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7-1
C H A P T E R 7
F R A C T U R E A N D F A T I G U E
7.1 FRACTURE 7.1.1 Stress Concentrat ion
7.1.2 Duct i le Fracture
7.1.3 Br i t t le Fracture
7.1.4 E f fects of Temperature & Stra in Rate: Duct i le - to-Br i t t le Trans i t ion
7.1.5 Impact Test ing
7.2 FATIGUE 7.2.1 Fat igue Mechanisms
7.2.2 Fat igue Test ing
7.2.3 Non-Destruct ive Test ing (NDT)
7-2
7.1 FRACTURE
• Fracture is the separation of a piece of material into two or
more pieces when subject to stress.
• Fracture involves the initiation and propagation (i.e.
growth) of a crack, and may occur in several ways, such as:
◆ slow application of external loads (e.g. tensile test)
◆ rapid application of external loads (impact)
◆ cyclic or repeated loading (fatigue)
◆ time-dependent deformation (creep)
7.1.1 Stress Concentration
• The theoretical fracture strength of a material is the stress
required to cause simultaneous breaking of all atomic
bonds across the fracture plane, resulting in the creation of
two new surfaces.
• However, actual fractures in bulk materials occur at stress
levels 10–1,000 times lower than their theoretical fracture
strengths. This is because real materials contain cracks,
which are unable to carry any tensile load; instead, the
applied load is transferred to regions around the cracks.
7-3
• For a long, thin crack, its geometry results in the amplifica-
tion or concentration of the applied stress at its crack tip,
such that the local stress near the tip is much higher that
the average applied stress (Fig. 7.1-1).
Fig. 7.1-1 Thin plate under uniaxial
tension with (a) edge crack of length a, and (b) centre crack of length 2a.
(c) Concentration of stress at the crack tip. (d) Photoelastic fringe pattern showing concentration of stress at a crack tip.
[Note: the closer the lines, the higher the stress]
(d)
• Cracks may be formed in a material during manufacture, or
through wear on the surface. In metals, cracks may also be
introduced during plastic deformation.
• The mechanism by which the crack propagates (grows)
determines whether the fracture is brittle or ductile.
7-4
7.1.2 Brittle Fracture
• Brittle fracture is sudden and catastrophic, with little or no
plastic deformation and low energy absorption (Fig. 7.1-2).
Materials that fail by brittle fracture have low toughness.
(a) (b)
Fig. 7.1-2 (a) Flat brittle fracture surface with little deformation. (b) Stress-strain behaviour of a brittle material. A brittle material exhibits a small strain at
fracture, and absorbs little energy prior to fracture (small area under the curve).
• The separation of the material tends to occurs through the
grains (transgranularly) along specific crystallographic
planes (Fig. 7-1-3a). This process of splitting is called cleavage.
• Cleavage fracture surfaces are relatively flat and shiny, with
a faceted texture (Fig. 7-1-3b & c), as the reflected light changes
with the orientation of cleavage planes in different grains.
• Brittle fracture can also occur along grain boundaries
(intergranularly), whose cohesive strength become lower
when the local chemistry is changed by the segregation of
harmful impurities or brittle second phases (Fig. 7-1-4). 7-5
Fig. 7.1-3 (a) Transgranular/cleavage fracture
– crack propagation through the interior of grains. (b) Typical grainy appearance of
cleavage fracture. Note the lack of macroscopic plastic deformation
(c) Cleavage fracture surface with a faceted (many small, shiny planes) appearance.
Fig. 7.1-4 (a) Intergranular fracture – decohesion (separation) along grain boundaries.
(b) Intergranular fracture surface revealing clearly outlined grains.
7-6
• The yield strength, σy, of a brittle material is higher than its
fracture strength. When the local stress, σlocal, near the
crack tip reaches the fracture strength, the atomic bonds
will break (Fig. 7.1-5). The crack grows rapidly between a pair
of atomic planes, giving rise to an atomically flat surface by
cleavage. Cleavage crack propagation reaches the speed of
sound, which is why brittle materials fail with a bang.
Fig. 7.1-5 Crack propagation by cleavage occurs once the local stress, σlocal, near the crack
tip reaches the theoretical fracture strength. Brittle fracture is sudden and catastrophic.
