Why is stainless steel stainless?
Stainless steels are defined as being characterized by particularly high resistance to chemical attack by aqueous media. In general, they contain at least 12% by weight of chrome and a maximum of 1.2% carbon. The reason of their high resistance to corrosion is a passive layer that forms on the surface. This consists of a metal oxide or hydroxide layer, rich in chrome, only a few Angstrom units thick, separating the actual metal from the attacking medium.

History of stainless steel
Steel is made corrosion-resistant by the addition of nickel and chromium. This fact was known as long ago as the 19th century, but at the time the steels alloyed in this way still left a great deal to be desired.
The breakthrough was made by Krupp in 1912. In addition to establishing the ideal nickel to chromium ratio to guarantee high corrosion resistance, the company also discovered that precisely controlled heat treatment decisively improved the mechanical properties of the material. This innovation opened up completely new perspectives for fabrication and laid the foundation stone for the success of stainless steel.
Today stainless steel is continuous optimization and the emergence of new applications ensure a shining future for this multipurpose material.

Types of stainless steel
There are five basic categories of stainless steel, Austenitic, Ferritic, Martensitic, Duplex and Precipitation Hardening.
The austenitic grades are non magnetic and in addition to chromium, typically at the 18% level, contain nickel which increases corrosion resistance. They are the most widely used group of stainless steels.
The ferritic grades are magnetic, have contain chromium as the main element, typically at the 17% level. They have good oxidation resistance.
The martensitic grades are magnetic, containing typically 13% chromium and a moderate level of carbon. They are hardenable by quenching and tempering.
The deplex grades have a mixed structure of Ferritic and Austenitic, chromium content varies from 18% to 28% and nickel from 4.5% to 8%. They find application particularly where high chloride content is present.
The precipitation hardening grades have chromium content at 17% typically with additions of nickel, copper and niobium. They can be hardened in a single ageing process.