Transcript Chapter 18: Electrical Properties

EED2013 Engineering Materials
Non-Mechanical Properties of
Materials
1
Overview
•
•
•
•
Electrical Properties
Magnetic Properties
Thermal Properties
Durability
2
Electrical Properties
ISSUES TO ADDRESS...
• How are electrical conductance and resistance
characterized?
• What are the physical phenomena that distinguish
conductors, semiconductors, and insulators?
• For metals, how is conductivity affected by
imperfections, temperature, and deformation?
• For semiconductors, how is conductivity affected
by impurities (doping) and temperature?
3
View of an Integrated Circuit
• Scanning electron micrographs of an IC:
Al
Si
(doped)
(d)
(d)
(a)
45 mm
0.5 mm
• A dot map showing location of Si (a semiconductor):
-- Si shows up as light regions.
(b)
• A dot map showing location of Al (a conductor):
-- Al shows up as light regions.
Fig. (d) from Fig. 12.27(a), Callister & Rethwisch 3e.
(Fig. 12.27 is courtesy Nick Gonzales, National
Semiconductor Corp., West Jordan, UT.)
(c)
Figs. (a), (b), (c) from Fig. 18.27, Callister
& Rethwisch 8e.
4
Electrical Conduction
• Ohm's Law:
V=IR
voltage drop (volts = J/C)
resistance (Ohms)
current (amps = C/s)
C = Coulomb
• Resistivity, :
-- a material property that is independent of sample size and
geometry
RA

l
• Conductivity, 



surface area
of current flow
current flow
path length
1

5
Electrical Properties
• Which will have the greater resistance?
2
R1 
D
2D


2
8

D 2 D2
  
2 



R1
R2 


2
2
8
2D  D
  
 2 
• Analogous to flow of water in a pipe
• Resistance depends on sample
 geometry and
size.
6
Definitions
Further definitions
J=
<= another way to state Ohm’s law
J  current density
current
I


surface area A
like a flux
  electric field potential = V/
J =  (V/ )
Electron flux
conductivity
voltage gradient
7
Conductivity: Comparison
• Room temperature values (Ohm-m)-1 = ( - m)-1
METALS
CERAMICS
conductors
-10
Silver
6.8 x 10 7
Soda-lime glass 10 -10-11
Copper
6.0 x 10 7
Concrete
10 -9
Iron
1.0 x 10 7
Aluminum oxide <10-13
SEMICONDUCTORS
POLYMERS
Polystyrene
Silicon
4 x 10 -4
Polyethylene
Germanium 2 x 10 0
GaAs
10 -6
semiconductors
-14
<10
10 -15-10-17
insulators
Selected values from Tables 18.1, 18.3, and 18.4, Callister & Rethwisch 8e.
8
Example: Conductivity Problem
What is the minimum diameter (D) of the wire so that V < 1.5 V?
 100 m
I = 2.5 A
Cu wire -
V

