Metals - Chemistry - page 165

Questions
1 What is rust? Explain how rust forms on structures made of
iron or steel.
2 Rusting is a redox reaction. Explain the process of rusting in
terms of oxidation and reduction (Chapter 5, p. 73).
3 Design an experiment to help you decide whether steel
rusts faster than iron.
4 Why do car exhausts rust faster than other structures made
of steel?

● Alloys
The majority of the metallic substances used today
are alloys . Alloys are mixtures of two or more metals
and are formed by mixing molten metals thoroughly.
It is generally found that alloying produces a metallic
substance that has more useful properties than the
original pure metals it was made from. For example,
the alloy brass is made from copper and zinc. The
alloy is harder and more corrosion resistant than
either of the metals it is made from.
Steel, which is a mixture of the metal iron and
the non-metal carbon, is also considered to be an
alloy. Of all the alloys we use, steel is perhaps the
most important. Many steels have been produced;
they contain not only iron but also carbon and
other metals. For example, nickel and chromium are
the added metals when stainless steel is produced
(Figure 10.27). The chromium prevents the steel
from rusting while the nickel makes it harder.

Figure 10.27 A stainless steel exhaust system. Why do you think more
people are buying these exhaust systems?

● Carbon is oxidised to carbon monoxide and carbon
dioxide, while sulfur is oxidised to sulfur dioxide.
These escape as gases.
● Silicon and phosphorus are oxidised to silicon( iv )
oxide and phosphorus pentoxide, which are solid
oxides.
● Some calcium oxide (lime) is added to remove
these solid oxides as slag. The slag may be skimmed
or poured off the surface.
● Samples are continuously taken and checked for
carbon content. When the required amount of
carbon has been reached, the blast of oxygen is
turned off.
The basic oxygen furnace can convert up to
300 tonnes of pig iron to steel per hour. Worldwide
production by this process is 430 million tonnes.
There are various types of steel that differ only
in their carbon content. The differing amounts of
carbon present confer different properties on the steel
and they are used for different purposes (Table 10.7).
If other types of steel are required then up to 30%
scrap steel is added, along with other metals (such as
tungsten), and the carbon is burned off.

Production of steel
The 'pig iron' obtained from the blast furnace
contains between 3% and 5% of carbon and other
impurities, such as sulfur, silicon and phosphorus.
These impurities make the iron hard and brittle. In
order to improve the quality of the metal, most of the
impurities must be removed and in doing this, steel is
produced.
The impurities are removed in the basic oxygen
process , which is the most important of the steel-
making processes. In this process:
● Molten pig iron from the blast furnace is poured
into the basic oxygen furnace (Figure 10.28
p. 166).
● A water-cooled 'lance' is introduced into the
furnace and oxygen at 5-15 atm pressure is blown
onto the surface of the molten metal.