1. Cathodic Protection System
WELCOME TO A
PRESENTATION ON
Corrosion Process
&
Cathodic Protection System
Presented By:
Sri Rajib Kumar Sarmah
Dy.SEE(Gen)
Date: Monday, April 22, 2019
Venue:
Free Powerpoint Templates

2. Overview of Presentation
What is Corrosion?
Harmful effects of corrosion
Components of a corrosion cell
Commonly Affected Structures
Factors Affecting Corrosion Cell Formation
Corrosion Prevention Methods
Coating Types  
Fundamentals of Cathodic Protection
Galvanic CP or Sacrificial Anode CP
Impressed Current CP System
Arrangement of Anodes for Impressed Current CP
Interference on Pipe Lines
Cathodic Protection of Storage Tanks

3. What is Corrosion?
Corrosion is the deterioration of materials by chemical interaction with their environment. The term corrosion is sometimes also applied to the degradation of plastics, concrete and wood, but generally refers to metals.
Metallic Corrosion: A process in which a metal is destroyed by a electrochemical reaction (oxidation) forming an oxide layer on the metal surface.
This process requires that the metal surface be exposed to oxygen, and is favoured in the presence of water.
In the case of iron and steel, corrosion is often referred to as rusting.

5. Some of the major harmful effects of corrosion can be summarised as follows:
Reduction of metal thickness leading to loss of mechanical strength and Structural Failure or breakdown. Considerable weakening may result from a small amount of metal loss.
Hazards or injuries to people arising from structural failure or breakdown (e.g. bridges, cars, aircraft, ships etc.)
Loss of time in availability of costly & dependable industrial equipment.
Reduced value of goods due to deterioration of appearance.
Contamination of fluids in vessels and pipes .
Perforation of vessels and pipes allowing escape of their contents and possible harm to the surroundings.
Loss of technically important surface properties of a metallic component.
Mechanical damage to valves, pumps, etc, or blockage of pipes by solid corrosion products.
Cost of equipments increased which needs to be designed to last for prolonged life.

6. Transco Pipeline failure 14th Sept, 2008 in Appomattox Virginia, US
Gas cloud ignited producing a large fireball and resulting in a 11m wide, 4.5m deep crater and a burn zone of 340m in diameter. Failure mainly due to corrosion.
10m section of pipe was blown out.

7. Mexico (Guadalajara) Sewer Explosion due to Corrosion
killed 215 people in Mexico, in April Besides the fatalities, the series of blasts damaged 1,600 buildings and injured 1,500 people
The first cause of the disaster was a galvanized steel pipeline that was close up in a humid environment with a steel gasoline pipeline. Both of them corroded, and gasoline leaked through the holes and got contaminated into the main sewer.

8. Bhopal Gas Tragedy
Bhopal is probably the site of the greatest industrial disaster in history.
Union Carbide India Limited (UCIL in Bhopal, was licensed to manufacture phosgene, mono-methylamine , methyl-isocyanate and the pesticide carbaryl.
On the night of the 2-3 December 1984 water inadvertently entered the methyl-isocyanate storage tank, where over 40 metric tons of MIC were being stored.
The addition of water to the tank caused a runaway chemical reaction, resulting in a rapid rise in pressure & temperature and the tank exploded.
It is been estimated that at least 3000 people died as a result of this accident, while figures for the number of people injured currently range from 200,000 to 600,000.

9. Components of a corrosion cell
Anode
–At anode metal goes into solution as metal ions (oxidation reaction) and Metal loss or corrosion occurs .
Cathode
– At cathode metal deposition or reduction of gases occur s(reduction reaction)
- Little or no corrosion occurs at the cathode
Return Circuit/Metallic Path
– Provides a path for electrons to flow, between the anode and cathode
Electrolyte
– Ionized solution capable of conducting electricity

10. Components of a corrosion cell
Anodic sites have a more negative potential than cathodic sites in the same electrolyte.
Current flow through the electrolyte is due to ion movement & current flow through the metal is due to movement of electron.
Thus any corrosion phenomenon is associated with passage of D.C. Current from anode to electrolyte.

