Stainless Steel, also called Stainless Steel or inox and popularly known as stainless steel, is an alloy of mainly iron, chromium, nickel and carbon. To be able to speak of stainless steel, a minimum of 10.5% chromium and a maximum of 1.2% carbon is required. Furthermore, many types of stainless steel also contain the elements molybdenum, titanium, manganese, nitrogen and silicon.
The first stainless steel was cast by Harry Brearley in the Brown-Firth laboratory on August 13, 1913, after being asked to conduct research for the arms industry in 1912. The American standardization is often used industrially:
AISI 304 (1.4301) consists of 18% chromium and 8% nickel. This alloy is neither magnetic nor hardenable.
A more corrosion resistant but more expensive type is AISI 316 (EN 1.4401) with 16% chromium and 10% nickel and 2% molybdenum. Type 316 is more resistant to salt corrosion and is widely used in the chemical industry.
The difference between stainless steel 304 and 316 is in the composition. Stainless steel 316 contains 2% molybdenum, which makes the material more resistant to crevice and stress corrosion and pitting corrosion. For outdoor applications or applications where the material can come into contact with chlorine or other acids, we always recommend stainless steel 316.
Inox is a stainless material because the chromium present in contact with oxygen forms an invisible protective layer, the oxide skin. This layer protects the underlying metal against further rusting. In addition, it repairs itself when damaged. Stainless steels are very sensitive to chlorine. City water, swimming pool water, bleach (NaOcl), hydrochloric acid (HCl) and iron trichloride (Fe2Cl3) are very aggressive to stainless steel.
Pitting corrosion is the corrosion where pits form on the surface. For example, if stainless steel AISI 304 comes into contact with chlorinated water, from drinking water or swimming pool water, for example, the chlorine will locally attack the protective layer of chromium oxide (oxide skin). This creates the beginning of a shallow well, where more chlorine ions collect again, so that the attack preferably continues at that location and the well becomes deeper. Ultimately, the material looks mostly undamaged, but with some pits across the surface.