chapter17 corrosion°radationofmaterials

8/19/2019 Chapter17 Corrosion&DegradationofMaterials

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Photograph of a 1936 Deluxe Ford Sedan

having a body that is made entirely of

unpainted stainless steel. Six of these cars were

manufactured to provide an ultimate test as to

the durability and corrosion resistance of stainless steels. Each automobile has logged

hundreds miles of everyday driving. Whereas

the surface finish on the stainless steel is

essentially the same as when the car left the

manufacture’s assembly line, other nonstainless

components such as the engine, shock

absorbers, brakers, springs, clutch,transmission, and gears have had to be

replaced; for example, one car has gone

through three engines.

Materials Science and Engineering 魏茂國Corrosion and Degradation of Materials

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By way of contrast, a classic automobile of the same vintage as the one above that is

rusting away in a field in Bodie, California. Its body is made of a plain-carbon steel

that at one time was painted. This paint offered limited protection for the steel, which

is susceptible to corrosion in normal atmospheric environments.

Materials Science and Engineering 魏茂國Corrosion and Degradation of Materials

3


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With a knowledge of the types of and an understanding of the mechanisms and

causes of corrosion and degradation, it is possible to take measures to prevent

them from occurring. For example, we may change the nature of the environment,

select a material that is relatively nonreactive, and/or protect the material from

appreciable deterioration.

Materials Science and Engineering 魏茂國Why Study Corrosion and Degradation of Materials?

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1. Distinguish between oxidation and reduction electrochemical reactions.

2. Describe the following: galvanic couple, standard half-cell, and standard hydrogen

electrode.

3. Compute the cell potential and write the spontaneous electrochemical reaction

direction for two pure metals that are electrically connected and also submerged insolutions of their respective ions.

4. Determine metal oxidation rate given the reaction current density.

5. Name and briefly describe the two different types of polarization, and specify the

conditions under which each is rate controlling.

6. For each of the eight forms of corrosion and hydrogen embrittlement, describe the

nature of the deteriorative process, and then note the proposed mechanism.

7. List five measures that are commonly used to prevent corrosion.8. Explain why ceramic materials are, in general, very resistant to corrosion.

9. For polymeric materials, discuss (a) two degradation processes that occur when

they are exposed to liquid solvents and (b) the causes and consequences of

molecular chain bond rupture.

Materials Science and Engineering 魏茂國Learning Objectives

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Deteriorative mechanisms

- Metals

In metals, there is actual material loss either by dissolution (corrosion) or by the

formation of nonmetallic scale or film (oxidation).

- Ceramics

Ceramic materials are relatively resistant to deterioration, which usually occurs at

elevated temperatures or in rather extreme environments; the process is frequently

called corrosion.

- Polymers

Polymers may dissolve when exposed to a liquid solvent, or they may absorb the

solvent and swell; also, electromagnetic radiation (UV) and heat may causealterations in their molecular structure. The processes are called degradation.

Materials Science and Engineering 魏茂國Introduction

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Corrosion of metals

- Corrosion is defined as the destructive and unintentional attack of a metal.

- Corrosion is a chemical reaction in which there is transfer of electrons from one

chemical species to another.

- Corrosion is electrochemical and ordinarily at the surface.

Importance of corrosion

Approximately 5% of an industrialized nation’s income is spent on corrosion

prevention and the maintenance or replacement of products lost or contaminated asa result of corrosion reactions.

Advantage of corrosion

- Etching procedures make use of the selective chemical reactivity of grain boundaries or various microstructural constituents.

- The current developed in dry-cell batteries is a result of corrosion processes.

Materials Science and Engineering 魏茂國Introduction

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Oxidation reaction

- Metal atoms characteristically lose or give up electrons in what is called an

oxidation reaction.

M Mn+ + ne- (17.1)

n: valence (number of valence electrons)- The site at which oxidation takes place is called the anode.

- Oxidation is sometimes called an anodic reaction.

- Examples

Fe Fe2+ + 2e- (17.2a)

Al Al3+ + 3e- (17.2b)

Materials Science and Engineering 魏茂國Electrochemical Considerations

8


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Reduction reaction

- The electrons generated from each metal atom that is oxidized must be transferred

to and become a part of another chemical species in what is termed a reduction

reaction.

- In acid solutions, which have a high concentration of hydrogen ions2H+ + 2e- H2 (17.3)

- For an acid solution having dissolved oxygen

O2 + 4H+ + 4e- 2H2O (17.4)

- For a neutral or basic aqueous solution in which oxygen is dissolved

O2 + 2H2O + 4e- 4(OH-) (17.5)

- Any metal ions present in the solution may be reduced

Mn+

+ e- M

(n-1)+

(17.6)or Mn+ + ne- M (17.7)

- The location at which reduction occurs is called cathode.

- It is possible for two or more of the reduction reactions to occur simultaneous.


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10


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Overall electrochemical reaction

- An overall electrochemical reaction must consist of at least one oxidation and one

reduction, and will be the sum of them.

- The individual oxidation and reduction reactions are termed half-reactions.

- The total rate of oxidation must equal the total rate of reduction, or all electronsgenerated through oxidation must be consumed by reduction.

Fig. 17.1 The electrochemical reactions associated

with the corrosion of zinc in an acid solution.

Zn Zn2+ + 2e-

2H+ + 2e- H2 (gas)

Zn + 2H+ Zn2+ + H2 (gas)

(17.8) oxidation

(17.9) reduction

(17.10) overall


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- Another example is the oxidation or rusting of iron in water, which contains

dissolved oxygen.

This process occurs in two steps; in the first, Fe is oxidized to Fe2+ [as Fe(OH)2],

Fe + ½O2 + H2O Fe2+ + 2OH- Fe(OH)2 (17.11)

in the second stage, to Fe3+

[as Fe(OH)3].2Fe(OH)2 + ½O2 + H2O 2Fe(OH)3 (17.12)


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Electrode potentials

- If the iron and copper electrodes are connected electrically, reduction will occur for

copper at the expense of the oxidation of iron.

