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Chapter 17 -
IMC207: CIENCIA DE LOSMATERIALES I
Corrosin y degradacin
a
Henry A. Colorado, PhD
Mechanical Engineering Department
Universidad de Antioquia
From, Materials science and engineering an introduction, William D. Callister (Wiley)Chapter 17 -
What is Corrosion?Corrosion is the oxidation of a metal due to anELECTROCHEMICAL reaction. The oxidizing agent ismost often O2 (atmospheric corrosion) or H
+ (chemicalcorrosion) or both.
Why is it a problem?Financial - $350 Billion Dollar Annual Problem in U.S.(4.25% of GNP) Department of Defense spends $68 Billion
Chapter 17 -
Corrosion Mechanism
Cathodic cell
discussion of emf
galvanic series
intergranular corrosion
oxidation-reduction of iron
salt effects
Chapter 17 -
Basics of Corrosion
EMF series is a numeric rating of potentialunder ideal conditions
Galvanic Series is a practical listing
Galvanic Protection
Chapter 17 -
EL COSTO DE LA CORROSION
Chapter 17 -
Recipe for corrosion
Active metal
Water
Oxygen(atmospheric corrosion)
Acid(chemical corrosion)
Salt
High temperature
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Chapter 17 -
Oxidation - Reduction Reduction
Anodic metal gives up electrons(oxidation)
Cathodic metal accepts electrons(reduction)
Or gases accept electrons (reduction)
Fe Fe e 2 2
Al Al e 3 3
Cu e Cu2 2
2 2 2H e H gas
( )
Chapter 17 -
Chemical vs. Atmospheric Corrosion(H+ vs. O2)
Anodic Reaction:
Fe0(s) Fe2+
(aq) + 2e- Deterioration of metal
Cathodic Reaction:
2H+(aq) + 2e- H2 ( g) Chemical
O2 ( g) + 2H2O (l) + 4e- 4OH-(aq) Atmospheric
O2 ( g) + 4H+
(aq) + 4e- 2H2O (l) Combination
Which of these wil l oxidize copper? Silver? Gold?
Overall Reaction:
Fe0(s) + 2H+
(aq) Fe2+
(aq) + H2 ( g) Chemical
2Fe0(s) + O2 (g) + 2H2O (l) 2Fe2+
(aq) + 4OH-(aq) Atmospheric
2Fe0(s) + O2 ( g) + 4H+
(aq) 2Fe2+
(aq) + 2H2O (l) Combination
Eo red(V)
Eocell (V)
-0.44
0.00
+0.40
+1.23
+1.67
+0.84
+0.44
Cu(s) Cu2+
(aq) + 2e-
+0.34
Ag(s) Ag+
(aq) + e-
+0.80
+1.50
Chapter 17 -
Why wont iron corrode in pure (degassed) water?
Anodic Reaction:
Fe0(s) Fe2+
(aq) + 2e-
Cathodic Reaction:
Eo (V)
-0.44
-0.83
Fe0(s) + 2H2O (l) Fe2+
(aq) + H2 ( g) + 2OH-
(aq)
Eocell (V)
-0.39
What metals will corrode in pure (degassed) water?
Any sufficiently active metal Eored < -0.83 V
(alkali metals, alkaline earth metals, aluminum, manganese)
Overall reaction:
2H2O (l) + 2e- H2 ( g) + 2OH
-(aq)
Chapter 17 -
Steel Corrosion
2 2 22 2 2Fe O H O Fe OH ( )
2 1
2
22 2 2 3Fe OH O H O Fe OH( ) ( )
Initial Oxidation Reaction
Secondary Oxidation Reaction
Rust
Chapter 17 -
Example of AtmosphericCorrosion
Corrosion on wing of Navy aircraft
Why does corrosion of anairplane occur primarilywhile the plane is on theground?
F/A-18C Hornet
Chapter 17 -
Example of Chemical CorrosionNuclear Reactor Vessel Head
Degradation February 16, 2002, Davis-Besse Nuclear Power Station in Oak Harbor,
Ohio Boric Acid leak fromcontrol rod drivemechanism led tochemical corrosion ofreactor vessel head
Serious potential for loss ofreactor coolant access
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Chapter 17 -
Corrosion of a Ships Hull
Anodic and Cathodic Regions
O2
O2
+ 2H2
O + 4e- 4OH-
OH-
Fe2+
Fe2+ + 2OH-
Fe(OH)2
4Fe(OH)2 + O2 2(Fe2O3H2O)+ 2H2O Fe Fe2+ + 2e-
Hull ofship
CathodicRegionRUST
AnodicRegion
Electrons Migratefrom Anodic toCathodic Region
e-
Chapter 17 -High pH (> 9)
Preventing Corrosion
Salt
Chapter 17 -
Preventing the Corrosion of Iron
(cathodic protection/sacrificial anode)
Sacrificial anode
Chapter 17 -
Applications of Cathodic Protection
Galvanized Steel
Zinc coating
Sacrificial Anodes
Ship Hulls
Subs (free flooding areas)
Los Angeles Class Sub
Arleigh-Burke Destroyer
Chapter 17 -
Impressed Current CathodicProtection
Power Supply
Shipboard Power
Controller
Shipboard Power
Insulation
Pt Anode
ReferenceElectrode
Paint
Layer
Hull
e-e-
Chapter 17 -
Galvanic Corrosion
(contact between unlike metals;
opposite of cathodic protection)
Copper Iron
H2OO2
Cathode:O2 + 2H2O + 4e
- 4OH-
Anode:Fe Fe2+ + 2e-
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Chapter 17 - 19
Corrosion:-- the destructive electrochemical attack of a material.--Al Capone's
ship, Sapona,off the coastof Bimini.
