Corrosion of Metals

1

• Corrosion of Metals

• Any metal corroding is simply that metal being
  oxidized.

• The result is often the formation of metallic
  oxides.

eg CuO

• Turquoise color of the roof of the Hotel
  Vancouver and Statue of Liberty)

eg Formation of rust
Fe ? Fe2 2e-
H2O ½O2 2e- ? 2OH-
Step 1
Fe H2O ½O2 ? Fe(OH)2
Fe(OH)2 H2O ½O2 ? Fe(OH)3
Step 2
Sun
Fe(OH)3 ? Fe2O3 3H2O
Step 3
Rust is both Fe2O3 and Fe(OH)3
2
Note Never store different metals in contact
with each other One metal will speed up
corrosion of the other.
eg
CuO H2O ? Cu(OH)2
then
Cu2 2e- ? Cu
Fe ? Fe2 2e-
H2O

• The presence of the Cu speeds up the oxidation of
  the iron (a simple electrochemical cell has been
  created)

• This technique could be used to protect the Cu
  from oxidizing if you dont care about losing
  Fe.(Cathodic protection)

3
Preventing Corrosion

• Paint surface.

• Galvanize use a thin layer of zinc to protect
  underlying metal.

eg galvanized nails are iron or steel nails
dipped in molten zinc.

• Cathodic protection one metal is sacrificed to
  protect another.

eg strips of zinc are used to protect iron
hulls of ships. The zinc is more easily oxidized
than the iron.(As zinc strips breakdown they are
replaced)
4

• Redox Titrations

• One substance is used to determine concentration
  of another.

eg
MnO4- 8H 5Fe2 ? Mn2 4H2O
5Fe3
Purple
Colourless
If 27.4mL of 0.020M KMnO4 is used to titrate
25.0mL of a solution of Fe2, what is the
concentration of Fe2?
Moles of MnO4-
0.0274L
0.020mol
5.5x10-4mol
L
Moles of Fe2
5.5x10-4mol MnO4-
5 mol Fe2
2.7x10-3mol
1 mol MnO4-
Fe2
2.7x10-3mol
0.11M
0.025L
5

• Breathalyser Function

• Redox reaction is used.

Orange
Green

• Alcohol (CH3CH2OH) from breath reacts with Cr2O72-

• The more alcohol that is present, the more Cr3
  is produced and reaction goes from orange to
  green.(monitored by computer)

The End