- Cu2+ ions will deposit (electrodeposit) as metallic copper on the copper electrode,

while iron dissolves (corrodes) on the other side of the cell and goes into solutionas Fe2+ ions. Fe Fe2+ + 2e- (oxidation) (17.14a)

Cu2+ + 2e- Cu (reduction) (17.14b)

Cu2+ + Fe Cu + Fe2+ (overall reaction) (17.13)

- When a current passes through the external circuit,

electrons generated from the oxidation of iron flow to

the copper cell in order that Cu2+ be reduced.

- There will be some net ion motion from each cell tothe other across the membrane.


13

Fig. 17.2 An electrochemical cell consisting of iron and

copper electrodes, each of which is immersed in a 1M

solution of its ion. Iron corrodes while copper electrodeposits.


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- Galvanic couple

Two metals electrically connected in a liquid electrolyte wherein one metal becomes

an anode and corrodes, while the other acts as a cathode.

- An electric potential or voltage will exist between the two cell halves, and its

magnitude can be determined if a voltmeter is connected in the external circuit.- Various electrode pairs have different voltages; the

magnitude of such a voltage may be thought of as

representing the driving force for the electrochemical

oxidation-reduction reaction.

Cu2+ + Fe Cu + Fe2+ (0.780V)

Fe2+ + Zn Fe + Zn2+ (0.323V) (17.15)

- Standard half-cell: a half-cell of a metal electrodeimmersed in a 1M solution of ions and at 25C.


14

Fig. 17.3 An electrochemical cell consisting of iron and

zinc electrodes, each of which is immersed in a 1M

solution of its ion. The iron electrodeposits while the zinccorrodes.


15/84

Fig. 17.4 The standard hydrogen reference half-cell.

Standard hydrogen reference half cell

It consists of an inert platinum electrode in a 1M solution of H+ ions, saturated with

hydrogen gas that is bubbled through the solution at a pressure of 1 atm and a

temperature of 25C. Standard electromotive force (emf) series

- It is generated by coupling to the standard hydrogenelectrode, standard half-cells for various metals and

ranking them according to measured voltage.

- Consider the generalized reactions involving the

oxidation of metal M1 and the reduction of metal M2

the overall cell potential V0

For this reaction to occur spontaneously, V0 must

be positive.

ne M M n11

22 M ne M n

2121 M M M M nn

0

1V 0

2V

0

1

0

2 V V


15

(17.16a)

(17.16b)

(17.17) (17.18)


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(oxidation)

Au3+ + 3e- AuO2 + 4H

+ + 4e- 2H2OPt2+ + 2e- Pt

Ag+ + e- AgFe3+ + e- Fe2+

O2 + 2H2O + 4e- 4(OH-)

Cu2+ + 2e- Cu2H+ + 2e- H2

Pb2+ + 2e- PbSn2+ + 2e- Sn Ni2+ + 2e- NiCo2+ + 2e- CoCd 2+ + 2e- Cd Fe2+ + 2e- FeCr 3+ + 3e- Cr Zn2+ + 2e- ZnAl3+ + 3e- AlMg2+ + 2e- Mg

Na+ + e- Na

K + + e- K

+1.420

+1.229

~+1.200

+0.800

+0.771

+0.401

+0.340

0.000

-0.126-0.136

-0.250

-0.277

-0.403

-0.440

-0.744

-0.763

-1.662

-2.363

-2.714

-2.924

Electrode reaction

(reduction)

Standard electrode

potential, V0 (V)

Increasingly inert

(cathodic)

Increasingly active

(anodic)

(reduction)

Table 17.1 The standard emf series.


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Influence of concentration & temperature on cell potential

- If M1 and M2 electrodes are pure metals.

- Altering temperature or solution concentration or using alloy electrodes instead of

pure metals will change the cell potential. According to the Nernst equation

R: gas constant, 8.31 J/molK

n: number of electrons participating in either of the half-cell reactions,

F : Faraday constant (96500 C/mol): molar ion concentrations][&][ 21

nn M M


17

(17.18)

11 M ne M n

ne M M n11

22 M ne M n

2121 M M M M nn

0

1V

0

1V 0

2V

0

1

0

2

0 V V V

n M nF

RT V V 1

0

11 ln

n

M nF

RT

V V 20

22 ln(17.19)

n

n

M

M

nF

RT V V V

V V V

2

10

1

0

2

12

ln


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- At 25C

to give V in volts.For reaction spontaneously, V must be positive.


18

(17.20)

V

M

M

nV V V

C

J

M

M

n

V V V

M M

mol

C n

K

K mol

J

V V V

M

M

nF

RT V V V

n

n

n

n

n

n

n

n

2

10

1

0

2

2

10

1

0

2

2

101

02

2

10

1

0

2

log0592.0

ln02566.0

ln96500

29831.8

ln


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One-half of an electrochemical cell consists of a pure nickel electrode in a solution of

Ni2+ ions; the other half is a cadmium electrode immersed in a Cd 2+ solution.

(a) If the cell is a standard one, write the spontaneous overall reaction and calculate the

voltage that is generated.

(b) Compute the cell potential at 25C if the Cd 2+

and Ni2+

concentrations are 0.5 and 10-3 M, respectively. Is the spontaneous reaction direction still the same as for the

standard cell?

(a) From Table 17.1

Cd 2+ + 2e- Cd V 0 = -0.403

Ni2+ + 2e- Ni V 0 = -0.250

Therefore

Cd Cd 2+ + 2e- V 0 = +0.403 Ni2+ + 2e- Ni V 0 = -0.250

Ni2+ + Cd Ni + Cd 2+ V

V = 0.403 - 0.250 = 0.153 (V)

073.0

5.0

10log

2

0592.0250.0403.0

][

][log

0592.0

3

2

200

Ni

Cd NiCd

M

M

n

V V V

(V)

(b) Cd Cd 2+ + 2e- V 0 = +0.403

Ni2+ + 2e- Ni V 0 = -0.250

Ni2+ + Cd Ni + Cd 2+

The same as for the standard cell.