100 m
D 2
4
Solve to get
+
R
< 1.5 V

V

A I
2.5 A
6.07 x 107 (Ohm-m)-1
D > 1.87 mm
9
Relative Permittivity, εr
• Property governs the electro-static charge
stored on an electric capacitor.
• The main equation that this is found is:
C = εoεrA/d
where C = capacitance in Farads
A = area of the capacitor plate
d = distance between the capacitor plates
εo = absolute permittivity (8.85 x 10-12)
10
Relative Permittivity, εr
• Here is a table of some example values of εr:
11
Band Structure Representation
Adapted from Fig. 18.3,
Callister & Rethwisch 8e.
12
Conduction & Electron Transport
• Metals (Conductors):
partly
filled
band
filled
band
filled states
- partially filled band
- empty band that
overlaps filled band
filled states
-- for metals empty energy states are adjacent to filled states.
-- thermal energy
Partially filled band
Overlapping bands
excites electrons
Energy
Energy
into empty higher
empty
energy states.
band
empty
-- two types of band
GAP
band
structures for metals
13
filled
band
filled
band
Energy Band Structures:
Insulators & Semiconductors
• Insulators:
• Semiconductors:
-- wide band gap (> 2 eV)
-- narrow band gap (< 2 eV)
-- few electrons excited
-- more electrons excited
across band gap
across band gap
empty
Energy
Energy
empty
conduction
conduction
band
band
filled
valence
band
filled
band
?
GAP
filled states
filled states
GAP
filled
valence
band
filled
band
14
Metals: Influence of Temperature and
Impurities on Resistivity
• Presence of imperfections increases resistivity
(10 -8 Ohm-m)
Resistivity, 
-- grain boundaries
-- dislocations
-- impurity atoms
-- vacancies
6
These act to scatter
electrons so that they
take a less direct path.
• Resistivity
5
increases with:
4
3
d
2
i
1
0
-- temperature
-- wt% impurity
-- %CW
t
-200
-100
0
T (ºC)
Adapted from Fig. 18.8, Callister & Rethwisch 8e. (Fig. 18.8
adapted from J.O. Linde, Ann. Physik 5, p. 219 (1932); and C.A.
Wert and R.M. Thomson, Physics of Solids, 2nd ed., McGraw-Hill
Book Company, New York, 1970.)
 = thermal
+ impurity
+ deformation
15
Estimating Conductivity
• Question:
180
160
140
125
120
100
21 wt% Ni
80
60
0 10 20 30 40 50
Resistivity, 
(10 -8 Ohm-m)
Yield strength (MPa)
-- Estimate the electrical conductivity  of a Cu-Ni alloy
that has a yield strength of 125 MPa.
wt% Ni, (Concentration C)
Adapted from Fig. 7.16(b), Callister & Rethwisch 8e.
From step 1:
CNi = 21 wt% Ni
Adapted from Fig.
18.9, Callister &
Rethwisch 8e.
50
40
30
20
10
0
0 10 20 30 40 50
wt% Ni, (Concentration C)
8
  30 x 10 Ohm  m
1
   3.3 x 106(Ohm  m)1

16
Charge Carriers in Insulators and
Semiconductors
Adapted from Fig. 18.6(b),
Callister & Rethwisch 8e.
Two types of electronic charge
carriers:
Free Electron
– negative charge
– in conduction band
Hole
– positive charge
– vacant electron state in
the valence band
Move at different speeds - drift velocities
17
Intrinsic Semiconductors
• Pure material semiconductors: e.g., silicon &
germanium
– Group IVA materials
• Compound semiconductors
– III-V compounds
• Ex: GaAs & InSb
– II-VI compounds
• Ex: CdS & ZnTe
– The wider the electronegativity difference between
the elements the wider the energy gap.
18
Intrinsic Semiconduction in Terms of
Electron and Hole Migration
• Concept of electrons and holes:
valence
electron
electron
hole
pair creation
Si atom
+ -
no applied
electric field
electron
hole
pair migration
applied
electric field
• Electrical Conductivity given by:
+
applied
electric field
Adapted from Fig. 18.11,
Callister & Rethwisch 8e.
# holes/m3
  n e me  p e m h
# electrons/m3
hole mobility
electron mobility
19
Number of Charge Carriers
Intrinsic Conductivity
  n e me  p e m h
• for intrinsic semiconductor n = p = ni
 = ni|e|(me + mh)

• Ex: GaAs

106 (  m) 1
ni 

e me  mh  (1.6x1019 C)(0.85  0.45 m2 /V  s)
For GaAs
For Si
ni = 4.8 x 1024 m-3
ni = 1.3 x 1016 m-3
20
Intrinsic Semiconductors:
Conductivity vs T
• Data for Pure Silicon:
--  increases with T
-- opposite to metals
  ni e me  mh 
E gap / kT


