1 Basic Corrosion and Basic Corrosion and Cathodic Protection for Cathodic Protection for Steel Steel - - Reinforced Reinforced Concrete Structures Concrete Structures Jeff Schramuk NACE CP Specialist #7695 www.cpsolutionsinc.net
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Basic Corrosion andBasic Corrosion andCathodic Protection forCathodic Protection for
SteelSteel--ReinforcedReinforcedConcrete StructuresConcrete Structures
Jeff SchramukNACE CP Specialist #7695
www.cpsolutionsinc.net
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Topics to be Covered
Why Should We Be Concerned about Corrosion?
Definitions and Terminology
Some Forms of Corrosion
Coatings and Cathodic Protection
Cathodic Protection using Magnesium Anodes
Advantages & Limitations of Galvanic Anode CP Systems
Impressed Current Cathodic Protection
Field Test Equipment
Cathodic Protection Criteria.
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Topics to be Covered
Why Should We Be Concerned about Corrosion?
Definitions and Terminology
Some Forms of Corrosion
Coatings and Cathodic Protection
Cathodic Protection using Magnesium Anodes
Advantages & Limitations of Galvanic Anode CP Systems
Impressed Current Cathodic Protection
Field Test Equipment
Cathodic Protection Criteria.
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As a Nation, How do We Manage our Highway Bridges?
*Source: “2005 Report Card for America's Infrastructure.” ASCE: Reston, VA.
The Future Dilemma: The U.S. faces a shortfall of $9.4 billion per year over the next 20 years to
eliminate all bridge deficiencies.
Present Condition: The U.S. earns a grade of “C”for maintenance of the nations’ nearly 600,000highway bridges, and
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Concrete Footing Deterioration
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Column Deterioration
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Beam Deterioration
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Ceiling Deterioration
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Parapet Wall Deterioration
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Hanging Concrete Delamination
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Leaking Expansion Joint & Patch Repairs
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Topics to be Covered
Why Should We Be Concerned about Corrosion?
Definitions and Terminology
Some Forms of Corrosion
Coatings and Cathodic Protection
Cathodic Protection using Magnesium Anodes
Advantages & Limitations of Galvanic Anode CP Systems
Impressed Current Cathodic Protection
Field Test Equipment
Cathodic Protection Criteria.
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Corrosion Can be Defined as:
Practical
Definition
Scientific
Definition
The Tendencyof a Metal toRevert to itsNative State
ElectrochemicalDegradation of Metal
as a Result of aReaction with itsEnvironment
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Four Basic Components of A Corrosion Cell
Anode – A metal electrode in contact with the
electrolyte which corrodes
Cathode - A metal electrode in contact with theelectrolyte which is protected against corrosion
Electrolyte – A solution or conducting mediumsuch as soil, water or concrete which containsoxygen and dissolved chemicals
Metal Path – An external circuit that connectsthe anode and the cathode
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Electron Flow vs. Conventional Current
Flow of conventional current is from positive (+) tonegative (-)
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
CathodicArea
(Protected)
Definitions - Anodes & Cathodes
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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 -1.00
Mild Steel (New) -0.70Mild Steel (Old) -0.50Cast / Ductile Iron -0.50Stainless Steel -0.50 to + 0.10Copper, Brass, Bronze -0.20
Gold +0.20Carbon, Graphite, Coke +0.40
* Potentials in Volts Versus a Saturated Cu-CuSO4 Electrode
L e s s
A c t i v e
M o r e
Practical Galvanic Series*
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Eliminating the Corrosion Cell
A n o d e
C a t h o d e
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Topics to be Covered
Why Should We Be Concerned about Corrosion?
Definitions and Terminology
Some Forms of Corrosion
Coatings and Cathodic Protection
Cathodic Protection using Magnesium Anodes
Advantages & Limitations of Galvanic Anode CP Systems
Impressed Current Cathodic Protection
Field Test Equipment
Cathodic Protection Criteria.
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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 andplaced into a corrosive environment.
Corrosion current is proportional to the differencein electrochemical energy between the twometals
Area 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 -1.00
Mild Steel (New) -0.70Mild Steel (Old) -0.50Cast / Ductile Iron -0.50Stainless Steel -0.50 to + 0.10Copper, Brass, Bronze -0.20
Gold +0.20Carbon, Graphite, Coke +0.40
* Potentials in Volts Versus a Saturated Cu-CuSO4 Electrode
L e s s
A c t i v e
M o r 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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Dissimilar Surface Conditions
Pipe(Cathode) Threads
Bright Metal
(Anode)
Scratches(Anode)
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Concrete is an alkaline environment thatpassivates the steel and protects it from corrosion
Deicing salts will accelerate the corrosion rate by
reducing the alkalinity of the concreteRebar corrosion (rust) has a volume 2.2 times as
great as the original steel that can lead to a “spall”
The spall delaminates the concrete and allowsadditional corrosion to occur on the steel rebar
Corrosion of Steel Rebar in Concrete
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Pitting of Coated Carbon Steel in Soil
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Pitting at a Coating Defect
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Coating Defects on Epoxy Coated Rebar
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Topics to be Covered
Why Should We Be Concerned about Corrosion?