)250.0,403.0( 00 V V V V NiCd

Materials Science and Engineering 魏茂國Example Problem 17.1

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20

A galvanic cell at 25C consists of an electrode of zinc in a 0.10 M ZnSO4 solution

and another of nickel in a 0.05 M NiSO4 solution. The two electrodes are separated

by a porous wall and connected by an external wire. What is the emf of the cell when

a switch between the two electrodes is just closed?

(For 1 M solutions, Zn2+

+ 2e-

Zn V0

= -0.763 V; Ni2+

+ 2e-

Ni V0

= -0.250 V)

Overall reaction: V V V E E V C Acell 505.0288.0793.0

)(288.005.0log2

0592.0250.0 V V C Cathode reaction:

Anode reaction: )(793.010.0log2

0592.0763.0 V V A

ionC

n

V V log0592.00

Materials Science and Engineering 魏茂國Example Problem


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21

One end of an iron wire is immersed in an electrolyte of 0.02 M Fe2+ ions and the other

in an electrolyte of 0.005 M Fe2+ ions. The two electrodes are separated by a porous

wall. (a) Which end of the wire will corrode? (b) What will be the potential difference

between the two ends of the wire when it is just immersed in the electrolytes?

(Fe2+

+ 2e-

Fe V0

= -0.440V)

(a) The end of the wire that will corrode will be the one immersed in the more dilute

electrolyte, which is the 0.005 M one. Thus, the wire end in the 0.005 M solution

will be the anode.

For 0.005 M solution: )(508.0005.0log0296.0440.0 V V A

For 0.02 M solution: )(490.002.0log0296.0440.0 V V C

(b) ionFe C V V log0296.00

2

V V V V V V C Acell 018.0490.0508.0



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Platinum

Gold

Graphite

Titanium

Silver

316 Stainless steel (passive)

304 Stainless steel (passive)Inconel (80Ni-13Cr-7Fe) (passive)

Nickel (passive)

Monel (70Ni-30Cu)

Copper-nickel alloys

Bronzes (Cu-Sn alloys)

Copper

Brasses (Cu-Zn alloys)

Inconel (active)

Nickel (active)

Tin

Lead

316 Stainless steel (active)

304 Stainless steel (active)Cast iron

Iron and steel

Aluminum alloys

Cadmium

Commercially pure aluminum

Zinc

Magnesium and magnesium alloys

Increasingly inert

(cathodic)

Increasingly active

(anodic)

Table 17.2 The galvanic series. Galvanic series

- This represents the relative reactivities

of a number of metals and commercial

alloys in seawater .

- The alloys near the top are cathodicand unreactive, whereas those at the

bottom are most anodic.

- Most metals and alloys are more stable

in an ionic state than as metals. In

thermodynamic term, there is a net

decrease in free energy in going from

metallic to oxidized states.- Essentially all metals occur in nature

as compounds. Two notable exceptions

are the noble metals gold and platinum.


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Corrosion rate

- The rate of material removal as a consequence of the chemical action.

- Real corroding systems are not at equilibrium; there will be a net flow electrons

from anode to cathode, which means that the half-cell potential parameters

(Table 17.1) cannot be applied.- Half-cell potentials: (1) the magnitude of a driving force, (2) determine

spontaneous reaction directions, (3) provide no information as to corrosion rates.

- Corrosion penetration rate (CPR )

The thickness loss of material per unit of time.

W : weight loss, t : exposure time, : density, A: exposed specimen area, K : constant,CPR: mils per year (mpy) or millimeters per year (mm/yr).

At

KW CPR

Materials Science and Engineering 魏茂國Corrosion Rates

23

(17.23)


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- There is an electric current associated with electrochemical corrosion reactions, we

can express corrosion rate in terms of this current density.

i: current density (C/m2

s, A/m2

),n: number of electrons associated with the ionization of each atom,

F : Faraday constant, 96500 C/mol, r : mol/m2s.

nF

ir


24

(17.24)


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25

Rate of uniform corrosion or electroplating of a metal in an aqueous solution

- The amount of metal uniformly corroded from an anode or electroplated on a

cathode in an aqueous solution in a time period can be determined by using

Faraday’s equation of general chemistry.

w: weight of metal (g) corroded or electroplated in an aqueous solution in time t (s),

I : current flow (A), M : atomic mass of the metal (g/mol),

n: number of electrons/atom produced or consumed in the process,

F : faraday’s constant (96500 C/mol, 96500 As/mol).

- Sometimes the uniform aqueous corrosion of a metal is expressed in terms of a

current density.

i: current density (A/cm2), A: area (cm2).

nF

ItM w

nF

iAtM w

Metal Metaln+ + ne-

M

w

nF

It



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Fig. 17.5 Electrochemical cell consisting of

standard zinc and hydrogen electrodes thathas been short-circuited.

Polarization & overvoltage

- The potentials of the two short-circuited electrodes are not at the values determined

from the standard emf series because the system is a nonequilibrium one. The

displacement of each electrode potential from its equilibrium value is termed

polarization, and the magnitude of this displacement is the overvoltage ( ).- Overvoltage is expressed in terms of plus or minus volts relative to the equilibrium

potential. - For example, suppose that the zinc electrode has a

potential of -0.621 V after it has been connected to

the platinum electrode. The equilibrium potential

is -0.763 V, therefore

= -0.621 - (-0.763) = +0.142 (V)

- Two types of polarizationActivation polarization

Concentration polarization

26

Materials Science and Engineering 魏茂國Prediction of Corrosion Rates


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Fig. 17.6 Schematic representation of possible

steps in the hydrogen reduction reaction, the rateof which is controlled by activation polarization.

Activation polarization

- Activation polarization refers to the condition wherein the reaction rate is

controlled by the one step in the series that occurs at the slowest rate.

- The term “activation” is applied to this type of polarization because an activation

energy barrier is associated with this slowest, rate-limiting step.1. Adsorption of H+ ions from the solution

onto the zinc surface.

2. Electron transfer from the zinc to form a

hydrogen atom.H+ + e- H

3. Combining of two hydrogen atoms to

form a molecule of hydrogen.

