eed2013 engineering materials non-mechanical properties of materials 1

EED2013 Engineering Materials

Non-Mechanical Properties of Materials

1

Overview

• Electrical Properties

• Magnetic Properties

• Thermal Properties

• Durability

2

3

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?

Electrical Properties

4

• Scanning electron micrographs of an IC:

Fig. (d) from Fig. 12.27(a), Callister & Rethwisch 3e. (Fig. 12.27 is courtesy Nick Gonzales, National Semiconductor Corp., West Jordan, UT.)

• A dot map showing location of Si (a semiconductor): -- Si shows up as light regions.

(b)

View of an Integrated Circuit

0.5 mm

(a)(d)

45 m

Al

Si (doped)

(d)

• A dot map showing location of Al (a conductor): -- Al shows up as light regions. (c)

Figs. (a), (b), (c) from Fig. 18.27, Callister & Rethwisch 8e.

5

Electrical Conduction• Ohm's Law: V = I R

voltage drop (volts = J/C) C = Coulomb

resistance (Ohms)current (amps = C/s)

1

• Conductivity,

• Resistivity, : -- a material property that is independent of sample size and geometry

RA

l

surface area of current flow

current flow path length

6

Electrical Properties

• Which will have the greater resistance?

• Analogous to flow of water in a pipe• Resistance depends on sample geometry and

size.

D

2D

R1 2

D2

2

8D2

2

R2

2D

2

2D2

R1

8

7

Definitions

Further definitions

J = <= another way to state Ohm’s law

J current density

electric field potential = V/

flux a like area surface

current

A

I

Electron flux conductivity voltage gradient

J = (V/ )

8

• Room temperature values (Ohm-m)-1 = ( - m)-1

Selected values from Tables 18.1, 18.3, and 18.4, Callister & Rethwisch 8e.

Conductivity: Comparison

Silver 6.8 x 10 7

Copper 6.0 x 10 7

Iron 1.0 x 10 7

METALS conductors

Silicon 4 x 10-4

Germanium 2 x 10 0

GaAs 10 -6

SEMICONDUCTORS

semiconductors

Polystyrene <10-14

Polyethylene 10-15-10-17

Soda-lime glass 10

Concrete 10-9

Aluminum oxide <10-13

CERAMICS

POLYMERS

insulators

-10-10-11

9

What is the minimum diameter (D) of the wire so that V < 1.5 V?

Example: Conductivity Problem

Cu wire I = 2.5 A- +

V

Solve to get D > 1.87 mm

< 1.5 V

2.5 A

6.07 x 107 (Ohm-m)-1

100 m

I

V

AR

4

2D

100 m

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

12

Band Structure Representation

Adapted from Fig. 18.3, Callister & Rethwisch 8e.

13

Conduction & Electron Transport• Metals (Conductors):-- for metals empty energy states are adjacent to filled states.

-- two types of band structures for metals

-- thermal energy excites electrons into empty higher energy states.

- partially filled band - empty band that overlaps filled band

filled band

Energy

partly filled band

empty band

GAP

fille

d st

ates

Partially filled band

Energy

filled band

filled band

empty band

fille

d st

ates

Overlapping bands

14

Energy Band Structures: Insulators & Semiconductors

• Insulators: -- wide band gap (> 2 eV) -- few electrons excited across band gap

Energy

filled band

filled valence band

fille

d st

ates

GAP

empty

bandconduction

• Semiconductors: -- narrow band gap (< 2 eV) -- more electrons excited across band gap

Energy

filled band

filled valence band

fille

d st

ates

GAP?

empty

bandconduction

15

Metals: Influence of Temperature and Impurities on Resistivity

• Presence of imperfections increases resistivity -- grain boundaries -- dislocations -- impurity atoms -- vacancies

These act to scatterelectrons so that theytake a less direct path.

• Resistivity increases with:

=

deformed Cu + 1.12 at%Ni

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.)

