Transcript The Science and Engineering of Materials, 4th ed Donald R

Objectives of Chapter 22
 To introduce the principles and mechanisms
by which corrosion and wear occur under
different conditions. This includes the
aqueous corrosion of metals, the oxidation
of metals, the corrosion of ceramics, and
the degradation of polymers.
 To give summary of different technologies
that are used to prevent or minimize
corrosion and associated problems.
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Chapter Outline
 22.1 Chemical Corrosion
 22.2 Electrochemical Corrosion
 22.3 The Electrode Potential in
Electrochemical Cells
 22.4 The Corrosion Current and Polarization
 22.5 Types of Electrochemical Corrosion
 22.6 Protection Against Electrochemical
Corrosion
 22.7 Microbial Degradation and
Biodegradable Polymers
 22.8 Oxidation and Other Gas Reactions
 22.9 Wear and Erosion
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Figure 22.2 Photomicrograph of a copper deposit in brass,
showing the effect of dezincification (x50).
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Figure 22.4 The anode and cathode reactions in typical
electrolytic corrosion cells: (a) the hydrogen electrode, (b) the
oxygen electrode, and (c) the water electrode.
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Section 22.3
The Electrode Potential in
Electrochemical Cells
 Electrode potential - Related to the tendency of a
material to corrode. The potential is the voltage
produced between the material and a standard
electrode.
 emf series - The arrangement of elements according to
their electrode potential, or their tendency to corrode.
 Nernst equation - The relationship that describes the
effect of electrolyte concentration on the electrode
potential in an electrochemical cell.
 Faraday’s equation - The relationship that describes the
rate at which corrosion or plating occurs in an
electrochemical cell.
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Figure 22.5 The half-cell
used to measured the
electrode potential of
copper under standard
conditions. The electrode
potential of copper is the
potential difference
between it and the
standard hydrogen
electrode in an open
circuit. Since E0 is great
than zero, copper is
cathodic compared with
the hydrogen electrode.
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Section 22.5
Types of Electrochemical Corrosion
 Intergranular corrosion - Corrosion at grain boundaries
because grain boundary segregation or precipitation
produces local galvanic cells.
 Stress corrosion - Deterioration of a material in which an
applied stress accelerates the rate of corrosion.
 Oxygen starvation - In the concentration cell, lowoxygen regions of the electrolyte cause the underlying
material to behave as the anode and to corrode.
 Crevice corrosion - A special concentration cell in which
corrosion occurs in crevices because of the low
concentration of oxygen.
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Example 22.5
Corrosion of a Soldered Brass Fitting
A brass fitting used in a marine application is joined by
soldering with lead-tin solder. Will the brass or the solder
corrode?
Example 22.5 SOLUTION
From the galvanic series, we find that all of the copper-based
alloys are more cathodic than a 50% Pb-50% Sn solder. Thus,
the solder is the anode and corrodes. In a similar manner, the
corrosion of solder can contaminate water in freshwater
plumbing systems with lead.
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Figure 22.6 Example of microgalvanic cells in two-phase alloys:
(a) In steel, ferrite is anodic to cementite. (b) In austenitic
stainless steel, precipitation of chromium carbide makes the low
Cr austenite in the grain boundaries anodic.
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Figure 22.7 Photomicrograph of intergranular corrosion in a
zinc die casting. Segregation of impurities to the grain
boundaries produces microgalvanic corrosion cells (x50).
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Figure 22.8 Examples of stress cells. (a) Cold work required to
bend a steel bar introduces high residual stresses at the bend,
which then is anodic and corrodes. (b) Because grain
boundaries have a high energy, they are anodic and corrode.
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Example 22.6
Corrosion of Cold-Drawn Steel
A cold-drawn steel wire is formed into a nail by additional
deformation, producing the point at one end and the head at
the other. Where will the most severe corrosion of the nail
occur?
Example 22.6 SOLUTION
Since the head and point have been cold-worked an additional
amount compared with the shank of the nail, the head and
point serve as anodes and corrode most rapidly.