• Cleavage fracture occurs in brittle ceramics, but is also
common in BCC and some HCP metals at low tempera-
tures (Sec. 7.1-4). True cleavage has not been observed in FCC
metals, due to their low shear stresses required for slip,
numerous slip systems, and the ability to cross-slip.
7-7
7.1.3 Ducti le Fracture
• Ductile fracture is characterized by extensive plastic
deformation prior to failure, which absorbs considerable
energy (Fig. 7.1-6b). Materials that fail by ductile fracture are
considered tough.
• A tensile specimen that has failed by ductile fracture in the
necked region typically exhibits a cup-and-cone
appearance (Fig. 7.1-6a, Fig. 7.1-7a).
(a) (b)
Fig. 7.1-6 (a) Typical cup-and-cone ductile fracture. (b) Stress-strain behaviour of a ductile material. A ductile material exhibits a large strain (a lot of plastic deformation) at fracture, and absorbs a large
amount of energy prior to fracture (large area under the curve).
• To the naked eye, the central region of the fracture surface
looks dull, with an irregular and fibrous appearance (Fig. 7.1-
7a). At high magnifications, “dimples” may be seen on the
fracture surface (Fig. 7.1-7b).
7-8
Fig. 7.1-7 (a) Typical cup-and-cone ductile fracture. (b) Dimples characteristic of ductile fracture. In some dimples, inclusions can be seen.
However, inclusions sometimes fall out of the dimples or they might be obscured by the surrounding matrix although they are still inside the dimples.
• The yield strength, σy, of a ductile material is less than its
fracture strength. Plastic flow will take place in the region
of the material ahead of the crack tip where the local
stress, σlocal, is greater than the σy (Fig. 7.1-8a). The size of this
plastic zone depends on the magnitude of σy: as σy
decreases, the plastic zone increases.
• The plastic flow ahead of the crack tip has the effect of
turning an initially sharp crack into a blunt crack, such that
the stress concentration at the crack tip decreases; i.e. σlocal
is reduced. At the same time, the plastic flow also strain-
hardens the material in the plastic zone (Fig. 7.1-8b). If the
crack becomes blunt enough, σlocal could drop below the
new higher yield strength at the crack tip, effectively
arresting crack growth unless a higher stress is applied. 7-9
• When σlocal is sufficiently high to match or exceed the yield
strength of the strain-hardened material at the crack tip,
crack propagation will continue via continual plastic
deformation ahead of the crack tip.
(a) (b)
Fig. 7.1-8 (a) Stress distribution ahead of the crack tip, showing a zone of plastic deformation where the local stress is higher than the yield strength; (b) plastic
deformation blunts crack tip, lowering local stress, while strain hardening in the plastic zone raises yield strength. Crack propagation will continue if the applied stress is high enough such that the local stress is greater than the strain-hardened yield strength.
• Well-annealed metals do not usually contain cracks, since
diffusion during annealing would have caused existing
cracks to close. However, most engineering alloys contain
second phase particles (inclusions or precipitates), at which
voids nucleate, typically through decohesion of matrix-
particle interface. Such voids grow and coalesce
(join/combine) to become a crack.
7-10
Fig. 7.1-9 Plastic zone ahead of crack tip where voids nucleate and grow.
• Within the plastic zone ahead of the crack tip, void
nucleation, growth and coalescence also occur (Fig. 7.1-9),
and the crack continues to grow to fracture by this
mechanism (Fig. 7.1-10).
Fig. 7.1-10 Stages in ductile fracture caused by void coalescence in the plastic zone.
• The “dimpled” appearance of a ductile fracture surface (Fig.
7.1-7b) shows the remnants of the voids, which were
separated by thin walls of material until these ruptured via
shear (slip) during crack growth. Sometimes, the particles
that nucleated the voids may even remain within the
dimples (Fig. 7.1-7b). 7-11
• The continual plastic deformation during ductile fracture
consumes a lot of energy. The larger the plastic zone
ahead of the crack tip, the more energy is absorbed, and
the tougher the material.
Brittle vs Ductile Fracture
• In a material under stress, there is a competition between
the processes of plastic deformation and fracture. If the
stress required to initiate plastic deformation by the
shearing of atomic planes (i.e. yield stress) is less than the
stress necessary to permanently separate atoms (i.e.
fracture stress), yielding occurs in preference to fracture
and vice versa.
