1. Metal Corrosion

2. The structure of metals
The arrangement of the atoms
Metals are giant structures of atoms held together by metallic bonds
Metallic bonds - atoms surrounded by delocalised electrons ("sea of electrons")

3. The structure of metals

4. The structure of metals
The electrons can move freely within the metallic bonds
each electron becomes detached from its parent atom - the electrons are delocalised
The metal is held together by the strong forces of attraction between the positive nuclei and the delocalised electrons

5. The structure of metals
The more electrons you can involve - the stronger the attractions
Transition metals tend to have particularly high melting points and boiling points
They can involve many delocalised electrons
Metallic character increases as we move to the right of the periodic table 

6. The Corrosion of Metals
Most metals react with their surroundings to form oxides and hydroxides
E.g. copper forms copper hydroxide
iron rusts to form iron oxide
Sodium corrodes in air to form a layer of sodium oxide on the metal surface

7. Metals That Don’t Corrode
Only a few metals can resist corrosion:
Gold & Platinum (don’t react with oxygen)
Stainless steel (iron + carbon + chromium (form stable film against corrosion)

8. Definitions
Corrosion - is the changing of the surface of a metal element into a compound (oxide)
Silver + oxygen silver oxide
Rusting - is the special name given to the corrosion of iron
Iron + oxygen iron (III) oxide

9. What Happens When Iron Rusts?
Rusting involves iron atoms (metal) losing electrons to form ions. It occurs over two steps
Fe(s) Fe2+(aq) + 2e-
Fe2+(aq) Fe3+(aq) + e-
The electrons lost from the iron are accepted by the water and oxygen
2H2O(l) + O2(g) + 4e OH-(aq)
oxidation
oxidation

10. Experiment: Conditions For Rusting
Dry air
Boiled water + oil
water
salty water
Experiment
Air
water
electrolyte
Did Rusting Occur? 1
yes no 2 3
Yes (small) 4

11. Experimental Conclusions
Water and oxygen are both required for rusting to take place
An electrolyte must also be present and the speed of rusting is increased
An electrolyte is an ionic substance dissolved in water and provides free ions to carry a current

12. Corrosion and the Reactivity series
Potassium
Sodium
Lithium
Calcium
Magnesium
Aluminum
Zinc
Iron
Tin
Lead
Copper
Mercury
Silver
Gold
The uses of metals depends on their position in the reactivity series
Why don’t we make nails from potassium or gold?
What are the benefits and disadvantage of having silver jewelry?
Why are bridges not built from stainless steel?
Increasing speed of corrosion

13. Corrosion and the Reactivity series

14. Cell Potentials
We can calculate the magnitude of electron flow by measuring the voltage e-
Anode (-)
Cathode (+)

15. Allows ion flow without mixing solutions
Allows ions to pass between solutions but doesn’t allow the solutions to mix

16. Zn(s) + Cu2+(aq) Zn2+(aq) + Cu(s)
Figure 21.5
A voltaic cell based on the zinc-copper reaction
Displacing electrons from zinc to copper
Over time, the anode will decrease in mass, and the cathode will increase. The Cu2+ solution will also become more clear as the Cu2+ ions are reduced into solid Cu
Cu2+(aq) + 2e Cu(s)
Zn(s) Zn2+(aq) + 2e-
Zn(s) + Cu2+(aq) Zn2+(aq) + Cu(s)

17. Summary of Corrosion and reduction potential
E cell = E anode – E cathode
Anode (-)
Cathode (+)
MX (s)  M+ (aq) + X- (aq)
Oxidation: lose electrons at anode (-)
M  M+ + e-
Reduction: gain electrons at cathode (+)
X + e-  X-

18. Displacement Reactions
Chemical reaction in which a less reactive element is replaced in a compound by a more reactive one
For example, the addition of zinc metal to a solution of copper(II) sulphate displaces copper metal:
Zn(s) + CuSO4(aq) → ZnSO4(aq) + Cu(s)
The copper is taken out of the solution and is deposited as a solid
In the electrochemical series an element can be displaced from a compound by any element above it in the series

19. Corrosion and the Reactivity series
The more negative the metal in the series the more reactive it is (its reaction is fast and more exothermic) - it wants to lose electrons to form an oxide
Therefore the reverse reaction becomes difficult (oxide -> pure metal)
Hard to extract a metal from its ore (stable)
The pure metal is also more susceptible to corrosion with oxygen and water

20. Electronegativity – the ability to gain or lose electrons

21. Atomic radius

22. Atomic radius
Elements with a large atomic radius are high on the reactivity series (lose electrons easily)
The number of protons increases across a period as does the effective nuclear charge
Electrons within a shell cannot shield each other from the attraction to protons
This causes the atomic radius to shrink (less reactive)

23. Summary of trends in the Reactivity Series
High on activity series (-)
Large atomic radius
LHS of periodic table (lose electrons)
Low on activity series (+)
Smaller atomic radius
RHS periodic table (gain electrons)

