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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.
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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.
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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)
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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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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.
Materials Science and Engineering 魏茂國Electrochemical Considerations
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.
Materials Science and Engineering 魏茂國Electrochemical Considerations
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.
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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
Materials Science and Engineering 魏茂國Electrochemical Considerations
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
Materials Science and Engineering 魏茂國Electrochemical Considerations
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.
Materials Science and Engineering 魏茂國Electrochemical Considerations
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
Materials Science and Engineering 魏茂國Example Problem
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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
Materials Science and Engineering 魏茂國Corrosion Rates
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(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
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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
Materials Science and Engineering 魏茂國Prediction of Corrosion Rates
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
Materials Science and Engineering 魏茂國Prediction of Corrosion Rates
(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
Materials Science and Engineering 魏茂國Prediction of Corrosion Rates
(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
Materials Science and Engineering 魏茂國Prediction of Corrosion Rates
LC i
i
nF
RT
1log
3.2
(17.27)
Prediction of Corrosion Rates
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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
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茂Prediction of Corrosion Rates
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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
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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
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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
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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
Materials Science and Engineering 魏茂國Prediction of Corrosion Rates
Materials Science and Engineering 魏茂國Example Problem
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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
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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
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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
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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
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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
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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
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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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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.
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Mechanism of stress-corrosion cracking
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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
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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
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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
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Oxidation
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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
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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
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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
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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.
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Swelling
- With swelling the liquid or solute diffuses into and is absorbed within the polymer;
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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.
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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.
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Table 17.4 Resistance to degradation by various environments for selected plastic materials.
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Scission
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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
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- 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.
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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.
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