• Ductile fracture is generally preferred because crack
growth is relatively slow and steady, and the presence of
plastic deformation gives warning that fracture is
imminent, allowing preventive measures to be taken.
• Brittle fracture, on the other hand, occurs suddenly and
catastrophically without any warning.
• Ductile fracture is also more desirable because of the
greater amount of energy absorbed prior to fracture.
7-12
7.1.4 Effects of Temperature & Strain Rate:
Ducti le-to-Brittle Transition
• Many materials exhibit a transition from ductile to brittle
modes of fracture as the temperature is lowered.
• Ductile fracture is accompanied by plastic deformation,
which involves dislocation motion. For materials in which
the critical shear stress is high (e.g. BCC metals [see Table 6.1-2,
pg 6-6]), thermal energy helps in overcoming the energy
barrier.
• At low temperatures, thermal activation for dislocation
motion is reduced. The result is that the yield strength
increases as temperature decreases (Fig.7.1-11), such that the
plastic zone at the crack tip shrinks until it becomes so
small that the fracture mechanism changes from void
coalescence to cleavage.
• Such materials also show a dependence on the strain rate
(i.e. how fast deformation is made to occur, which
depends on the loading rate), because thermal activation is
less effective at a faster rate of deformation. A higher strain
rate would raise the yield strength but lower the
elongation at fracture.
7-13
Fig. 7.1-11 Engineering stress-strain behaviour of iron at different temperatures. The strength and ductility of iron (BCC) show a strong temperature dependence.
• In general, slip systems in BCC metals and some HCP
metals become active only when there is sufficient thermal
energy for dislocation motion; such metals exhibit a
ductile-to-brittle transition (Fig. 7.1-12).
Fig. 7.1-12 The effects of temperature on the fracture behaviour of various materials in impact tests.
7-14
7.1.5 Impact Testing
• The ductile-to-brittle transition behaviour of materials may
be ascertained through impact testing. Impact testing may
also be used to compare the relative toughness of
materials.
• An impact test imposes a stringent set of conditions – a
pre-existing notch (crack) and high loading rates (fast
strain rates) – which tend to promote brittle fracture rather
than plastic flow.
• The Charpy test is one of the most widely used impact
tests (Fig. 7.1-13). A notched specimen is positioned at the
base of the machine. A heavy pendulum is released from a
known height, h, striking the specimen on its downward
swing, fracturing it. The pendulum continues its swing,
rising to a maximum height h’, which is lower than h. The
energy absorbed by the fracture is measured by the
difference between h and h’.
• A ductile material will absorb greater impact energy than a
brittle material. This ability of a material to withstand an
impact blow is often referred to as impact toughness.
7-15
Fig. 7.1-13 Schematic diagram of the Charpy impact tests.
• While impact tests are useful for qualitative assessment of
material toughness, the results obtained; namely, the
energy to fracture and the appearance of the fracture
surfaces, are not useful in design problems. Designing for
fracture requires the use of fracture toughness, a material
property derived from fracture mechanics.
7-16
7.2 FATIGUE
• If a component or structure is subjected to repeated stress
cycles, it may fail at stresses well below the ultimate tensile
strength, and often even below the yield strength of the
material.
• The processes leading to this type of failure are known as
fatigue, and it is estimated that fatigue accounts for
approximately 90% of all metallic failures.
• Fatigue fracture resembles brittle fracture in that failure is
sudden and catastrophic, with very little visible plastic
deformation, even in normally ductile metals.
7.2.1 Fatigue Mechanisms
• In fatigue, cracks grow slowly under stresses less than the
yield strength until the cracks become so large that the
remaining cross-sectional area can no longer support the
load, and sudden and catastrophic failure occurs.
• Fatigue failures generally start at the surface (where
bending or torsional stresses are highest). Fatigue cracks
may be pre-existing, or initiated through plastic
deformation at stress raisers (Figs. 7.2-1 & 7.2-2), including rough
surfaces left by tools or grinding. 7-17
Fig. 7.2-1 Fatigue crack initiation at stress raisers.
Fig. 7.2-2 Local plastic deformation under a cyclic stress can roughen a surface, in such a way that the “valleys’ concentrate stress and initiate fatigue cracks.
• Under a cyclic stress (Fig. 7.2-3), tensile stress produces a small
plastic zone at the crack tip, stretching open the crack tip
by an amount δ, creating new surface πδ/2 there (Fig. 7.2-4).
