Material of the month - stainless steel
Maria Felice examines the history and many applications of this diverse alloy.
In the same year that the Titanic dragged more than 24,000 tonnes of steel to the bottom of the ocean, a new version of the alloy was born. Celebrating its centenary this year is stainless steel, a material that we are all familiar with and come across in a vast array of everyday items – from cutlery, jewellery and razor blades, to washing machines and cars. It is also a much-photographed material in iconic structures, including the Jin Mao Building in Shanghai, the Chrysler Building in New York City and the Thames Barrier in London. Indeed, in developed countries there is over 100kg of stainless steel stock per capita.
The term ‘stainless’ was coined early in the development of the material for cutlery applications that did not corrode or lose their lustre. Indeed one of the earliest trade names for stainless steel was Staybrite. The main requirement for stainless steels is that they should be corrosion-resistant for a specified application or environment. For example, some types of stainless steel do not resist corrosion in the presence of chlorine – hence problems arise if they are erroneously selected for indoor swimming pool structures. The technical requirement for a steel to be classed as stainless is a minimum weight of 10.5% chromium. This can be more than doubled for harsh environments, and other alloying elements such as nickel are sometimes added to enhance its structure and properties.
Stainless steel is also known as Inox – from the French term ‘acier inoxydable’, which literally translates as ‘steel that is not oxidised’. Actually, stainless steel must be used in an oxygenated environment and it is the chromium that oxidises to form a layer of Cr2O3. This layer has three very different attributes to the iron-oxide film that forms on unprotected carbon steels that we commonly know as rust. First, the layer of chromium oxide is too thin to be seen. Second, there is sufficient chromium to form a passive film of oxide, which prevents further surface corrosion and its spread into the bulk material. Finally, since the atomic chromium and chromium oxide molecules are of similar size and therefore bond well, the layer does not flake off.
While iron has been used for several millennia, chromium was also harnessed by the Qin Dynasty thousands of years ago, to strengthen their weapons and prevent them from corroding. However, it was not until 1797 that the element chromium was discovered by Louis Nicolas Vauquelin. In the first decade of the 20th Century, Leon Alexandre Guillet published the findings of his research on iron, nickel and chromium alloys, which would nowadays be known as stainless steels. In the years that followed, a number of scientists developed and patented various versions of the alloy, but it was in 1912 that austenitic stainless steel was patented by Krupp engineers Benno Strauss and Eduard Maurer. In 1913, Portland Works in Sheffield, UK, became the first place in the world to make stainless steel cutlery.
Being 100% recyclable, stainless steel has become a big player in sustainability. About 60% of stainless steel used in products today is recycled – some from items that have reached their end-of-life, while some is scrap from manufacturing processes used to make other stainless steel products. Austenitic stainless steel accounts for about three quarters of the stainless steel used, with 17Mt produced in 2004. This saw an energy saving of 33% when compared with using raw materials alone, with 3.6t of CO2 released per 1t of stainless steel produced instead of 5.3t per 1t.
There are countless modern applications of stainless steel, but I have chosen to mention a few that contribute directly to the care and protection of human life. There are simple examples like stainless steel studs in pavements and walkways to guide blind people. Then there are more complex examples, such as magnetic resonance imaging (MRI) machinery, which is made of austenitic stainless steel. As well as being easy to sterilise due to its corrosion resistance and lack of porosity, it is nonmagnetic – a requirement of MRI machinery.
Stainless steel is the material of choice for both delicate products, such as flexible spectacle frames (cheaper than titanium), as well as bulky products like cars – specifically in the crumple zones. It is used in medicine containers, as it will not corrode and cause contamination of the contents. For similar reasons stainless steel is also used inside the human body, in implants such as hip joints.
I wonder what applications stainless steel will have been used for when people are celebrating its bicentenary?