corrosion of reinforcing steel-2018-icri-3

Corrosion of Reinforcing Steel November 7, 2018

Prof Ken Hover, P.E., Ph.D.Cornell University

607-280-3004 / [email protected] 1

2016 ICRI Kick‐Off Party | February 1, 20162018 Fall Convention | | November 7-9 | Omaha, Nebraska

Why Does Rebar Corrode in Concrete?

(in plain language)

Ken Hover, P.E.,

Cornell University

Corrosion of Reinforcing SteelSequence of Deterioration

• Penetration of oxygen, salt, CO2, water

• Volume expansion of rust

• Concrete cracking

• Accelerated corrosion

• Loss of steel cross section

Rust

Cracks Expansion of corrosion products

"Piling-Bedworth Ratio": volume of rust occupies 4 to 10 times the volume of parent metal.

• Bursting stresses develop from rust expansion




Fe

FeO

Fe3O4

Fe2O3

Fe(OH)2

Fe(OH)3

Fe(OH)3 3H2O

0 1 2 3 4 5 6 7

Volume (ml)

Pilling–Bedworth Ratio Principal Causes

• Chloride induced corrosion:– Deicing salts

– Seawater

– Chloride admixtures

• Carbonation effects

Impact of corrosion

• Rust-stains, reduction in ride-quality

• Loss of Serviceability– Cracking, increased penetration

• Loss of load capacity and reliability– Loss of concrete cover

– Loss of bond

– Loss of steel cross section




Loss of Steel Section

• Can be critical for shear

• Critical for non-redundant sections

Background

Rodents!

Latin: Rōdere to gnaw,Corrodere-to gnaw through.

water

Peppermint

Stick

Sugar Dissolves in Water




water

Iron (steel)

Iron Dissolves in Water

water

Iron (steel)

Fe+++

Fe+++

Fe+++

Fe++

Fe++

Fe++

Positive metal ions go into solution

EPA Drinking Water Standard:0.3 mg Iron / Liter = 0.3g / 1000 Liters

Red/Brown Tinge

At 0.3 mg/Liter

Corrosion is an electrochemical process

• Corroding element = anode

• Non corroding element = cathode

• Anode and Cathode in electrical contact

• Immersed in an Electrolyte

• Cathode reaction determines rate

Iron

Steel

http://www.sciencemaster.co.uk/category/atomic-structure/element/

Iron Atom with 4 Free Electrons

Iron can lose some of all of them

Water

(electrolyte)




water

Iron (steel)

Fe++

Fe++

Fe++

Fe++

Fe++

Fe++

e-

e-

e-e-

e-

e-

e-

Negative charge builds on metal

water

Iron (steel)

Fe++

Fe++

Fe++

Fe++

Fe++

Fe++

Further dissolution is inhibited by negative charge

e-

e-

e-e-

e-

e-

e-

water

More Active Metal Less Active Metal

(Loses electrons more readily)

(Has a better gripon its electrons)

Iron Copper

Galvanic Series

in Seawater

Metals shift order in different corrosive

environments More “Active” = More

susceptible to corrosion

Less “Active” = More “Noble” =

Less susceptible to

corrosion

water


Fe++

Fe++

Fe++

Fe++

Fe++

Fe++

e-

e-

e-e-

e-

e-

e-

(Loses electrons more readily)

(Has a better gripon its electrons)

Iron Copper

water


Fe++

Fe++

Fe++

Fe++

Fe++

Fe++

e-

e-

e-

e-

e-

e-

e-e-

Electron transfer to less active metal

Iron Copper




water

Anode Cathode

Fe++

Fe++

Fe++

Fe++

Fe++

Fe++

e-

e-

e-

e-

e-

e-

e-e-

Oxygen reduction at the cathodeO2 + 2H2O + 4e- 4 OH-

e-

e-

e-e-

Iron Copper

water

Anode Cathode

Fe++

Fe++

Fe++

Fe++

Fe++

Fe++

e-

e-

e-

e-

e-

e-

e-e-

Metal oxidation at the anodeFe Fe++ 2e-

e-

e-

e-e-

Iron Copper

water

Anode Cathode

Fe++

Fe++

Fe++

Fe++

Fe++

Fe++

e-

e-

e-

e-

e-

e-

e-e-

A complete “Corrosion Cell”

e-

e-

e-

e-OH-

Fe++

Ele

ctro

lyte

Ele

ctric

al

Con

nect

ion

Iron Copper

Required Components

• Iron (steel)

• Water

• Oxygen




Low Salt Concentration

High Salt Concentration

• Crystallization pressure

• Osmotic pressure

• Thermal shock

• Chemical attack

• Increased electrical conductivity

• Destabilization of steel passivity

Influence of Salt (Sodium Chloride or Calcium Chloride)

Steel “Passivates”

with increased oxygen

Corrosion rate with 165 ppm CaCl2

Chloride Limit (new constr.)

