Basic Corrosion & Cathodic Protection
Basic Corrosion
and
Cathodic Protection
Jeff Schramuk
NACE CP Specialist #7695
www.cpsolutionsinc.net
1
Topics to be Covered
Why Should We Be Concerned about Corrosion?
Definitions and Terminology
Forms of Corrosion
Pipe Coatings and Cathodic Protection
Cathodic Protection using Magnesium Anodes
Advantages & Limitations of Galvanic Anode CP Systems
Impressed Current Cathodic Protection
Measurement and Testing of CP Systems
Field Test Equipment
Cathodic Protection Criteria.
2
Basic Corrosion & Cathodic Protection
Why Should We Be Concerned about Corrosion?
Definitions and Terminology
Forms of Corrosion
Pipe Coatings and Cathodic Protection
Cathodic Protection using Magnesium Anodes
Advantages & Limitations of Galvanic Anode CP Systems
Impressed Current Cathodic Protection
Measurement and Testing of CP Systems
Field Test Equipment
Cathodic Protection Criteria.
3
Effects of Infrastructure Corrosion
Regulatory
Life Safety Compliance
Economics Environmental 4
Basic Corrosion & Cathodic Protection
Why Should We Be Concerned about Corrosion?
Definitions and Terminology
Forms of Corrosion
Pipe Coatings and Cathodic Protection
Cathodic Protection using Magnesium Anodes
Advantages & Limitations of Galvanic Anode CP Systems
Impressed Current Cathodic Protection
Measurement and Testing of CP Systems
Field Test Equipment
Cathodic Protection Criteria.
5
Corrosion Can be Defined as:
The Tendency
Practical of a Metal to
Revert to its
Definition Native State
Electrochemical
Scientific Degradation of Metal
Definition as a Result of a
Reaction with its
Environment
6
Corrosion - A Natural Process
IRON OXIDE REFINING MILLING
IRON CORROSION IRON OXIDE
7
Four Basic Parts of a Corrosion Cell
Anode – A metal electrode in contact with the
electrolyte which corrodes
Cathode - A metal electrode in contact with the
electrolyte which is protected against corrosion
Electrolyte – A solution or conducting medium
such as soil, water or concrete which contains
oxygen and dissolved chemicals
Metal Path – An external circuit that connects
the anode and the cathode
8
Electron Flow vs. Conventional Current
Flow of conventional current is from positive (+) to
negative (-)
Conventional current flow from (+) to (-) will be
from the cathode to the anode in the metal path
Conventional current flow from (+) to (-) will be
from the anode to the cathode in the electrolyte.
9
Definitions - Anodes & Cathodes
Cathodic
Area
(Protected) DC Current
Anodic Area
(Metal Loss)
10
The Simplified Corrosion Cell
1. Anode
Steel at -600mV
at -200mV
2. Cathode
mV
Steel at -600 mV
at -200
3. Electrolyte
Copper
4. Metal Path Copper
11
Components of a Familiar Corrosion Cell
CARBON ROD
(Cathode)
ZINC CASE I
(Anode)
I
I
NH4 and Cl- Paste
(Electrolyte)
I
WIRE I e-
(Metallic Path)
12
Practical Galvanic Series*
Material Potential*
Pure Magnesium -1.75
Magnesium Alloy -1.60
Zinc -1.10
Less Active More
Aluminum Alloy -1.00
Mild Steel (New) -0.70
Mild Steel (Old) -0.50
Cast / Ductile Iron -0.50
Stainless Steel -0.50 to + 0.10
Copper, Brass, Bronze -0.20
Gold +0.20
Carbon, Graphite, Coke +0.40
* Potentials With Respect to Saturated Cu-CuSO4 Electrode
13
Corrosion Reaction and Ohm’s Law
Ohm’s Law States that: I = ∆E/R where:
∆E = Driving Potential (EA minus EC)
EA = Anode Potential (measured in volts)
EC = Cathode Potential (measured in volts)
I = Current Flow (measured in amperes)
R = Resistance (measured in ohms)
14
Some Common Electrical Quantities
Current Flow: 1 ampere (A) = 1000 milliamps (mA)
Examples:
A sacrificial anode’s output is measured in mA
A CP rectifier’s output is can be up 100 A
Voltage: 1 volt (V) = 1000 millivolts (mV)
Examples:
A magnesium anode’s potential is ~1.6 V (1600 mV)
A CP rectifier can have a DC voltage of up to 100 V 15
Corrosion Cell - Anodic Reactions
I
Fe++
Steel at -600mV
e-
at -200mV OH-
Fe++
Cathode
OH-
Anode
Copper
OH-
Fe++
16
Corrosion Cell - Cathodic Reactions
Steel at -600mV
e- at -200mV
Cathode
e- H+
e- H+
Anode
Copper
e- H+
e- H+
17
Corrosion Cell – Combined Reactions
Steel at -600mV
e-
e- at -200mV H2
Fe2(OH)3
Cathode
H2
Anode
Copper
H2 Fe2(OH)3
H2
Fe2(OH)3
18
Basic Corrosion & Cathodic Protection
Why Should We Be Concerned about Corrosion?
