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Corrosion of Reinforcing Steel November 7, 2018
Prof Ken Hover, P.E., Ph.D.Cornell University
607-280-3004 / kch7@cornell.edu 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
Corrosion of Reinforcing Steel November 7, 2018
Prof Ken Hover, P.E., Ph.D.Cornell University
607-280-3004 / kch7@cornell.edu 2
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
Corrosion of Reinforcing Steel November 7, 2018
Prof Ken Hover, P.E., Ph.D.Cornell University
607-280-3004 / kch7@cornell.edu 3
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
Corrosion of Reinforcing Steel November 7, 2018
Prof Ken Hover, P.E., Ph.D.Cornell University
607-280-3004 / kch7@cornell.edu 4
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)
Corrosion of Reinforcing Steel November 7, 2018
Prof Ken Hover, P.E., Ph.D.Cornell University
607-280-3004 / kch7@cornell.edu 5
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
More Active Metal Less Active Metal
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
More Active Metal Less Active Metal
Fe++
Fe++
Fe++
Fe++
Fe++
Fe++
e-
e-
e-
e-
e-
e-
e-e-
Electron transfer to less active metal
Iron Copper
Corrosion of Reinforcing Steel November 7, 2018
Prof Ken Hover, P.E., Ph.D.Cornell University
607-280-3004 / kch7@cornell.edu 6
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
Corrosion of Reinforcing Steel November 7, 2018
Prof Ken Hover, P.E., Ph.D.Cornell University
607-280-3004 / kch7@cornell.edu 7
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
Corrosion of Reinforcing Steel November 7, 2018
Prof Ken Hover, P.E., Ph.D.Cornell University
607-280-3004 / kch7@cornell.edu 8
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
Corrosion of Reinforcing Steel November 7, 2018
Prof Ken Hover, P.E., Ph.D.Cornell University
607-280-3004 / kch7@cornell.edu 9
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
Corrosion of Reinforcing Steel November 7, 2018
Prof Ken Hover, P.E., Ph.D.Cornell University
607-280-3004 / kch7@cornell.edu 10
Oxygen
Salt and water
Anode Cathode
Anode
Corrosion of Reinforcing Steel November 7, 2018
Prof Ken Hover, P.E., Ph.D.Cornell University
607-280-3004 / kch7@cornell.edu 11
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.
Corrosion of Reinforcing Steel November 7, 2018
Prof Ken Hover, P.E., Ph.D.Cornell University
607-280-3004 / kch7@cornell.edu 12
Ohio Steel Fabricators Fiasco
Steel pipe for Oxygen
Ohio Steel Fabricators Fiasco
Steel pipe for Oxygen
Pressure-Test with water
Ohio Steel Fabricators Fiasco
Steel pipe for Oxygen
Drain water (mostly) after successful test
Ohio Steel Fabricators Fiasco
Steel pipe for Oxygen
Pressurize with Oxygen
Ohio Steel Fabricators Fiasco
Steel pipe for 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
Corrosion of Reinforcing Steel November 7, 2018
Prof Ken Hover, P.E., Ph.D.Cornell University
607-280-3004 / kch7@cornell.edu 13
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
Corrosion of Reinforcing Steel November 7, 2018
Prof Ken Hover, P.E., Ph.D.Cornell University
607-280-3004 / kch7@cornell.edu 14
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
Corrosion of Reinforcing Steel November 7, 2018
Prof Ken Hover, P.E., Ph.D.Cornell University
607-280-3004 / kch7@cornell.edu 15
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
kch7@cornell.edu