C8.01 The reactivity of aluminium and
anodising
Aluminium is high in the reactivity series but does not corrode in air or
water.
The reason is that aluminium covers itself with a thin layer of aluminium oxide
which protects it from further corrosion.
Anodising is the industrial process of coating aluminium objects with a thicker
layer of aluminium oxide. The aluminium object forms the anode (positive
electrode) in a sulfuric acid electrolyte during electrolysis. Oxygen
atoms form at the anode and join with the aluminium. This is oxidation.
Task C8.01 Draw a labelled diagram for apparatus suitable
for anodising an aluminium rod. Show the electrodes, the electrolyte, and
the cell.
C8.02 The need for alloying aluminium to increase its strength
An alloy is a mixture of metals. Adding copper to aluminium makes it
harder and adding magnesium makes it more corrosion resistant. e.g.
duralumin Al 95%, Cu 4%, Mg 1%
magnalium 83% Al, 17% Mg
Task C8.02 Explain duralumin use in aircraft instead of
aluminium. Explain magnalium use in window frames instead of aluminium.
C8.03 Important uses of aluminium and its alloys
| metal or alloy |
Use |
Property |
| aluminium | overhead power cables | good electrical conductor, low density |
| aluminium | drinks cans | Does not react with water |
| aluminium | cooking pots | good heat conductor |
| duralumin | aircraft and bicycle parts | high strength, low density and hard |
| magnalium | aircraft parts |
high strength, low density and
corrosion resistance |
C8.04 Chemical reactions in
different parts of the blast furnace
(high tier)
Iron is made in the blast furnace.
Task C8.04 Write equations for each of the reactions in the blast
furnace.
C8.05 The limited uses of pure
iron and impure iron from the blast furnace
Impure iron from the blast furnace (only 93% pure) is called cast iron.
Pure or wrought iron is not
now mass produced but is still available.
| type | properties | uses |
| cast iron | brittle, high compression strength | car engine blocks, man hole covers, gas stoves. |
| wrought iron | soft, bends easily, easily worked, low corrosion | nails, bolts, chains, garden gates, decorative ironwork. |
C8.06 The production of mild steel
Iron from the blast furnace contains impurities like carbon, sulfur, silicon
and phosphorus. These are removed in the basic oxygen process. In
this process a water cooled lance is put into impure molten iron and pure oxygen
is blown through it. The impurities are changed to oxides which come
out as gases such carbon dioxide, and sulfur dioxide. Solid oxides formed
react with added calcium oxide to form a slag which floats on top of the
iron. When the amount of carbon drops to about 1 or 2% the process is
stopped and the result is called mild steel.
C8.07 The uses of mild steel
Mild steel containing a small amount of carbon eg 0.5% is has the hardness
and strength for making machines, rails, ship's plates and girders for bridges
and buildings.
C8.08 The uses of alloy steels
The properties of steels can be controlled by carefully changing the amounts
of carbon removed and amounts of other metals put into them.
| steel | composition | properties | uses |
| stainless steel | 74% Fe, 18% Cr, 8% Ni | tough, does not corrode | cutlery, sinks, surgical knives. |
| titanium stainless steel | 72% Fe, 18% Cr, 8% Ni, 1.6% Ti | keeps structure on heating, high strength low weight | stainless steels that can be welded aircraft engine parts |
| manganese steel | 87% Fe, 13% Mn | Tough and springy | drill bits and springs |
| tungsten steel | 95% Fe, 5% W | tough and hard even at high temperatures | tip of high speed drill |