T (ºC)-200 -100 0

1

2

3

4

5

6

Res

istiv

ity,

(10

-8 O

hm

-m)

0

Cu + 1.12 at%Ni

“Pure” Cu

d -- %CW

+ deformation

i

-- wt% impurity

+ impurity

t

-- temperature

thermal

Cu + 3.32 at%Ni

16

Estimating Conductivity

Adapted from Fig. 7.16(b), Callister & Rethwisch 8e.

• Question:-- Estimate the electrical conductivity of a Cu-Ni alloy that has a yield strength of 125 MPa.

mOhm10 x 30 8

16 )mOhm(10 x 3.31

Yie

ld s

tren

gth

(M

Pa

)

wt% Ni, (Concentration C)0 10 20 30 40 50

6080

100120140160180

21 wt% Ni


wt% Ni, (Concentration C)R

esi

stiv

ity,

(10

-8 O

hm

-m)

10 20 30 40 500

10203040

50

0

125

CNi = 21 wt% Ni

From step 1:

30

17

Charge Carriers in Insulators and Semiconductors

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

18

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.

19

Intrinsic Semiconduction in Terms of Electron and Hole Migration


electric field electric field electric field

• Electrical Conductivity given by:

# electrons/m3 electron mobility

# holes/m3

hole mobilityhe epen

• Concept of electrons and holes:

+-

electron hole pair creation

+-

no applied applied

valence electron Si atom

applied

electron hole pair migration

20

Number of Charge Carriers

Intrinsic Conductivity

)s/Vm 45.085.0)(C10x6.1(

m)(10219

16

hei e

n

For GaAs ni = 4.8 x 1024 m-3

For Si ni = 1.3 x 1016 m-3

• Ex: GaAs

he epen

• for intrinsic semiconductor n = p = ni

= ni|e|(e + h)

21

Intrinsic Semiconductors: Conductivity vs T

• Data for Pure Silicon: -- increases with T -- opposite to metals


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.

ni e Egap / kT

ni e e h

22

• 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

Intrinsic vs Extrinsic Conduction

3+

• p-type Extrinsic: (p >> n)

no applied electric field

Boron atom

4+ 4+ 4+ 4+

4+

4+4+4+4+

4+ 4+ hep

hole

• n-type Extrinsic: (n >> p)

no applied electric field

5+

4+ 4+ 4+ 4+

4+

4+4+4+4+

4+ 4+

Phosphorus atom

valence electron

Si atom

conductionelectron

een

Adapted from Figs. 18.12(a) & 18.14(a), Callister & Rethwisch 8e.

23

Extrinsic Semiconductors: Conductivity vs. Temperature

• Data for Doped Silicon: -- increases doping -- reason: imperfection sites lower the activation energy to produce mobile electrons.

• Comparison: intrinsic vs extrinsic conduction... -- 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"

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.)

Co

nd

uct

ion

ele

ctro

n

con

cen

tra

tion

(1

021

/m3 )

T (K)6004002000

0

1

2

3

fre

eze

-ou

t

ext

rin

sic

intr

insi

c

doped

undoped

24

• 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.

-- 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.

p-n Rectifying Junction

++

++

+- ---

-p-type n-type

+ -

++ +

++

--

--

-

p-type n-typeAdapted from Fig. 18.21 Callister & Rethwisch 8e.

+++

+

+

---

--

p-type n-type- +

25

Properties of Rectifying Junction

Fig. 18.22, Callister & Rethwisch 8e. Fig. 18.23, Callister & Rethwisch 8e.

26

• 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

Summary

27


• What are the important magnetic properties?

• How does magnetic memory storage work?

Magnetic Properties

28

• Created by current through a coil:

• Computation of the applied magnetic field, H:

H N I

Generation of a Magnetic Field -- Vacuum

I = current (ampere)H

I

B0 N = total number of turns

B0 = 0Hpermeability of a vacuum(1.257 x 10-6 Henry/m)

• Computation of the magnetic flux density in a vacuum, B0:

= length of each turn (m)

H = applied magnetic field (ampere-turns/m)B0 = magnetic flux density in a vacuum (tesla)

29

• A magnetic field is induced in the material

Generation of a Magnetic Field -- within a Solid Material

current I

B = Magnetic Induction (tesla) inside the materialapplied

magnetic field H B = H

permeability of a solid

r 0

• Relative permeability (dimensionless)

B

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

31

Types of Magnetism

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.

