chapter 18: corrosion and degradation of materials
TRANSCRIPT
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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 18:Corrosion and Degradation of Materials
• 01 Introduction• 02 Electrochemical Considerations• 03 Corrosion Rates• 04 Prediction of Corrosion Rates• 05 Passivity• 06 Environmental Effects• 07 Forms of Corrosion• 08 Corrosion Environments• 09 Corrosion Prevention• 10 Oxidation• 11 Swelling and Dissolution• 12 Bond Rupture• 13 Weathering
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01 Corrosion
• Corrosion is a natural process, which converts a refined metal to a more stable form, such as its oxide, hydroxide, or sulfide. It is the gradual destruction of materials (metals) by chemical reaction with their environment.
• Corrosion engineering is the field dedicated to controlling and stopping corrosion.
• Deteriorative mechanisms: – In metals, there is actual material loss either by dissolution
or by oxidation. – Ceramics, resistant to deterioration, but at elevated
temperatures or in rather extreme environments– Polymers, degradation
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• Corrosion:-- the destructive
electrochemical attack of a material.
-- Ex: Rusting of automobiles and other equipment
• Cost:-- 4 to 5% of the Gross
National Product (GNP)*-- in the U.S. this amounts
to just over $400 billion/yr**
THE COST OF CORROSION
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oto
Figun_18_p638a
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• Two reactions are necessary:-- oxidation reaction:-- reduction reaction:
• Other reduction reactions in solutions with dissolved oxygen:-- acidic solution -- neutral or basic solution
02 ELECTROCHEMICAL CORROSION
Zinc
Oxidation reactionZn Zn2+
2e-Acid solution
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
Fig_18_02
An electrochemical cell consisting of iron and copper Electrodes
Iron corrodes(Oxidation)
Copper electrodeposits(Reduction)
Anode Cathode
Fig_18_03
An electrochemical cell consisting of iron and zincElectrodes
Zinc corrodes
Iron electrodeposits
Cathode Anode
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STANDARD HYDROGEN ELECTRODE• Two outcomes:
(relative to Pt)
Standard Electrode Potential
-- Corrosion
-- Metal is the anode (-)
Plat
inum
met
al, M Mn+
ions
ne- H2(gas)
25°C1M Mn+ sol’n 1M H+ sol’n
2e-
e-e-
H+H+
-- Electrodeposition
-- Metal is the cathode (+)
Mn+ions
ne-
e- e-
25°C1M Mn+ sol’n 1M H+ sol’n
Plat
inum
met
al, M
H+H+
2e-
(relative to Pt)
H2(gas)
Fig_18_04
The standard hydrogen reference half-cell
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STANDARD ElectoMotive Force (EMF) SERIES
metalo
• Metal with smallerV corrodes.
• EMF series
AuCuPbSnNiCoCdFeCrZnAlMgNaK
+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 18.1, Callister 9e.
mor
e an
odic
mor
e ca
thod
ic
∆V = 0.153V
o
-
1.0 M Ni2+ solution
1.0 M Cd2+ solution
+
25°C NiCd
Cdo
Nio
• Ex: Cd-Ni cellV < V ∴ Cd corrodes
Table_18_01
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CORROSION IN A GRAPEFRUIT
Zn2+
2e- oxidation reaction
AcidH+ H+
H+
H+
H+
H+
H+-+
Zn (anode)Cu (cathode)
reduction reactions
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EFFECT OF SOLUTION CONCENTRATION AND TEMPERATURE
• Ex: Cd-Ni cell withstandard 1 M solutions
-
Ni
1.0 M Ni2+ solution
1.0 M Cd2+ solution
+
Cd 25°C
• Ex: Cd-Ni cell withnon-standard solutions
n = #e-
per unitoxid/redreaction(= 2 here)F = Faraday'sconstant= 96,500C/mol.
• Reduce VNi - VCd by-- increasing X-- decreasing Y-- increasing T
- +
Ni
Y M Ni2+ solution
X M Cd2+ solution
Cd T
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GALVANIC SERIES• Ranking of the reactivity of metals/alloys in seawater
PlatinumGoldGraphiteTitaniumSilver316 Stainless Steel (passive)Nickel (passive)CopperNickel (active)TinLead316 Stainless Steel (active)Iron/SteelAluminum AlloysCadmiumZincMagnesium
mor
e an
odic
(act
ive)
mor
e ca
thod
ic(in
ert)
The Galvanic Series: The relative reactivities in seawater
15Table_18_02
• 01 Introduction• 02 Electrochemical Considerations• 03 Corrosion Rates• 04 Prediction of Corrosion Rates• 05 Passivity• 06 Environmental Effects• 07 Forms of Corrosion• 08 Corrosion Environments• 09 Corrosion Prevention• 10 Oxidation• 11 Swelling and Dissolution• 12 Bond Rupture• 13 Weathering
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03 Corrosion rate• Corrosion penetration rate (CPR) or thickness
loss per unit time — as a function of specimen weight loss, density, area, and time of exposure– CPR = KW / ρAt (mm/yr)
• Expression relating corrosion rate and current density– r = I /nF (mole/m2.s)
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Fig_18_05
Electrochemical cell consisting of standard zinc and hydrogen electrodes that has been sort-circuited
04 Prediction of corrosion rate
Fig_18_06
Schematic representation of possible steps in the hydrogen reduction process, controlled by activation polarization.