Cost:-- 4 to 5% of the Gross National Product (GNP)*-- this amounts to just over $400 billion/yr**
* H.H. Uhlig and W.R. Revie, Corrosion and Corrosion Control: An Introductionto Corrosion Science and Engineering, 3rd ed., John Wiley and Sons, Inc.,1985.**Economic Report of the President (1998).
Photos courtesy L.M. Maestas, SandiaNational Labs. Used with permission.
EL COSTO DE LA CORROSION
Chapter 17 - 20
Se requieren dos reacciones:-- oxidacion:-- reduccion:
Zn Zn2
2e
2H
2e H2(gas)
Other reduction reactions:-- in an acid solution -- in a neutral or base solution
O2 4H
4e 2H2O O2 2H2O4e
4(OH)
Adapted from Fig. 17.1, Callister 7e.(Fig. 17.1 is from M.G. Fontana,Corrosion Engineering, 3rd ed.,McGraw-Hill Book Company, 1986.)
COROSION DE ZINC EN ACIDO
Zinc
Oxidation reactionZn Zn2+
2e-Acidsolution
reduction reaction
H+H+
H2(gas)
H+
H+
H+
H+
H+
flow of e-
in the metal
Chapter 17 -
STANDARD HYDROGEN (EMF) TEST
Chapter 17 - 22
STANDARD HYDROGEN (EMF) TEST
Two outcomes:
0ometalV (relative to Pt)
Standard Electrode Potential
--Metal sample mass
--Metal is the cathode (+)
Mn+
ions
ne-
e- e-
25C1M Mn+ soln 1M H+ soln
Platinum
metal,M
H+
H+2e-
Adapted from Fig. 17.2, Callister 7e.
0ometalV (relative to Pt)
--Metal sample mass
--Metal is the anode (-)
Platinum
metal,M Mn+
ions
ne-H2(gas)
25C1M Mn+ soln 1M H+ soln
2e-
e-e-
H+
H+
Chapter 17 - 23
STANDARD EMF SERIES EMF series
Au
CuPbSnNiCoCdFeCrZnAlMgNaK
+1.420 V
+0.340- 0.126- 0.136- 0.250- 0.277- 0.403- 0.440- 0.744- 0.763- 1.662- 2.363- 2.714- 2.924
metal Vmetalo
Data based on Table 17.1,Callister 7e.
moreanodic
morecathodic
metalo
Metal with smallerV corrodes.
Ex: Cd-Ni cell
DV =0.153V
o
Adapted from Fig. 17.2, Callister 7e.
-
1.0 M
Ni2+ solution
1.0 M
Cd2+ solution
+
25C NiCd
Chapter 17 -
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Chapter 17 -
Potentiodynamic Polarization
Curve
Chapter 17 -
Corrosion potential calculation
Reduction Reaction must have higherpotential than the oxidation reaction orthey will not form a cathodic cell
Fe e Fe
2
2 -0.440 V
Zn e Zn
2
2 -0.763 V
DV V . . .440 763 0 323 Relative measure ofcorrosion
Chapter 17 - 27
GALVANIC SERIES Ranks the reactivity of metals/alloys in seawater
Based on Table 17.2, Callister7e. (Source of Table 17.2 isM.G. Fontana, CorrosionEngineering, 3rd ed., McGraw-Hill Book Company, 1986.)
PlatinumGoldGraphiteTitaniumSilver316 Stainless SteelNickel (passive)CopperNickel (active)TinLead316 Stainless SteelIron/Steel
Aluminum AlloysCadmiumZincMagnesium
moreanodic
(active)
morecathodic
(inert)
Chapter 17 -
Chapter 17 - Chapter 17 -
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Chapter 17 - Chapter 17 -
Acceleration of Corrosion
Physical Characteristics
exposed area (less, increases corrosion rate)
time of exposure (more time, more corrosion)
Environmental Characteristics acidic environment
sulfur gas environment
temperature (high temps, more corrosion)
moisture (oxygenated moisture)
Chapter 17 -
Passivation
A protective film in oxidizing atmospheres
chromium,nickel, titanium, aluminum
Metal oxide layer adheres to parent metal
barrier against further damage
self-healing if scratched
Sensitive to environmental conditions
passivated metal may have high corrosionrates
Chapter 17 - 34
Uniform AttackOxidation & reductionoccur uniformly oversurface.