2H H24. The coalescence of many hydrogen

molecules to form a bubble.

The slowest of the steps determines

the rate of the overall reaction.

27


P di i f C i R


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Fig. 17.7 For a hydrogen electrode, plot

of activation polarization overvoltage

versus logarithm of current density for both oxidation and reduction reactions.

0

logi

ia

- For activation polarization, the relationship between overvoltage and current density

is

a: overvoltage, : constant, i: current density, i0: exchange current density.

- For the standard hydrogen cell2H+ + 2e- H2 rate: r red H2 2H+ + 2e- rate: r oxd

- At equilibrium, there is no net reaction.

The value for i0 is determined

experimentally and will vary from systemto system.

nF

ir r oxid red

0

28


(17.25)

(17.26)

P di ti f C i R t


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0

logi

ia

- According to Equation 17.25, when overvoltage is plotted as a function of the

logarithm of current density, straight-line segments results (Fig. 17.7).

The line segment with a slope of + corresponds to the oxidation half-reaction,whereas the line with a - is for reduction.

- Also worth noting is that both line

segments originates at i0 (H2/H+),

the exchange current density, and

at zero overvoltage, because at

this point the system is at

equilibrium and there is no netreaction.

29


(17.25)

P di ti f C i R t


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Concentration polarization

- Concentration polarization exists when the reaction rate is limited by diffusion in

the solution.

- When the reaction is low and/or the concentration of H+ is high, there is always an

adequate supply of hydrogen ions available in the solution at the region near theelectrode interface (Fig. 17.8a).

- At high rates and/or low H+ concentrations, a depletion zone may be formed in the

vicinity of the interface, inasmuch as the H+ ions are not replenished at a rate

sufficient to keep up with the reaction (Fig. 17.8b). Thus, diffusion of H+ to the

interface is rate controlling, and the system is concentration polarized.

- It generally occurs only for reduction reactions because for oxidation, there is

virtually an unlimited supply of metal atoms at the corroding electrode interface.- It may be noted that overvoltage is independent of current density until i

approach i L (the limiting diffusion current density); at this point, C decreases

abruptly in magnitude.

30


LC i

i

nF

RT

1log

3.2

(17.27)

Prediction of Corrosion Rates


31/84

Fig. 17.8 For hydrogen reduction, schematic representations of H+ distribution in the

vicinity of the cathode for (a) low reaction rates and/or high concentrations, and (b)

high reaction rate and/or low concentrations wherein a depletion zone is formed that

gives rise to concentration polarization. 31


茂Prediction of Corrosion Rates


32/84

Fig. 17.9 For reduction reactions, schematic plots

of overvoltage versus logarithm of current density

for (a) concentration polarization, and (b) combined

activation-concentration polarization.

Both activation and concentration polarization

Both concentration and activation polarization are possible for reduction reactions.

The total overvoltage is just the sum of both overvoltage contributions (Fig. 17.9b).

32


魏茂國Prediction of Corrosion Rates


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Corrosion rates from activation polarization data

- In the first case, both oxidation and reduction reactions are rate limited by

activation polarization (Fig. 17.1).

The potentials of the uncoupled hydrogen and zinc half-cells,

V (H+/H2) and V (Zn/Zn2+), respectively, are indicated, along

with their respective exchange current densities, i0(H+/H2)

and i0(Zn/Zn2+).

Upon immersion, both hydrogen and zinc experience activation polarization alongtheir respective lines. Also, oxidation and reduction rates must be equal, which is

only possible at the intersection of the two line segments; this intersection occurs

at the corrosion potential, designated V C , and the corrosion current density iC . The

corrosion rate of zinc (which also corresponds to the rate of hydrogen evolution)

may thus be computed by insertion of this iC value into Equation 17.24.

33


i l S i d i i 魏茂國Prediction of Corrosion Rates


34/84

Fig. 17.10 Electrode kinetic behavior of zinc in

an acid solution; both oxidation and reductionreactions are rate limited by activation polarization.

nF

ir

H iiV V H H H H 0

)/(log

2

Zni

iV V Zn Zn Zn Zn

0)/(

log2

Zn H V V

34


M t i l S i d E i i 魏茂國Prediction of Corrosion Rates


35/84

Fig. 17.11 Schematic electrode kinetic

behavior for metal M; the reduction reaction

is under combined activation-concentration

polarization control.

- In the second case, both concentration and activation polarization control the

reduction reaction, whereas only activation polarization is important for oxidation.

Fig. 17.11 shows both polarization curves; corrosion potential and corrosion current

density correspond to the point at which the oxidation and reduction lines intersect.

35




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36

A copper electroplating process uses 15 A of current by chemically dissolving

(corroding) a copper anode and electroplating a copper cathode. If it is assumed that

there are no side reactions, how long will it take to corrode 8.50 g of copper from the

anode? (Atomic mass of Cu: 63.5 g/mol, F : 96500 As/mol.)

IM

wnF t M

nF

It w

222 neCuCu

min7.281722

/5.6315

/9650025.8

s

molg A

mols Agt




37/84

37

A mild steel cylindrical tank 1 m high and 50 cm in diameter contains aerated water to

the 60 cm level and shows a loss in weight due to corrosion of 304 g after 6 weeks.

Calculate (a) the corrosion current and (b) the current density involved in the corrosion

of the tank. Assume uniform corrosion on the tank’s inner surface and that the steel

corrodes in the same manner as pure iron. ( M Fe

: 55.85 g/mol, F : 96500 As/mol.)

tM

wnF I

nF

ItM w

222 neFeFe

Amolghsdayhweek daysweek

mols Ag I 289.0

/5.63/3600/24/76

/965002304

(a)

(b) area

I i

222 113802/506050 cmcmcmcmr Dharea

25

2/1053.2

11380

289.0cm A

cm

Ai

50

40

60




38/84

38

The wall of a steel tank containing aerated water is corroding at a rate of 54.7 mdd.

