Insight From Top Steel Suppliers UK – Why is Stainless Steel Stainless?
As respected stainless steel suppliers in the UK, it’s our job to know our subject inside out. If you’ve ever wondered exactly what makes stainless steel stainless, here’s the answer.
It’s a chromium thing
Put simply, stainless steel contains at least ten and a half percent chromium, according to the British and European standard, and that’s what makes it stainless. But look deeper and it isn’t quite that simple. The thing is, the metal isn’t made of a specific grade of steel but a broad family of iron alloys that all contain the 10.5% of chromium required by the EU.
If you add more than 10.5% chromium the stainless properties are even better. Most grades of steel we supply have 17% or so chromium and some steels come with as much as 26% chromium, designed for particularly harsh environments.
Atomic magic – The world of the very, very small
Look at stainless steel from an atomic perspective and things get even more interesting. Chromium and iron atoms are about the same diameter, about 0.25 nanometres. This means they can intermingle easily, with the chromium uniformly dispersed through the metal. When they’re exposed to oxygen the chromium atoms form a thin oxidised layer called a passive film, around 15 nanometres thick. This sticks fast and is remarkably inert. And whenever there’s oxygen present, any damage to the layer is instantly repaired to deliver a stainless surface.
Completely stainless? Not always…
Having said all that, in a way the term ‘stainless’ steel is a bit of a misnomer, since it can actually stain. The protective chromium oxide layer than forms on the surface of the metal can be scratched and damaged. It usually repairs itself by reacting with oxygen. But when it comes in contact with an aggressive reagent or is stored in a non-oxidising environment, staining can happen. You can also use other alloys to make stainless steel. Nickel, for example, creates different properties, making the metal non-magnetic without having to heat treat it.
If you heat stainless steel it can also discolour, this time thanks to uneven oxidisation. Sometimes you get an attractive rainbow effect because of interference to reflected light, something you also get in Titanium, loved by jewellers for its remarkable beauty.
How was stainless steel discovered?
As it happens, the discovery of this incredibly useful metal was a happy accident. Roll back time to the years just prior to the First World War, when a man called Harry Brearley was busy searching for ways to make a very high temperature, wear-resistant steel for making gun barrels, since the corrosive explosive cordite in gunpowder kept wearing them out.
Harry tried alloying iron with chromium but it was a frustrating task – whatever he did, he ended up with an annoyingly shiny surface that resisted every attempt to scratch it with acid to reveal the crystalline structure below. It took a while for him to have that magical Eureka moment, but he eventually realised he’d unwittingly created a steel it was almost impossible to stain. And his discovery changed our world.
Today stainless steel is used for cutlery, microwaves, washing machine drums, cladding, chemical tankers in shipping, catering gear, water treatment, nuts and bolts, exhaust systems, surgical implants, and countless more essential applications.
If you’re looking for expert steel suppliers in the UK, you’ve found us. We’ll be happy to help with all your metal supply needs.