Why is stainless steel, er, stainless?
鈥� The simple answer is that it contains at least 10.5 per cent chromium by mass, the sole requirement laid down by the British/European standard for stainless steel.
The more detailed metallurgical answer is that stainless steel is not one specific grade of steel, but a whole family of iron alloys with one thing in common: they all contain at least 10.5 per cent chromium by mass. It is this additional element that enhances their resistance to corrosion. Increasing the chromium content above 10.5 per cent improves this resistance, with the most common grades of stainless steel containing 17 or 18 per cent.
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鈥淭he film on stainless steel is extremely adherent and inert, and is self-repairing in oxygen鈥�
So, how does the protective mechanism work? The chromium and iron atoms that make up the alloy have very similar diameters (about 0.25 nanometres) so they readily intermingle on a common, cubic lattice. In practical terms, this means that the chromium atoms 鈥� which comprise at least one in every 10 atoms in stainless steel 鈥� are uniformly dispersed throughout the solid metal.
When these iron/chromium alloys are exposed to oxygen, including ambient air, the highly reactive chromium atoms immediately form a thin, chromium-rich oxide layer on the metal surface that is commonly known as a passive film. Additional chromium atoms migrate from the main body of the steel to the surface to build up this oxide layer, resulting in a protective film that is around 13 to 15 nanometres thick. The passive film is extremely adherent and inert, and, most importantly, is self-repairing in any environment containing oxygen.
Colin Honess, Former UK representative on the European standards committee for stainless steels Sheffield, South Yorkshire, UK
鈥� Stainless steel was discovered by accident a little over 100 years ago, just before the first world war. The British army had a problem: its new explosive 鈥� cordite 鈥� rapidly wore out gun barrels. The search was on for a high temperature, wear-resistant steel 鈥� and where better to hunt than in Sheffield, the 鈥溾�� in the north of England, where Harry Brearley was in charge of the laboratories selected for this work. He followed a promising line, alloying iron with chromium.
The standard way to examine the structure of a new alloy is to polish a sample and then etch the surface with acid so that the crystal structure can be seen under a microscope. Brearley tried to etch his new alloy with nitric acid but found he still had a polished surface. Somewhat annoyed, he tried another acid, but that was no better. After trying several other acids, frustration turned to elation as he realised he had discovered a corrosion-resistant steel.
Being Sheffield born and bred, Brearley knew he was in the right place to exploit this discovery. Sheffield made cutlery. Silver was expensive and too soft for making a knife with a sharp edge, and ordinary steel rusted or stained. The new steel was just what the cutlers wanted. They called it stainless steel.
Robert Senior, Retired from British Steel Oakham, Rutland, UK
鈥� The name is a bit of a misnomer because stainless steel can be stained.
The apparent stainless effect comes from adding chromium, with a composition of 18 per cent being common for most household usage, but one of up to 26 per cent being best for harsher environments. Depending on the desired properties, other metals may be added, such as nickel, which reduces brittleness at low temperatures. Nickel also makes the steel non-magnetic without the need for heat treatment.
When the chromium oxide layer that forms on the surface is broken or scratched, further chromium reacts with oxygen and the layer is repaired. However, staining can occur if the protective layer is broken by aggressive reagents or in a non-oxidising environment.
Heating can also cause discoloration as it leads to uneven oxidation. The varying oxide thickness results in rainbow-like colouring because of interference within reflected light 鈥� an effect also seen in titanium.
Joe Geesin, Stourport-on-Severn Worcestershire, UK
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This article appeared in print under the headline 鈥淚t鈥檚 a steel鈥