High silicon iron
• Lead-platinum
• Lead-silver
• Graphite
• Iron
• Cast iron

46. GALVANIC ANODE CURRENT OUTPUT
The output current from a sacrificial anode can be
calculated by Ohm’s law as follows:
• Ia = ^V / R
• Ia = anode out current (A)
• V= driving voltage (V)
• R = Current resistance
• V is the voltage difference between the potential of the
cathodically protected surface and the potential of the
anode. The circuit resistance comprises resistance of
anode to earth, resistance of protected structure to earth
and resistance of cables and structure itself.

47. Anode Bed
anode lead
anode
backfill
canister

48. Backfill
• Effective size of anode increases
• Anode loss partly shared by bedding
material
• Porous surrounding for gases to escape
Carbonacious backfill – coal coke breeze
Calcined petroleum coke breeze

49. Ground Bed
– A system of anode beds
• Swallow / deep ground bed
• Horizontal / vertical type ground bed
• Distributed / closely spaced ground bed
• Close / remote ground bed

50. CP Field Surveys
• Pearson survey
• Current attenuation Test (CAT) survey
• Direct Current Voltage Gradient survey
• Close Interval Potential Logging (CPL)
survey

51. Sources of Power Supply
• AC power grid
• Solar power
• Wind power
• Thermo electric generator
• CCVT (closed circuit vacuum turbine)

52. Controls
• DC output control
Manual output voltage control
Constant voltage control
Constant current control
Constant PSP control
• Multi reference control

53. Annunciations
• Reference fail
• Overprotection
• Under protection

54. Stray Current Interface
Stray current: current through path other
than the intended circuit
Sources:
– DC traction system
– Geomagnetic / telluric current
– HVDC interference
– EHV AC interference

55. Anodic Interference
power supply
anode design current p/l
stray current
metallic object

56. Cathodic Interference (more
critical)
power supply
anode design current p/l
stray current
metallic
object

57. Geomagnetic Current
• Also called telluric current
• Caused by the effect of solar winds
• Varies from time to time

58. Safe Potential
Etouch = Ib×{Rb+Rfeet}/√duration of shock
Rb = 1000 Ω human body (adult)
Ib = Safe current 15mA let go
117mA ventricular fibrillation

59. CP of Offshore Structures
• Resistivity is extremely low (16-40 Ω-cm)
• CP current requirement is very high
(thousands of amperes)
• Depolarisation due to temperature, oxygen,
sea currents
• Steel corrodes at the rate of 0.13 mm/year
under laboratory conditions

60. Galvanic Anode material
• Aluminium
Low driving potential
Extensively used for marine applications
• Magnesium
Low efficiency
Can affect rapid polarisation of offshore
structures in combination with aluminium

61. Galvanic Anode Material
• Zinc
Low driving potential
High efficiency
Suitable for low resistivity soils, fresh water
and marine applications (widely used)
Protection of ship hulls

62. Choice of Half Cell
• Silver / Silver Chloride half cell is most
common
0.8V w.r.t. Ag/AgCl electrode in aerated
seawater, 0.9V under anaerobic conditions
• Zinc / Zinc Chloride half cell can also be
used
• Cu/CuSO4 half cell will polarise due to the
presence of chloride ions

63. Thank You