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Chapter 17 - 1
ISSUES TO ADDRESS...
How does corrosion occur?
Which metals are most likely to corrode?
What environmental parameters affectcorrosion rate?
How do we prevent or control corrosion?
Chapter 17:
Corrosion and Degradation of
Materials
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Chapter 17 - 2
DESTRUCTIVE MECHANISM
In metal
there is an actual loss either by dissolution(corrosion) or by formation of non-metalic scale or film
(oxidation).
Ceramic materialrelatively resistant to deterioration,
usually occurs at elevated temperature or rather extreme
environments. Also called corrosion.
In polymerthe term used is degradation. Polymer may
dissolve when exposed to a liquid solvent or they mayabsorb the solvent and swell also electromagnetic radiation
(primarily ultraviolet) and heat may cause alterations in
their molecular structures
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Chapter 17 - 3
Corrosion in metal:- the destructive electrochemical attack on metal.
- ordinarlily begins at the surface- -- Ex:Al Capone's
ship, Sapona,off the coastof Bimini.
Cost:-- 4 to 5% of the Gross National Product (GNP)* spend
in corrosion prevention, maintenance and replacement-- in the U.S. 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.
THE COST OF CORROSION
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Chapter 17 - 4
Because metals want to go back to
their stable states.
Usually, metals are stable when they
bind with others.
In the presence of a corrosive
environment, metals tend to reactswith oxygen or other corrosive ions.
WHY CORROSION HAPPEN?
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Chapter 17 -
CORROSION INGREDIENTS
Water Oxygen
Electrolyte (ion transport in water) orElectrical conductor
To prevent corrosion remove one of theingredient
5
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Chapter 17 -
Corrosive Environments
All environments are corrosive to some degree.
Typical corrosive environments:
Humidity
SteamSeawater
Alkalis
Acids
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Chapter 17 -
Corrosive Environments Include the atmosphere, aqueous solutions,
soils, acids, bases, inorganic solvents, moltensalts, liquid metals.
Atmospheric corrosion accounts for thegreatest losses.
Moisture contain dissolve oxygen, and othercompound such as sodium chloride.
Freshwater contained dissolve oxygen and
other minerals. Sea water is more corrosive than freshwater.
Soil contain moisture, oxygen salt content,
alkalinity and acidity 7
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Chapter 17 -
Effect of pH on Corrosion
Corrosion accelerated under acidic condition Secondary reaction, prevent the oxide from
accumulating in the surface thus drive thereaction into the product
Pitting cause the thermodynamic activity ofFe further accelerating the corrosion process
8
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Chapter 17 - 9
Two reactions are necessary:-- oxidationreaction (give up electron)-- reductionreaction (receive electron)
Zn Zn2 2e
2H2e H2(gas)
Other reductionreactions in solutions with dissolved oxygen:
-- acidic solution -- neutral or basic solution
O2 4H4e 2H2O O2 2H2O4e
4(OH)
Adapted from Fig. 17.1,Callister & Rethwisch 8e.(Fig. 17.1 is from M.G.Fontana, CorrosionEngineering, 3rd ed., McGraw-Hill Book Company, 1986.)
ELECTROCHEMICAL CORROSION
Zinc
Oxidation reactionZn Zn2+
2e-Acidsolution
reduction reaction
H+H+
H2(gas)
H+
H+
H+
H+
H+
flow of e-in the metal
Ex: consider the corrosion of zinc in an acid solution
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Chapter 17 -10
STANDARD HYDROGEN ELECTRODE
Two outcomes:
0o
metalV (relative to Pt)
Standard Electrode PotentialAdapted from Fig. 17.2,Callister & Rethwisch 8e.
-- Corrosion
-- Metal is the anode (-)
Platinum
metal,M
Mn+
ions
ne-H2(gas)
25C1M Mn+ soln 1M H+ soln
2e-
e-e-
H+
H+
-- Electrodeposition
-- Metal is the cathode (+)
Mn+ions
ne-
e- e-
25C1M Mn+ soln 1M H+ soln
Platinum
metal,M
H+
H+2e-
0o
metal V (relative to Pt)
H2(gas)
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Chapter 17 - 11
STANDARD EMF (electromotive force) SERIES
metal
o Metal with smaller
V corrodes.
EMFseries
AuCuPbSn
NiCoCdFeCr
ZnAlMgNaK
+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
metalV
metal
o
Data based on Table 17.1,Callister 8e.
moreano
dic
more
cathodic
DV =0.153V
o
Adapted from Fig. 17.2,Callister & Rethwisch 8e.
-
1.0 M
Ni2+ solution
1.0 M
Cd2 + solution
+
25C NiCd
Cdo
Nio
Ex: Cd-Ni cell
V < V Cd corrodes
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Chapter 17 -12
GALVANIC SERIES Ranking of the reactivity of metals/alloys in seawater
Based on Table 17.2, Callister &Rethwisch 8e. (Source of Table17.2 is M.G. Fontana, CorrosionEngineering, 3rd ed., McGraw-Hill Book Company, 1986.)
PlatinumGoldGraphiteTitaniumSilver
316 Stainless Steel (passive)Nickel (passive)CopperNickel (active)TinLead
316 Stainless Steel (active)Iron/SteelAluminum AlloysCadmiumZincMagnesium
moreanodic
(active)
morecathodic
(inert)
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Chapter 17 -
Corrosion Rates
13
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Chapter 17 -
PASSIVITY
Some normally active metal and alloys underparticular environment; condition lose their chemicalreactivity and become extremely inert.Passivity
Results from the formation of a highly adherent andvery thin oxide form on metal surface which serve as
protective barrier to further corrosion.
