Please Sit with Your Group

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Please Sit with Your Group

• Please be sure each member of your team has a
  copy of
• Corrrosion and Electrochemical Processes Lecture
  Notes
• Todays reporter is the person who is the
  tallest.
• Next reading assignment
• Zumdahl Chapter 11

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Corrosion and Electrochemical Processes

• Edward A. Mottel
• Integrated, First-Year Curriculum in Science,
  Engineering and Mathematics

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Corrosion and Electrochemical Processes

• Corrosion
• Chemical Analysis
• Chemical Production

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Comparison of Metal Oxidation
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Comparison of Various Metals and Their Oxides
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Lithium Oxidation
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Lithium Oxidation
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Lithium Oxidation
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Iron Oxidation

• Unlike lithium, iron oxide has a larger volume
  than iron metal.

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Iron Oxidation
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Iron Oxidation
Why are rusty nuts and bolts hard to disassemble?
Iron oxide expands.
The iron oxide crusts over the metal stopping
the nut from moving through the threads.
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Aluminum Oxidation

• Does aluminum metal develop pinhole leaks or does
  aluminum oxide form a protective coating on the
  metal?

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Aluminum Oxidation
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Comparison of Iron Oxide and Aluminum Oxide
Why does iron rust away, but aluminum seems to
stay the same?
Aluminum is less reactive than iron.
There is something different about the oxide
coatings.
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When Oxidation is Occurring,What is Really
Happening?
atmosphere
O2(g) 4 e 2 O2
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When Oxidation is Occurring,What is Really
Happening?

• Electrons must be transferred between the metal
  and oxygen.
• Ions must move to prevent charge imbalance.

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When Oxidation is Occurring,What is Really
Happening?
iron oxide is an electrical conductor FeO, Fe2O3,
Fe3O4
O2(g) 4 e 2 O2
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When Oxidation is Occurring,What is Really
Happening?
aluminum oxide is an electrical insulator Al2O3
O2(g) 4 e 2 O2
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Corrosion Prevention

• Coatings
• paint, plating

• Sacrificial electrode
• A more reactive metal is electrically connected
  to a metallic object which is being protected.
• underground steel tanks
• ship hulls

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Corrosion Prevention

• Steel is often plated with various metals
• Zinc on steel pipes (galvanized iron)
• Tin on tin cans

What is the purpose of each of these coatings?
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Standard Oxidation Potential Table
Which of these metals is the most reactive?
Zinc oxidizes before iron. Galvanized iron
doesnt rust.
Iron oxidizes before tin.
Why put tin on cans?
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Chemical Analysis

• Glass (pH) electrodes
• Ion selective electrodes
• Deposition
• Reduction potential/oxidation potential analysis

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Glass (pH) Electrode
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Specific Ion Electrode
Orion liquid membrane calcium ion electrode
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Chemical Production

• Chemical Reduction
• Electrochemical Reduction

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Chemical Reduction

• Reduction with carbon
• Fe3O4(s) 2 C(s) 3 Fe(l) CO2(g)

• Reduction with hydrogen
• Fe3O4(s) 4 H2(g) 3 Fe(l) 4 H2O(g)
• Reduction with active metals
• CaCl2(s) 2 Na(s) Ca(s) 2 NaCl(s)

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Electrochemical Reduction Aluminum Production

• Produced by electrolysis of molten bauxite ore
  (hydrated aluminum oxides)
• Bauxite melting point gt 2000 C
• Addition of cryolite ore (Na3AlF6) lowers melting
  point and saves energy.

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Electrolytic Production of Aluminum
Al2O3 in molten cryolite
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Aluminum Production Example

• A current of 10.40 A flows in an electrolytic
  cell in which aluminum metal is produced by the
  reduction of aluminum ion. Electricity costs
  0.071 per kilowatt-hours.
• What is the minimum electrical cost to produce
  one pound of metal?

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Game Plan
pounds of Al moles of Al moles of e- charge
charge time power energy cost
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Aluminum Production

• 1.000 lb (453.6 g/lb) (1 mol/26.98 g) 16.81
  mol
• mol of e- required 3 16.81 mol 50.43 mol e-
  
• charge required 50.43 mol 96,487 C/mol
  4.866 x 106 C
  4.866 x 106 As
• time 4.866 x 106 As /10.40 A 4.68 x
  105 s 129.9 h

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Aluminum Production

• time 4.866 x 106 As /10.40 A 4.68 x
  105 s 129.9 h
• Power V i 2.06 V 10.40 A
  21.4 W
• Energy P t 21.4 W 129.9 h
  2.78 kWh
• Cost 2.78 kWh x 0.071 kWh-1 0.20

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Aluminum Production

• It takes 7 kWh to produce a pound of aluminum.
  What factors could account for the difference in
  the above answer and the actual value?
• Conversion from AC to DC and inefficiencies in
  converting all of the electrical energy into
  work. Some energy may be converted into heat.

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Recycling

• Compare the energy required to produce a pound of
  aluminum electrolytically to the energy involved
  in remelting scrap aluminum.
• The enthalpy of fusion for aluminum is 10.7
  kJmol-1.
• 1 watt 1 Js-1

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Sodium Hydroxide Production

• Electrolysis of aqueous sodium chloride

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