56525094 basic corrosion cp
DESCRIPTION
1TRANSCRIPT
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Jeff SchramukNACE CP Specialist #7695
www.cpsolutionsinc.net
Basic Corrosion Basic Corrosion andand
Cathodic ProtectionCathodic Protection
Basic Corrosion & Cathodic Protection
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Topics to be Covered
Why Should We Be Concerned about Corrosion?Definitions and TerminologyForms of CorrosionPipe Coatings and Cathodic ProtectionCathodic Protection using Magnesium AnodesAdvantages & Limitations of Galvanic Anode CP SystemsImpressed Current Cathodic Protection Measurement and Testing of CP SystemsField Test EquipmentCathodic Protection Criteria.
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Why Should We Be Concerned about Corrosion?Definitions and TerminologyForms of CorrosionPipe Coatings and Cathodic ProtectionCathodic Protection using Magnesium AnodesAdvantages & Limitations of Galvanic Anode CP SystemsImpressed Current Cathodic Protection Measurement and Testing of CP SystemsField Test EquipmentCathodic Protection Criteria.
Basic Corrosion & Cathodic Protection
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Effects of Infrastructure Corrosion
Life Safety
Economics Environmental
Regulatory Compliance
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Why Should We Be Concerned about Corrosion?Definitions and TerminologyForms of CorrosionPipe Coatings and Cathodic ProtectionCathodic Protection using Magnesium AnodesAdvantages & Limitations of Galvanic Anode CP SystemsImpressed Current Cathodic Protection Measurement and Testing of CP SystemsField Test EquipmentCathodic Protection Criteria.
Basic Corrosion & Cathodic Protection
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Corrosion Can be Defined as:
Practical Definition
Scientific Definition
The Tendency of a Metal to Revert to its Native State
Electrochemical Degradation of Metal as a Result of a Reaction with its Environment
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Corrosion - A Natural Process
IRON OXIDE REFINING MILLING
IRON CORROSION IRON OXIDE
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Four Basic Parts of a Corrosion Cell
Anode – A metal electrode in contact with the electrolyte which corrodesCathode - A metal electrode in contact with the electrolyte which is protected against corrosionElectrolyte – A solution or conducting medium such as soil, water or concrete which contains oxygen and dissolved chemicalsMetal Path – An external circuit that connects the anode and the cathode
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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.
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Anodic Area(Metal Loss)
DC Current
Cathodic Area
(Protected)
Definitions - Anodes & Cathodes
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Cop
per a
t -20
0mV
Ste
el a
t -60
0mV
The Simplified Corrosion Cell
Cop
per
at -2
00 m
V
Ste
el a
t -60
0 m
V
1. Anode
2. Cathode
3. Electrolyte
4. Metal Path
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Components of a Familiar Corrosion Cell
CARBON ROD(Cathode)
ZINC CASE(Anode)
NH4 and Cl- Paste(Electrolyte)
WIRE(Metallic Path)
I
I
I
I
I
e-
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Material Potential*Pure Magnesium -1.75Magnesium Alloy -1.60Zinc -1.10Aluminum Alloy -1.00Mild Steel (New) -0.70Mild Steel (Old) -0.50Cast / Ductile Iron -0.50Stainless Steel -0.50 to + 0.10Copper, Brass, Bronze -0.20Gold +0.20Carbon, Graphite, Coke +0.40* Potentials With Respect to Saturated Cu-CuSO4 Electrode
Less
Act
ive
Mor
e
Practical Galvanic Series*
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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)
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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
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Corrosion Cell - Anodic Reactions
Cop
per a
t -20
0mV
Ste
el a
t -60
0mV
Cat
hode
Ano
deI
e-Fe++
Fe++
Fe++
OH-
OH-
OH-
I
I
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Corrosion Cell - Cathodic Reactions
Cop
per a
t -20
0mV
Ste
el a
t -60
0mV
Cat
hode
Ano
de
I
e-H+
H+
H+
H+
e-
e-
e-
e-
I
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Corrosion Cell – Combined Reactions
Cop
per a
t -20
0mV
Ste
el a
t -60
0mV
Cat
hode
Ano
de
I
e-H2
H2
e-
H2
H2
Fe2(OH)3
Fe2(OH)3
Fe2(OH)3
I
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Why Should We Be Concerned about Corrosion?Definitions and TerminologyForms of CorrosionPipe Coatings and Cathodic ProtectionCathodic Protection using Magnesium AnodesAdvantages & Limitations of Galvanic Anode CP SystemsImpressed Current Cathodic Protection Measurement and Testing of CP SystemsField Test EquipmentCathodic Protection Criteria.
