1. Corrosion Fundamentals
BOPINDER (“BOP”) PHULL
Corrosion-Control & Materials-Selection Consultant

2. WHAT IS CORROSION?
CORROSION IS THE DETERIORATION OF A MATERIAL BY REACTION WITH ITS ENVIRONMENT
METALS
POLYMERS
CONCRETE
WOOD
WE WILL BE TALKING ABOUT METALS IN A PIPELINE ENVIRONMENT

3. ENERGY CYCLE OF STEEL REFINING PROCESS CORROSION PROCESS IRON OXIDE
BLAST FURNACE
REFINING
PIPE MILL
STEEL PIPE
PIPE CORRODING
REFINING PROCESS
CORROSION PROCESS
ENERGY CYCLE OF STEEL

4. Natural process which returns metal to its native state
CORROSION OF STEEL
Natural process which returns metal to its native state

5. GOLD in NATURAL ENVIRONMENTS
GOLD is usually found in its stable, metallic state in nature rather than in ore form
Hence resists corrosion in natural environments

6. High input impedance DC voltmeter
Electrode Potential
Potential of Metal measured using a Reference Electrode (RE) and a DC Voltmeter V + -
High input impedance DC voltmeter
Metal
Electrolyte
Reference Electrode
RE (half-cell)

7. Reference Electrodes Various types Examples:
Copper-Copper Sulfate (Cu/CuSO4)
Silver-Silver Chloride (Ag/AgCl)
Saturated Calomel (SCE)
Standard Hydrogen (SHE)
Measured values can be converted from one scale to another

8. Practical Reference Electrodes
Examples:

9. Meters
Digital
Analog

10. Examples of Potential MeasurementV
Buried or submerged pipeline
Storage tank interior
Ship or boat
Rebar in concrete

11. Galvanic Series
Ranking of Metals and Alloys according to their Electrode Potentials
Potentials Measured in Bulk Electrolyte Environments,
e.g. Soil, Water
Measured with respect to a Reference Electrode (half-cell)

12. PRACTICAL GALVANIC SERIES
MATERIAL POTENTIAL in VOLTS (vs. Cu/CuSO4 RE)
PURE MAGNESIUM
MAGNESIUM ALLOY
ZINC
ALUMINUM ALLOY
MILD STEEL (NEW)
MILD STEEL (OLD)
CAST / DUCTILE IRON
STAINLESS STEEL to
COPPER, BRASS, BRONZE -0.20
GOLD
CARBON, GRAPHITE, COKE +0.30
More Anodic or Active end
More Cathodic, Passive or Noble end

13. Practical Galvanic Series of Metals and Alloys in Neutral Soils and Water
Material
Potential (Volts)
vs. Cu/CuSO4
Pure Magnesium
- 1.75
Magnesium Alloy
(Mg-6Al-3Zn-0.15Mn)
- 1.6
Zinc
- 1.1
Aluminum Alloy (5% Zn)
- 1.05
Commercially pure Aluminum
- 0.8
Low-carbon steel
(clean and shiny)
- 0.5 to - 0.8
Low-carbon steel (rusty)
- 0.2 to - 0.5
Gray cast iron
(not graphitized)
- 0.5
Lead
(in uncontaminated concrete)
- 0.2
Copper, Brass, Bronze
High-silicon cast iron
Mill-scale on steel
Carbon, Graphite, Coke
+ 0.3
More Anodic or Active end
More Cathodic, Passive or Noble end

14. Practical Galvanic Series of Metals and Alloys in Flowing Seawater (13 ft/s, 240C )
Material
Potential (Volts)
vs. SCE
Magnesium
- 1.55
Zinc
- 1.03
Aluminum Alloy 3003-H
- 0.79
Ductile Cast iron
- 0.61
Carbon steel
Copper alloy
(Naval brass)
- 0.40
Copper
- 0.36
Copper Alloy
(Admiralty brass)
- 0.29
Type 316 SS (active)
- 0.18
Titanium
- 0.10
Ni-Cr-Mo Alloy C
- 0.08
Type 316 SS (passive)
-0.05
Platinum
+ 0.15
Graphite
+ 0.25
More Anodic or Active end
More Cathodic, Passive or Noble end

15. Galvanic Series
Ranking of Metals and Alloys according to their Electrode Potentials
Potentials Measured in Bulk Electrolyte Environments,
e.g. soil, water
Measured with respect to a Reference Electrode (half-cell)

16. NATURALLY OCCURRING CORROSION
DISSIMILAR METALS
CINDERS (conductive carbon – nonmetal)
DISSIMILAR SURFACES
DISSIMILAR SOILS
DIFFERENTIAL AERATION
STRESS

17. DISSIMILAR METAL CORROSION
Also called Galvanic Corrosion
Different Metals have Different Electrode Potentials
Potential of metal measured using a DC voltmeter and a reference electrode (half-cell) in electrolyte of interest
Galvanic Corrosion Cell created when dissimilar metals are electrically coupled to each other in the same electrolyte
ANODE – Metal with more Active or more Electronegative potential
CATHODE – Metal with more Noble or more Electropositive potential
Potential Difference between the two metals is the driving force the corrosion cell

18. DISSIMILAR METAL POTENTIALS
Open-Circuit Potential (OCP) or Corrosion Potential
METAL - B
METAL - A V V RE
No Current Flow between the Metals

19. DISSIMILAR METAL (GALVANIC) CORROSION
Metals Electrically Connected by Metallic Path
METAL - B
METAL - A V V RE
Current Flow between the Metals.
Metal-A is CATHODE and METAL-B is ANODE