B (

tesl

a)

H (ampere-turns/m)

vacuum (m = 0)

(1) diamagnetic (m ~ -10-5)e.g., Al2O3, Cu, Au, Si, Ag, Zn

(3) ferromagnetic e.g. Fe3O4, NiFe2O4

(4) ferrimagnetic e.g. ferrite(), Co, Ni, Gd(m as large as 106 !)

(2) paramagnetic ( e.g., Al, Cr, Mo, Na, Ti, Zr

m ~ 10-4)

32

Magnetic Responses for 4 Types

Adapted from Fig. 20.5(a), Callister & Rethwisch 8e.

No Applied Magnetic Field (H = 0)

Applied Magnetic Field (H)

(1) diamagnetic

none

oppo

sing


(2) paramagnetic

rand

om

alig

ned


(3) ferromagnetic(4) ferrimagnetic

alig

ned

alig

ned

33

• As the applied field (H) increases the magnetic domains change shape and size by movement of domain boundaries.

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.)

Domains in Ferromagnetic & Ferrimagnetic Materials

Applied Magnetic Field (H)

Mag

netic

in

duct

ion

(B)

0

Bsat

H = 0

H

H

H

H

H

• “Domains” with aligned magnetic moment grow at expense of poorly aligned ones!

34


Hysteresis and Permanent Magnetization

H

Stage 1. Initial (unmagnetized state)

B

Stage 4. Coercivity, HC

Negative H needed to demagnitize!

• The magnetic hysteresis phenomenon

Stage 2. Apply H, align domains Stage 3. Remove H, alignment

remains! => permanent magnet!

Stage 5. Apply -H, align domains

Stage 6. Close the hysteresis loop

35

Hard and Soft Magnetic Materials

Hard magnetic materials:-- large coercivities-- used for permanent magnets-- add particles/voids to inhibit domain wall motion-- example: tungsten steel -- Hc = 5900 amp-turn/m)

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.)

H

B

Har

d

So

ft

36

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

Fig. 20.23, Callister & Rethwisch 8e.

-- “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

37

Magnetic Storage Media Types

Fig. 20.25, Callister & Rethwisch 8e. (Fig. 20.25 from Seagate Recording Media)

-- 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

• Hard disk drives (granular/perpendicular media):

• Recording tape (particulate media):

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

38

• 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)

Summary

39


• 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?

Thermal Properties

40

• Quantitatively: The energy required to produce a unit rise in temperature for one mole of a material.

heat capacity(J/mol-K)

energy input (J/mol)

temperature change (K)

Heat Capacity

• Two ways to measure heat capacity:Cp : Heat capacity at constant pressure.

Cv : Heat capacity at constant volume.Cp usually > Cv

• Heat capacity has units of

Fmollb

Btu

Kmol

J

dT

dQC

The ability of a material to absorb heat

41

• Heat capacity... -- increases with temperature -- for solids it reaches a limiting value of 3R

• From atomic perspective: -- Energy is stored as atomic vibrations. -- As temperature increases, the average energy of atomic vibrations increases.

Dependence of Heat Capacity on Temperature


R = gas constant 3R = 8.31 J/mol-K

Cv = constant

Debye temperature (usually less than Troom )

T (K)D00

Cv

42

Atomic VibrationsAtomic vibrations are in the form of lattice waves or phonons


43

incr

eas

ing

cp • Why is cp significantly

larger for polymers?


• PolymersPolypropylene Polyethylene Polystyrene Teflon

cp (J/kg-K)

at room T

• CeramicsMagnesia (MgO)Alumina (Al2O3)Glass

• MetalsAluminum Steel Tungsten Gold

1925 1850 1170 1050

900 486 138 128

cp (specific heat): (J/kg-K)

Material

940 775 840

Specific Heat: Comparison

Cp (heat capacity): (J/mol-K)

44

Thermal Expansion

Materials change size when temperature is changed

l final l initial

l initial

l (Tfinal Tinitial)

linear coefficient ofthermal expansion (1/K or 1/ºC)

Tinitial

Tfinal

initial

final

Tfinal > Tinitial

45

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

46

Coefficient of Thermal Expansion: Comparison

• Q: Why does generally decrease with increasing bond energy?