As the tensile stress is removed or reduced, the crack closes
and the new surface folds forward, extending the crack by
πδ/4. The process is repeated on the next tensile cycle, and
so the crack inches forward slowly and steadily.
7-18
Fig. 7.2-3 (a) Sinusoidal, and (b) random variable stress cycles.
(a)
(b) Fig. 7.2-4 (b) Tensile stress
causes plastic deformation at the crack tip, creating
new surface of length πδ/2, which, (c) when folded forwards as the tensile
(c) stress is relaxed, grows the fatigue crack by πδ/4.
(d)
• As the crack grows, the cross-sectional area that supports
the load is decreased and the stress in this section
increases, until the component fails catastrophically in the
next stress cycle due to overload (Fig. 7.2-5). 7-19
Fig. 7.2-5 The stages of fatigue failure: after crack initiation, the crack grows slowly
and steadily until the remaining cross section can no longer support the load, resulting in rapid and catastrophic failure.
• Fatigue fracture surfaces typically exhibit beachmarks (so-
called because they resemble ripples in the sand on a
beach), or clamshell markings (Fig. 7.2-6), which are visible to
the naked eye.
Fig. 7.2-6 Beachmarks or clamshell markings typical of fatigue fracture.
7-20
• Microscopic examination of the beachmarks reveal similar
markings on a finer scale, called striations (Fig. 7.2-7). The
spacing between the striations is a measure of the slow
crack advance per stress/strain cycle.
Fig. 7.2-7 Fatigue fracture striations (crack propagation is from left to right).
• Although fatigue is associated with slip, it is of such a
localized nature that very little overall plastic deformation
is produced during the development of a crack. It is this
lack of visible distortion that makes fatigue cracks difficult
to detect in service prior to final catastrophic failure.
7-21
7.2.2 Fatigue Testing • The resistance of a material to failure by fatigue may be
determined through a fatigue test, in which a specimen is
subjected to a stress cycling through simultaneous bending
and rotation (Fig. 7.2-8).
Fig. 7.2-8 Fatigue testing.
• The stress, in general, varies sinusoidally with time, and is
characterized by a stress amplitude, σa or S, that
alternates about a mean stress, σm (Fig 7.2-9).
σa =
€
σmax - σmin2
; σm =
€
σmax + σmin2
Fig. 7.2-9 A typical stress cycle.
7-22
• From the fatigue test, the stress amplitude, S, measured for
some constant mean stress, σm, is plotted against the
number of cycles to failure, N.
• For some alloys (e.g. steels and titanium alloys), the S-N curve becomes horizontal below a certain level of S (Fig. 7.2-
10). This is called the fatigue limit (or endurance limit). Below this stress amplitude, fatigue failure would not occur regardless of the number of cycles.
Fig. 7.2-10 Stress amplitude, S, versus the number of cycles to fatigue failure, N
for a material that displays a fatigue limit.
• Many nonferrous alloys (e.g. Al, Cu, Mg) do no have a well-defined fatigue limit. The S-N curve continues to slope
downward with increasing N values (Fig. 7.2-11). For such
materials, the fatigue strength is defined as the stress level at which failure would occur for some specified number of
cycles (e.g. 107 cycles). Fatigue life is the number of cycles
to cause failure at a specified stress level. 7-23
Fig. 7.2-11 Stress amplitude, S, versus the number of cycles to fatigue failure, N
for a material that does not display a fatigue limit.
• Apart from the dependence on stress amplitude, fatigue
life is also affected the mean stress, σm, about which the
stress cycle alternates. Increasing the mean stress level has
the effect of lowering the S-N curve to lower S values, thus
leading to a shorter fatigue life (Fig. 7.2-12).
Fig. 7.2-12 Effect of mean stress on S-N fatigue behaviour.
7-24
• The endurance limit / fatigue strength may be correlated
with strength (Fig. 7.2-13), since a stronger material has higher
resistance to slip at the crack tip during each tensile cycle.
Fig. 7.2-13 Relationship between endurance limit / fatigue strength and strength.
• Fatigue life may be improved by selecting a stronger
material, increasing surface hardness (e.g. carburizing in
steels [Fig. 7.2-14]), reducing stress raisers through proper
design and improved surface finish, as well as inducing
compressive surface stresses through heat treatment or
mechanical operations (e.g. shot peening [Fig. 7.2-15]).