Selective LeachingPreferred corrosion ofone element/constituent(e.g., Zn from brass (Cu-Zn)).
Stress corrosionStress & corrosionwork togetherat crack tips.
GalvanicDissimilar metals arephysically joined. The
more anodic onecorrodes.(see Table17.2) Zn & Mgvery anodic.
Erosion-corrosionBreak down of passivatinglayer by erosion (pipeelbows).
FORMS OF CORROSION
Formsof
corrosion
Crevice Between twopieces of the same metal.
Fig. 17.15, Callister 7e. (Fi g. 17.15 iscourtesy LaQue Center for CorrosionTechnology, Inc.)
Rivet holes
IntergranularCorrosion alonggrain boundaries,often where specialphases exist.
Fig. 17.18, Callister 7e.
attackedzones
g.b.prec.
PittingDownward propagationof small pits & holes.
Fig. 17.17, Callister 7e.(Fig. 17.17 from M.G.Fontana, CorrosionEngineering, 3rd ed.,McGraw-Hill BookCompany, 1986.)
Chapter 17 -
Forms of Corrosion
Uniform corrosion of a single metal
usually an electrochemical reaction atgranular level
relatively slow and predictable
rusting of exposed steel, tarnishedsilver
easily corrected with coatings andregular maintenance
Chapter 17 -
Forms of Corrosion
Galvanic Corrosion
2 dissimilar metals, electrolyte,electrical connection and oxygen
Pitting Corrosion
Localized corrosion forming holes orindentations
Difficult to initially detect
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Chapter 17 -
Forms of Corrosion
Crevice Corrosion
narrow crevice filled with ionizedsolution
Oxygen-rich on the outside, oxygen-poor on the inside
metals oxidize with salt anions FeCl2and pH rises in cathodic zone
H+ may destroy passivity
Chapter 17 -
Forms of Corrosion
Intergranular Corrosion
corrosion along grain boundaries atmicroscopic level
stainless steels and heat treated high-strength steels
carbides precipitate along grainboundaries leaving these areas with noalloyed Chromium
Welds can have this same depletioneffect
Chapter 17 -
Forms of Corrosion
Cavitation and Erosion in Pipe
particulate matter
turbulent flow
abrades away the corrosion product
abrasion of zinc coatings
Chapter 17 -
Forms of Corrosion
Stress Corrosion Cracking
tensile stress and corrosiveenvironments
cracks are initiated at corrosion areas
tensile stresses propagate the crack
corrosion further deteriorate crack
etc.....
Chapter 17 -
Reinforcement Corrosion
Chapter 17 -
Corrosion of Metals in ConcreteReinforcing Steel & Prestressing
Steel
Concrete is Normally Highly Alkaline
Protects Steel from Rusting if Properly
Embedded
If Corrosion Occurs, the Reaction Products
are Greater in Volume Than the
Original Steel
Corrosion Initiation and Rate Depends On
Amount of Concrete Cover, Quality of Concrete
Details of Construction, & Exposure to Chlorides
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Chapter 17 - Chapter 17 - 44
Self-protecting metals!-- Metal ions combine with O
to form a thin, adhering oxide layer that slows corrosion.
Reduce T(slows kinetics of oxidation and reduction)
Add inhibitors-- Slow oxidation/reduction reactions by removing reactants
(e.g., remove O2 gas by reacting it w/an inhibitor).-- Slow oxidation reaction by attaching species to
the surface (e.g., paint it!).
CONTROLLING CORROSION
Metal (e.g., Al,stainless steel)
Metal oxide
Adapted from Fig. 17.22(a),Callister 7e. (Fi g. 17.22(a) isfrom M.G. Fontana, CorrosionEngineering, 3rd ed., McGraw-HillBook Co., 1986.)
steelpipe
Mganode
Cu wiree-
Earth
Mg2+
e.g., Mg Anode
Cathodic (or sacrificial) protection-- Attach a more anodic material to the one to be protected.
Adaptedfrom Fig.17.23,Callister
7e. steel
zinczinc
Zn2+
2e- 2e-
e.g., zinc-coated nail
Chapter 17 -
Corrosion Prevention
Coatings
Barrier films
Inhibitive Pigments
Sacrificial treatments
Paint
Active CathodicProtection
Chapter 17 -
Activation polarization
Chapter 17 - Chapter 17 -
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Chapter 17 - Chapter 17 -
Chapter 17 -
Recommended