Polypropylene 145-180 Polyethylene 106-198 Polystyrene 90-150 Teflon 126-216

• Polymers

• CeramicsMagnesia (MgO) 13.5Alumina (Al2O3) 7.6Soda-lime glass 9Silica (cryst. SiO2) 0.4

• MetalsAluminum 23.6Steel 12 Tungsten 4.5 Gold 14.2

(10-6/C)at room T

Material


Polymers have larger

values because of

weak secondary bonds

incr

eas

ing

47

Thermal Expansion: Example

Ex: A copper wire 15 m long is cooled from 40 to -9ºC. How much change in length will it experience?

16.5 x 10 6 (C) 1• Answer: For Cu

mm 12m 012.0

)]C9(C40[)m 15)](C/1(10 x5.16[ 60

T

rearranging Equation 19.3b

48

The ability of a material to transport heat.

temperaturegradient

thermal conductivity (J/m-K-s)

heat flux

(J/m2-s)

• Atomic perspective: Atomic vibrations and free electrons in hotter regions transport energy to cooler regions.

T2 T2 > T1

T1

x1 x2heat flux

Thermal Conductivity

dx

dTkq

Fourier’s Law

49

Thermal Conductivity: Comparisonin

crea

sing

k

• PolymersPolypropylene 0.12Polyethylene 0.46-0.50 Polystyrene 0.13 Teflon 0.25

vibration/rotation of chain molecules

• CeramicsMagnesia (MgO) 38Alumina (Al2O3) 39 Soda-lime glass 1.7 Silica (cryst. SiO2) 1.4

atomic vibrations

• MetalsAluminum 247Steel 52 Tungsten 178 Gold 315

atomic vibrations and motion of free electrons

k (W/m-K)Energy Transfer

MechanismMaterial


50

• Occur due to: -- restrained thermal expansion/contraction -- temperature gradients that lead to differential

dimensional changes

Thermal Stresses

E(T0 Tf ) ET

Thermal stress

-- 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?

Example Problem

T0

0

Solution:

Original conditions

room

thermal (Tf T0)Tf

Step 1: Assume unconstrained thermal expansion

0

Step 2: Compress specimen back to original length

0

compress room

thermal

51

52

Example Problem (cont.)

0

The thermal stress can be directly calculated as

E(compress)

E(thermal ) E(Tf T0) E(T0 Tf )

Noting that compress = -thermal and substituting gives

20 x 10-6/ºCAnswer: 106ºC 100 GPa

Tf T0

E

20ºC

Rearranging and solving for Tf gives

-172 MPa (since in compression)

53

• Occurs due to: nonuniform heating/cooling• Ex: Assume top thin layer is rapidly cooled from T1 to T2

Tension develops at surface

E(T1 T2)

Critical temperature difference

for fracture (set = f)

(T1 T2)fracture f

Eset equal

• Large TSR when is large

fk

E

Thermal Shock Resistance

Temperature difference thatcan be produced by cooling:

kTT

rate quench)( 21

rapid quench

resists contraction

tries to contract during cooling T2

T1

(quench rate)for fracture Thermal Shock Resistance (TSR)fkE

•

54

• Application:

Space Shuttle Orbiter

• Silica tiles (400-1260ºC):-- large scale application -- microstructure:

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.)

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 CeramicsSystems, Sunnyvale, CA.)

Thermal Protection System

reinf C-C (1650ºC)

Re-entry T Distribution

silica tiles(400-1260ºC)

nylon felt, silicon rubbercoating (400ºC)

~90% porosity!Si fibersbonded to oneanother duringheat treatment.

100 m

Chapter-opening photograph, Chapter 23, Callister 5e (courtesy of the National Aeronautics and Space Administration.)

55

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

Summary

fk

E

56


• How does corrosion occur?