7-25
Fig. 7.2-14 Carburized steel gear teeth showing gradual increase in hardness
from core to surface due to increasing carbon content in the steel.
Fig. 7.2-15 In shot peening, tiny steel, glass or ceramic balls shot at high speeds
onto metal surfaces creates plastic deformation and induces compressive stresses on the surface.
7-26
7.2.3 Non-Destructive Testing (NDT) • Since many alloys (e.g. Al, Cu, Mg) do not have endurance
limits, critical engineering components must be inspected
periodically for fatigue cracks, which must not be allowed
to grow to critical sizes for final fracture.
• Several techniques are available to detect surface and
internal flaws during manufacture and in-service (Table 7.2-1).
7-27
8-1
C H A P T E R 8
C O R R O S I O N
8.1 ELECTROCHEMICAL REACTIONS
8.2 FORMS OF CORROSION 8.2.1 Galvanic Corros ion
8.2.2 Di f ferent ia l Aerat ion Corros ion 8.2.3 Other Concentrat ion Cel l s
8.3 PROTECTION AGAINST CORROSION
8-2
• The performance of a material must be evaluated in the
context of its interaction with its service environment. Such
interactions may be mechanical (friction and wear), or
electrochemical (oxidation and corrosion).
• These interactions usually result in the degradation of the
material (i.e. deterioration in mechanical and/or physical
properties, or appearance), leading to huge economic
losses (5% or more of the GDP of industrialized countries),
as well as posing threats to human safety.
NITRICACID
PETROL
(a) Fresh water (b) Salt water
(c) Acids and alkalis (d) Organic solvents
(e) Oxidation (f) UV radiation
Fig. 8.1-1 Materials undergo electrochemical interactions with water, acids and alkalis, organic solvents, oxygen and radiation.
8-3
Corrosion occurs in an aqueous environment, and
together with wear, are the main causes of metal
degradation at ambient temperatures. Generally, corrosion
will occur in air with relative humidity (RH) greater than
60-70%, which forms an invisible thin film of moisture on
the surface of metal.
8.1 ELECTROCHEMICAL REACTIONS
• When immersed in an electrolyte, most metals have
varying degrees of tendency to dissolve into the electrolyte
as positively-charged metal ions, leaving behind the
negatively-charged electrons in the metal (Fig. 8.1-2a).
• The metal, A, now negatively-charged, will attract positive
ions in the electrolyte in its vicinity, such that the metal
ions that have just dissolved in the electrolyte tend to
revert back to solid metal atoms again.
• In the absence of external influences, an equilibrium is
reached in which metal atoms and ions are continually
entering and leaving the electrolyte at the metal surface.
Overall, there is no net change in the system, but the metal
has developed an electrochemical potential.
8-4
• The magnitude of this potential depends on the tendency
of the metal to dissolve into the electrolyte. The stronger
the tendency, the more negative the potential.
• When a different metal, C, is added to the same electrolyte
(Fig. 8.1-2b), it also develops its own potential, depending on
its tendency for metal dissolution.
• If the two metals were connected by a wire (Fig. 8.1-2c), the
excess electrons would try to minimize the potential
difference by distributing themselves around the system.
Electrons flow from the metal with more excess electrons
(more negative potential), A, to the metal with fewer
electrons (less negative potential), C.
Fig. 8.1-2 (a) Metal A at equilibrium with positive metal ions in solution, adjacent to the electrons left behind. (b) Metal C has less tendency to dissolve in the electrolyte, leaving less negative charge in the metal. (c) A wire conncection allows electrons to flow from the more negative to the less negative metal. As electrons flow, metal A becomes less
negative and metal C becomes more negative.
8-5
• Since metal A now becomes less negatively-charged, there is less attraction between the metal and the positive ions in the electrolyte. Some of the metal ions drift away, upsetting the equilibrium around the metal surface and allowing further dissolution of metal A, i.e. A corrodes.
• Metal A is known as the anode, where electrons are produced through the following reaction:
M → Mn++ ne– (anodic/oxidation reaction)
• On the other hand, metal C has gained electrons to become more negatively-charged and its positive ions in the electrolyte are thus more strongly attracted to the metal and may even convert back into solid metal atoms.
