18'1 Galvanic series

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18.1 Galvanic series
Electromotive force DC voltage needed to halt
the ionization of a metal in a standard
electrolyte.
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Electromotive force series
Reactive metals
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Electromotive force series
Noble metals
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18.2 Galvanic two-metal corrosion
Fe anode, corrodes
Cu cathode, Does not corrode
Introduction to Materials Science for Engineers,
J.F. Shackelford, Prentice-Hall, 1994.
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Galvanic cell

• Fe dissolves in electrolyte because of Fes low
  electrode potential (-0.44 V), leaving electrons
  on the anode.
• Cu deposits on the cathode because of Cus high
  electrode potential (0.33 V), consuming electrons
  that flow to the cathode.
• Voltage driving dissolution of Fe 0.77 V.

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Electro-chemical corrosion

• Involves two different metals.
• The two metals are in electrical contact
• The two metals are immersed in a electrolyte.

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Example of Galvanic corrosion
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Importance of relative areas

• Slow corrosion of plate

• Rapid corrosion of bolt

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18.3 Galvanic series in sea water
Pt?Au ? Ti ? Ag ? Sn ? Pb ? High Ni cast iron ?
SS ? Cast iron ? PC steel ? 2024 Al ? Zn ? Mg
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18.4 Microscopic Galvanic cell

• Two phase alloys (e.g. Cu-Zn).
• Microscopic chemical inhomogeneity.
• Grain boundaries.

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18.5 Gaseous reduction corrosion
O2 depleted region anode
O2 rich region cathode
Introduction to Materials Science for Engineers,
J.F. Shackelford, Prentice-Hall, 1994.
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O2 concentration cell

• Anode low oxygen. Fe dissolves as Fe, leaving
  electrons on the anode.
• Cathode high oxygen. Oxygen reacts with H2O to
  form hydroxide ions OH-, consuming an electron
• O2 2H2O 4e- ? 4OH-
• Electrons are pulled from O2-depleted region to
  O2-rich region.

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Examples of gaseous corrosion
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Examples of gaseous corrosion
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Examples of gaseous corrosion
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Materials Selection Project

• Start Oct. 11
• White paper (5) due Oct. 18
• Report due Nov. 22
• One week delay for off-campus
• No Late submission.

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White paper

• Less than one page single space
• Title
• Name and concentration.
• Engineering issue
• Importance of materials and selection
• Proposed activities.

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Content of project

• Focus on selection and use of materials in one
  engineering issue in your area of specialization.
• In-depth examination of one material.
• Comparative study of competing materials.

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Content of project

• Discussion of properties covered in this course
• i.e. mechanical, thermal, treatment, failure
  (oxidation and corrosion, friction and wear,
  fatigue, fracture)
• Do not discuss properties not covered in this
  course.
• e.g. optical, biological, electrical, magnetic,
  liquids, gases.

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Misc on report

• Check course web for grading guide of report.
• Must be individual effort and shown to be so.
• No late submission

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Possible selection topics

• Landing gear
• Fuel tanks
• Wings
• Turbine blades
• Helicopter rotor blades
• Shuttle exterior panel
• Brakes

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Possible selection topics

• Body panel
• Engine block
• Engine valve
• Exhaust system
• Brakes
• Bearings
• Transmission shaft
• Steering wheel