05 Passivity• A number of metals and alloys
passivate, or lose their chemical reactivity, under some environmental circumstances. – involve the formation of a
thin protective oxide film – Stainless steels and
aluminum alloys• The active-to-passive behavior
may be explained by the alloy's S-shaped electrochemical potential-versus-log current density curve
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Fig_18_12
• 01 Introduction• 02 Electrochemical Considerations• 03 Corrosion Rates• 04 Prediction of Corrosion Rates• 05 Passivity• 06 Environmental Effects• 07 Forms of Corrosion• 08 Corrosion Environments• 09 Corrosion Prevention• 10 Oxidation• 11 Swelling and Dissolution• 12 Bond Rupture• 13 Weathering
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06 Environmental Effects• The variables in the corrosion environment, which
include fluid velocity, temperature, and composition, can have a decided influence on the corrosion properties of the materials that are in contact with it.– increasing fluid velocity enhances the rate of
corrosion due to erosive effects, – Rates of reactions rise with increasing temp., conc.
of corrosive species (e.g., H+ ions in acids)
• Cold working or plastically deforming ductile metals is more susceptible to corrosion than the annealed state.
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• Uniform AttackOxidation & reductionreactions occur uniformly over surfaces.
• Selective LeachingPreferred corrosion ofone element/constituent[e.g., Zn from brass (Cu-Zn)].
• Stress corrosionCorrosion at crack tips when a tensile stress is present.
• GalvanicDissimilar metals arephysically joined in the presence of an electrolyte. Themore anodic metalcorrodes.
• Erosion-corrosionCombined chemical attack and mechanical wear (e.g., pipe elbows).
07 FORMS OF CORROSION
• Crevice Narrow and confined spaces.
Rivet holes
• IntergranularCorrosion alonggrain boundaries,often where precip.particles form.
attacked zones
g.b. prec.
• PittingDownward propagationof small pits and holes.
• Hydrogen embritlementa significant reduction in ductility that accompanies the penetration of atomic hydrogen into a metal/alloy
Fig_18_14
Fig_18_15
Immersed in seawater, crevice corrosion has occurred at the regions that were covered by washers.
Fig_18_16
Mechanism of crevice corrosion
The pitting of a 304 stainless steel plate by an acid chloride solution
Fig_18_18
Intergranular corrosion of stainless steel
Fig_18_19
Weld decay in a stainless steel
Impingement failure of an elbow that was part of steam condensate line
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Fi 18 21
Stress corrosion of a bent steel bar in sea water
Fig_18_22
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-- Use metals that passivate- These metals form a thin,
adhering oxide layer that slows corrosion.
• Lower the temperature (reduces rates of oxidation and reduction)
09 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 the corrosion environment -- e.g., Ni in basic solutions
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• Add inhibitors (substances added to solution that decrease its reactivity)-- 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.
CORROSION PREVENTION (ii)
Using a sacrificial anode
steel pipe
Mg anode
Cu wiree-
Earth
Mg2+
• Cathodic (or sacrificial) protection-- Attach a more anodic material to the one to be protected.
Adapted from Fig. 18.24, Callister & Rethwisch 9e. steel
zinczinc
Zn2+
2e- 2e-
e.g., zinc-coated nail
Galvanized Steel
e.g., Mg Anode
Fig_18_23
Fig_18_24
• 01 Introduction• 02 Electrochemical Considerations• 03 Corrosion Rates• 04 Prediction of Corrosion Rates• 05 Passivity• 06 Environmental Effects• 07 Forms of Corrosion• 08 Corrosion Environments• 09 Corrosion Prevention• 10 Oxidation• 11 Swelling and Dissolution• 12 Bond Rupture• 13 Weathering
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10 Oxidation
• Oxidation of metallic materials by electrochemical action is also possible in dry, gaseous atmospheres
• An oxide film forms on the surface that may act as a barrier to further oxidation if the volumes of metal and oxide film are similar
• The kinetics of film formation may follow parabolic, linear, or logarithmic rate laws.
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Fig_18_25
Fig_18_26
Oxidation film growth curves
Table_18_03
Ao (AM)is molecular weight of the oxide (metal)ρo (ρM) is density of the oxide (metal)
= Vo/VM =(Ao/ ρo) / (AM/ ρM)
Corrosion of ceramics
• Ceramic materials, being compounds of metallic and nonmetallic elements, may be thought of as having already been corroded
• Ceramic materials, being inherently corrosion resistant, are frequently used at elevated temperatures and/or in extremely corrosive environments.
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• 01 Introduction• 02 Electrochemical Considerations• 03 Corrosion Rates• 04 Prediction of Corrosion Rates• 05 Passivity• 06 Environmental Effects• 07 Forms of Corrosion• 08 Corrosion Environments• 09 Corrosion Prevention• 10 Oxidation• 11 Swelling and Dissolution• 12 Bond Rupture• 13 Weathering
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11-13 Degradation of polymers• Deteriorate by noncorrosive processes. • Upon exposure to liquids, they may experience degradation
by swelling or dissolution.– solute molecules actually fit into the molecular structure.– occur when the polymer is completely soluble in the liquid.
• Scission, or the severance of molecular chain bonds, may be induced by radiation, chemical reactions, or heat. – Results in a reduction of molecular weight and a deterioration of the
physical and chemical properties of the polymer.
• Weathering: any resultant degradation caused by exposure to outdoor conditions
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• Metallic corrosion involves electrochemical reactions-- electrons are given up by metals in an oxidation reaction-- these electrons are consumed in a reduction reaction
• Metals and alloys are ranked according to their corrosiveness in standard emf and 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