Why does stainless steel not rust?
Why does stainless steel not rust?

This explanation is offered by Michael L. Free, University of Utah's metallurgical engineer.

Because of the interactions between the alloying elements and their environment, stainless steel does not rust or remain stainless. Stainless steel is composed of iron, chromium and manganese as well as carbon, and in some cases significant amounts (of nickel and molybdenum) These elements combine with oxygen from water and air to form thin, stable films that are made up of corrosion products such as metal oxides or hydroxides. This corrosion product film is formed primarily by the reaction of chromium with oxygen. All stainless steels contain at least 10% chromium.

Stable film acts as a barrier to prevent further corrosion and limits oxygen and water access at the underlying metal surface. The film forms quickly and tightly so that even a few atomic layers can reduce corrosion rates to very low levels. It is difficult to see the film without modern instruments because it is thinner than the wavelength light. The steel appears to be stainless even though it has been corroded at the atomic level. The common inexpensive steel reacts with oxygen from the water to form an iron oxide/hydroxide layer that is relatively unstable and continues to grow as time passes and more exposure to air and water. This film, also known as rust is thick enough to be easily visible soon after being exposed to water and air.

The bottom line is that stainless steel doesn't rust. It is sufficiently reactive to form a passive corrosion product coating, which protects itself against further attack. Passive film formation is also used to protect other important metals like aluminum and titanium from corrosion. Stainless steel's durability and aesthetic appeal make it a popular choice for many products, including kitchen sinks, bank vaults, and eating utensils.

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