24. Protection Against Corrosion

25. What Happens When Metals Corrode?
Corrosion is a chemical reaction
It involves the metal atoms losing electrons
Fe(s) Fe e-
Metals corroding are examples of oxidation reactions
Cu(s) Cu e-

26. Methods of Protection
There are two main methods of protecting metals from corrosion:
Physical protection - placing a barrier to water and oxygen on the surface of the metal
Chemical Protection - providing the metal with a source of electrons to prevent oxidation

27. Physical Protection
Plastic coating
Oil and grease
Paint
Tin plating

28. Questions Plastic Oil and grease Paint Tin-plating
What are the advantages and disadvantage of each type of physical protection:
Plastic
Oil and grease
Paint
Tin-plating

29. Physical Protection
Electro-plating - coating the iron with a less reactive metal
Gold, silver, chromium, and copper are a few of the metals that can be used as coatings
The metal item to be plated is used as the negative (-) electrode
The item is placed in a solution of the metal coating
The metal in solution is reduced from ions to atoms and deposited on the metal item
Power pack + -
gold electrode
item to be coated
gold ion solution
Au2+(aq) + 2e- Au(s)

30. Problems with Electroplating
When the electroplating is broken an electrochemical cell is set up
If iron is higher in the electrochemical series than the coating metal electrons flow away from the iron
Rusting is speeded up
Electron flow V
gold
iron
electrolyte solution

31. Physical Protection Galvanizing - coating the iron with zinc
The iron or steel is dipped in molten zinc
The zinc provides a physical coating on the surface
If the zinc coating is damaged sacrificial protection occurs. This also is a form of chemical protection (see later)
This is expensive and requires special equipment to achieve

32. Chemical Protection
When a metal corrodes it loses electrons Mg Mg e-
Chemical protection supplies the metal with a flow of electrons by two methods:
1. Direct current power supply
2. Metal higher in the reactivity series

33. Direct Electrical Protection
This involves connecting the iron to the negative terminal of a battery or power supply
Connecting the negative terminal of a car battery to the car body slows corrosion
Ocean liners use direct electrical protection when docked

34. Sacrificial Protection
What happens when you connect a more reactive metal to a less reactive metal in a simple cell?
Electron flow
The electrons flow from the metal higher up the electrochemical series to the metal lower
zinc iron
The flow of electrons prevents the iron from rusting V
zinc
iron
electrolyte solution
Zn e- Zn

35. Sacrificial ProtectionMg
steel pipeline
To protect underground pipes from rusting the pipe is connected to scrap magnesium by a wire
The magnesium sacrificially corrodes giving electrons to the iron pipe
Rusting is slowed down
The scrap magnesium needs regularly replaced

36. Questions What happens to a metal when it corrodes?
How can we prevent this loss of electrons from a corroding metal?

37. Questions Name the two types of corrosion prevention.
What two chemicals are required for rusting to occur?
What else is required for rusting to occur?
Clue: It contains charged particles that allow electrons to be lost more easily from the iron

38. Questions What metals are used to plate steel and iron?
Where do most of these metals sit in the electrochemical series?
What terminal on the power pack is the metal to be coated connected?

39. Questions What metal is used in galvanising?
What type of protection does this offer the iron/steel?
Give 3 examples of galvanising being used to prevent rusting.
Which metal is higher in the electrochemical series - iron or zinc?

40. Questions When does electro-plating prevent rusting?
When does electro-plating cause rusting to occur faster?
When rusting occurs what metal is losing electrons?
What metal is being protected from corrosion?

41. Questions
What do you use to provide direct-electrical protection to a metal surface?
Are there any disadvantages to this method?
Give two examples of where this is used

42. Questions
Where must the metal used for sacrificial protection be on the E.C. series?
What is the next most suitable metal for sacrificial protection after zinc?
Which metal would provide the best protection out of these two?

43. Testing For Protection Against Rusting
Set up the following experiments:
Half fill the petri dish with warm agar solution.
Add 5 drops of ferroxyl indicator and 3 drops of phenolpthalein indicatior into the dish.
Gently stir.
Place a nail in each dish as shown below.
Ferroxyl indicator + iron nail
Ferroxyl indicator + iron nail wrapped in magnesium
Ferroxyl indicator + iron nail wrapped in copper

44. Testing For Protection Against Rusting
After leaving the experiment for minutes, draw a before and after diagram. What did you observe happening?
Ferroxyl indicator + iron nail
Ferroxyl indicator + iron nail wrapped in magnesium
Ferroxyl indicator + iron nail wrapped in copper

45. Testing For Protection Against Rusting
Set up the following experiments:
Half fill the petri dish with warm agar solution.
Add 5 drops of ferroxyl indicator and 3 drops of phenolpthalein indicatior into the dish.
Gently stir.
Place a nail in each dish as shown below.
Ferroxyl indicator + iron nail
Ferroxyl indicator + iron nail wrapped in magnesium
Ferroxyl indicator + iron nail wrapped in copper