Week 10
Corrosion/Degredation & Economic, Social and Environmental Issues
Callister Chapters 17 & 22
You Should Know/Be Able to:
I. Corrosion
 Explain the electrochemistry involved in the corrosion of metals as expressed in
oxidation-reduction reactions (including what reactions are likely at the anode and
cathode).
 Name thermodynamic driving force for the corrosion reaction
 Name the four factors required for corrosion.
 For the types of corrosion (galvanic - macro and micro, selective leaching, erosion
corrosion, hydrogen damage, pit & crevice corrosion, stress corrosion cracking)
 Describe conditions for each type and name possible preventive techniques
 Given a situation, state which type of corrosion is most likely
 Name methods to reduce the problems of corrosion, especially the use of cathodic
protections (sacrificial anodes).
 Name the causes for degradation of polymers (radiation, solvents, ozone ...)
II.
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Economic, Social and Environmental Issues
List the phases of the material life cycle as well as the inputs and outputs at each phase
State alternatives to disposal in the material life cycle.
Define sustainability from the perspective of the material life cycle
Explain the conflict between material consumption and achieving a more sustainable
material cycle
Define the term Life Cycle Analysis (LCA) and list pros and cons to this type of analysis
Explain the significance of embodied energy in the materials selection for “green
design”
Compare and contrast the ability to and feasibility of recycling or reusing each class of
material (metals, ceramics/glasses, polymers and composites) from an economic, social
and environmental perspective.
Discuss advantages and disadvantages of using alternative sources (ex. biomass) to
produce polymers
Discuss advantages and disadvantages of degradable/compostable polymers
Vocabulary
Chapt 17
activation polarization
anode
cathode
cathodic protection
concentration
polarization
corrosion
crevice corrosion
degradation
Chapt 22
material life cycle
life cycle analysis
electrolyte
emf series
erosion corrosion
galvanic corosion
galvanic series
hydrogen
embrittlement
inhibitor
intergranular corrosion
oxidation
passivity
pitting
polarization
reduction
sacrificial anode
scission
selective leaching
stress corrosion
embodied energy
degradable
recyclable
Week 10
Corrosion/Degredation & Economic, Social and Environmental Issues
Callister Chapters 17 & 22
I. Corrosion
Conductor allows travel of
electrons from
anode to cathode
Electrolyte Provides Ions
for reduction
Anode Lose Electron Oxidation
Zn
2+
Zn
Cu
-
Zn + 2e
HCl
Cathode Gain Electron Reduction
2H++ 2e-
2H
Which metal will be the
Anode?
 standard EMF
(electromotive force)
series
 galvanic series (alloys
in sea water)
 active metals (Mg, Al,
Zn) tend to be
anodic, passive
metals (Au, Cu, Pt)
cathodic
Polarization
Activation Polarization - reaction rate controlled by some physical or electrical factor (ex. hydrogen
film at surface of cathode can act as barrier)
Concentration Polarization - diffusion rate controls reaction rate (ex. if concentration of electrolyte
is low, fewer ions must travel further and rate slows)
Types of Corrosion
Type
Uniform Attack
Galvanic Corrosion
Intergranular
Erosion-Corrosion
Hydrogen
Embrttlement
Crevice Corrosion
Pitting
Selective Leaching
Stress-Corrosion
Characteristics
Most common for bare metals
dissimilar metals
anodic metal corrodes
along grain boundaries due to
concentration of precipitates
(dissimilar metals at micro level)
combined wear and corrosion
small element diffuses in and
reduces ductility
concentration cell forms in regions
of stagnation
pH decrease in crevice results in
surface damage
very similar mechanism to crevice
corrosion but no crevice is needed.
microscopic loss of one component
in alloy
combination of stress and a
particular environment
Example
rust
galvanized steel (Fe,Zn)
“sensitized” stainless steel
impellers; bends, elbows in pipes
high strength steels under stress
threads, gaskets, deposits
metals that rely on passive films
(Al, S/S)
metals that rely on passive films
dezincification of brass
graphitization of CI
brass and ammonia
stainless steel and chlorides
Week 10
Corrosion/Degredation & Economic, Social and Environmental Issues
Callister Chapters 17 & 22
Corrosion Prevention
 Get rid of one of the four factors (Anode, Cathode, Electrolyte, Conductor)
 In general-avoid differences (differences in material, in concentration, in cold work) If parts are
identical, there is no anode/cathode
 Barriers prevent contact with electrolyte (coatings-paint, anodizing)
 Prevent electrical connection between anode and cathode (dielectric connections in plumbing)
 Inhibitors - chemicals that slow or stop the process (boilers, power plants)
 Cathodic Protection (sacrificial anodes) (in galvanized steel, the anodic Zn is lost and the
cathodic Fe is protected.)
