Why doesn't stainless steel rust?

Michael L. Free, a metallurgical engineer at the University of Utah, gives this explanation

Stainless steel remains stainless or rust-free due to the interaction between its alloying elements and the environment. Stainless steel contains iron, chromium, manganese, silicon, carbon, and in many cases significant amounts of nickel and molybdenum. These elements react with water and oxygen in the air to form a very thin and stable film composed of corrosion products such as metal oxides and hydroxides. Chromium plays a dominant role in reacting with oxygen to form this corrosion product film. In fact, by definition, all stainless steel contains at least 10% chromium.

The presence of the stabilizing film acts as a barrier, limiting the access of oxygen and water to the underlying metal surface, thereby preventing additional corrosion. Because the film forms so easily and tightly, even with only a few atomic layers, the corrosion rate can be reduced to very low levels. In fact, the film is much thinner than the wavelength of light, making it difficult to see without the aid of modern instruments. Therefore, although the steel corrodes at the atomic level, it appears to be stainless steel. In contrast, regular cheap steel reacts with oxygen in water to form a relatively unstable iron oxide/iron hydroxide film that continues to grow over time and when exposed to water and air. As a result, the film (also called rust) reaches a sufficient thickness that it can be easily observed soon after exposure to water and air. water and air.

In summary, stainless steel does not rust because it is reactive enough to protect itself from further attack by forming a layer of passivating corrosion products. (Other important metals like titanium and aluminum also rely on the formation of passive films for corrosion resistance.) Stainless steel is used in a wide range of products for its durability and beauty, from tableware to bank vaults to kitchen sinks.