chemistry 128

December 6, 2002

see Interlude, Chapter 8 of McMurray and Fay (text book)

• A number of metals form protective oxide coatings.
  • the oxide coating often forms spontaneously when fresh metal comes into contact with air
  • the oxide forms a thin coating that adheres very well to the metal
  • the oxide coating is generally not reactive and provides chemical protection for the metal
  • the oxide is thin and transparent so the metal still looks shiny
  • the oxide layer, if thin enough, continues to conduct electricity

• Aluminum is perhaps the most important example
  • It's reduction potential (Eo = -1.66) says this is an extremely reactive element
    • Aluminum should react strongly with dilute acids
    • It should react with oxygen much more readily than Iron
    • It should even react with boiling water to produce H2 gas...
  • Aluminum is an important structural metal
    • Al cooking foil, light poles, cooking pots, window screens, door and window frames...
    • It wouldn't be useful if it reacted under normal environmental conditions
    • The oxide coating seals off the metal and Aluminum is normally considered a fairly inert metal

• The usual oxide film is fairly thin, but it is thick enough to prevent further oxidation
  • this film is relatively easily scratched
• Thicker oxide films add important features
  • Al₂O₃ is an extremely hard material
    • used for making metal and stone cutting wheels, "sandpaper"
    • single crystals take form of ruby and sapphire
  • A thin Aluminum object can be much stronger if the outer surface is Al₂O₃
    • the strength coming from the hard outer shell more than the underlying metal
  • Coated Al surfaces glide freely against each other
    • (untreated aluminum tends to scratch and surfaces stick without sliding

• We can build much thicker oxide coatings by a process called Anodizing
  • We make the Aluminum the anode in an electrochemical cell
    • The Anode reaction is Al + H₂O --> Al2O3 + OH-
      • The cathode can be any relatively inert material (lead, stainless steel, more Aluminum or graphite)
      • The cathode reaction reduces water to H2 gas
    • Electrical current is typically 10-50 mA per square centimeter
    • Anodizing time is typically 20-60 minutes

• One valuable property of the oxide film
  • when fresh, it absorbs dyes readily
  • as it ages, the film seals and the dyes are locked in the surface coating
    • Most Aluminum windows and door frames are anodized and dyed

• In lab we will anodize some aluminum sheet
  • we will use tape to mask off parts of the surface
  • we will let the surface absorb dye

• Other metals with protective oxide layers
  1. stainless steel
  2. chromium
  3. nickel
  4. titanium

• Titanium has an even more interesting characteristic
  • shared with a few less common but more expensive metals like tantalum and niobium

• The oxide layer has a very high refractive index
  • helps intensify several properties involving light
  • layers create optical interference
    • user sees brilliant spectral -like colors (actually complementary colors)
    • often used in Jewelry since the colors are much brighter than we can get from paints or other surface coatings.
  • layer thickness easily controlled by voltage applied

• In lab we will demonstrate the Anodizing of Titanium metal
  • The voltages can be as high as 140V so this is much more hazardous than the low voltages used to anodize Aluminum.
  • We will begin by anodizing a metal strip at 20-30 Volts
    • Then we will raise the strip (partly out of the solution) and raise the voltage
    • This is repeated until we have anodized at perhaps 10 different voltages
  • As the layer gets thicker the interference effects eliminate light of longer wavelength
    • At some thickness, the color pattern repeats again
    • It's analogous to the second order spectrum from a grating or the second ring of a rainbow