ni  e
material
Si
Ge
GaP
CdS
band gap (eV)
1.11
0.67
2.25
2.40
Selected values from Table 18.3,
Callister & Rethwisch 8e.
Adapted from Fig. 18.16,
Callister & Rethwisch 8e.
21
Intrinsic vs Extrinsic Conduction
• Intrinsic:
-- case for pure Si
-- # electrons = # holes (n = p)
• Extrinsic:
-- electrical behavior is determined by presence of impurities
that introduce excess electrons or holes
-- n ≠ p
• n-type Extrinsic: (n >> p)
• p-type Extrinsic: (p >> n)
Phosphorus atom
4+ 4+ 4+ 4+
  n e me
4+ 5+ 4+ 4+
4+ 4+ 4+ 4+
Adapted from Figs. 18.12(a)
& 18.14(a), Callister &
Rethwisch 8e.
no applied
electric field
Boron atom
hole
conduction
electron
4+ 4+ 4+ 4+
valence
electron
4+ 4+ 4+ 4+
Si atom
4+ 3+ 4+ 4+
no applied
electric field
  p e mh
22
Extrinsic Semiconductors: Conductivity
vs. Temperature
• Data for Doped Silicon:
--  increases doping
-- reason: imperfection sites
-- extrinsic doping level:
1021/m3 of a n-type donor
impurity (such as P).
-- for T < 100 K: "freeze-out“,
thermal energy insufficient to
excite electrons.
-- for 150 K < T < 450 K: "extrinsic"
-- for T >> 450 K: "intrinsic"
1
extrinsic
2
intrinsic
3
freeze-out
extrinsic conduction...
concentration (1021/m3)
• Comparison: intrinsic vs
undoped
Conduction electron
lower the activation energy to
produce mobile electrons.
doped
0
0
200
400
600
T (K)
Adapted from Fig. 18.17, Callister & Rethwisch
8e. (Fig. 18.17 from S.M. Sze, Semiconductor
Devices, Physics, and Technology, Bell
Telephone Laboratories, Inc., 1985.)
23
p-n Rectifying Junction
• Allows flow of electrons in one direction only (e.g., useful
to convert alternating current to direct current).
• Processing: diffuse P into one side of a B-doped crystal.
+ p-type
+ +
+ +
-- No applied potential:
no net current flow.
-- Forward bias: carriers
flow through p-type and
n-type regions; holes and
electrons recombine at
p-n junction; current flows.
-- Reverse bias: carriers
flow away from p-n junction;
junction region depleted of
carriers; little current flow.
n-type
-
-
-
Adapted from
Fig. 18.21
Callister &
Rethwisch
8e.
-
p-type
+
-
+ - n-type
+
++- - + -
+ p-type
+ +
+ +
n-type
-
-
-
-
+
24
Properties of Rectifying Junction
Fig. 18.22, Callister & Rethwisch 8e.
Fig. 18.23, Callister & Rethwisch 8e.
25
Summary
• Electrical conductivity and resistivity are:
-- material parameters
-- geometry independent
• Conductors, semiconductors, and insulators...
-- differ in range of conductivity values
-- differ in availability of electron excitation states
• For metals, resistivity is increased by
-- increasing temperature
-- addition of imperfections
-- plastic deformation
• For pure semiconductors, conductivity is increased by
-- increasing temperature
-- doping [e.g., adding B to Si (p-type) or P to Si (n-type)]
• Other electrical characteristics
-- ferroelectricity
-- piezoelectricity
26
Magnetic Properties
ISSUES TO ADDRESS...
• What are the important magnetic properties?
• How does magnetic memory storage work?
27
Generation of a Magnetic Field -Vacuum
• Created by current through a coil:
B0
N = total number of turns
 = length of each turn (m)
I = current (ampere)
H = applied magnetic field (ampere-turns/m)
B0 = magnetic flux density in a vacuum
(tesla)
H
I
• Computation of the applied magnetic field, H:
H
NI
• Computation of the magnetic flux density in a vacuum, B0:

B0 = m0H
permeability of a vacuum
(1.257 x 10-6 Henry/m)
28
Generation of a Magnetic Field -within a Solid Material
• A magnetic field is induced in the material
B
applied
magnetic
field H
current I
B = Magnetic Induction (tesla)
inside the material
B = mH
permeability of a solid
m
• Relative permeability (dimensionless) mr 
m0
29
Relative Permeability, μr
• Property that governs the magnetic strength of a
material.
• The main equation that this property occurs is
B/H = μoμr
where B = flux density in Tesla
H = magnetising force
μo = absolute permeability (12.566 x 10-7)
30
B (tesla)
Types of Magnetism
(3) ferromagnetic e.g. Fe3O4, NiFe2O4
(4) ferrimagnetic e.g. ferrite(), Co, Ni, Gd
( cm as large as 106 !)
(2) paramagnetic ( cm ~ 10-4)
e.g., Al, Cr, Mo, Na, Ti, Zr
vacuum (cm = 0)
(1) diamagnetic (cm ~ -10-5)
e.g., Al2O3, Cu, Au, Si, Ag, Zn
H (ampere-turns/m)
Plot adapted from Fig. 20.6, Callister & Rethwisch 8e.
Values and materials from Table 20.2 and discussion in
Section 20.4, Callister & Rethwisch 8e.
31
Magnetic Responses for 4 Types
none
opposing
Adapted from Fig.
20.5(a), Callister &
Rethwisch 8e.
(2) paramagnetic
random
aligned
Adapted from Fig.
20.5(b), Callister &
Rethwisch 8e.
(3) ferromagnetic
(4) ferrimagnetic
aligned
Applied
Magnetic Field (H)
aligned
No Applied
Magnetic Field (H = 0)
Adapted from Fig.
20.7, Callister &
Rethwisch 8e.
(1) diamagnetic
32
Domains in Ferromagnetic &
Ferrimagnetic Materials
• As the applied field (H) increases the magnetic domains
change shape and size by movement of domain boundaries.
B sat
H
Magnetic
induction (B)
H
H
H
H
0
• “Domains” with
aligned magnetic
moment grow at
expense of poorly
aligned ones!
Adapted from Fig. 20.13,
Callister & Rethwisch
8e. (Fig. 20.13 adapted
from O.H. Wyatt and D.
Dew-Hughes, Metals,
Ceramics, and
Polymers, Cambridge
University Press, 1974.)
Applied Magnetic Field (H)
H=0
33
Hysteresis and Permanent
Magnetization
• The magnetic hysteresis phenomenon
B
Stage 3. Remove H, alignment
remains! => permanent magnet!
Stage 2. Apply H,
align domains
H
Stage 4. Coercivity, HC
Negative H needed to
demagnitize!
Stage 5. Apply -H,
align domains
Adapted from Fig. 20.14,
Callister & Rethwisch 8e.
Stage 1. Initial (unmagnetized state)
Stage 6. Close the
hysteresis loop
34
Hard and Soft Magnetic Materials
B
-- large coercivities
-- used for permanent magnets
-- add particles/voids to
inhibit domain wall motion
-- example: tungsten steel -Hc = 5900 amp-turn/m)
Soft
Hard magnetic materials:
H
Soft magnetic materials:
-- small coercivities
-- used for electric motors
-- example: commercial iron 99.95 Fe
Adapted from Fig. 20.19, Callister & Rethwisch
8e. (Fig. 20.19 from K.M. Ralls, T.H. Courtney,
and J. Wulff, Introduction to Materials Science
and Engineering, John Wiley and Sons, Inc.,
1976.)
35
Magnetic Storage
• Digitized data in the form of electrical signals are transferred to
and recorded digitally on a magnetic medium (tape or disk)
• This transference is accomplished by a recording system that
consists of a read/write head
-- “write” or record data by applying a
magnetic field that aligns domains
in small regions of the recording
medium
-- “read” or retrieve data from
medium by sensing changes
in magnetization
Fig. 20.23, Callister &
Rethwisch 8e.
36
Magnetic Storage Media Types
-- CoCr alloy grains (darker regions)
separated by oxide grain boundary
segregant layer (lighter regions)
-- Magnetization direction of each
grain is perpendicular to plane of
disk
80 nm
• Hard disk drives (granular/perpendicular media):
Fig. 20.25, Callister
& Rethwisch 8e.
(Fig. 20.25 from
Seagate Recording
Media)
• Recording tape (particulate media):
~ 500 nm
~ 500 nm
Fig. 20.24, Callister
& Rethwisch 8e.
(Fig. 20.24
courtesy Fuji Film
Inc., Recording
Media Division)
-- Acicular (needle-shaped)
ferromagnetic metal alloy
particles
-- Tabular (plate-shaped)
ferrimagnetic barium-ferrite
particles
37
Summary
• A magnetic field is produced when a current flows
through a wire coil.
• Magnetic induction (B):
-- an internal magnetic field is induced in a material that is
situated within an external magnetic field (H).
-- magnetic moments result from electron interactions with
the applied magnetic field
• Types of material responses to magnetic fields are:
-- ferrimagnetic and ferromagnetic (large magnetic susceptibilities)
-- paramagnetic (small and positive magnetic susceptibilities)
-- diamagnetic (small and negative magnetic susceptibilities)
• Types of ferrimagnetic and ferromagnetic materials:
-- Hard: large coercivities
-- Soft: small coercivities
• Magnetic storage media:
-- particulate barium-ferrite in polymeric film (tape)
-- thin film Co-Cr alloy (hard drive)
38
Thermal Properties
ISSUES TO ADDRESS...
• How do materials respond to the application of heat?
• How do we define and measure...
-- heat capacity?
-- thermal expansion?
-- thermal conductivity?
-- thermal shock resistance?
• How do the thermal properties of ceramics, metals,
and polymers differ?
39
Heat Capacity
The ability of a material to absorb heat
• Quantitatively: The energy required to produce a unit rise in
temperature for one mole of a material.
heat capacity
(J/mol-K)
dQ
C
dT
energy input (J/mol)
temperature change (K)
• Two ways to measure heat capacity:
Cp : Heat capacity at constant pressure.
Cv : Heat capacity at constant volume.
Cp usually > Cv
J
Btu