% of cement

Test Method

Acid Sol. Water Soluble

Category ASTM C1152

ASTM C1218

Soxhlet

Pretressed 0.08 0.06 0.06

Reinf wet 0.10 0.08 0.08

Reinf dry 0.20 0.15 0.15

ACI 222R

Corrosion Cells Applied to Concrete Structures

• Macro cells

• Micro cells




Anode

Cathodes

Micro-Cell

Macro-Cell

Cathode

Anode

AnodesCathodes

Rate-Limiting Step

• Oxygen Reduction

• Anode Size

• Oxygen partial pressure

• Oxygen permeability

• Temperature (accumulate salt in winter, corrode in summer) Warm garage, warm sea water, hot water heater

0.00

1.00

2.00

3.00

4.00

5.00

6.00

7.00

8.00

50 70 90 110 130

Corrosion Rate (m

m/year)

Temperature (F)Rate DoublesEach 22F Increase

Types of Cells

• Differential– Metals Steel water tank / Copper pipe

– Salt Concentration Parking garage

– Oxygen Concentration Bridge deck

– Temperature

• Stray Current

• Pitting

• Under-film




Differential metal cells

• Metal forms & embedments

• Galvanized components

• Bar Deformationswater

Anode Cathode

Fe++

Fe++

Fe++

Fe++

Fe++

Fe++

e-

e-

e-

e-

e-

e-

e-e-

A complete “Corrosion Cell”

e-

e-

e-

e-OH-

Fe++

Copper (Cathode)

Steel (Anode)

Ironis more “Active”

than Copper

Ironis more “Active”

than Copper

Differential concentration cells

• Variations in chloride content

• Variations in moisture content

• Variations in Oxygen concentrationwater

Anode Cathode

Fe++

Fe++

Fe++

Fe++

Fe++

Fe++

e-

e-

e-

e-

e-

e-

e-e-

e-

e-

e-

e-OH-

Fe++

Hig

her

Chl

orid

e C

once

ntra

tion

Hig

her

Oxy

gen

Con

cent

ratio

n




Oxygen

Salt and water

Anode Cathode

Anode




Under-film Corrosion

Under-film Corrosion

Pitting Corrosion

Pitting Corrosion

Corrosion Rate Determined by Cathode Reaction

• Large cathode-small most rapid corrosion.

• The bottom mat cathode; top mat anode.

• Pinholes in epoxy coatings become small anodes; uncoated steel becomes large cathode.




Ohio Steel Fabricators Fiasco

Steel pipe for Oxygen



Pressure-Test with water



Drain water (mostly) after successful test



Pressurize with Oxygen



Pressurize with Oxygen

Large Cathode

Small Anode

Continuous Perf. Of Pipe

Yet Another Corrosion Cellof Sorts

Sometimes we feed the anode,

Sometimes we feed the cathode




Tidal Zones

Sea Salt in the air diffuses inland to a distance of about 10 km (6 miles)

Monitoring Ongoing Electrochemical

Corrosion Activity

• Electrical potential

ASTM C876 - 15

Standard Test Method for Corrosion Potentials

of Uncoated Reinforcing Steel in Concrete

water

Anode Cathode

Fe++

Fe++

Fe++

Fe++

Fe++

Fe++

e-

e-

e-

e-

e-

e-

e-e-

e-

e-

e-

e-OH-

Fe++

Unknown Activity at the Anode

Standard Copper-in-Copper Sulfate Cathode

V




Connect ground line and half-cell lines to voltmeter

Shift half-cell to all points on grid, record elec. potential

Expose rebarMake electrical ground connection

Cathodic Protection

General principles

• Negatively charge steel

• Generate OH- at cathode

• Sacrificial Anodes

• Impressed current

water

Anode Cathode

Fe++

Fe++

Fe++

Fe++

Fe++

Fe++

e-

e-

e-

e-

e-

e-

e-e-

Corrosion at Anode, Protection at Cathode

e-

e-

e-

e-OH-

Fe++

AnodeCathode

Impressed Current




Impressed Current

Impressed Current

Impressed Current

Impressed Current

Sacrificial Anodes 2016 ICRI Kick‐Off Party | February 1, 20162018 Fall Convention | | November 7-9 | Omaha, Nebraska

Thank youvery much!

Ken Hover, P.E.,

Cornell University

[email protected]

corrosion of reinforcing steel-2018-icri-3

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