• The electrons from A are consumed through various reactions at C, called the cathode, depending on the electrolyte. Some cathodic/reduction reactions are:
Table 8.1-1 Some common cathode reactions for aqueous galvanic cells.
8-6
• In order for corrosion to occur in a system, a potential
difference must exist between different regions, which
must also be electrically connected, such that electrons
produced at the anode can be consumed at the cathode.
An electrolyte must also be present for the conduction of
ions. If any of these elements were missing, the corrosion
reaction would stop.
• The potentials of various metals under standard conditions
have been measured relative to a hydrogen electrode and
are listed in the electrochemical series (Table 8.1-2). A metal
that is lower on the electrochemical series has a greater
tendency to lose electrons and become metal ions in
solution (i.e. corrode).
Table 8.1-2 Standard electrochemical potentials.
8-7
• The potential of a metal depends on many factors, such as
temperature, type of electrolyte, concentration of metal
ions and other species (e.g. oxygen) in the electrolyte, etc.
The electrochemical series was generated under highly
idealized conditions; in most practical situations, the
potential of a metal differs from its value in this series.
• It is thus more useful to refer to the experimental galvanic
series, which is a simple, qualitative ranking of the relative
reactivities of a number of metals and commercial alloys in
a particular environment, e.g. seawater (Table 8.1-3).
Table 8.1-3 The galvanic series in aerated seawater at 25°C.
8-8
8.2 FORMS OF CORROSION
8.2.1 Galvanic Corrosion
• Galvanic corrosion occurs when two metals of different
compositions are electrically connected in an electrolyte.
• The alloy with the more negative potential becomes the
anode and corrodes, while the less negative metal acts as
the cathode. Generally, the greater the potential
difference, the more severe corrosion would be.
• Since the same total number of electrons must flow
through the anodic and cathodic regions during corrosion,
the ratio of anodic to cathodic surface area will affect the
rate of corrosion. Small anodes in contact with large
cathodes will corrode more severely than large anodes in
contact with small cathodes (Fig. 8.2-1).
Fig. 8.2-1 Effects of anode-to-cathode area:
(a) A large cathode (Cu) area consumes a large number of electrons during the cathodic reaction, which would have to be provided by the severe corrosion of the small anode (Fe).
(b) Corrosion is less severe when the ratio of the anode to cathode surface area is large (i.e. large anode–small cathode is favourable).
8-9
• Uniform corrosion is a special case of galvanic corrosion occurring at the microscopic level within the same metal. Even a nominally homogeneous metal surface contains tiny variations in composition and structure, giving rise to microscopic local anodes and cathodes. With time, the locations of the anodic and cathodic regions change randomly so that the result is a uniform loss of metal over the entire exposed surface.
• Alloys containing more than one phase (Sec. 9.1) would similarly undergo microscopic galvanic corrosion (Fig 8.2-2), since each phase has a different composition. Single-phase alloys are generally more resistant to corrosion than multi-phase ones.
Fig. 8.2-2 Microscopic galvanic corrosion in steel (a two-phase alloy).
Fig. 8.2-3 Intergranular corrosion due to compositional differences between grain boundaries and grain interior.
• Galvanic couples may develop at grain boundaries, leading to intergranular corrosion (Fig 8.2-3). Differences in composition and structure due to segregation of impurities to, or the formation of precipitates at grain boundaries could make these regions anodic to the bulk of the grains.
8-10
8.2.2 Differential Aeration Corrosion
• In many instances, oxygen is required at the cathode in
order for corrosion to occur [See Table 8.1-1, cathode reactions 3 and 4].
If there is a difference in oxygen concentration between
two regions, the area of low oxygen concentration will
become the anode, while the region of high oxygen
concentration will act as the cathode.
• Differential aeration is responsible for crevice corrosion
and pitting corrosion (Fig. 8.2-5). In these forms of localized
corrosion (“localized” as opposed to “general” or “uniform” corrosion, which
occurs everywhere on the metal surface), the oxygen in small crevices
and pits are consumed during uniform corrosion. Since
there is restricted exchange of electrolyte from within the
confined spaces of these locations, these regions become
lower in oxygen concentration than the rest of the material
that is exposed to a large volume of electrolyte. As a result,
corrosion occurs preferentially in the crevices and pits.
