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7.3 Stainless steel

Steel is an alloy of iron and carbon. But it can also have other elements added to it to enhance its properties. Stainless steel has a minimum of 12 per cent chromium added to improve its corrosion resistance. The corrosion resistance of stainless steel arises from the formation of a thin protective film of chromium oxide on its surface. This film is highly resistant to chemical attack and if scratched (in air), the oxide layer rapidly reforms. The drawbacks to stainless steels are that they are readily attacked by chloride ions (in salt for example), and that the oxide layer does not form if oxygen is not present (not a problem for most domestic applications!).

There is a number of different stainless steels, and they are classified by the alloying elements that are present and the heat treatment that the steel has received. Stainless steel is widely used for cutlery and different cutlery has different compositions. One example contains the following alloying elements: 14 wt% chromium, 0.04 wt% carbon and 0.45 wt% manganese (these figures tell us the percentage, by weight, of the alloying additions in stainless steel, the balance of course being iron). This alloy of steel is widely used for producing forks and spoons where a high hardness is not required, as there is no need for a cutting edge, and the steel produced is ductile, formable and easily shaped from thin sheets.

A slightly more expensive type of stainless steel is known as 18/8 stainless steel. This has the composition 18 wt% chromium, 8.5 wt% nickel, 0.8 wt% manganese and 0.05 wt% carbon, (the 18/8 'label' comes from the chromium/nickel composition). This type of steel is more expensive because nickel is a costly alloying addition. This steel is non-magnetic (if you go through your cutlery with a magnet, you should be able to differentiate between the two types of steel by seeing which of your knives and forks are magnetic!). 18/8 steel is ductile and can be formed into shape by cold-forming, which also has the benefit of increasing the strength through work hardening.

The type of stainless steel used for kitchen knives would have a typical composition of 13 wt% chromium, 0.3 wt% carbon and 0.4 wt% manganese. The higher carbon content means that this steel can be hardened and then tempered to give a Vickers hardness in the range 500–700 HV. A small amount of the element molybdenum is added to this steel, as well as to 18/8 steel, which improves the resistance to attack by dishwashing powders and salt water. The Thames Barrier is made from an 18/8 stainless steel (as are many ocean-going yacht fittings) with 3 per cent molybdenum added.