The mystery of Stainless Steel
Stainless steel got its name from one of its most notable properties, namely that it is highly resistant to corrosion, tarnishing, and rust. The finished surface of the metal is less likely to become “stained” or develop any kind of patina under normal circumstances. This is in contrast with ordinary steel, which as an alloy of iron and carbon is highly susceptible to rust in particular.
The key element: Chromium
The reason why stainless steel is "rust-proof" is that it contains a sufficient amount of chromium (Cr). International standards stipulate that the mass fraction of chromium must be no less than 10.5%. Steel can acquire stable corrosion resistance only when the chromium content reaches or exceeds this critical value.
Chromium reacts with oxygen to form a passivation film
When the surface of chromium-rich alloys is exposed to oxygen, chromium atoms will react with oxygen preferentially. A very thin, dense and firmly adhering chromium oxide film to the surface is formed, which is known as the "passivation film". This film is amorphous, pore-free and highly chemically stable. It is like an invisible and continuous wall, effectively blocking the contact between corrosive media such as oxygen, water molecules and chloride ions from the outside and the internal iron substrate.
The self-repairing ability of stainless steel
The most outstanding feature of the passivation film is its dynamic self-healing property. If the local film layer is damaged due to mechanical scratches, wear or cleaning, as long as there is a trace amount of oxygen in the environment, the exposed fresh chromium will immediately recombine with oxygen, automatically repair the damaged area and rebuild the protective layer. This enables stainless steel to resist daily wear and scratches.
The synergistic effect of multiple alloys
Modern stainless steel, by adding other elements, has optimized various properties while maintaining corrosion resistance, thus forming different "families".
|
Alloy Element |
Major Function | Typical Application Examples |
|
Ni |
Stabilize the austenite structure, significantly enhancing toughness, ductility, weldability and corrosion resistance in reducing acids | 304 stainless steel (18%Cr-8%Ni), kitchenware, architectural decoration. |
|
Mo |
Significantly enhance resistance to pitting and crevice corrosion caused by chlorides. Make the passivation film more stable. | 316 stainless steel (containing 2-3%Mo), used in coastal buildings, chemical equipment, and ship components. |
|
N |
Improve the stability of austenite (can partially replace nickel), increase strength, and enhance the resistance to pitting corrosion. | It is commonly used in 200 series or high-strength austenitic stainless steel. |
|
C |
Increase the strength, but if it is too high, it will cause chromium carbide to precipitate, creating "chromium-poor zones" and reducing the corrosion resistance. Strict control is required. | Low-carbon models such as 304L are used in situations where welding is required. |
Can stainless steel rust?
Stainless steel does not never rust; rather, it has a high degree of corrosion resistance. Under specific and harsh conditions, its passivation film may disappear.
Correct use and maintenance
Pitting corrosion: In an environment rich in chloride ions (such as seawater and de-icing salts), the local film layer is punctures, forming deep and small corrosion pits. Molybdenum is a key element in resisting pitting corrosion.
Crevice corrosion: It occurs in narrow slits where oxygen circulation is restricted, such as at bolt connections and beneath gaskets, due to the difficulty in maintaining the local passivation film.
Correct use and maintenance
Rational material selection: Choose the appropriate grade based on the environment (corrosiveness, temperature, and force conditions) (for example, 316 for coastal areas and 304 for the food industry).
Keep clean: Remove surface stains, salts and chemical residues in a timely manner to maintain the integrity of the passivation film.
Conclusion
The stainless nature of stainless steel lies in the chemical reactivity of chromium, which transforms it into a stable, dense and self-healing chromium oxide passivation film. It is not a static protective film, but a dynamic defense system. This characteristic, combined with the precise control of multi-alloying, has enabled stainless steel to move from the kitchen to the deep sea, from medical devices to skyscrapers, and become a cornerstone material of modern industrial civilization.
Post time: Dec-26-2025