Environmental Degradation of Polymers
Solvents - some polymers are attacked (dissolved) by some chemicals. (ex. latex and petroleum
products, PMMA and acetone)
Radiation - many photons (esp. UV) have sufficient energy to break atomic bonds. If a side group
bond is broken, cross-linking can occur. If a backbone bond is broken, chain scission occurs and
chains get shorter. (ex. old wrinkled sunbathers)
Ozone and other radicals- O3 has a strong drive to bond and can rupture bonds and cause problems
similar to radiation.
II. Economic, Social and Environmental Issues
Material Consumption
Fig 20.1 from Materials, Ashby, Shercliff, Cebon, B-H (2007)
Material life cycle and assessment
Material Cycle
 Oil and concrete are used in the largest
quantities, followed by steel, wood and
other construction materials.
 The consumption of most materials is
growing at an exponential rate, due to
growth of population and living
standards.
 Many of these materails, esp oil and oilderived polymers, are limited
resources.
 Natural resources are processed in
the material production phase.
Energy, feedstocks and tranportation
are “inputs” of each phase.
 Energy and materials are consumed
at each phase
 Stressors or other wastes are
“outputs” of each phase.
 A “sustainable material cycle”
minimizes the inputs (esp. natural
resources) and outputs (esp. waste).
Fig 20.3 from Materials, Ashby, Shercliff, Cebon,
B-H (2007)
Week 10
Corrosion/Degredation & Economic, Social and Environmental Issues
Callister Chapters 17 & 22
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Embodied Energy – the energy required to create 1 kg of usable material (often in units of
MJ/kg). These values are assessed through an input-output analysis. For example, to produce
PETE pellets, energy is inputted to the process from oil derivatives (the feedstock), the
transportation to deliver the feedstock, and the power used in the refining of the feedstock.
The output is PETE pellets. Note that the embodied energy of the feedstock come from the
drilling, transportation and refining of crude oil.
Life Cycle Analysis (LSA) – a technique to assess every impact associated with all the phases of a
process from cradle to grave, including processing of raw materials, manufacturing, distribution,
use, and disposal. The stressors that are assessed include greenhouse gases, ozone layer
depletion, toxicity to humans and the environment, habitat destruction, and the depletion of
minerals and fossil fuels among many others.
o Advantage: LSA is very thorough and accurate in the quantification of stressors for
comparison of existing products.
o Disadvantages: LSA is time-consuming, costly, requires knowledge of the detailed
history of a product, can only realistically be done on an existing product (not useful at
the design stage),
End-of-life potential – the possible utility of a material at the end of its intended use
Recycling
Strict definition – processing used materials/waste to produce a supply of the raw material from
which it came (this implies that it can be made into the same product again).
General definition – processing used materials/waste into new products (this includes “reusing”
a material)
Down-cycling
Definition – processing used materials/waste into raw material for a lower-grade application
(this implies that the material properties have been diminished, compromised or the supply has
been contaminated)
Degradation (usually refers to polymers)
Definition – the chemical breakdown of a material by environment via hydrolysis or oxidation
resulting in physical disintegration and large reduction in molecular weights (chain length).
Heat, moisture, enzymes or other environmental condition initiate the degradation by breaking
long polymer chains into smaller molecules. Microbes can then consume and digest the smaller
chains to further degrade the material into water, biomass and CO2. Degradation rates vary
between polymers and many materials require proper disposal, such as well managed
composting facilities, for degradation to occur.
Combustion
Definition – the controlled burning of waste to produce energy. Combustion issues involve the
possible formation of air pollutants (NOx, SOx) and hazardous ash, which requires further
disposal or recycling.
