Stainless steel (also known as inox steel) is a steel alloy with a chromium content higher than 13%. Stainless steel is notable for its anti-rust properties. In fact, chromium turns into chromium oxide (CrO2) when it gets into contact with oxygen, which creates a superficial very adherent and resistant layer on the underlying material. This “screen” prevents the surface to oxyde even more (this phenomenon is known as passivation).
Stainless steel have been industrially produced for the first time in 1913 by H. Brearley and, thanks to the subsequent advances in metallurgy between 1940s and 1960s, it has increased its development and applications. Nowadays it is still improved and adapted for the requests of the various industrial sectors, such as oil and petrochemical sector, mining, energy, nuclear and food sector (18/10 stainless steel - containing 18% chromium and 10% nickel - is highly used in the food sector).
The steel used for the watch cases, bracelets and external components are mostly stainless.
Most used steel types in watchmaking
Comparison between 904L steel used by Rolex and 316L used by other watchmakers
316L steel is a very good material to be used for cases and bracelets for sports wristwatches, thanks to its resistance to high temperatures and oxidizing agents. Moreover, it has a lower Nickel content than 904L, which makes it more suitable for those with contact allergies and dermatitis. It is used by most of the medium but also high-end watchmakers. For example, Audemars Piguet uses 316L steel for its Royal Oak.
So, why has Rolex decided to use 904L steel? Of course we do not know the company policies but what we can say is that despite its higher price and greater processing difficulty than 316L, 904L steel is more resistant to acid attack and to oxidizing agents in general. It has a harder surface and it better reflects light appearing shinier. People that are allergic to Nickel may have more problems with watches and bracelets made with this 904L because its content is higher than in 316L.