Definitions and Terminology
Forms of Corrosion
Pipe Coatings and Cathodic Protection
Cathodic Protection using Magnesium Anodes
Advantages & Limitations of Galvanic Anode CP Systems
Impressed Current Cathodic Protection
Measurement and Testing of CP Systems
Field Test Equipment
Cathodic Protection Criteria.
19
General Corrosion
Corrosive environment is uniform around the
structure
Anode area is uniformly distributed over the structure
Corrosion rate is usually constant over the structure
Environments where uniform attack can occur
Atmospheric, Aqueous, Concrete
20
True Uniform Corrosive Attack
21
Galvanic Corrosion
When two different metals are connected and
placed into a corrosive environment.
Corrosion current is proportional to the difference
in electrochemical energy between the two
metals
Area Effect
Avoid small anode connected to a large cathode
Distance Effect
Area closest to anode will have the greatest corrosion
22
Practical Galvanic Series*
Material Potential*
Pure Magnesium -1.75
Magnesium Alloy -1.60
Zinc -1.10
Less Active More
Aluminum Alloy -1.00
Mild Steel (New) -0.70
Mild Steel (Old) -0.50
Cast / Ductile Iron -0.50
Stainless Steel -0.50 to + 0.10
Copper, Brass, Bronze -0.20
Gold +0.20
Carbon, Graphite, Coke +0.40
* Potentials With Respect to Saturated Cu-CuSO4 Electrode
23
Galvanic Corrosion Bimetallic Connection
24
Old-New Pipe Corrosion Cell
Old Pipe New Pipe
(Cathode) (Anode)
25
Steel in Concrete-Soil
Concrete Note: Arrows Indicate Direction of DC Current Flow
Encasement
Pipe in Soil
Corrodes
Cathodic Anodic
Zone Zone
26
Dissimilar Surface Conditions
Pipe Scratches
(Cathode) Threads (Anode)
Bright Metal
(Anode)
27
Concentration Cell Corrosion
Due to differences in the environment
Differential Soil Aeration – Very common
28
Differential Soil Aeration
Aerated Soil
O2 O2
Oxygen diffusing through
backfill sustains corrosion to
cathodic (top) area of pipe
Cathodic Zone
Clay soil Clay soil
Anodic Zone
Lack of oxygen at bottom of
pipe creates relative corrosion
cell to (top) area of pipe
29
Differential Aeration on Cast Iron Pipe
Cathodic Zone
Anodic Zone
30
Differential Soil Aeration
Pavement
Sandy Loam Clay (moist Sandy Loam
(well drained, low oxygen) (well drained,
high oxygen) high oxygen)
Cathode Anode Cathode
Factors contributing to an increased corrosive
attack are de-icing salts and agricultural fertilizers31
Pitting Corrosion
Random and highly localized
Depth greater than area of attack
Most destructive form of corrosion
Pit location and growth difficult to predict
32
Pitting of Coated Carbon Steel in Soil
33
External Pitting: Ductile Iron Water Main
34
Selective Leaching Corrosion
Selective Leaching
Graphitization (Gray Cast Iron)
Dezincification (Brass)
35
Dealloying Corrosion (Graphitization)
36
Basic Corrosion & Cathodic Protection
Why Should We Be Concerned about Corrosion?
