What makes stainless steel stainless?

Stainless steel, a magical metal that is ubiquitous in our daily lives, is strong and bright, as if it will never experience the traces of time. So, have you ever wondered why stainless steel can be so durable and seems to never rust? What makes stainless steel stainless? In this article, we’ll try to answer these questions.

First, let’s figure out why iron rusts.

Why does iron rust?

It is well known that among natural metals, except for gold and platinum that exist in the free metal state, other metals such as iron (Fe) exist stably in the form of ores such as magnetite (Fe3O4) and limonite (Fe2O3·xH2O).

Through metallurgy, people turn iron ores into steel, which is to change the stable state of steel from iron oxide (ore) to an unstable state. All things in nature have a strong tendency to “return” from the unstable state to the stable state. This is a natural law, and rusting of steel is a natural reflection of this “return” phenomenon.

Rusting is the effect of iron and oxygen in the atmosphere on the surface, forming Fe2＋, Fe3＋, which are not protected by a loose and easily detachable iron-rich oxide or hydroxide, so that the steel returns to “ore”.

The rusting of iron is the process of iron changing from metal to Fe3＋ ionization, which is a typical corrosion phenomenon. To prevent steel from rusting (corrosion), people only take measures such as surface painting artificially to prevent the atmosphere from contacting with steel. Once the paint is damaged, the steel will continue to rust.

What makes stainless steel stainless?

The non-rusting of stainless steel, once considered a “miraculous” phenomenon, has been well understood by people through long-term research and practice: the non-rusting of stainless steel and the rusting of iron belong to the same natural phenomenon.

Research has found that as the content of chromium in steel increases, the corrosion resistance of the steel improves. As can be seen from the curve below, when the chromium content in steel exceeds 11-12%, the corrosion resistance of the steel in the atmosphere undergoes a sudden change, from rusting to non-rusting.

This is because chromium (Cr) in the steel combines with oxygen (O2) in the air to automatically form a very thin, colorless, transparent and smooth layer of chromium-rich oxide film on the steel surface. This layer of film is called the passivation film, which protects the stainless steel from rusting and keeps it as bright as new.

Why can chromium oxide protect steel from rusting, while iron oxide corrodes faster?

Because the size of chromium atoms is similar to that of chromium oxide, they can be arranged neatly on the metal surface to form a stable layer that is only a few atoms thick. On the other hand, iron atoms are much smaller than iron oxides, forming loose oxides that are easily detached.
The invisible passivation film on the surface of stainless steel, though extremely thin (about 2-5 nanometers), is continuously and firmly adhered to the surface, very stable, and difficult to detach. As long as oxygen is present, the passivation film can be spontaneously formed, and if it is damaged, it can be quickly repaired by itself.

If the chromium content is higher, the passivation film is stronger. As the chromium content in steel increases to ≥19%, the passivation film changes from a crystalline film to an amorphous film. Because the amorphous film has fewer defects and a more uniform structure, the chromium element is more easily enriched. Therefore, it has better strength and corrosion resistance than ordinary crystalline films.

Conclusion: What makes stainless steel stainless?

The non-rusting property of stainless steel is determined by the chromium content in the steel. Without chromium, there is no stainless steel. A large number of domestic and international studies and practices have shown that chromium is the only element that can passivate steel and make it non-rusting, corrosion-resistant, and useful for industrial applications. There is no such thing as chrome-free stainless steel.

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