11. CORROSION REACTIONS
 AT ANODE (-ve terminal) M  M +z + Ze ( z– valency of metals)
For Iron
Fe  Fe++ +2e
Fe H-Fe (OH)2
4Fe (OH)2 + O2 + 2H2 O  4Fe (OH) (in presence of excess oxygen)
 Fe(OH)3  FeOOH+H2O (Hydrated Ferric oxide, Red Rust)
Or Fe(OH) 2 + CO2  Fe(OH)(HCO3) (Ferrous Hydroxyl Carbonate)
AT CATHODE (+ve terminal)
  (Acid) H + +2e  H2
(Aerated wet acidic soil) O2+ 4H+ + 4e  2 H2O
(Neutral sea water or Basic solutions) O 2+ 2H2O+ 4e  4OH -

12. Galvanic Series
Example: Connecting magnesium to copper will produce a corrosion cell with a potential of about 1.5 volts.
METAL VOLTS (CSE)‏
Commercially Pure Magnesium
Magnesium Alloy
Zinc
Aluminum Alloy
Commercially Pure Aluminum
Mild Steel (clean & shiny) to -0.80
Mild Steel (rusted) to -0.50
Cast Iron (not graphitized)
Lead
Mild Steel in Concrete
Copper, Brass, Bronze
High Silicon Cast Iron
Carbon, Graphite, Coke

13. Commonly Affected Structures
Buried Piping
Storage Tanks - Above Ground / Underground
Reinforcing Steel in Concrete
Steel Piles
Ships/Boats
Structural Steel Towers

14. Corrosion Type
Uniform or near uniform - Corrosion attacks all areas of the metal at the same or similar rate.
Localized - Some areas of the metal corrode at different rates due to heterogeneities in the metal or environment. This type of attack can approach pitting.
Pitting - Very highly localized attack resulting in small pits that may penetrate to perforation.
Erosion: Removal of surface material by the action of numerous individual impacts of solid or liquid particles
Stress cracking: Service failures in materials that occur by slow environmentally induced crack propagation

15. Factors Affecting Corrosion Cell Formation:
Cell Caused by Different Soil Resistivity
Cell Caused by Different Concentrations of Oxygen
Cell Caused by Different Concentrations of soil water
Cell Caused by Non-Homogeneous Soil

16. Factors Affecting Corrosion Cell Formation:
Cell Caused by Concrete and Soil Electrolytes
Galvanic Corrosion Cell Caused by Different Metals
Corrosion Cell Caused by Old and New Steel
Corrosion Cell Caused by Stressed and Scratched Surfaces

18. Methods to Control Corrosion
Use of Corrosion resistant materials (plastic, stainless alloys, fiberglass).
Use of the same or similar metals per the galvanic series.
Altering the environment (utilizing homogeneous high resistivity backfill or inhibitors).
Utilize coatings and linings that electrically insulate the structure from the electrolyte (paints, plastic films, etc).
Use of Cathodic Protection.

19. Corrosion Prevention Methods for Metal against Electrolyte Corrosion
Coating
Painting of surface by Anti-corrosive primers. Primers may be lead-based or lead-free zinc rich primer types.
Finishing with paints/enamels.
Coating by Coal Tar Enamel, Fusion Bonded Epoxy (FBE), Polyethylene or Polyurethane are primary methods of external corrosion protection.
Cathodic protection
Protection by coating alone is not recommended due to rapid attack of metal at coating holidays (areas of coating defects).
Application of coating drastically reduces cathodic protection current requirement, since C.P. Current is required at defect areas of coating as well as preventing degradation in healthy areas.