Basic Corrosion & Cathodic Protection
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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
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True Uniform Corrosive Attack
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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 metalsArea Effect
Avoid small anode connected to a large cathode Distance Effect
Area closest to anode will have the greatest corrosion
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Material Potential*Pure Magnesium -1.75Magnesium Alloy -1.60Zinc -1.10Aluminum Alloy -1.00Mild Steel (New) -0.70Mild Steel (Old) -0.50Cast / Ductile Iron -0.50Stainless Steel -0.50 to + 0.10Copper, Brass, Bronze -0.20Gold +0.20Carbon, Graphite, Coke +0.40* Potentials With Respect to Saturated Cu-CuSO4 Electrode
Less
Act
ive
Mor
e
Practical Galvanic Series*
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Galvanic Corrosion Bimetallic Connection
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Old Pipe (Cathode)
New Pipe (Anode)
Old-New Pipe Corrosion Cell
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Steel in Concrete-Soil
Cathodic Zone
Anodic Zone
Concrete Encasement
Pipe in Soil Corrodes
Note: Arrows Indicate Direction of DC Current Flow
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Dissimilar Surface Conditions
Pipe(Cathode) Threads
Bright Metal (Anode)
Scratches (Anode)
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Concentration Cell Corrosion
Due to differences in the environment
Differential Soil Aeration – Very common
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Aerated Soil
Differential Soil Aeration
Oxygen diffusing through backfill sustains corrosion to cathodic (top) area of pipe
Lack of oxygen at bottom of pipe creates relative corrosion cell to (top) area of pipe
Clay soil Clay soil
Anodic Zone
Cathodic Zone
O2 O2
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Differential Aeration on Cast Iron Pipe
Cathodic Zone
Anodic Zone
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Clay (moist low oxygen)
Sandy Loam (well drained, high oxygen)
Anode CathodeCathode
Differential Soil Aeration
Factors contributing to an increased corrosive attack are de-icing salts and agricultural fertilizers
Pavement
Sandy Loam (well drained, high oxygen)
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Pitting Corrosion
Random and highly localized
Depth greater than area of attack
Most destructive form of corrosion
Pit location and growth difficult to predict
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Pitting of Coated Carbon Steel in Soil
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External Pitting: Ductile Iron Water Main
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Selective Leaching Corrosion
Selective LeachingGraphitization (Gray Cast Iron)
Dezincification (Brass)
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Dealloying Corrosion (Graphitization)
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Why Should We Be Concerned about Corrosion?Definitions and TerminologyForms of CorrosionPipe Coatings and Cathodic ProtectionCathodic Protection using Magnesium AnodesAdvantages & Limitations of Galvanic Anode CP SystemsImpressed Current Cathodic Protection Measurement and Testing of CP SystemsField Test EquipmentCathodic Protection Criteria.
Basic Corrosion & Cathodic Protection
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Eliminating the Corrosion Cell
Ano
de
Cat
hode
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Apply a Bonded Tape Wrapping
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Pitting at a Coating Defect
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Coat the Structure & Electrically Isolate It
What’s Wrong Here?
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Encase the Pipe in a “Corrosion Barrier”
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Why Should We Be Concerned about Corrosion?Definitions and TerminologyForms of CorrosionPipe Coatings and Cathodic ProtectionCathodic Protection using Magnesium AnodesAdvantages & Limitations of Galvanic Anode CP SystemsImpressed Current Cathodic Protection Measurement and Testing of CP SystemsField Test EquipmentCathodic Protection Criteria.
Basic Corrosion & Cathodic Protection
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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.
How Cathodic Protection Works
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Current is obtained from a metal of a higher energy level.
Galvanic Anode Cathodic Protection
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Material Potential*Pure Magnesium -1.75Magnesium Alloy -1.60Zinc -1.10Aluminum Alloy -1.00Mild Steel (New) -0.70Mild Steel (Old) -0.50Cast / Ductile Iron -0.50Stainless Steel -0.50 to + 0.10Copper, Brass, Bronze -0.20Gold +0.20Carbon, Graphite, Coke +0.40* Potentials With Respect to Saturated Cu-CuSO4 Electrode
Less
Act
ive
Mor
e
Practical Galvanic Series*
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Cop
per
-200
mV
Ste
el -
600m
V
Mag
nesi
um -
1.7V
1. Anode
2. Cathode
3. Electrolyte
4. Metal Path
Galvanic Corrosion – No C.P. Benefit
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Cat
hode
Cat
hode
Ano
de
1. Anode
2. Cathode
3. Electrolyte
4. Metal Path
Galvanic Corrosion - Mitigated w/CP
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CP Performance - Can Be Verified
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Sacrificial Anode on a Buried Pipeline
Sacrificial Anode
Coating Defect
Connection to Pipe
Grade
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Coating Defect
Connection to Pipe
Grade
Sacrificial Anode
Sacrificial Anode w/Test Station
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CP Test Station - Terminal Board
structure lead wire
anode lead wire
insulated terminal board calibrated
shunt resistor
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Magnesium Anodes
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Proper distance of anode from pipeAt 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.