• Heat capacity has units of
mol  K  lb  mol  F 
40
Dependence of Heat Capacity on
Temperature
• Heat capacity...
-- increases with temperature
-- for solids it reaches a limiting value of 3R
R = gas constant 3R
= 8.31 J/mol-K
0
Cv = constant
0
qD
• From atomic perspective:
T (K)
Adapted from Fig. 19.2,
Callister & Rethwisch 8e.
Debye temperature
(usually less than T room )
-- Energy is stored as atomic vibrations.
-- As temperature increases, the average energy of
atomic vibrations increases.
41
Atomic Vibrations
Atomic vibrations are in the form of lattice waves or phonons
Adapted from Fig. 19.1,
Callister & Rethwisch 8e.
42
Specific Heat: Comparison
increasing cp
Material
• Polymers
Polypropylene
Polyethylene
Polystyrene
Teflon
cp (J/kg-K)
at room T
1925
1850
1170
1050
• Ceramics
Magnesia (MgO)
Alumina (Al2O3)
Glass
940
775
840
• Metals
Aluminum
Steel
Tungsten
Gold
900
486
138
128
cp (specific heat): (J/kg-K)
Cp (heat capacity): (J/mol-K)
• Why is cp significantly
larger for polymers?
Selected values from Table 19.1,
Callister & Rethwisch 8e.
43
Thermal Expansion
Materials change size when temperature
is changed
Tinitial
 initial
Tfinal
 final
l
f inal
l
l
initial
Tfinal > Tinitial
  l (Tf inal Tinitial )
initial
linear coefficient of
thermal expansion (1/K or 1/ºC)