How long will it take for the wall thickness to decrease by 0.50 mm? ( Fe: 7.87 g/cm3)

daycm

g

daycm

gmdd

2

4

2

3

1047.5)10(

107.547.54

daysdaycm

cmt 719

/1095.6

1050.05

1

daycmcmg

daycmg /1095.6/87.7/1047.5 53

24

depth of corrosion per day =

mdd: milligram weight loss per square decimeter per day.




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39

A sample of zinc corrodes uniformly with a current density of 4.2710-7 A/cm2 in an

aqueous solution. What is the corrosion rate of the zinc in milligrams per decimeter

per day? The reaction for the oxidation of zinc is Zn Zn2+ + 2e-.

nF

tM areai

nF

ItM w

222 ne Zn Zn

gmols A

molghshcmcm Aw

3227

1025.1/965002

/38.65/360024100/1027.4

The corrosion rate is 1.25 mdd.

Materials Science and Engineering 魏茂國p



40/84

Zinc experiences corrosion in an acid solution according to the reaction

Zn + 2H+ Zn2+ + H2The rates of both oxidation and reduction half-reactions are controlled by activation

polarization.

(a) Compute the rate of oxidation and reduction of Zn (mol/cm2

s) given thefollowing activation polarization data:

(b) Compute the value of the corrosion potential.

V(Zn/Zn2+) = -0.763 V

i0 = 10-7 A/cm2

= 0.09

V(H+/H2) = 0 V

i0 = 10-10 A/cm2

= -0.08

For Zn For Hydrogen

H iiV V H H H H 0

)/(log

2

For hydrogen reduction (a)

Zni

iV V Zn Zn Zn Zn

0)/(

log2 For zinc oxidation

Zn H V V At equilibrium 40

Materials Science and Engineering 魏茂國p



41/84

4924.3

710

00)/()/(

00)/()/(

0)/(0)/(

0)/(

0)/(

1019.110

924.310log09.010log)08.0()763.0(0)08.0(09.0

1log

loglog1log

loglogloglog

loglogloglog

loglog

22

22

22

2

2

C

C

Zn H Zn Zn H H H Zn

C

C H C Zn Zn H Zn Zn H H

ZnC Zn Zn Zn H C H H H

C Zn Zn Zn

C H H H

i

i

iiV V i

iiiiV V

iiV iiV

i

iV

i

iV

Zn H

Zn H

Zn H

Zn H

(A/cm2

)

104

1017.6965002

1019.1

nF

ir C (mol/cm2s)

(b)

486.010

1019.1log)08.0(0

log

10

4

0)/( 2

C

C H H H C

V

i

iV V

H

(V)

41

g g 茂國p

Materials Science and Engineering 魏茂國Passivity


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Passivity

- Some normally active metals and alloys, under particular environmental conditions,lose their chemical reactivity and become extremely inert. This phenomenon is

termed passivity.

- Passivity is displayed by Cr, Fe, Ni, Ti, and many of their alloys.

- This passive behavior results from the formation of a highly adherent and very thin

oxide film on the metal surface, which serves as a protective barrier to further

corrosion. If damaged, the protective film normally reforms very rapidly.

- Electrochemical potential vs current density

1. At low potential values, within the “active” region the behavior is like a normal

metal.

2. With increasing potential, the current density suddenly decrease to a very low

value that remains independent of potential; this is termed the “passive” region.

3. At even higher values, the current density again increases with potential in the

“transpassive” region.

42

g g



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Fig. 17.12 Schematic polarization curve for a metal that displays an active-passive

transition. 43



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Fig. 17.13 Demonstration of how an active-

passive metal can exhibit both active and

passive corrosion behavior.

Influence of corrosion environment

- Curve 1 intersects the oxidation polarization curve in the active region at point A,

yielding a corrosion current density iC (A).

- The intersection of curve 2 at point B is in the passive region and at current density

iC (B).

- The corrosion rate of metal M in solution

1 is greater than in solution 2 since iC (A)

is greater than iC (B) and the rate is

proportional to current density.

44

Materials Science and Engineering 魏茂國Environmental Effects


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Fluid velocity

In most instances, increasing fluid velocity enhances the rate of corrosion due toerosive effects.

Temperature

For the great majority of corrosion situations, the rates rise with increasingtemperature.

Composition

- In many situations, increasing the concentration of the corrosive species producesa more rapid rate of corrosion.

- For materials capable of passivation, raising the corrosive content may result in

an active-to-passive transition, with a considerable reduction in corrosion.

Cold work

A cold-worked metal is more susceptible to corrosion than the same material in an

annealed state.

45


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Materials Science and Engineering 魏茂國Forms of Corrosion


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Uniform attack (most common form of corrosion)

- Uniform attack is a form of electrochemical corrosion that occurs with equivalentintensity over the entire exposed surface and often leaves behind a scale or deposit.

In a microscopic sense, the oxidation and reduction reactions occur randomly over

the surface.

- Example: general rusting of steel and iron, tarnishing of silverware.

47



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Galvanic corrosion

- It occurs when two metals or alloys having different compositions are electricallycoupled while exposed to an electrolyte.

- The less noble or more reactive metal in the particular environment will experience

corrosion; the more inert metal, the cathode, will be protected from corrosion.

- Example: steel screws corrode when in contact

with brass in a marine environment.

- When two alloys are coupled in seawater, the

one lower in the galvanic series (Table 17.2) willexperience corrosion.

- The rate of galvanic attack depends on the

relative anode-to-cathode surface areas that are

exposed to the electrolyte, and is related directly

to the cathode-anode area ratio. The reason is

that corrosion rate depends on current density.

48Fig. 17.14 Galvanic corrosion of a magnesium shell

that was cast around a steel core.



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49

Anodic-cathodic behavior of steel with zinc and tin outside layers exposed to the

atmosphere. (a) Zinc is anodic to steel and corrodes (V 0 for Zn and Fe are -0.763 V

and -0.440 V, respectively. (b) Steel is anodic to tin and corrodes (the tin layer was

perforated before the corrosion began) (V 0

for Sn is -0.136 V).



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50

Cu SteelCuSteel

Effect of area relationships between cathode and anode for copper-steel couplesimmersed in seawater. (a) Small cathode (copper rivets) and large anode (steel

plates) cause only slight damage to steel. (b) Small anode (steel rivets) and large

cathode (copper plates) cause severe corrosion of steel rivets.


