Download - Applied Electrochemistry
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Applied Electrochemistry
Dept. Chem. & Chem. Eng.
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Lecture 14Metal Electrochemistry
Dept. Chem. & Chem. Eng.
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Outline
Electrodeposition 1
Corrosion 2
Industrial electrolytic process
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DefinitionCorrosion is the deterioration of materials by chemical interaction with their environment. The term corrosion is sometimes also applied to the degradation of plastics, concrete and wood, but generally refers to metals.
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Env
ironm
ents
in C
orro
sion
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1Sheir, L.L., R.A. Jarman, and G.T. Burstein, eds. Corrosion. 3rd ed. Vol. 1. 2000, Butterworth-Heinemann: Oxford.
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* H.H. Uhlig and W.R. Revie, Corrosion and Corrosion Control: An Introduction to Corrosion Science and Engineering, 3rd ed., John Wiley and Sons, Inc., 1985.**Economic Report of the President (1998).
Photos courtesy L.M. Maestas, Sandia National Labs. Used with permission.
Corrosion: --the destructive electrochemical attack of a material. --Al Capone's ship, Sapona, off the coast of Bimini.
Cost: --4 to 5% of the Gross National Product (GNP)* --this amounts to just over $400 billion/yr**
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Cost of Corrosion(2004) in billion US$5
940 (approximately)510.14Global
.....
.....
.....
3.38Canada
3.53Poland
3.78India
6.75Belgium
7.32Australia
8.51UK
49.26Germany
55.01Former USSR
59.02Japan
200 (approximately)303.76USA
Indirect Cost Direct CostCountry
5Bhaskaran, R., N. Palaniswamy, and N.S. Rengaswamy, Global Cost of Corrosion—A Historical Review, in Corrosion: Materials, Vol 13B, ASM Handbook. 2005, ASM International.
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4Fe + 6H2O + 3O2 4Fe(OH)3
gives ferric hydroxide 2Fe(OH)3 Fe2O3 3H2O
gives iron oxide (rust) and water
The Rusting Mechanism (Peel)
Basic “rusting” or corrosion requirements1. The metal is oxidized at the anode of an electrolytic cell2. Some ions are reduced at the cathode3. There is a potential or voltage difference between the anode and cathode4. An electrolyte (fluid) must be present5. The electrical path must be completed
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• Two reactions are necessary: -- oxidation reaction: Zn → Zn2+ + 2e-
-- reduction reaction: 2H+ + 2e → H2(gas)
• Other reduction reactions:-- in an acid solution -- in a neutral or base solution
O2 4H 4e 2H2O O2 2H2O 4e 4(OH)
Zinc
oxidation reactionZn Zn2+
2e-Acid solution
reduction reaction
H+H+
H2(gas)
H+
H+
H+
H+
H+
flow of e- in the metal
Adapted from Fig. 17.1, Callister 6e. (Fig. 17.1 is from M.G. Fontana, Corrosion Engineering, 3rd ed., McGraw-Hill Book Company, 1986.)
Corrosion of zinc in acid
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• EMF series • Metal with smaller V corrodes.• Ex: Cd-Ni cell
metalo
more
anodic
more
cath
odic 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- 2.262- 2.714- 2.924
metal Vmetalo
V = 0.153V
o
Data based on Table 17.1, Callister 6e.
Standard EMF
-
1.0 M
Ni2+ solution
1.0 M
Cd2+ solution
+
25°C NiCd
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Thermodynamic Driving Force
Like all chemical reactions – ThermodynamicsWhat is the driving force for the reaction? (otherwise stated as what is the electrochemical potential for the reaction)
Dissimilar metals Different cold work states Different grain sizes Difference in local chemistry Difference in the availability of species for a reaction (concentration cells) Differential aeration cells
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eFeFe 22
tsreac
productsoo
a
aRTGQRTGG
tan
ln)ln(
)(22 2 gHeH
For:
)(2 22 gHFeHFe
2
2
22
2
][
][ln
)()(
)()(ln
H
FeG
HaFea
HfFeaGG oo
1
1
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)ln(
)ln(
QRTnFEnFE
or
QRTnFEnFE
o
o
Introduce: The total electropotential is G = -nFE
Where: F = Faraday’s constant (total charge on Avogad
ro’s number of electrons)n = the number of electrons transferredE = The electrode potential
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)ln(QnF
RTEE o
For the Given Example:
2
2
][
][ln
H
Fe
nF
RTEE o
Note: pH = -log10[H+]
Nernst Equation:
The Basic equation which describes ALL corrosion reactions
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- +
Ni
Y M
Ni2+ solution
X M
Cd2+ solution
Cd T
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• Ex: Cd-Ni cell with standard 1M solutions
• Ex: Cd-Ni cell with non-standard solutions
VNio VCd
o 0.153 VNi VCd VNi
o VCdo
RTnF
lnXY-
Ni
1.0 M
Ni2+ solution
1.0 M
Cd2+ solution
+
Cd 25°C
n = #e-
per unitoxid/redreaction(=2 here)F = Faraday'sconstant=96,500C/mol.