Definitions and Terminology
Forms of Corrosion
Pipe Coatings and Cathodic Protection
Cathodic Protection using Magnesium Anodes
Advantages & Limitations of Galvanic Anode CP Systems
Impressed Current Cathodic Protection
Measurement and Testing of CP Systems
Field Test Equipment
Cathodic Protection Criteria.
37
Eliminating the Corrosion Cell
Cathode
Anode
38
Apply a Bonded Tape Wrapping
39
Pitting at a Coating Defect
40
Coat the Structure & Electrically Isolate It
What’s Wrong
Here?
41
Encase the Pipe in a “Corrosion Barrier”
42
Basic Corrosion & Cathodic Protection
Why Should We Be Concerned about Corrosion?
Definitions and Terminology
Forms of Corrosion
Pipe Coatings and Cathodic Protection
Cathodic Protection using Magnesium Anodes
Advantages & Limitations of Galvanic Anode CP Systems
Impressed Current Cathodic Protection
Measurement and Testing of CP Systems
Field Test Equipment
Cathodic Protection Criteria.
43
How Cathodic Protection Works
Corrosion occurs where current discharges
from metal to electrolyte
The objective of cathodic protection is to
force the entire surface to be cathodic to the
environment.
44
Galvanic Anode Cathodic Protection
Current is obtained from a metal of a higher
energy level.
45
Practical Galvanic Series*
Material Potential*
Pure Magnesium -1.75
Magnesium Alloy -1.60
Zinc -1.10
Less Active More
Aluminum Alloy -1.00
Mild Steel (New) -0.70
Mild Steel (Old) -0.50
Cast / Ductile Iron -0.50
Stainless Steel -0.50 to + 0.10
Copper, Brass, Bronze -0.20
Gold +0.20
Carbon, Graphite, Coke +0.40
* Potentials With Respect to Saturated Cu-CuSO4 Electrode
46
Galvanic Corrosion – No C.P. Benefit
1. Anode
2. Cathode
Copper -200mV
Magnesium -1.7V
Steel -600mV
3. Electrolyte
4. Metal Path
47
Galvanic Corrosion - Mitigated w/CP
1. Anode
2. Cathode
Anode
Cathode
Cathode
3. Electrolyte
4. Metal Path
48
CP Performance - Can Be Verified
49
Sacrificial Anode on a Buried Pipeline
Grade
Sacrificial Anode Connection to Pipe
Coating
Defect
50
Sacrificial Anode w/Test Station
Grade
Sacrificial Anode Connection to Pipe
Coating
Defect
51
CP Test Station - Terminal Board
insulated
terminal board calibrated
shunt resistor
anode structure
lead wire lead wire
52
Magnesium Anodes
53
Packaged Magnesium Anode
Natural Gas PL
Proper distance of anode from pipe
At least 3’ from a coated pipe
At least 6’ from bare steel
At least 1’ deeper than pipeline
Evaluate pipe coating
Install anode carefully – don’t lift by the lead wire
Tamp earth firmly around anode package.
54
Packaged Magnesium Anode
Natural Gas PL (cont.)
Leave slack in the anode lead wire
Wet area thoroughly around anode
Make a secure electrical connection to the pipe (e.g.
exothermic weld)
Repair pipe coating to match original
Place test box where it is protected from damage and
can be easily located
Do not allow any foreign pipeline contacts.
55
Packaged Magnesium Anode
Natural Gas PL (cont.)
56
*Detail courtesy of Midwest Energy Association
Basic Corrosion & Cathodic Protection
Why Should We Be Concerned about Corrosion?
Definitions and Terminology
Forms of Corrosion
Pipe Coatings and Cathodic Protection
Cathodic Protection using Magnesium Anodes
Advantages & Limitations of Galvanic Anode CP Systems
Impressed Current Cathodic Protection
Measurement and Testing of CP Systems
Field Test Equipment
Cathodic Protection Criteria.
57
Galvanic Anode CP Advantages
No external AC power is required
Effective utilization of protective current
Simple and inexpensive to install on new
underground structures
Seldom cause stray DC interference
Minimal maintenance requirements.
58
Galvanic Anode CP Limitations
Limited driving potential Æ ∆E = (Ea – Ec)
Limited current output Æ I = ∆E / Rt
Large number of anodes will be required on
bare or poorly coated structures
Ineffective in high-resistivity soil
environments Æ (Rt ).