Landfill
Landfill disposal issues involve minimizing the contamination of the land, air or water through
material toxicity (ex. lead or mercury) or harmful material degradation products (ex. acid
leeching from batteries).
Week 10
Corrosion/Degredation & Economic, Social and Environmental Issues
Callister Chapters 17 & 22
Recycling Issues
Recycling Challenges
Industrial scrap is often very easy to recycle and often has very high recycling rates; however here
are many challenges to recycling consumer products including:
 Collecting, sorting, and decontaminating are time consuming and expensive
 Products have many parts that are made from different materials (disassembly required for
recycling)
 Painting, printing, plating or coating contaminate the material
 Use of the product can result in contamination
 Collected recycled materials sometimes end up in landfills if there is not sufficient demand.
There are economic advantages to recycling some materials, metals especially. A few materials are
commonly recycled because their disposal is regulated due to toxicity (ex. lead). For many other
common materials, including glass and plastics, there is little economic incentive to recycle.
Consumer demand and environmental consciousness, rather than economics, may help drive the
recycling of such materials. Each class of material is considered in the following sections.
Polymer recycling
Table 20.1 from Materials, Ashby, Shercliff, Cebon, B-H (2007)
Polymer
Embodied Energy
Price
(MJ/kg)
($/kg)
Virgin
Recycled Virgin
Recycled
HDPE
82
40
1.9
0.9
PP
82
40
1.8
1.0
PETE
85
55
2.0
1.1
PS
101
45
1.5
0.8
PVC
66
37
1.4
0.9
 Less energy is needed to create new polymeric “raw material” from recycled material compared
to creating virgin material from crude oil.
 However, the properties of recycled polymers are usually diminished, resulting in a lower price
for the recycled polymer. These materials are often down-cycled into less demanding
applications.
 Very little (<10%) of the polymer in use comes from recycled materials (see chart below for
details)
 A comparison of metals and polymers in
terms of how much of the material currently
in use is from recycled stock.
 Metals do not experience significant
decreases in properties when recycled.
 Polymers properties are more sensitive to
contamination, polymer raw materials are
cheap, and the energy savings is not as great
as that of recycled metals.
Fig 20.3 from Materials, Ashby, Shercliff, Cebon, B-H (2007)
Week 10
Corrosion/Degredation & Economic, Social and Environmental Issues
Callister Chapters 17 & 22
Metal recycling
Material
Energy Ratio
 For many metals, much less energy is required to
process recycled metal that is necessary to mine and
Recycled/Virgin (%)
refine ore.
Aluminum
5-10 %
 In general, an energy savings is accompanied by
Steel
25-30 %
reduced air and water pollution and reduced water
Copper
15-85 %
usage.
Zinc
5-40 %
* varying sources report different values
Ceramics/Glass recycling
Recycled glass, called cullet, is generally more expensive than the raw materials needed to
produce new glass. The energy savings from the processing of cullet over that of raw
materials is small, especially compared to many metals.
Recycling of other types of ceramics is fairly limited. Some “traditional” ceramics, such as
bricks, tiles and concrete, are crushed and reused as filler or drainage materials (like gravel).
A few companies are beginning to recycle green (unfired) engineering ceramic waste. For
example, a company called Ceramatec, an R&D company, is working to recycle green
alumina waste from other ceramic manufacturing companies. Due to contamination/purity
issues as well as low consumption (compared to “construction” ceramics), recycling of
engineering ceramics is challenging, and as such, very limited information is available on
efforts to reuse or recycle engineering ceramic waste.
Alternative sources for polymers
The vast majority of polymers are produced from crude oil derivatives, which contain only
hydrogen and carbon (hydrocarbons). Plant material, however, also contains oxygen, which
complicates the processing to form polymeric materials. Many of the polymers that are
derived from plants do not have the same chemical structure or properties as common oilderived polymers (PE, PP, PETE, etc). Some polymers made from biomass sources are also
degradable, though this is not necessarily the case.
Information from various sources, including:
Jon Evans, “Alternative Feedstocks,” Plastics Engineering, 67:2, Feb 2011.