• Reduce VNi - VCd by --increasing X --decreasing Y
EFFECT OF SOLUTION CONCENTRATION
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Kinetics Describes Rate of Reaction Evan’s DiagramKinetics Describes Rate of Reaction Evan’s Diagram
CURRENT DENSITY, A/cm²
BB
A
0
0
E
E
io Fe ANODIC
ANODIC
Fe F
e+2 +2e-
H 2H+ +2e-
2
PO
TE
NT
IAL
VO
LT
S (
SH
E)
-0.7
-0.6
-0.5
-0.4
-0.3
-0.2
-0.1
0.0
0.1
10 10 10 10 10 10 10 10 10-10 -9 -8 -7 -6 -5 -4 -3 -2
CATHODIC
CATHODIC
Fe +2 +2e - Fe
2H + +2e - H2
A
for H2on Fe
io
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M
M+ +
e-
iaO2
Ac
ioH+ Aa
½ O2 + H
2O + 2e - 2OH -
AaioO2
CreviceEffect
No CreviceEffect
PLUS CreviceEffect
icorr in very aggresive environment
Log
E
EM/M+
EO2 /OH +
= Area Inside Crevice (Anodic)= Area Outside Crevice (Cathodic)
Aa << Ac
Aa
Ac
i
+oi H Ac
Area Effects
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Effect of Oxidizer Concentration (e.g., Oxygen) on the Electrochemical Behavior of an Active - Passive Metal
Log i
M M+
[Fontanna and Greene, Corrosion Engineering, McGraw-Hill, 1967]
Increasing OxidantConcentration
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Passivity is defined as corrosion resistance due to formation of thin surface films under oxidizing conditions with high anodic polariza tion.
Passivity
For example Fe is passivated in concentrated nitric acid.
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V o lts : S a tu ra te d C a lo m e l H a lf- C e ll R e fe re n c e E le c tro d e
G a lv a n ic S e rie s - C o n c e n tra te d H y d ro c h lo ric A c id a t 2 5 °C [C ru m a n d S c a rb e rry, C o rro s io n o f N ic ke l B a s e A llo y s C o n fe re n c e P ro c e e d in g s - A S M 1 9 8 5 ]
+0.4 +0.3 +0.2 +0.1 0 -0.1 -0.2 -0.3 -0.4 -0.5 -0.6 -0.7 -0.8 -0.9 -1.0 -1.1 -1.2 -1.3 -1.4 -1.5 -1.6 -1.7 -1.8M a g n e s iu mM a n g a n e seA lu m in u mZin cC a d m iu mLe a dA llo y 2 5 5 (F e rra liu m )H ig h P u rity Iro nC o p p e rA llo y P E 6 2A llo y 2 6 - 1 , 2 6 - 1 1 / 4C a rb o n S te e lM O N E L a llo y 4 5 1TinA llo y 1 8 - 1 8 - 2A llo y 3 R E 6 0A llo y 1 7 - 4 p HIN C O LO Y a llo y 8 4 0 , 5 0 N i - 5 0 C rB ra ss A llo y sN ic ke l S ilv e r9 0 - 1 0 C o p p e r- N ic ke lB ro n z e A llo y s7 0 - 3 0 C o p p e r- N ic ke lA llo y 2 3 0 (C o rro n e l)A llo y 7 0 C b 3S ta in le ss S te e l 3 0 4 , 3 1 6 , 3 1 6 L, 3 1 7A llo y 2 0C a s t Iro nN i- R e s is t 2A llo y 2 5 4 S L XA llo y 9 0 4 LIN C O N E L a llo y s 6 0 0 , 6 0 1 , 6 9 0 , 7 0 2 , 7 4 8 , X 7 5 0IN C O LO Y a llo y 8 2 5A llo y B , P, P D (Illiu m )A llo y GA llo y 6 X (H A )M O N E L a llo y s 4 0 0 , 4 0 4 , 4 0 5 R , K 5 0 0N ic ke l 2 0 0 , 2 7 0A llo y 7 0 0 (J e s so p )A llo y 6 XS ilv e rA llo y GIN C O LO Y a llo y 8 0 0IN C O N E L a llo y s 6 1 7 , 6 1 8 E , 6 2 5A lu m in u m A llo y 5 0 5 2S ta in le ss S te e l 4 3 0Tita n iu m+ 0 .4 - 0 . 4 8 V P la t in u m 9 2 3 4 5 r1