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Fig. 17.15 On this plate, which was immersed in seawater, crevice corrosion has

occurred at the regions that were covered by washers.

Crevice corrosion

- Electrochemical corrosion may occur as a consequence of concentrationdifferences of ions or dissolved gases in the electrolyte solution, and between two

regions of the same metal piece. For such a concentration cell, corrosion occurs in

the locale that has the lower concentration. Corrosion preferentially occurring at

these positions is called crevice corrosion.

- The crevice must be wide enough for the solution to penetrate, yet narrow enough

for stagnancy; usually the width is tenths of meters.

52



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OH eO H O 442 22Fig. 17.16 Schematic

illustration of the mechanism

of crevice corrosion between two

riveted sheets. 53

- After oxygen has been depleted within the crevice, oxidation of the metal occurs at

the crevice (Fig. 17.16). Electrons from this electrochemical reaction are conducted through the metal to adjacent external regions, where they are consumed by reduction.



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54

- In many aqueous environments, the solution of H+ and Cl- ions, which are

especially corrosive.

Prevention of crevice corrosion

- Using welded instead of riveted or bolted joints.

- Using nonabsorbing gaskets when possible.- Removing accumulated deposits frequently.

- Designing containment vessels to avoid stagnant areas and ensure complete

drainage.

Pi i



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Fig. 17.17 The pitting of a

304 stainless steel plate byan acid-chloride solution.

Pitting

- A form of very localized corrosion attack in which small pits or holes form.- The pits or holes ordinarily penetrate from the top of a horizontal surface

downward in a nearly vertical direction.

Mechanism for pitting (next page)It is probably the same as for crevice corrosion in that oxidation occurs within the pit

itself, with complementary reduction at the surface. It is supposed that gravity causes

the pits to grow downward, the solution at the pit tip

becoming more concentrated and dense as pit growth

progresses.

Improvement of pitting-resistance

- Specimens polished surfaces display a greater resistance to

pitting corrosion.

- For stainless steel, alloying with about 2% molybdenum

enhances their resistance significantly. 55

The propagation of a pit is believed to involve the



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56

- The propagation of a pit is believed to involve the

dissolution of the metal in the pit while maintaining ahigh degree of acidity at the bottom of the pit.

The anodic reaction of the metal at the bottom of the

pit is M Mn+ + ne-.

The cathodic reaction takes place at the metal surface surrounding the pit and is the

reaction of oxygen with water and the electrons from the anodic reaction:

O2 + 2H2O + 4e- 4OH-.

Thus, the metal surrounding the pit is cathodically protected.

The increased concentration of metal ions in the pit brings in chloride ions to

maintain charge neutrality. The metal chloride then reacts with water to produce the

metal hydroxide and free acid as

In this way a high acid concentration builds up at the bottom of the pit, which makes

the anodic reaction rate increase, and the whole process becomes autocatalytic.

Cl H MOH O H Cl M 2

I t l i



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Fig. 17.18 Schematic illustration of chromium

carbide particles that have precipitated

along grain boundaries in stainless steel, and

the attendant zones of chromium depletion.

Intergranular corrosion

- It occurs preferentially along grain boundaries for some alloys and in specificenvironments. A macroscopic specimen disintegrates along its grain boundaries.

- When some stainless steel are heated to temperatures between 500 and 800C for

sufficiently long time periods, they become sensitized to intergranular attack.

Mechanism of intergranular corrosion

This heat treatment permits the formation of small precipitate particles of chromium

carbide (Cr 23C6). These particles form along the grain boundaries. Both the

chromium and the carbon atom must diffuse to the grain boundaries to form the

precipitates, which leaves a chromium-depleted zone adjacent to the grain boundary.

This grain boundary is now highly susceptible to

corrosion.

57

Weld decay



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Weld decay

Intergranular corrosion is an especially severe problem in the welding of stainlesssteels, when it is often termed weld decay (Fig. 17.19).

Protection from intergranular corrosion

- Subjecting the sensitized material to ahigh-temperature heat treatment in which

all the chromium carbide particles are

redissolved.

- Lowering the carbon content below 0.03

wt% so that carbide formation is minimal.

- Alloying the stainless steel with another

metal such as niobium or titanium, whichhas a greater tendency to form carbides

than does chromium so that Cr remains

in solid solution.

58

Fig. 17.19 Weld decay in a stainless steel.

The regions along which the grooves have

formed were sensitized as the weld cooled.

Selective leaching



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Selective leaching

- Selective leaching is found in solid solution alloys and occurs when one element or constituent is preferentially removed as a consequence of corrosion processes.

- Example: dezincification of brass.

- The mechanical properties of the alloy are significantly impaired, because only a

porous mass of copper remains in the region that has been dezincified.

59

Erosion-corrosion



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Fig. 17.20 Impingement failure of an elbow that was part of

a steam condensate line.

Erosion-corrosion

- Erosion-corrosion arises from the combined action of chemical attack and mechanical abrasion or wear as a consequence of fluid motion.

- It is especially harmful to alloys that passivate by forming a protective surface film;

the abrasive action may erode away the film, leaving exposed a bare metal surface.

- Erosion-corrosion is commonly found in piping, especially at bends, elbows, and

abrupt changes in pipe diameter-positions where the fluid changes direction or flow

suddenly becomes turbulent.

- Increasing fluid velocity normally enhances the rate of

corrosion.

- A solution is more erosive when bubbles and suspended

particulate solids are present.

60



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Reduction of erosion-corrosion

- Changing the design to eliminate fluid turbulent and impingement effects.

- Utilizing of other materials that inherently resist erosion.

- Removal of particulates and bubbles from the solution.

61

Erosion-corrosion wear pattern of silica slurry in mild-steel pipe.

Stress corrosion (cracking)



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Fig. 17.21 Photomicrograph showing inter-

granular stress corrosion cracking in brass.