44
Atomic Perspective: Thermal
Expansion
Asymmetric curve:
-- increase temperature,
-- increase in interatomic
separation
-- thermal expansion
Symmetric curve:
-- increase temperature,
-- no increase in interatomic
separation
-- no thermal expansion
Adapted from Fig. 19.3, Callister & Rethwisch 8e.
45
Coefficient of Thermal Expansion:
Comparison
Material
increasing 
• Polymers
Polypropylene
Polyethylene
Polystyrene
Teflon
• Metals
Aluminum
Steel
Tungsten
Gold
• Ceramics
Magnesia (MgO)
Alumina (Al2O3)
Soda-lime glass
Silica (cryst. SiO2)
 (10-6/C)
at room T
145-180
106-198
90-150
126-216
23.6
12
4.5
14.2
13.5
7.6
9
0.4
Polymers have larger
 values because of
weak secondary bonds
• Q: Why does 
generally decrease
with increasing
bond energy?
Selected values from Table 19.1,
Callister & Rethwisch 8e.
46
Thermal Expansion: Example
Ex: A copper wire 15 m long is cooled from
40 to -9ºC. How much change in length will it
experience?
• Answer: For Cu
  16.5 x 106 ( C)1
rearranging Equation 19.3b
     0 T  [
16.5 x 10 6 (1/ C)](15 m)[ 40C  ( 9C)]
  0.012 m  12 mm
47
Thermal Conductivity
The ability of a material to transport heat.
Fourier’s Law
heat flux
(J/m2-s)
dT
q  k
dx
temperature
gradient
thermal conductivity (J/m-K-s)
T2
T1
x1
heat flux
T2 > T1
x2
• Atomic perspective: Atomic vibrations and free electrons in
hotter regions transport energy to cooler regions.
48
increasing k
Thermal Conductivity: Comparison
Material
k (W/m-K)
• Metals
Aluminum
247
Steel
52
Tungsten
178
Gold
315
• Ceramics
Magnesia (MgO)
38
Alumina (Al2O3)
39
Soda-lime glass
1.7
Silica (cryst. SiO2)
1.4
• Polymers
Polypropylene
0.12
Polyethylene
0.46-0.50
Polystyrene
0.13
Teflon
0.25
Energy Transfer
Mechanism
atomic vibrations
and motion of free
electrons
atomic vibrations
vibration/rotation of
chain molecules
Selected values from Table 19.1, Callister & Rethwisch 8e.
49
Thermal Stresses
• Occur due to:
-- restrained thermal expansion/contraction
-- temperature gradients that lead to differential
dimensional changes
Thermal stress 
 E (T0 Tf )  E T

50
Example Problem
-- A brass rod is stress-free at room temperature (20ºC).
-- It is heated up, but prevented from lengthening.
-- At what temperature does the stress reach -172 MPa?
Solution:
T0
Original conditions
0
Step 1: Assume unconstrained thermal expansion
0


Tf
 thermal   (Tf T0 )
room
Step 2: Compress specimen back to original length
0




compress 

 thermal
room
51
Example Problem (cont.)
0


The thermal stress can be directly
calculated as
  E(compress)
Noting that compress = -thermal and substituting gives
  E(thermal )  E
 (Tf T0 )  E (T0 Tf )
Rearranging and solving for Tf gives

20ºC
Tf  T0 
Answer: 106ºC

100 GPa

E
-172 MPa (since in compression)
20 x 10-6/ºC
52
Thermal Shock Resistance
• Occurs due to: nonuniform heating/cooling
• Ex: Assume top thin layer is rapidly cooled from T1 to T2
rapid quench
tries to contract during cooling
T2
resists contraction
T1

Tension develops at surface
  E (T1 T2 )
Critical temperature difference
for fracture (set  = f)
Temperature difference that
can be produced by cooling:
(T1  T2 ) 
quench rate
k