• Differential aeration is also the mechanism behind
waterline corrosion. Oxygen is able to reach metal near
the waterline than regions further away; thus, the metal
just inside the waterline will become cathodic while the
remaining immersed metal will become anodic (Figs. 8.2-6 & 7).
8-11
(a) (b)
Fig. 8.2-5 (a) Crevice corrosion, and (b) pitting corrosion, due to differential aeration.
Fig. 8.2-6 Waterline corrosion due to falling levels of dissolved oxygen
below the liquid surface.
Fig. 8.2-7 “Waterline” corrosion beneath a drop of water.
8-12
• An environment in which oxygen is severely limited will
tend to reduce corrosion significantly (Fig. 8.2-8), since
oxygen is required for cathodic reactions in all solutions
[Table 8.1-1, cathode reactions 3 and 4].
Fig. 8.2-8 Corrosion is more severe in (a) sandy soil than in (b) clay, due to the permeability of sand to oxygen.
• Corrosion of all forms also increases in severity with
temperature, because all electrochemical reactions are
thermally activated.
8-13
8.2.3 Other Concentration Cells
• A metal that has been cold worked contains a high density
of dislocations. Since dislocations are associated with
higher energy, these highly-stressed, cold-worked regions
in a metal will act as anodes to less-stressed cathodic areas
(Fig. 8.2-9).
Fig. 8.2-9 The regions of a nail that were stressed during fabrication or use
are anodic and will corrode locally.
Fig. 8.2-10 The higher energy of grain boundaries makes them anodic and
more susceptible to corrosion.
• Similarly, grain boundaries, with their higher energy, are
anodic to the bulk of the grains and tend to corrode more
severely (Fig. 8.2-10; see also Sec. 4.7 on Metallography). However, unlike
intergranular corrosion (Sec. 8.2-1), the rate of corrosion at
grain boundaries that do not contain segregated impurities
or precipitates is not significantly higher than the grain
interior, since the potential difference between the
boundaries and interior is only minimal.
8-14
8.3 PROTECTION AGAINST CORROSION
• Corrosion can be retarded or prevented if either the anodic
or cathodic reaction can be stifled.
• Many corrosion problems can be eliminated through
proper design (Fig. 8.3-1) and material selection to minimize
concentration or galvanic differences. Materials that
produce protective oxide layers, e.g. aluminium alloys
(Al2O3) or stainless steels (Cr2O3), may be used instead.
Fig. 8.3-1 Reducing crevice corrosion through improved design.
8-15
• Protective coatings prevent contact between the metal
and electrolyte (e.g. paints, chrome-plating). Furthermore,
if the coating is anodic to the underlying metal (e.g. Zn on
steel or galvanized steel), should the coating become
scratched, the metal will continue to be protected since
the coating will corrode instead (Fig. 8.3-2).
Fig. 8.3-2 Zinc is anodic to steel, such that should the Zn coating become disrupted, the
large anode (Zn) to small cathode (steel) area would ensure continued protection. However, steel is anodic to tin, which would result in a small anode-to-cathode area ratio if the coating
is scratched; this is undesirable since it would cause severe corrosion of the steel.
• Inhibitors may be used in closed systems. These are
chemicals added to the electrolyte that usually form
protective layers either on the surfaces of the anode or
cathode. Other inhibitors may retard the cathodic reaction,
thereby reducing rate of corrosion at the anode.
• Cathodic protection works by “forcing” the corroding
metal to become the cathode instead. One way is to use a
sacrificial anode by electrically connecting a more anodic
metal to the metal to be protected. The more anodic metal
corrodes in preference to the original metal (Fig. 8.3-3 & 4a).
The sacrificial anode would need to be replaced
periodically.
8-16
Fig. 8.3-3 Cathodic protection through sacrificial zinc anodes on the steel hulls of ships.
Fig. 8.3-4 Cathodic protection of underground pipelines using
(a) sacrificial zinc anode, and (b) impressed current.
• Another cathodic protection method similar to the
sacrificial anode method is the use of an impressed
current. Since an anode corrodes when electrons flow
away, the anode can be made cathodic by supplying a
larger counter-current to neutralize the corrosion current
(Fig 8.3-4b). The major advantage of this method is that it is
possible to use non-consumable anodes. Another
advantage is that the supply may be controlled to regulate
the current provided.