20. Coating Types  
2. Fusion Bonded Epoxy (FBE)
Powder Coating
1. Internal Lining
Internal coating using a two component liquid epoxy based paint.
4. Bitumen / Asphalt Enamel (AE) Coating
3. Dual Fusion Bonded Epoxy (D-FBE ) coating
FBE primer (first layer), FBE topcoat (top layer

21. Coating Types 5. Three Layer Polypropylene (3LPP) Coating
6. Three Layer Polyethylene (3LPE) Coating
7. Concrete Weight Coating (CWC)
8. Polyurethane Insulation Coating

22. 1824: Earliest practical use of cathodic protection
Sir Humphrey Davy’s work on protecting the copper sheathing on wooden hulls in the British Navy by sacrificial zinc or iron anodes is generally considered to be the earliest example of practical cathodic protection.

23. Fundamentals of Cathodic Protection
Cathodic Protection (CP): Minimize corrosion by utilizing an external source of electrical current (DC).
The method consists of supplying electrons from external source to the corroding metal so as to convert all anodic sites of the corroding metal to cathode where by only reduction reaction occurs by consuming the supplied electrons.
Cathodic protection being electro-chemical technique, arrests all forms of corrosion (Uniform Attack, Galvanic, Pitting, Stress Corrosion Cracking etc excepting H2 damage.
The magnitude of external current is such as to depress the metal potential to negative side to prevent dissolution of metal and current direction being opposite to corrosion current.

24. Galvanic CP or Sacrificial Anode CP
When metals, which are more electronegative than protected metal (such as magnesium or zinc) are placed in the same environment and kept in contact with the protected metal (such as steel,) a current flows from the more active anode (zinc) to the noble cathode (steel), protecting the cathode.
Anodes Materials Used for Sacrificial Anode CP
Sacrificial anodes have fixed driving voltage to protected metal which is in range of volts.
Magnesium – It is often used in soil to protect small electrically isolated structures, such as underground storage tanks, and well coated pipelines.
Zinc – It is often used in marine environments. They are commonly found on boats.
Aluminum – It can be used for a variety of marine applications.

25. Galvanic Sacrificial Anode-Installation

26. Keys to obtaining enough cathodic protection
Determination of amount of current required
Theoretical calculations based on coating quality and environment.
Perform physical testing of current requirement.
Calculation of expected from of anode and determination of number of anodes required.

28. Sacrificial Anode CP Advantages : Inexpensive Little Maintenance cost
No external Power source
Robust system, reduced maintenance
Typically work best with electrically isolated structures.
Dis-advantages :
Limited driving voltage of 0.25 V to 1.25 V (driving potential based on the galvanic series)‏
Limited output makes it ineffective when trying to protect large uncoated surfaces.
Require a low resistivity electrolyte (e.g sea water) to function well. Not suited for media with high resistive soil.
Require more no of anodes for protection
Require more quantity of anode material for securing long operating time.

29. Impressed Current CP System Components
Power Supplies
Control Amplifier
Anodes
Metal surface of protected material (Cathode)
Reference Electrode
Objective #4

30. Impressed Current CP System
Advantages:
High driving voltage (30 V to 75 V)
Capable of protecting large steel structures, when designed properly.
Requires less anodes then a galvanic system.
Output can be controlled using a permanent reference electrode, desirable when the electrolyte resistivity is known to change due to seasonal changes.
Disadvantages:
Initial costs can be more expensive
Vulnerable components, such as Anode wires can be susceptible to damage
Requires an external DC power source along with an AC supply.
Need for regulation/control system
Risk of overprotection of highly charged materials
Risk of Coating damages – cathodic disboardment due to O2 release.
Need for/recommended protection shield around the anodes
System requires routine maintenance and monitoring.

31. Impressed Current CP Rectifier
A rectifier converts available AC power to low voltage DC power. Most cathodic protection rectifiers are provided with a means to vary the DC output voltage in small increments, or in some cases offer complete control from zero to 100% of rated DC output.
Controller consist of
D.C. Power source:
Automatic potential controlled transformer-rectifier of rating 25A/ 25V, 50A/ 50V, 75A/75V, 50A/75V, 100A/ 12V etc.
Battery bank with potential controller for uninterrupted cathodic protection.
It Amplifies the difference between level of cathodic protection of the metal as determined from the reference electrode with the desired level of protection, as set by the operator; which in turn controls the amount of current delivered to the anodes.

