What is Stainless Steel and Why is it Stainless?

What Is Stainless Steel and Why Is it Stainless? In 1913, English metallurgist Harry Brearly, working on a project to improve rifle barrels, accidentally discovered that adding chromium to low carbon steel gives it stain resistance. In addition to iron, carbon, and chromium, modern stainless steel may also contain other elements, such as nickel, niobium, molybdenum, and titanium. Nickel, molybdenum, niobium, and chromium enhance the corrosion resistance of stainless steel. It is the addition of a minimum of 12% chromium to the steel that makes it resist rust, or stain ‘less’ than other types of steel. The chromium in the steel combines with oxygen in the atmosphere to form a thin, invisible layer of chrome-containing oxide, called the passive film. The sizes of chromium atoms and their oxides are similar, so they pack neatly together on the surface of the metal, forming a stable layer only a few atoms thick. If the metal is cut or scratched and the passive film is disrupted, more oxide will quickly form and recover the exposed surface, protecting it from oxidative corrosion. (Iron, on the other hand, rusts quickly because atomic iron is much smaller than its oxide, so the oxide forms a loose rather than tightly-packed layer and flakes away.) The passive film requires oxygen to self-repair, so stainless steels have poor corrosion resistance in low-oxygen and poor circulation environments. In seawater, chlorides from the salt will attack and destroy the passive film more quickly than it can be repaired in a low oxygen environment.

Stainless steel is not a single specific material – it is the name given to a group of corrosion-resistant steels containing a minimum of 11% chromium – varying additions of nickel, molybdenum, titanium, niobium and other elements may also be present. The mechanical properties and behavior in service of each type of steel depends upon its composition and consequently a carefully considered choice of steel is vital.

There are four classes of stainless steel:

• The Martensitic types may be strengthened and hardened by heat treatment, in the same way as plain carbon steels, but the Ferritic steels, which in general have lower carbon and higher Chromium contents, do not respond in this way. Martensitic and Ferritic steels are magnetic.
• Ferritic stainless steels contain chromium usually in the range 11 to 18%. They are magnetic, have moderate corrosion resistance and are not susceptible to stress corrosion.
• Austenitic stainless steels contain nickel as well as chromium, and are sometimes referred to by the generic title 18/8, i.e. 18% chromium, 8% nickel, although the actual composition may vary widely from these figures. They are non-magnetic and cannot be hardened by heat treatment, although they strain harden rapidly when cold worked. All stainless steels except hardened Martensitic steels are ductile and therefore can be formed, but the Austenitic types are outstanding in this respect. They are also amongst the most highly corrosion-resistant materials available to the designer and engineer.
• Duplex stainless steels combine the optimum properties of austenitic and ferritic types. They contain 18 – 26% chromium plus 4.5 to 6.5% nickel and have good resistance to stress corrosion cracking.