Packaged Magnesium Anode Natural Gas PL
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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.
Packaged Magnesium Anode Natural Gas PL (cont.)
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Packaged Magnesium Anode Natural Gas PL (cont.)
*Detail courtesy of Midwest Energy Association*Detail courtesy of Midwest Energy Association
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Why Should We Be Concerned about Corrosion?Definitions and TerminologyForms of CorrosionPipe Coatings and Cathodic ProtectionCathodic Protection using Magnesium AnodesAdvantages & Limitations of Galvanic Anode CP SystemsImpressed Current Cathodic Protection Measurement and Testing of CP SystemsField Test EquipmentCathodic Protection Criteria.
Basic Corrosion & Cathodic Protection
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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.
Galvanic Anode CP Advantages
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Limited driving potential ∆E = (Ea – Ec)
Limited current output I = ∆E / Rt
Large number of anodes will be required on bare or poorly coated structures
Ineffective in high-resistivity soil environments (Rt ).
Galvanic Anode CP Limitations
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Why Should We Be Concerned about Corrosion?Definitions and TerminologyForms of CorrosionPipe Coatings and Cathodic ProtectionCathodic Protection using Magnesium AnodesAdvantages & Limitations of Galvanic Anode CP SystemsImpressed Current Cathodic ProtectionMeasurement and Testing of CP SystemsField Test EquipmentCathodic Protection Criteria.
Basic Corrosion & Cathodic Protection
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Rectifier
Anode Groundbed
( - ) ( + )
Pipeline(Structure)
Surface (Horizontal) Anode System
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Deep Anode (Vertical) Anode System
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Continuous Linear Anode System
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Impressed Current Transformer Rectifier
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Why Should We Be Concerned about Corrosion?Definitions and TerminologyForms of CorrosionPipe Coatings and Cathodic ProtectionCathodic Protection using Magnesium AnodesAdvantages & Limitations of Galvanic Anode CP SystemsImpressed Current Cathodic ProtectionMeasurement and Testing of CP SystemsField Test EquipmentCathodic Protection Criteria.
Basic Corrosion & Cathodic Protection
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Have you checked your rectifier lately?
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Monitoring Data for a CP Rectifier
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Can you locate your test stations?
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Potential Profile Survey Technique
Reference Cells
Test StationVoltmeter-Computer
Wire Dispenser & Distance Chainer
Pipeline
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Why Should We Be Concerned about Corrosion?Definitions and TerminologyForms of CorrosionPipe Coatings and Cathodic ProtectionCathodic Protection using Magnesium AnodesAdvantages & Limitations of Galvanic Anode CP SystemsImpressed Current Cathodic Protection Measurement and Testing of CP SystemsField Test EquipmentCathodic Protection Criteria.
Basic Corrosion & Cathodic Protection
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CP Test Equipment - Multi-Meters
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Multi-Meter Characteristics
Basic FunctionsReads AC & DC Volts
Reads Ohms (optional diode checker)
Reads AC and DC Amps (be careful here!)
Performance CriteriaField rugged, water/drop resistant
High input impedance (min. 20 M-Ω)
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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
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Reference Electrode Basic Components
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Reference Electrode - Maintenance
Periodically verify cell against a known standardKeep porous plug covered when not usedClean and refill the reference cell annually
Clean copper rod with a non-metallic abrasive padReplace w/fresh Cu/CuSO4 solution (½ full at all times)Some Cu/CuSO4 crystals should always remain in suspensionWash hands after using – Cu/CuSO4 solution is hazardous
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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.
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Meter Connections
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Why Should We Be Concerned about Corrosion?Definitions and TerminologyForms of CorrosionPipe Coatings and Cathodic ProtectionCathodic Protection using Magnesium AnodesAdvantages & Limitations of Galvanic Anode CP SystemsImpressed Current Cathodic Protection Measurement and Testing of CP SystemsField Test EquipmentCathodic Protection Criteria.
Basic Corrosion & Cathodic Protection
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Cathodic Protection Criteria-0.85 V (w/IR-drop consideration)-0.85 V Instant-Off100 mV polarization decayOther criteria determined to be “appropriate” by regulatory authority
DOT Standard – Part 192.463
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NACE International – CP Criteria
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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.
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Do We Have a Good Reading?
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Questions?Questions?
Jeff SchramukNACE CP Specialist #7695
www.cpsolutionsinc.net
Basic Corrosion & Cathodic Protection