59
Basic Corrosion & Cathodic Protection
Why Should We Be Concerned about Corrosion?
Definitions and Terminology
Forms of Corrosion
Pipe Coatings and Cathodic Protection
Cathodic Protection using Magnesium Anodes
Advantages & Limitations of Galvanic Anode CP Systems
Impressed Current Cathodic Protection
Measurement and Testing of CP Systems
Field Test Equipment
Cathodic Protection Criteria.
60
Surface (Horizontal) Anode System
Rectifier
(-) (+)
Anode
Groundbed
Pipeline
(Structure)
61
Deep Anode (Vertical) Anode System
62
Continuous Linear Anode System
63
Impressed Current Transformer Rectifier
64
Basic Corrosion & Cathodic Protection
Why Should We Be Concerned about Corrosion?
Definitions and Terminology
Forms of Corrosion
Pipe Coatings and Cathodic Protection
Cathodic Protection using Magnesium Anodes
Advantages & Limitations of Galvanic Anode CP Systems
Impressed Current Cathodic Protection
Measurement and Testing of CP Systems
Field Test Equipment
Cathodic Protection Criteria.
65
Have you checked your rectifier lately?
66
Monitoring Data for a CP Rectifier
67
Can you locate your test stations?
68
Potential Profile Survey Technique
Voltmeter-Computer
Test Station
Wire Dispenser &
Distance Chainer
Pipeline
Reference Cells
69
Basic Corrosion & Cathodic Protection
Why Should We Be Concerned about Corrosion?
Definitions and Terminology
Forms of Corrosion
Pipe Coatings and Cathodic Protection
Cathodic Protection using Magnesium Anodes
Advantages & Limitations of Galvanic Anode CP Systems
Impressed Current Cathodic Protection
Measurement and Testing of CP Systems
Field Test Equipment
Cathodic Protection Criteria.
70
CP Test Equipment - Multi-Meters
71
Multi-Meter Characteristics
Basic Functions
Reads AC & DC Volts
Reads Ohms (optional diode checker)
Reads AC and DC Amps (be careful here!)
Performance Criteria
Field rugged, water/drop resistant
High input impedance (min. 20 M-Ω)
72
Test Equipment Quality Assurance
Perform pre-test operational checks in accordance with
the manufacturer instructions
Verify the battery strength (if so equipped)
Initiate corrective action for equipment out of specification
Have the equipment calibrated each year
73
Reference Electrode Basic Components
74
Reference Electrode - Maintenance
Periodically verify cell against a known standard
Keep porous plug covered when not used
Clean and refill the reference cell annually
Clean copper rod with a non-metallic abrasive pad
Replace w/fresh Cu/CuSO4 solution (½ full at all times)
Some Cu/CuSO4 crystals should always remain in
suspension
Wash hands after using – Cu/CuSO4 solution is
hazardous
75
P/S Potential Readings
Connect voltmeter to pipe and reference
Ensure reference cell plug has good contact with moist
soil – not pavement
Place reference cell away from anodes
Read P/S on DCV scale
Record P/S reading using standard forms
If polarity is positive, notify corrosion dept.
76
Meter Connections
77
Basic Corrosion & Cathodic Protection
Why Should We Be Concerned about Corrosion?
Definitions and Terminology
Forms of Corrosion
Pipe Coatings and Cathodic Protection
Cathodic Protection using Magnesium Anodes
Advantages & Limitations of Galvanic Anode CP Systems
Impressed Current Cathodic Protection
Measurement and Testing of CP Systems
Field Test Equipment
Cathodic Protection Criteria.
78
DOT Standard – Part 192.463
Cathodic Protection Criteria
-0.85 V (w/IR-drop consideration)
-0.85 V Instant-Off
100 mV polarization decay
Other criteria determined to be “appropriate” by
regulatory authority
79
NACE International – CP Criteria
80
DOT Standard – Part 192.465
Monitoring of Cathodic Protection
Potentials tested every 12 months at intervals not
exceeding 15 months, or
10% per year to sample entire line every 10 years
Rectifiers and critical bonds checked every 2 months
at intervals not exceeding 2-1/2 months.
81
Do We Have a Good Reading?
82
Basic Corrosion & Cathodic Protection
Questions?
Jeff Schramuk
NACE CP Specialist #7695
www.cpsolutionsinc.net
83