20. CORROSION REACTIONS ANODE:
More Active (i.e. more electronegative) potential
Metal atoms lose electrons and become ions
Electrons flow in metallic path from Anode to Cathode
CATHODE:
More Noble (i.e. more electropositive) potential
Certain species in the electrolyte accept those electrons

21. CORROSION REACTIONS At the ANODE: At the CATHODE: M – 2e- M2+
2 H e H2
(hydrogen ions reduced to gaseous hydrogen, e.g. in acids)
½ O H2O e OH-
(dissolved oxygen reduced to hydroxyl ions, e.g. in neutral-pH waters)

22. CORROSION REACTIONS Corrosion of ANODE is Accelerated
Corrosion of CATHODE is slowed down or stopped
Species produced by reactions at Anode and Cathode may react to form insoluble corrosion products, e.g.,
M OH M(OH)2

23. CORROSION CELL Requirements of a Corrosion Cell
ANODE – where corrosion (oxidation) occurs and metal ions and electrons are generated
CATHODE – where reduction of certain species in the electrolyte occurs by consumption of electrons that were generated at the ANODE
ELECTROLYTE – which allows passage of ionic current
METALLIC PATH – which allows passage of electrons from ANODE to CATHODE

24. Conventional Current vs. Electron Flow
ANODE
CATHODE
Electron flow ( e- )

25. Basic Corrosion Cell

26. SELF-CORROSION Occurs on same metal
E.g. due to local metallurgical differences, species concentration differences in the electrolyte, differences in stresses, etc.
Conventional Current
Electrolyte
(Soil or Water)
Electron Flow
ANODE
CATHODE

27. THE CORROSION PROCESS ANODE ELECTROLYTE CATHODE METALLIC PATH
REDUCTION
O2 + 2H2 O + 4e - ---> 4OH -
OXIDATION
Fe ---> Fe e -
I a
ANODE
e -
METAL
CATHODE
I c

28. Basic Corrosion Cell

29. Corrosion by Cinders

30. Corrosion by Cinders

31. Galvanic Corrosion – Dissimilar Metal

32. Pipe Leak

33. Galvanic Corrosion
Bronze valve
(cathode)
Steel Pipe
(anode)

34. Galvanic Corrosion – Effect of Anode-to-Cathode Area Ratio

35. GALVANIC CORROSION

37. SURFACE CONDITION VARIATIONS

39. PITTING AT BREAKS IN MILL SCALE

41. Galvanic Corrosion - Used to Advantage
In Galvanic Cathodic Protection Systems
In Batteries
Mg or Zn anode

42. VARIATIONS IN SOIL TYPES

43. Dissimilar Soils De-icing salts? Fertilizers? Pavement Clay Sandy Loam
Cathode
Anode
Cathode
De-icing salts?
Fertilizers?

44. CORROSION DUE TO CONCENTRATION CELLS

45. CONCENTRATION-CELL CORROSION

47. STRESS-CELL CORROSION

48. STRESS CORROSION Lower Stress Area (Cathode) Pipe Threaded Bolt
Higher Stress Area
(Anode)
Metallic Coupling

49. MICROBIOLOGICALLY INFLUENCED CORROSION (MIC)
Many different types of Bacteria
Sulfate-Reducing Bacteria (SRB) - Usually most problematic
Require sulfate (SO42-)
Require anaerobic conditions
Require Organic carbon food source
SRB activity produces hydrogen sulfide (H2S) which is corrosive to many metals. On steel corrosion product is iron sulfide
Certain aerobic bacteria can also be problematic

50. Corrosion of Steel by SRB

51. Examples of Corrosion Attack by SRB

52. Corrosion by Aerobic Bacteria

53. AMPHOTERIC METALS
Metals that corrode in acidic as well as alkaline environments
E.g. Aluminum, Zinc, Tin, Lead, Antimony

54. STRAY CURRENT CORROSION
Corrosion caused by current that deviates (strays) from its intended path

55. SOURCES of STRAY CURRENTS
DYNAMIC
ELECTRIC TRANSIT SYSTEMS
MINE RAILROADS
WELDING OPERATIONS
STATIC
IMPRESSED CURRENT CATHODIC PROTECTION (ICCP)
HIGH VOLTAGE POWER TRANMISSION

56. STRAY CURRENT CORROSION – SOURCE DC TRANSIT SYSTEM

57. STRAY CURRENT CORROSION – SOURCE DC MINING CARS

58. STRAY CURRENT CORROSION – SOURCE DC WELDING

59. STRAY CURRENT CORROSION – SOURCE ICCP SYSTEM

61. STRAY CURRENT CORROSION – HVDC SYSTEM

63. FARADAY’S LAW (Rate of Corrosion)
W = K x I x T
Where:
W = Weight Loss in One Year
K = Electrochemical Equivalent in Pounds
Per Ampere Per Year
I = Corrosion Current in Amperes
T = Time in Years

64. METAL CORROSION LOSS

65. Current Density at Coating Holidays
Current Discharge
Current Discharge
ELECTROLYTE
COATING
HOLIDAY
HOLIDAY
SUBSTRATE
Anodic current density at coating holidays will be very high. According to Faraday’s Law the metal penetration (corrosion) rate will also be proportionately high.

66. Corrosion Failure Consequences

67. Corrosion Failure Consequences

68. Corrosion Failure Consequences

69. Corrosion Failure Consequences