Display by chromium, nickel, titanium, iron, stainlesssteel, aluminum and their alloys.
Aluminum form a tough oxide Al2O3which prevent
further oxidation
Unfortunately Fe2O3 is a flaky , not tough
However, a change in the character of theenvironment may cause a passivated material to
revert to an active state. 14
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Chapter 17 -15
Uniform AttackOxidation & reductionreactions occur uniformlyover surfaces.
Selective LeachingPreferred corrosion of
one element/constituent[e.g., Zn from brass (Cu-Zn)].
Stress corrosionCorrosion at crack tips
when a tensile stressis present.
GalvanicDissimilar metals arephysically joined in thepresence of anelectrolyte. Themore anodic metal
corrodes.
Erosion-corrosionCombined chemical attack andmechanical wear (e.g., pipeelbows).
FORMS OF CORROSION
Formsof
corrosion
Crevice Narrow andconfined spaces.
Fig. 17.15, Callister & Rethwisch 8e. (Fig. 17.15is courtesy LaQue Center for Corrosion
Technology, Inc.)
Rivet holes
IntergranularCorrosion alonggrain boundaries,
often where precip.particles form.
Fig. 17.18, Callister &
Rethwisch 8e.
attackedzones
g.b.prec.
PittingDownward propagation
of small pits and holes.
Fig. 17.17, Callister &Rethwisch 8e. (Fig. 17.17from M.G. Fontana,Corrosion Engineering,3rd ed., McGraw-Hill BookCompany, 1986.)
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Chapter 17 -16
-- Use metals that passivate
- These metals form a thin,adhering oxide layer thatslows corrosion.
Lower the temperature (reduces rates of oxidation and
reduction)
CORROSION PREVENTION (i)
Metal (e.g., Al,stainless steel)
Metal oxide
Apply physical barriers -- e.g., films and coatings
Materials Selection-- Use metals that are relatively unreactive in thecorrosion environment -- e.g., Ni in basic solutions
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Chapter 17 -17
Add inhibitors (substances added to solution that decreaseits reactivity)-- Slow oxidation/reduction reactions by removing reactants
(e.g., remove O2gas by reacting it w/an inhibitor).-- Slow oxidation reaction by attaching species to
the surface.
CORROSION PREVENTION (ii)
Adaptedfrom Fig.17.22(a),Callister &
Rethwisch
8e.
Using a sacrificial anode
steelpipe
Mganode
Cu wiree-
Earth
Mg2+
Cathodic (or sacrificial) protection-- Attach a more anodic material to the one to be protected.
Adaptedfrom Fig.17.23,Callister &
Rethwisch
8e. steel
zinczinc
Zn2+
2e- 2e-
e.g., zinc-coated nail
Galvanized Steel
e.g., Mg Anode
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Chapter 17 -18
CORROSION OF CERAMIC
MATERIALS
Extremely immune to corrosion by almost all
environments especially at room temperature.
Corrosion of ceramic materials generally
involve simple chemical dissolution in contact
to electrochemical process found in metals.
Frequently used because of their resistance to
corrosion. Eg. glass is often used to contain liquids for
this reason.
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Chapter 17 -19
DEGRADATION OF POLYMERS
Whereas most metallic corrosions areelectrochemical, polymeric degradation isphysiochemical: that it involve physical as well aschemical phenomena.
Polymers deteriorate by swelling and dissolution. With swelling, the liquid or solute diffuses into and is
absorbed within the polymer: the small solutemolecules fit into and occupy positions amongpolymer molecules thus the macromolecules are
forced apart. This increase in chain separationresults in a reduction of secondary intermolecularbonding forces; as a consequences, the materialbecome softer and more ductile.
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Chapter 17 -20
DEGRADATION OF POLYMERS
Scissionthe severance or rupture of molecular chain bonds.This
courses a separation of chain segments at the point of scission anda reduction in the molecular weight. Bond rapture may result from exposure to radiation or to heat and
from chemical reaction. Certain type of radiationpossess sufficient energy to penetrate a
polymer specimen and interact with the constituent atoms or theirelectrons.
Oxygen, ozone and other substances can causes or acceleratechain scission as a result of chemical reaction
Thermal degradationcorresponds to the scission of molecular chainat elevated temperature; as a consequence some polymer undergo
chemical reactions in which gaseous species are produced. Weatheringwhen exposed outdoor condition deterioration is a
result of oxidation which initiated by ultraviolet radiation from thesun and ,water absorption.
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Chapter 17 -21
Metallic corrosion involves electrochemical reactions
-- electrons are given up by metals in an oxidationreaction-- these electrons are consumed in a reductionreaction
Metals and alloys are ranked according to their
corrosiveness in standard emfand galvanic series. Temperature and solution composition affect corrosion
rates. Forms of corrosion are classified according to mechanism
Corrosion may be prevented or controlled by:-- materials selection
-- reducing the temperature-- applying physical barriers-- adding inhibitors-- cathodic protection
SUMMARY
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Chapter 17 -22
a) Explain the differences between oxidation and reduction
electrochemical reaction
b)Which reaction occurs at the anode and which at the cathode
Quiz
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Chapter 17 -23
a) Explain 5 different form of corrosion and in which
condition it will occur.b) Explain three methods to prevent corrosion
Home work
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Chapter 17 -24
Reading:
Core Problems:
Self-help Problems:
ANNOUNCEMENTS