(T1 T2 ) f racture 
f
E
set equal
• (quench rate) f or f racture  Thermal
Shock Resistance ( TSR) 
• Large TSR when
f k
is large
E
f k
E
53
Thermal Protection System
• Application:
Re-entry T
Distribution
Space Shuttle Orbiter
Chapter-opening photograph, Chapter 23, Callister 5e
(courtesy of the National Aeronautics and Space
Administration.)
• Silica tiles (400-1260ºC):
-- large scale application
reinf C-C
silica tiles
(1650ºC) (400-1260ºC)
nylon felt, silicon rubber
coating (400ºC)
Fig. 19.2W, Callister 6e. (Fig. 19.2W adapted from L.J.
Korb, C.A. Morant, R.M. Calland, and C.S. Thatcher, "The
Shuttle Orbiter Thermal Protection System", Ceramic
Bulletin, No. 11, Nov. 1981, p. 1189.)
-- microstructure:
~90% porosity!
Si fibers
bonded to one
another during
heat treatment.
100 mm
Fig. 19.3W, Callister 5e. (Fig. 19.3W courtesy the
National Aeronautics and Space Administration.)
Fig. 19.4W, Callister 5e. (Fig. 219.4W courtesy
Lockheed Aerospace Ceramics
Systems, Sunnyvale, CA.)
54
Summary
The thermal properties of materials include:
• Heat capacity:
-- energy required to increase a mole of material by a unit T
-- energy is stored as atomic vibrations
• Coefficient of thermal expansion:
-- the size of a material changes with a change in temperature
-- polymers have the largest values
• Thermal conductivity:
-- the ability of a material to transport heat
-- metals have the largest values
• Thermal shock resistance:
-- the ability of a material to be rapidly cooled and not fracture
-- is proportional to
f k
E
55
Durability:
Corrosion and Degradation of Materials
ISSUES TO ADDRESS...
• How does corrosion occur?
• Which metals are most likely to corrode?
• What environmental parameters affect
corrosion rate?
• How do we prevent or control corrosion?
56
THE COST OF CORROSION
• Corrosion:
-- the destructive electrochemical attack of a material.
-- Ex: Al Capone's
ship, Sapona,
off the coast
of Bimini.
Photos courtesy L.M. Maestas, Sandia
National Labs. Used with permission.
• Cost:
-- 4 to 5% of the Gross National Product (GNP)*
-- in the U.S. this amounts to just over $400 billion/yr**
* 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).
57
ELECTROCHEMICAL CORROSION
Ex: consider the corrosion of zinc in an acid solution
• Two reactions are necessary:
-- oxidation reaction:
-- reduction reaction:
Zn  Zn2  2e
2H  2e  H2 (gas)
H+
Oxidation reaction
 Zn Zn2+
H+
Zinc
flow of e
2ein the metal
H+
H+ +
H
H+
H2(gas)
H+
reduction reaction
Acid
solution
Adapted from Fig. 17.1,
Callister & Rethwisch 8e.
(Fig. 17.1 is from M.G.
Fontana, Corrosion
Engineering, 3rd ed., McGrawHill Book Company, 1986.)
• Other reduction reactions in solutions with dissolved oxygen:
-- acidic solution
O2  4H  4e  2H2O
-- neutral or basic solution
O2  2H2O  4e  4(OH)
58
STANDARD HYDROGEN ELECTRODE
• Two outcomes:
-- Electrodeposition
H2(gas)
Mn+ H+
ions
H+
e-
25ºC
e-
ne -
2e -
metal, M
metal, M
ne -
e-
Platinum
e-
Mn+
ions
H+ 2e H+
H2(gas)
Platinum
-- Corrosion
25ºC
1M Mn+ sol’n 1M H + sol’n
1M Mn+ sol’n 1M H+ sol’n
-- Metal is the anode (-)
-- Metal is the cathode (+)
o
Vmetal
 0 (relative to Pt)
o
Vmetal
 0 (relative to Pt)
Standard Electrode Potential
Adapted from Fig. 17.2,
Callister & Rethwisch 8e.
59
STANDARD EMF SERIES
more anodic
more cathodic
• EMF series
metal
Au
Cu
Pb
Sn
Ni
Co
Cd
Fe
Cr
Zn
Al
Mg
Na
K
o
Vmetal
• Metal with smaller
o
Vmetal
corrodes.
+1.420 V
• Ex: Cd-Ni cell
+0.340
o
o
V
<
V
 Cd corrodes
Ni
Cd
- 0.126
- 0.136
+
- 0.250
V o =
- 0.277
0.153V
- 0.403
- 0.440
Cd
Ni
25ºC
- 0.744
- 0.763
- 1.662
1.0 M
1.0 M
- 2.363
Cd 2+ solution Ni 2+ solution
- 2.714
Adapted from Fig. 17.2,
Data based on Table 17.1,
Callister & Rethwisch 8e.
- 2.924 Callister 8e.
60
CORROSION IN A GRAPEFRUIT
Cu (cathode)
Zn (anode)
+
H+