Definitions and Terminology
Some Forms of Corrosion
Coatings and Cathodic Protection
Cathodic Protection using Magnesium Anodes
Advantages & Limitations of Galvanic Anode CP Systems
Impressed Current Cathodic Protection
Field Test Equipment
Cathodic Protection Criteria.
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Corrosion occurs where current dischargesfrom metal to electrolyte
The objective of cathodic protection is toforce 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 -0.20
Gold +0.20Carbon, Graphite, Coke +0.40
* Potentials in Volts Versus a Saturated Cu-CuSO4 Electrode
L e s s
A c t i v e
M o r e
Practical Galvanic Series*
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C o p p e r - 2
0 0 m V
S t e e l - 6 0 0 m V
M a g n e s i u m
- 1 . 7 V
1. Anode2. Cathode
3. Electrolyte
4. Metal Path
Galvanic Corrosion – No C.P. Benefit
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C a t h
o d e
C a t h o
d e
A n
o d e
1. Anode
2. Cathode
3. Electrolyte
4. Metal Path
Galvanic Corrosion - Mitigated w/CP
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Topics to be Covered
Why Should We Be Concerned about Corrosion?
Definitions and Terminology
Some Forms of Corrosion
Coatings and Cathodic Protection
Cathodic Protection using Magnesium Anodes
Advantages & Limitations of Galvanic Anode CP Systems
Impressed Current Cathodic Protection
Field Test Equipment
Cathodic Protection Criteria.
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No external AC power is requiredSimple to install on new structures
Seldom causes stray DC interference
Minimal maintenance requirements.
Galvanic Anode CP Systems - Advantages
G l i A d CP S Li i i
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Limited driving potential ∆E = (Ea – Ec)
Limited current output I = ∆E / Rt
Large number of anodes will be required on
large mats of bare reinforcing steel
Galvanic Anode CP Systems - Limitations
T i t b C d
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Topics to be Covered
Why Should We Be Concerned about Corrosion?
Definitions and TerminologySome Forms of Corrosion
Coatings and Cathodic Protection
Cathodic Protection using Magnesium Anodes
Advantages & Limitations of Galvanic Anode CP Systems
Impressed Current Cathodic Protection
Field Test Equipment
Cathodic Protection Criteria.
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CP for Rebar Must be Electrically Continuous
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Scarified & Patched Deck Awaits Anode Mesh
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Rolling Out the Anode Mesh on the Bridge Deck
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Drilling Holes for Anode Hold-Down Clips
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Cutting Back Anode Mesh at Expansion Joint
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Anode Conductor (Current Distributor) Bar
Question!
Why theDouble
Conductor
Bar?
R ifi A i C ll
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Rectifier Automatic Controllers
A d & St t C ti
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Anode & Structure Connections
R f El t d d G d C ti
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Reference Electrode and Ground Connections
Topics to be Covered
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Topics to be Covered
Why Should We Be Concerned about Corrosion?
Definitions and TerminologySome Forms of Corrosion
Coatings and Cathodic Protection
Cathodic Protection using Magnesium AnodesAdvantages & Limitations of Galvanic Anode CP Systems
Impressed Current Cathodic Protection
Field Test EquipmentCathodic Protection Criteria.
Test Equipment Quality Assurance
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Test Equipment Quality Assurance
Perform pre-test operational checks inaccordance with the manufacturer instructions
Verify battery strength (if required)
Initiate corrective action for equipment out of
specification
Have the equipment calibrated annually
CP Test Equipment - Multi-Meters
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CP Test Equipment Multi Meters
Multi-Meter Characteristics
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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)
Meter Connections
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Potential Measurements
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Connect voltmeter to rebar and reference
Ensure reference cell plug has good contact with
damp pavement (use wet sponge, if necessary)
Place reference cell away from anodes
Read potential on DCV scale
Record reading using standard forms
If polarity is positive, verify proper connections
Reference Electrode – Basic Components
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p
Reference Electrode – Ensuring Accuracy
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g y
Reference Cell Placement
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A A BB
Current Flow & IR Drop Error
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p
IR Drop in P/S Measurements
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Reference Electrode - Maintenance
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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 emery clothReplace 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
Topics to be Covered
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Why Should We Be Concerned about Corrosion?
Definitions and TerminologySome Forms of Corrosion
Coatings and Cathodic Protection
Cathodic Protection using Magnesium AnodesAdvantages & Limitations of Galvanic Anode CP Systems
Impressed Current Cathodic Protection
Field Test EquipmentCathodic Protection Criteria.
CP Criteria for Steel in Concrete
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Example of the 100 mV polarization decaycriterion:
ON Potential = -650 mV
I-O Potential = -500 mV
The Static (Off) Potential must be -400 mV or
less after no more than 4 hours
CP Performance - Can Be Verified
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CP Performance Can Be Verified