• Which metals are most likely to corrode?

• What environmental parameters affect corrosion rate?

• How do we prevent or control corrosion?

Durability:Corrosion and Degradation of Materials

57

• Corrosion: -- the destructive electrochemical attack of a material. -- Ex: Al Capone's ship, Sapona, off the coast of Bimini.

• 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).

Photos courtesy L.M. Maestas, Sandia National Labs. Used with permission.

THE COST OF CORROSION

58

• Two reactions are necessary: -- oxidation reaction: -- reduction reaction:

Zn Zn2 2e

2H 2e H2(gas)

• Other reduction reactions in solutions with dissolved oxygen:

-- acidic solution -- neutral or basic solution

O2 4H 4e 2H2O

O2 2H2O 4e 4(OH)

Adapted from Fig. 17.1, Callister & Rethwisch 8e. (Fig. 17.1 is from M.G. Fontana, Corrosion Engineering, 3rd ed., McGraw-Hill Book Company, 1986.)

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

59

STANDARD HYDROGEN ELECTRODE• Two outcomes:

0ometal V (relative to Pt)

Standard Electrode PotentialAdapted from Fig. 17.2, Callister & Rethwisch 8e.

-- Corrosion

-- Metal is the anode (-)

Pla

tinum

met

al,

M

Mn+ ions

ne- H2(gas)

25ºC 1M Mn+ sol’n 1M H+ sol’n

2e-

e-e-

H+

H+

-- Electrodeposition

-- Metal is the cathode (+)

Mn+ ions

ne-

e- e-

25ºC1M Mn+ sol’n 1M H+ sol’n

Pla

tinum

met

al,

M

H+H+

2e-

0ometal V (relative to Pt)

H2(gas)

60

STANDARD EMF SERIES

metalo

• Metal with smaller V 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 17.1, Callister 8e.

mor

e an

odic

mor

e ca

thod

ic

V = 0.153V

o


-

1.0 M

Ni2+ solution

1.0 M

Cd2+ solution

+

25ºC NiCd

Cdo

Nio

• Ex: Cd-Ni cell V < V Cd corrodes

61

CORROSION IN A GRAPEFRUIT

Zn2+

2e- oxidation reaction

Acid

H+ H+H+

H+

H+

H+

H+-+

Zn (anode)Cu (cathode)

O2 4H 4e 2H2O

2H 2e H2(gas)

reduction reactions

Zn Zn2+ 2e

62

EFFECT OF SOLUTION CONCENTRATION AND TEMPERATURE

• Ex: Cd-Ni cell with standard 1 M solutions

VNio VCd

o 0.153 V-

Ni

1.0 M

Ni2+ solution

1.0 M

Cd2+ solution

+

Cd 25ºC

• Ex: Cd-Ni cell with non-standard solutions

Y

Xln

nF

RTVVVV o

Cdo

NiCdNi

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

63

GALVANIC SERIES• Ranking of the reactivity of metals/alloys in seawater

Based on Table 17.2, Callister & Rethwisch 8e. (Source of Table 17.2 is M.G. Fontana, Corrosion Engineering, 3rd ed., McGraw-Hill Book Company, 1986.)

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)

64

• 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., pipeelbows).

FORMS OF CORROSION

Formsof

corrosion

• Crevice Narrow and confined spaces.

Fig. 17.15, Callister & Rethwisch 8e. (Fig. 17.15 is courtesy LaQue Center for Corrosion Technology, Inc.)

Rivet holes

• IntergranularCorrosion alonggrain boundaries,often where precip.particles form.

Fig. 17.18, Callister &

Rethwisch 8e.

attacked zones

g.b. prec.

• PittingDownward propagationof small pits and holes.

Fig. 17.17, Callister & Rethwisch 8e. (Fig. 17.17 from M.G. Fontana, Corrosion Engineering, 3rd ed., McGraw-Hill BookCompany, 1986.)

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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)

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)

Adapted from Fig. 17.22(a), Callister & Rethwisch 8e.

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. 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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• 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

eed2013 engineering materials non-mechanical properties of materials 1

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