32. Anodes for Impressed Current CP –
Reference electrode – mounted next to the or through the metal surface to be protected.
Anodes for Impressed Current CP –
Anodes generally used are Platinised Titanium , Platinised Niobium or Platinised Tantalum etc.
Anodes are immersed in the corrosion medium, mounted through walls of tank or suspended from pier.

33. PSP Measurement taken by placing the electrode in the electrolyte and measuring the potential between structure and electrode with a high resistance voltmeter .

34. Requirement of Cathodic Voltage for Protection:
Steel structures exposed to soil, fresh water and sea water are fully cathodically protected when their interface potential to electrolyte is
Minimum volts (-850 mV) w.r.t. Cu/ CuSO4 for soil / fresh water and
Minimum volts (-800 mV) w.r.t. Ag/AgCl in sea water.
For large bare structure drawing high current, V criteria may not be achieved, for such cases minimum 100 mv potential shift is recommended as protection criteria
Potential Shift = Instant ‘Off’ Potential - Natural Pot
(w.r.t Cu/CuSO4) (w.r.t Cu/CuSO4)
This is done by temporarily switching ‘OFF’ the C.P. Current and measure potential within 1 sec from switching off. Potential so measured is termed as instant ‘OFF’ potential (see figure). Typical switching cycle is 12 sec ‘ON’, 3 sec ‘OFF’.

35. Typical Current Requirements for Cathodic Protection of Steel (Bare Surface)

36. Arrangement of Anodes for Impressed Current CP

37. Measurement of Pipe to Soil Potential (PSP)
By Structure to Electrolyte Potential or By Line Current Measurement

39. Pipe line Casing & Casing Insulation Testing
If the two potential measurements are significantly different (over 10 mV), the casing is not shorted to the pipeline.
Under normal conditions, the carrier pipeline
should be at a potential more negative than volts DC, and the casing should be
between approximately and volts DC (a difference of between 200 to 500 mV).

40. Interference On Pipe Lines (Stray Current)
Stray current or interference current can be classified as being static or dynamic.
Stray current can be either AC Current or DC Current

41. Dynamic stray current:
Dynamic Interference
Dynamic stray current:
- Vary with amplitude or change with direction in the direction of current flow
-can be man made or natural origin
SOURCES OF DYNAMIC STRAY CURRENT:
Transit systems (AC or DC Traction System)
Mining
DC welding machines
Electric power transmission
Industrial plants with arc furnaces
Telluric Current (due to Earth Magnetism)

42. DC Interference (Dynamic)

43. DC Interference (Static)

44. DC Interference (Static)

45. AC INTERFERENCE
Pipelines may share a common right-of-way with parallel high voltage AC power lines resulting in mutual interference effects.

46. AC INTERFERENCE
There are three basic method by which ac voltage and current appear in metallic structure near ac power line :
Electrostatic or Capacitive Coupling: Structure acts as one side of capacitor with respect to ground. This is only of concern when the structure is above ground.(i.e pipeline supported on skids , during construction or maintenance of pipeline care to be taken).
Resistive Coupling :
During power line faults to ground, a large AC current can be transmitted to the earth through resistance coupling and then flows on and off the underground structure.
These current which can be several thousand amperes, can cause substantial physical damage to structure coatings, in extreme case where ac density is high, steel piping has been known to melt. Normally these fault occur infrequently and are of short duration

47. Interference due to Electromagnetic Induction:
Structure acts as the single turn secondary of an air core transformer in which over head power line is primary. This type of induction may occur when the structure is either above ground or below ground .
The induced voltage does not directly proportional to power line voltage. Hence, relatively low voltage ac power line can produce electromagnetically induced current.
When a pipeline closely runs parallel to power line for some distance , induced voltage peak would be expected where the pipeline and power line separates.

