Mild steel is an excellent structural
material - cheap, easily formed and strong mechanically. But at low
temperatures it rusts, and at high, it oxidises rapidly. There is a
demand, for applications ranging from kitchen sinks via chemical
reactors to superheater tubes, for a corrosion-resistant steel.
In response to this demand, a range of
stainless irons and steels has been developed. When mild steel is
exposed to hot air, it oxidises quickly to form FeO (or higher
oxides).
A considerable quantity of this foreign
element is needed to give adequate protection. The best is chromium,
18% of which gives a very protective oxide film: it cuts down the
rate of attack at 900°C, for instance, by more than 100 times.
Other elements, when dissolved in
steel, cut down the rate of oxidation, too. A1203 and SiOz both form
in preference to FeO and form protective films. Thus 5% A1 dissolved
in steel decreases the oxidation rate by 30 times, and 5% Si by 20
times.
The same principle can be used to
impart corrosion resistance to other metals. We shall discuss nickel
and cobalt in the next case study - they can be alloyed in this way.
So, too, can copper; although it will not dissolve enough chromium to
give a good Cr,03 film, it will dissolve enough aluminium, giving a
range of stainless alloys called 'aluminium bronzes'.
Even silver can be prevented from
tarnishing (reaction with sulphur) by alloying it with aluminium or
silicon, giving protective A1,03 or Si02 surface films. And
archaeologists believe that the Delhi Pillar – an ornamental pillar
of cast iron which has stood, uncorroded, for some hundreds of years
in a particularly humid spot - survives because the iron has some 6%
silicon in it.
Ceramics themselves are sometimes
protected in this way. Silicon carbide, Sic, and silicon nitride,
Si3N4 both have large negative energies of oxidation (meaning that
they oxidise easily). But when they do, the silicon in them turns to
SiO, which quickly forms a protective skin and prevents further
attack.
This protection-by-alloying has one
great advantage over protection by a surface coating (like chromium
plating or gold plating): it repairs itself when damaged. If the
protective film is scored or abraded, fresh metal is exposed, and the
chromium (or aluminium or silicon) it contains immediately oxidises,
healing the break in the film.
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