corrosion protection of steel in concrete structures

8/3/2019 Corrosion Protection of Steel in Concrete Structures

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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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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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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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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



L e s s

A c t i v e

M o r e




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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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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



L e s s

A c t i v e

M o r e




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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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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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Definitions and TerminologySome Forms of Corrosion









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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




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Cathodic Protection using Magnesium AnodesAdvantages & Limitations of Galvanic Anode CP Systems


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




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Cathodic Protection using Magnesium AnodesAdvantages & Limitations of Galvanic Anode CP Systems


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



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Questions?

Jeff SchramukNACE CP Specialist #7695

www.cpsolutionsinc.net

corrosion protection of steel in concrete structures

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