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Figure 22.9 Concentration cells: (a) Corrosion occurs beneath a
water droplet on a steel plate due to low oxygen concentration
in the water. (b) Corrosion occurs at the tip of a crevice
because of limited access to oxygen.
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Example 22.7
Corrosion of Crimped Steel
Two pieces of steel are joined mechanically by crimping
the edges. Why would this be a bad idea if the steel is
then exposed to water? If the water contains salt, would
corrosion be affected?
Example 22.7 SOLUTION
By crimping the steel edges, we produce a crevice. The
region in the crevice is exposed to less air and moisture,
so it behaves as the anode in a concentration cell. The
steel in the crevice corrodes.
Salt in the water increases the conductivity of the
water, permitting electrical charge to be transferred at a
more rapid rate. This causes a higher current density and,
thus, faster corrosion due to less resistance polarization.
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Figure 22.10 (a) Bacterial
cells growing in a colony
(x2700). (b) Formation of
a tubercule and a pit
under a biological colony.
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Section 22.6
Protection Against Electrochemical
Corrosion
 Inhibitors - Additions to the electrolyte that preferentially
migrate to the anode or cathode, cause polarization, and
reduce the rate of corrosion.
 Sacrificial anode - Cathodic protection by which a more
anodic material is connected electrically to the material
to be protected. The anode corrodes to protect the
desired material.
 Passivation - Producing strong anodic polarization by
causing a protective coating to form on the anode
surface and to thereby interrupt the electric circuit.
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Figure 22.11 Alternative methods for joining two pieces of
steel: (a) Fasteners may produce a concentration cell, (b)
brazing or soldering may produce a composition cell, and (c)
welding with a filler metal that matches the base metal may
avoid the formation of galvanic cells (for Example 22.8)
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Figure 22.12 Zinc-plated steel and tin-plated steel are
protected differently. Zinc protects steel even when the
coating is scratched, since zinc is anodic to steel. Tin does
not protect steel when the coating is disrupted, since steel is
anodic with respect to tin.
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Figure 22.13 Cathodic protection of a buried steel pipeline:
(a) A sacrificial magnesium anode assures that the galvanic
cell makes the pipeline the cathode. (b) An impressed voltage
between a scrap iron auxiliary anode and the pipeline assures
that the pipeline is the cathode.
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Section 22.7
Microbial Degradation and
Biodegradable Polymers
 Simple polymers (such as polyethylene, polypropylene,
and polystyrene), high-molecular-weight polymers,
crystalline polymers, and thermosets are relatively
immune to attack.
 However, certain polymers—including polyesters,
polyurethanes, cellulosics, and plasticized polyvinyl
chloride (which contains additives that reduce the degree
of polymerization)—are particularly vulnerable to
microbial degradation.
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Section 22.8
Oxidation and Other Gas Reactions
 Oxidation - Reaction of a metal with oxygen to produce a
metallic oxide. This normally occurs most rapidly at high
temperatures.
 Pilling-Bedworth ratio - Describes the type of oxide film
that forms on a metal surface during oxidation.
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Section 22.9
Wear and Erosion
 Adhesive wear - Removal of material from surfaces of
moving equipment by momentary local bonding, then
bond fracture, at the surfaces.
 Abrasive wear - Removal of material from surfaces by
the cutting action of particles.
 Cavitation - Erosion of a material surface by the
pressures produced when a gas bubble collapses within a
moving liquid.
 Liquid impingement - Erosion of a material caused by the
impact of liquid droplets carried by a gas stream.
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Figure 22.18 The
asperities on two rough
surfaces may initially be
bonded. A sufficient force
breaks the bonds and the
surfaces slide. As they
slide, asperities may be
fractured, wearing away
the surfaces and
producing debris.
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Figure 22.19 Abrasive wear, caused by either trapped or freeflying abrasives, produces troughs in the material, piling up
asperities that may fracture into debris.
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Figure 22.20 Two steel sheets joined by an
aluminum rivet (for Problem 22.25).
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