Stress corrosion (cracking)

- Stress corrosion results from the combined action of an applied tensile stress and acorrosive environment; both influence are necessary.

- Some materials that are virtually inert in a particular corrosive medium become

susceptible to stress corrosion when a stress is applied. Small cracks form and then

propagate in a direction to the stress.

- Failure behavior is characteristic of that for a

brittle material, even though the metal alloy is

intrinsically ductile.- Cracks may form at relatively low stress

levels, significantly below the tensile strength.

62

- The stress that produces stress corrosion cracking need not be externally applied; it



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e st ess t at p oduces st ess co os o c ac g eed ot be e te a y app ed; t

may be a residual one that results from rapid temperature changes and unevencontraction, or for a two-phase alloys in which each phase has a different

coefficient of expansion.

Prevention from stress corrosion- Reducing the stress.

- Annealing

63Stress-corrosion cracks in a pipe.

Materials Science and Engineering 魏茂國

Mechanism of stress-corrosion cracking

Types of Corrosion


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64

- Most SCC mechanisms involve crack initiation and propagation stages.

- In many cases the crack initiates at a pit of other discontinuity on the metal surface.

After the crack has been started, the tip can advance (Fig. 13.27).

A high stress builds up at the tip of the crack

due to tensile stresses acting on the metal.

Anodic dissolution of the metal takes place

by localized electrochemical corrosion

at the tip of the crack as it advances. The

crack grows in a plane perpendicular to

the tensile stress until the metal fractures.

- If either the stress or the corrosion is stopped, the

crack stops growing.

- Tensile stress is necessary for both the initiation

and propagation of crack and is important in the rupturing of surface films.

Hydrogen embrittlement (hydrogen-induced cracking)



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y g ( y g g)

- Some steels experience a significant reduction in ductility and tensile strength whenatomic hydrogen (H) penetrates into the material. This phenomenon is hydrogen

embrittlement.

- Hydrogen in its atomic form diffuses interstitially through the crystal lattice, and

concentration as low as several parts per million can lead to cracking.

- Hydrogen-induced cracks are most often transgranular.

- In hydrogen embrittlement, a normally ductile metal experiences brittle fracture

when exposed to both a tensile stress and a corrosive atmosphere.- The presence of what are termed “poisons” such as sulfur (i.e., H2S) and arsenic

compounds accelerates hydrogen embrittlement.

Reduction of hydrogen embrittlement- Reducing the tensile strength of the alloy via a heat treatment.

- Removal of the source of hydrogen, “baking” the alloy at an elevated temperature

to drive out any dissolved hydrogen.

- Substitution of a more embrittlement resistant alloy. 65

Corrosive environments

Materials Science and Engineering 魏茂國Corrosion Environments


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- Atmosphere: oxygen dissolved moisture- Aqueous solutions: freshwater (oxygen), seawater (sodium chloride)

- Soils

- Acids

- Bases

- Inorganic solvents

- Molten salts

- Liquid metals- Human body

Materials

- For freshwater useCast iron, aluminum, copper, brass, some stainless steels.

- For seawater use

Titanium, brass, some bronzes, copper-nickel alloys, nickel-chromium-

molybdenum alloys. 66

Corrosion prevention

Materials Science and Engineering 魏茂國Corrosion Prevention


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- Material selection: the most common and easiest way.- Environmental alteration: lowering the fluid temperature and/or velocity, many

times increasing or decreasing concentration of some species, adding inhibitors in

relatively low concentration to the environment.

- Design: easy washing and shutdown, provision for the exclusion of air.

- Coatings

The coating must be nonreactive in the corrosive environment and resistant to

mechanical damage that exposes the bare metal to the corrosive environment.- Cathodic protection

Inhibitor

- Substances that, when added in relatively low concentration to the environment,decrease its corossiveness.

- Inhibitors are normally used in closed systems such as automobile radiators and

steam boilers.67


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- Impressed current



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Fig. 17.22b Cathodic protection of an

underground tank using an impressed

current.

The source of electrons is an impressed current from an external dc power source(Fig. 17.22b) for an underground tank. The negative terminal of the power source is

connected to the structure to be protected. The other terminal is joined to an inert

anode (often graphite), which is buried in the soil; high-conductivity backfill

material provides good electrical contact between the anode and surrounding soil.

A current path exists between the cathode and anode through the intervening soil,

completing the electrical circuit.

69

- Galvanizing (Fig. 17.23)



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Fig. 17.23 Galvanic protection of steel as

provided by a coating of zinc.

The process of galvanizing is simply one in which a layer of zinc is applied to thesurface of steel by hot dipping. In the atmosphere and most aqueous environments,

zinc is anodic to and will cathodically protect the steel if there is any surface damage.

Any corrosion of the zinc coating will proceed at an extremely slow rate because the

ratio of the anode-to-cathode surface area is quite large.

70

Materials Science and Engineering 魏茂國Oxidation

Oxidation


71/84

Fig. 17.15 Schematic representation of

processes that are involved in gaseous

oxidation at a metal surface.71

Oxidation of metal alloys is possible in gaseous atmospheres, normally air, whereinan oxide layer or scale forms on the surface of the metal. This phenomenon is

frequently termed scaling, tarnishing, or dry corrosion.

Mechanisms- Oxidation half-reaction occurs at the

metal-scale interface

- Reduction half-reaction occurs at the

scale-gas interface

- For divalent metal, the process of

oxide layer formation is an electro-

chemical one.

e M M 22

22 22

1OeO

MOO M 221

(17.29)

(17.28)

(17.31)

- For the oxide layer to increase in thickness, it is necessary that electrons be



72/84

conducted to the scale-gas interface; in addition, M2+

ions must diffuse away fromthe metal-scale interface, and/or O2- ions must diffuse toward this same interface.

- The oxide scale serves both as an electrolyte through which ions diffuse and as an

electrical circuit for the passage of electrons.

- The scale may protect the metal from rapid oxidation when it acts as a barrier to

ionic diffusion and/or electrical conduction; most metal oxides are highly

electrically insulative.