DISSOLVED O2 (Mg/H2O)
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30
25
20
15
10
5
0
CO
RR
OSIO
N R
ATE (
MPY))
DISSOLVED O2 (PPM)
0 1 2 3 4 5 6 7 8 9 10 11
0 0.7 1.4 2.1 2.8 3.5 4.2 4.9 5.6 6.3 7.0 7.7
Effect of Temperature and Dissolved O2
LEGEND
FRESH WATER @ 50°FFRESH WATER @ 90°FFRESH WATER @ 120°F
VELOCITY = 2.5 FPS
pH=7, R=100M-0HMpH=7, R=2500M-0HM
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Forms of
corrosion
• Uniform AttackOxidation & reductionoccur uniformly oversurface.
• Selective LeachingPreferred corrosion ofone element/constituent(e.g., Zn from brass (Cu-Zn)).
• IntergranularCorrosion alonggrain boundaries,often where specialphases exist.
• Stress corrosionStress & corrosionwork togetherat crack tips.
• GalvanicDissimilar metals arephysically joined. Themore anodic onecorrodes.(see Table17.2) Zn & Mgvery anodic.
• Erosion-corrosionBreak down of passivatinglayer by erosion (pipeelbows).
• PittingDownward propagationof small pits & holes.
• Crevice Between twopieces of the same metal.
Rivet holes
attacked zones
g.b. prec.
Fig. 17.6, Callister 6e. (Fig. 17.6 is courtesy LaQue Center for Corrosion Technology, Inc.)
Fig. 17.9, Callister 6e.
Fig. 17.8, Callister 6e.(Fig. 17.8 from M.G.Fontana, CorrosionEngineering, 3rd ed.,McGraw-Hill BookCompany, 1986.)
Forms of corrosion
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Types of Aqueous Corrosion Cells
a. General Corrosionb. Localized Corrosion
Pitting Crevice Corrosion Under-deposit Corrosion MIC
c. Tuberculationd. Galvanic Corrosion
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• Material properties• Metallurgical factors• Passivity• Environment
Metallurgical factors• Chemical segregation• Presence of multiple
phases• Inclusions• Cold Work• Non-uniform stresses
Passivity• Example with steel in
nitric acid…dilute solutions will cause rapid attack, strong solutions have little visible effect.
• Surface film can be formed
• Some types of steel may do this with rust
• Aluminum does this• Need to watch
passive film, but can be used for simple protection
Factors Affecting Corrosion
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• Self-protecting metals! --Metal ions combine with O2
to form a thin, adhering oxide layer that slows corrosion.
Metal (e.g., Al, stainless steel)
Metal oxide
• 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!).• Cathodic (or sacrificial) protection --Attach a more anodic material to the one to be protected.
Adapted from Figs. 17.13(a), 17.14 Callister 6e. (Fig. 17.13(a) is from M.G. Fontana, Corrosion Engineering, 3rd ed., McGraw-Hill Book Co., 1986.)
steel
zinczincZn2+
2e- 2e-
e.g., zinc-coated nail
steel pipe
Mg anode
Cu wiree-
Earth
Mg2+
e.g., Mg Anode
Controlling corrosion
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Introduction
Corrosion Processes
Corrosion of Metals
Passivity of Metals
Atmospheric Corrosion of Steels