48. Problems of AC Interference
The Capacitive, Ohmic or Inductive AC Coupling between power lines and pipelines may result in:
Danger to the safety of personnel under normal operation (15 volt limit)
Danger to the safety of personnel under fault conditions
Danger to the pipeline integrity under fault conditions.
Risk of ac-enhanced corrosion under normal operation.
Risk of damage to the coating from electrical stress under fault conditions

49. AC Interference mitigation methods include:
Significant separation between pipe and HVAC system.
Ground pipe with using distributed galvanic anodes.
Ground pipe with using a metal such as Zn, Mg, Steel and Cu with DC De-coupler Device (e.g Capacitor, Polarisation cell). A De-coupler allow to pass ac current and block dc current.
Protecting devices for electrical isolation such as flanges insulation kits or joints.
Step and touch protection systems (gradient mats or grid).

50. INTERFERENCE MITIGATION
Pipeline modification involve:
Installation of insulating joint
Installation of Magnesium anode on the pipeline at the location where pipeline is going into stray current discharge.
If magnesium anode is installed where the pipeline both pick up and discharge stray current, installation of diode with magnesium anode is necessary to assure that anode discharge stray current but do not collect stray current.
Installation of potentially control rectifier and impressed current gound bed where pipeline is going into stray current discharge.

51. DC INTERFERENCE MITIGATION

52. DC INTERFERENCE MITIGATION

53. Cathodic Protection of Over ground Storage Tanks

54. Cathodic System for Existing Tanks
-  Shallow vertical distributed close anodes (local protection).-
- Distributed anodes are arranged around tank periphery at 5-10 m separation from tank periphery (see Figure) 
-  Each bed having 1-2 anodes. 

55. Cathodic System for Existing Tanks

56. Cathodic Protection System for New Tanks
- For new tanks under construction best configuration is distributed anode under the tank bottom plate  
-    Anode to plate separation = 900 mm. 
-     Most uniform current/potential distribution & Minimum current requirement is achieved
-     Minimum voltage gradient across the tank diameter as in case of close distributed anodes around periphery. 
-    Anodes are metal oxide coated titanium mesh/ ribbon or wire anodes in coke backfill laid on sand pad before tank bottom plate is placed in position. 
-  A Dielectric liner is placed below anode to minimize current draw by tank foundation reinforcement steel.
- Current rating of the wire anode is 40mA/ M2

57. Anode Loop

58. Cathodic Protection System for Underground Tanks

59. Impressed Current - Design
Calculate total area to be protected (Ap)
Determine current density (ρ)
Calculate total protection current (Ip)
Calculate total anode needed (N)
Initial
Lifetime
Calculate total anode resistance
(Rtotal = f(N, ρ, d, L, spacing))
Calculate rectifier specification (Vdc, Idc, Pdc = f(Rtot,Ip)

60. Thanking You
Have A Nice Day

61. Corrosion Rate for Uniform Attack :
Corrosion rate expressed as mm /yr or mils/year (MPY) or mg/dm2/day (MDD). The above units represent average rate of metal penetration or weight loss of metal excluding any adherent or non adherent corrosion products.
For Mild Steel in soil rate is = mm/ yr.
For Mild Steel in sea water rate is = mm/ yr.
CLASSIFICATION ON BASIS OF CORROSION RATE:
a)   0.15 mm/yr – EXCELLENT CORROSION RESISTANCE.
b)   mm/yr – GOOD
c)    0.5 – 1.0 mm/yr – FAIR
d)    1.0 mm/yr – UNACCEPTABLE.

62. Resistance of Common Electrolytes
Soils – High resistivity soil reduces the corrosion rate, while low resistivity soil increases the corrosion rate.
CLASSIFICATION
ELECTROLYTE
RESISTIVITY
(ohm-cm)‏
ANTICIPATED
CORROSIVITY
Low Resistance
0 to 2,000
Severe
Medium
2,000 to 10,000
Moderate
High
10,000 to 30,000
Mild
Very High
Above 30,000
Increasingly Less

63. Resistance of Common Electrolytes
Water – Approximate resistivity values
Water Resistivity Ohms-cm
Open sea
Tea water (coastal)
River water ,000
Tap water ,000-10,000
Rain water ,000
Distilled water ,000
Pure water ,000,000