72

Pilling-Bedworth ratio (V O /V M )



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- Rate of oxidation and the tendency of the film to protect the metal from further oxidation are related to the relative volumes of the oxide and metal (Pilling-

Bedworth ratio).

AO: molecular weight of the oxide, A M : atomic weight of the metal,

O: oxide density, M : metal density.

- P-B ratio < 1, the oxide film tends to be porous and unprotective because it is

insufficient to fully cover the metal surface.- P-B ratio > 1, compressive stresses result in the film as it forms.

- P-B ratio > 2~3, the oxide coating may crack and flake off, continually exposing a

fresh and unprotectived metal surface.

- P-B ratio 1~2, protective coatings normally form for metals.

73

P-B ratio =0

0

M

M

A

A(17.32)

Table 17.3 Pilling-Bedworth ratios Factors for protective coatings



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Ce 1.16

Al 1.28

Pb 1.40

Ni 1.52Be 1.59

Pd 1.60

Cu 1.68

Fe 1.77

Mn 1.79

Co 1.99

Cr 1.99

Si 2.27

K 0.45

Li 0.57

Na 0.57

Cd 1.21Ag 1.59

Ti 1.95

Ta 2.33

Sb 2.35

Nb 2.61

U 3.05

Mo 3.40

W 3.40

Protective Nonprotectivefor a number of metals.- P-B ratios: 1~2.

- High adherence between film and metal.

- Comparable thermal expansion

coefficients for metal and oxide.

- A relatively high melting point.

- Good high-temperature plasticity.

74

Kinetics (Fig. 17.25)



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- When the oxide that forms is nonporous and adheres to the metal surface, the rateof layer growth is controlled by ionic diffusion. A parabolic relationship exists

between the weight per unit area W and the time t as follows:

K 1 & K 2: time-independent constants at a given temperature.

- In the oxidation of metals for which the scale is porous or flakes off, the oxidation

rate expression is linear:

K 3: constant.

- For very thin oxide layers (< 100 nm) that form at relatively low temperatures, the

dependence of weight gain on time is logarithmic:

K 4 & K 5 & K 6 : constants.

75

21

2 K t K W

t K W 3

6542 log K t K K W

(17.34)

(17.35)

(17.36)



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Fig. 17.25 Oxidation film growth curves for linear, parabolic, and

logarithmic rate laws.

76

Ceramic materials

Materials Science and Engineering 魏茂國Corrosion of Ceramic Materials


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- Ceramic materials, being compounds between metallic and nonmetallic elements,may be thought of as having already been corroded.

- Ceramic materials are frequently utilized because of their resistance to corrosion.

- Ceramic materials are much better suited to withstand most of severe environments

for reasonable time periods than are metals.

Corrosion of ceramic materials

- Ceramic materials are exceedingly immune to corrosion by almost all

environments, especially at room temperature.

- Corrosion of ceramic materials generally involves simple chemical dissolution, in

contrast to the electrochemical processes found in metals.

77

Degradation of polymers

- Polymeric degradation is physiochemical; it involves physical as well as chemical

Materials Science and Engineering 魏茂國Degradation of Polymers


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Polymeric degradation is physiochemical; it involves physical as well as chemical

phenomena.

- Polymers may deteriorate by swelling and dissolution.

- Covalent bond rupture.

- Chemical reactions.- Radiation.

78

Swelling

- With swelling the liquid or solute diffuses into and is absorbed within the polymer;

Materials Science and Engineering 魏茂國Swelling & Dissolution


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With swelling, the liquid or solute diffuses into and is absorbed within the polymer ;

the small solute molecules fit into and occupy positions among the polymer

molecules. Thus the macromolecules are forced apart such that specimen expands.

- This increase in chain separation results in a reduction of the secondary

intermolecular bonding forces; as a consequence, the material becomes softer and more ductile.

- The liquid solute lowers the glass transition temperature of polymers.

- Swelling may be considered to be a partial dissolution process in which there is

only limited solubility of the polymer in the solvent.

Dissolution

- Dissolution, which occurs when the polymer is completely soluble, may be thought

of as just a continuation of swelling.

- The greater the similarity of chemical structure between the solvent and polymer,

the greater is the likelihood of swelling and/or dissolution.

79

Swelling & dissolution



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- In general, increasing molecular weight, increasing degree of crosslinking and crystallinity, and decreasing temperature result in a reduction of deteriorative

processes.

- In general, polymers are much more resistant to attack by acidic and alkaline

solutions than are metals.

Table 17.5 Resistance to degradation by various environments for selected elastomeric materials.

80

Table 17.4 Resistance to degradation by various environments for selected plastic materials.



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81

Scission

Materials Science and Engineering 魏茂國Bond Rupture


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- Scission is the severence or rupture of molecular chain bonds. This causes aseparation of chain segments at the point of scission and a reduction in the

molecular weight.

- Bond rupture may result from exposure to radiation or to heat, and from chemical

reaction.

Radiation effects

- One reaction is ionization, in which the radiation removes an orbital electron from

a specific atom, converting that atom into a positively charged ion. As a

consequence, one of the covalent bonds associated with the specific atom is broken,

and there is a rearrangement of atoms or groups of atoms at that point.

- This bond breaking leads to either scission or crosslinking at the ionization site,

depending on the chemical structure of the polymer and also on the dose of

radiation.

- Stabilizers may be added to protect polymers from ultraviolet damage.82

Chemical reaction effects

Materials Science and Engineering 魏茂國Bond Rupture


83/84

- Oxygen, ozone, and other substances can cause or accelerate chain scission as aresult of chemical reaction.

Thermal effects on bond rupture

Thermal degradation corresponds to the scission of molecular chains at elevated

temperature; as a consequence, some polymers undergo chemical reactions in which

gaseous species are produced.

83

Weathering

Materials Science and Engineering 魏茂國Weathering


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- Many polymeric materials serve in applications that require exposure to outdoor conditions. Any resultant degradation is termed weathering.

- The deterioration is primarily a result of oxidation, which is initiated by ultraviolet

radiation from the sun.

- The fluorocarbons are virtually inert under these conditions.

84

chapter17 corrosion°radationofmaterials

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