Corrosion

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Corrosion
Intermediate 2 Unit 3(c)
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WHAT IS CORROSION?
Potassium
REATIVITY SERIES

• When the surface of a metal changes from being an
  element into a compound
• The surface goes from being shiny to dull
• Nearly all metals corrode
• They dont all corrode at the same rate
• Suggest a metal which does NOT corrode

Iron
Gold
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CHEMICALS FOR RUSTING
Rusting requires OXYGEN and WATER Rusting is
speeded up by SALT
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RUSTING

• The corrosion of iron is called rusting
• Iron is the only metal which rusts
• View Rusting practical demo

Why did the water rise up the tube? How would the
rate of the water rising compare if the wool had
been soaked in water instead of acid?
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CHEMISTRY OF RUSTING

• Iron metal reacts with oxygen to produce
  iron(III) oxide (rust)
• Write a word equation and a chemical equation for
  this process

Iron oxygen
Iron(III) oxide

• This is an example of a REDOX reaction

Writing ion-electron equation practice Sodium
atoms losing electrons to form sodium
ions Sulphur atoms gaining electrons to become
sulphide ions Copper(II) ions being
reduced Bromine atoms being reduced
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CHEMISTRY OF RUSTING 2

• Corrosion occurs when metals lose electrons
  OXIDATION
• When iron rusts, there are two steps.

Iron atoms into iron(II) ions Iron(II)
ions into iron(III) ions Write ion-electron
equations for both steps
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REDOX REACTIONS

• Whenever there is oxidation there must also be
  reduction
• What two other chemicals are involved in the
  rusting of iron?

Oxygen and water
The reduction step involves these chemicals
2H2O O2 4e 4OH-
GAIN of electrons REDUCTION

• Dissolved chemicals in the water called
  ELECTROLYTES help the electrons to be transferred
• Give 2 examples of these dissolved chemicals

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DETECTING RUSTING

• Ferroxyl indicator can be used to detect rusting
• The chemical changes colour in the presence of
  Fe2 ions
• It changes from yellow/green to blue

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TUTORIAL QUESTIONS

• True or false
• Corrosion is the rusting of iron
• pH indicator should be used to test for rusting
• Rusting takes place in the presence of oxygen
  only
• Rusting will not take place if water is not
  present
• Salt is required for rusting to take place
• Rusting will occur faster in sea water than tap
  water

• Problem
• Rusting occurs faster in salt solution than in
  pure water.
• Write an ion-electron equation for the rusting of
  iron
• Why does rusting occur faster in salt solution
  than pure water?
• Suggest why rusting also occurs faster in acid
  solution than in pure water

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Practical
Set up the experiment shown bellow, using two
metals of your choice. Use a U-tube instead of a
beaker.
A
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TUTORIAL QUESTION
Relative rate of rusting of iron
Concentration of dissolved electrolytes

1. Suggest units for the horizontal axis
2. What is the relationship between the rate of
   rusting of iron and the concentration of
   dissolved electrolytes.
3. Suggest why the graph does not start from a rate
   of zero.

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FLOWING ELECTRONS

• A chemical cell can be used to show electrons
  moving away from iron when it rusts

A
Oxidation
Reduction

• Electrons leave the iron atoms when it rusts

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MORE REDOX

• When a cell is made from two metals, ELECTRONS
  FLOW FROM THE METAL HIGHEST IN THE
  ELECTROCHEMICAL SERIES TO THE METAL LOWER IN THE
  SERIES.
• Which metal in this cell loses electrons?
• Which metal is oxidised?
• Describe what happens to
• The colour of the solution around the iron
• The mass of the iron in the cell
• Does the iron rust?

A
Iron
Tin
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MORE REDOX 2

• When a cell is made from two metals, ELECTRONS
  FLOW FROM THE METAL HIGHEST IN THE
  ELECTROCHEMICAL SERIES TO THE METAL LOWER IN THE
  SERIES.
• Which metal in this cell loses electrons?
• Which metal is oxidised?
• Describe what happens to
• The colour of the solution around the iron
• The mass of the iron in the cell
• Does the iron rust?

A
Iron
Magnesium
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MORE REDOX 3
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TUTORIAL QUESTIONS

• Write an ion-electron equation to show the
  rusting of iron.
• A cell is made using an iron nail and a carbon
  rod, in a solution of ferroxyl indicator. Decide
  which statements are true
• SG textbook, p118-119

Electrons move from the carbon rod to the
iron Electrons flow through the solution A blue
colour appears at the iron electrode The mass of
the iron electrode decreases The iron is oxidised
A
Iron
Carbon
Set up an experiment to answer the above
questions.
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PREVENTING CORROSION

• There are two types of way to help prevent
  corrosion
• Physical protection
• Chemical protection
• PHYSICAL PROTECTION
• Stops oxygen and water coming into contact with
  the metal
• Paint
• Oil or grease
• Plastic coating
• Tin plating covering in molten tin
• Galvanising covering in molten zinc (more
  expensive)
• Cathodic protection
• Electroplating

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ELECTROPLATING
The metal to be coated is at the NEGATIVE
terminal POSITIVE ions of the OTHER METAL are in
the solution These ions are attracted to the
negative terminal, and when they meet it turn
into atoms So the copper is coated in gold atoms
DC
-ve
ve
Gold
Copper
Gold ions, Au
Write an ion-electron equation for the reaction
occurring at the negative electrode Is this
oxidation or reduction?
Check-test 12.2 ext.
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Cathodic Protection

• When a metal corrodes it . electrons
• If electrons were forced back to the metal, it
  would not corrode
• How could these be supplied?
• Connecting to NEGATIVE terminal of any electrical
  supply
• - Used in cars

Carry out Nails in Contact practical (7.3 core)
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CHEMICAL PROTECTION

• Sacrificial protection

A metal high in the electrochemical series is
connected to the metal to be protected and is
sacrificed - This is used in underground pipes
A
Iron
Magnesium
Mg
In terms of electrons, explain how sacrificial
protection works.
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• Galvenising

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TUTORIAL QUESTIONS

• 1. a) Why does coating steel with plastic
  prevent corrosion?
• b) Chromium-plated steel corrodes quickly if
  scratched. What does this tell you about the
  reactivity of chromium?
• 2. Explain how zinc gives sacrificial protection
  to steel.
• 3. a) Why should a copper roof not be held in
  position with iron nails?
• b) Suggest a metal which the nails should be
  made of. Why?
• 4. Explain what is meant by electroplating
• 5. What name is given to the process where steel
  is protected by coating it with zinc?
• 6. Assessment test 12.1 12.2

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PRACTICE EXAM QUESTIONS
SG textbook, p120-121 Questions for SG textbook,
p43-44