Stainless steel is an iron-based alloy that contains chromium, which helps it resist rust and corrosion. It became a case material in the 1930s. Most steel watches are made from grade 316L.
Why doesn’t stainless steel rust?
Stainless steel resists corrosion only when it contains at least 10.5 percent chromium. The protection itself comes from a thin passive film that sits on the surface of the metal. This film acts as a barrier. It stops oxygen from reaching the steel underneath. Without that contact, the metal is protected from rust and corrosion.
Where it comes from
Harry Brearley discovered the alloy by accident in 1913. He was looking for a better low-carbon alloy steel at the time. In English, the new metal was first called rustless steel. The name stainless came later. Watchmaking took it up in the 1930s. Before then, watch cases were usually made of gold, silver or platinum. Taubert & Fils of Geneva developed the methods for making cases from stainless steel. The steel most closely linked with these early cases is Staybrite. It was covered by a Swiss patent granted in March 1930.
What is the difference between 316L and 904L?
316L is the grade used for most watches. The L stands for low carbon. Less carbon makes the steel more stable at high temperatures and more resistant to corrosion. Rolex has used 904L since 1985. It is a more highly alloyed grade. Compared with titanium, steel is much denser, so a steel watch is heavier than a titanium one. Grade 5 titanium is also harder than 316L steel on the Vickers scale.
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