O2  4H  4e  2H2O
Zn 2+
2e -
reduction reactions
2H  2e  H2 (gas)
H+
-
H+
H+
oxidation reaction
H+
Acid
H+
Zn  Zn2+  2e
H+

61
EFFECT OF SOLUTION CONCENTRATION AND
TEMPERATURE
• Ex: Cd-Ni cell with
standard 1 M solutions
o
VNio VCd
 0.153 V
-
Cd
+
25ºC
Ni
1.0 M
1.0 M
Cd 2+ solution Ni 2+ solution
• Ex: Cd-Ni cell with
non-standard solutions
RT X
o
o
VNi  VCd  VNi  VCd 
ln
nF Y
-
Cd
+
T
Ni
XM
YM
Cd 2+ solution Ni 2+ solution
• Reduce VNi - VCd by
-- increasing X
-- decreasing Y
-- increasing T
n = #eper unit
oxid/red
reaction
(= 2 here)
F=
Faraday's
constant
= 96,500
C/mol.
62
GALVANIC SERIES
more anodic
(active)
more cathodic
(inert)
• Ranking of the reactivity of metals/alloys in seawater
Platinum
Gold
Graphite
Titanium
Silver
316 Stainless Steel (passive)
Nickel (passive)
Copper
Nickel (active)
Tin
Lead
316 Stainless Steel (active)
Iron/Steel
Aluminum Alloys
Cadmium
Zinc
Magnesium
Based on Table 17.2, Callister &
Rethwisch 8e. (Source of Table
17.2 is M.G. Fontana, Corrosion
Engineering, 3rd ed., McGrawHill Book Company, 1986.)
63
FORMS OF CORROSION
• Stress corrosion
Corrosion at crack tips
• Uniform Attack
when a tensile stress • Erosion-corrosion
Oxidation & reduction
Combined chemical attack and
is present.
reactions occur uniformly
mechanical wear (e.g., pipe
over surfaces.
elbows).
• Selective Leaching
• Pitting
Preferred corrosion of
one element/constituent
[e.g., Zn from brass (Cu-Zn)].
• Intergranular
Corrosion along
grain boundaries,
often where precip.
particles form.
g.b.
prec.
attacked
zones
Fig. 17.18, Callister &
Rethwisch 8e.
Forms
of
corrosion
• Galvanic
Downward propagation
of small pits and holes.
Fig. 17.17, Callister &
Rethwisch 8e. (Fig. 17.17
from M.G. Fontana,
Corrosion Engineering,
3rd ed., McGraw-Hill Book
Company, 1986.)
• Crevice Narrow and
Dissimilar metals are confined spaces.
Rivet holes
physically joined in the
presence of an
electrolyte. The
Fig. 17.15, Callister & Rethwisch 8e. (Fig. 17.15
more anodic metal
is courtesy LaQue Center for Corrosion
Technology, Inc.)
corrodes.
64
CORROSION PREVENTION (i)
• Materials Selection
-- Use metals that are relatively unreactive in the
corrosion environment -- e.g., Ni in basic solutions
-- Use metals that passivate
- These metals form a thin,
adhering oxide layer that
slows corrosion.
Metal oxide
Metal (e.g., Al,
stainless steel)
• Lower the temperature (reduces rates of oxidation and
reduction)
• Apply physical barriers -- e.g., films and coatings
65
CORROSION PREVENTION (ii)
• 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.
• Cathodic (or sacrificial) protection
-- Attach a more anodic material to the one to be protected.
Galvanized Steel
Adapted
from Fig.
17.23,
Callister &
Rethwisch
8e.
Using a sacrificial anode
Zn 2+
zinc
zinc
2e - 2e steel
e.g., zinc-coated nail
steel
pipe
e-
Cu wire
Mg Mg 2+
anode
Earth
e.g., Mg Anode
Adapted
from Fig.
17.22(a),
Callister &
Rethwisch
8e.
66
SUMMARY
• 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
67