Objectives%20of%20Chapter%2022

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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.
• Faradays 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,
  low-oxygen 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 polymersincluding 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 free-flying 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).