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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