Is there such a strange building around you: it is a solid steel structure, but it is covered with thick red and black rust, as if it has been standing in the wind and rain for hundreds of years.

(Weathering steel appearance)

Maybe you will also be curious: can such a rusted building still be used?

I can tell you unequivocally: be safe! Not only is it safe, but it is more durable than ordinary steel and will not rust even after decades of exposure to the air.

This surface rusted, but actually durable material is actually a special kind of steel called weathering steel.

(1) Although weathering steel is "ugly", the "threshold" for using it is not low

Weathering steel, also known as atmospheric corrosion-resistant steel, is a low-alloy steel between ordinary steel and stainless steel. It is made of ordinary carbon steel with a certain amount of copper added, and corrosion-resistant elements such as phosphorus, chromium, nickel, titanium, and vanadium. It not only has the characteristics of easy ductility, high strength and fatigue resistance of ordinary steel, but also can reach 2-8 times of ordinary carbon steel in terms of corrosion resistance.

Weathering steel is expensive due to its excellent properties. Compared with ordinary steel, the price of weathering steel with similar properties is about 60% higher. Therefore, don't look at the ugly weathering steel, the general building can't use it.

Weathering steel is only used for structures such as containers, railway vehicles, oil derricks, and drilling platforms that are used in harsh environments and require high corrosion resistance of steel. As for the reinforced concrete used in ordinary buildings, because the steel bars buried in the concrete are less in contact with oxygen and water, engineers are reluctant to use weathering steel here!

(2) Why can weathering steel rust but not rot?

Pure iron is not easy to rust, but ordinary steel usually contains impurities such as copper and carbon. These impurities are less active than iron, and form a galvanic battery with iron in water-containing air (that is, a current that generates current through redox reactions. A device) that separates the oxidation and reduction reactions and provides a "highway" for the corrosion of steel.

The rust produced by the corrosion of steel is a loose and porous structure with many micro-cracks connecting the pores to each other. In this way, the rust acts like a sponge and can continue to absorb moisture from the air, allowing the steel to rust further until it is completely rusted through.

Weathering steel is different from ordinary steel, in the beginning, it will rust on the surface like ordinary steel. This process is even faster than normal steel due to its higher degree of alloying. However, due to the more complex lattice inside the weathering steel, a dark black dense rust layer will grow under the loose rust on the surface. This rust layer consists of nanoparticles of α-FeOOH. In this dense rust layer, nickel atoms replace part of the iron atoms, which makes the rust layer have cation selectivity and inhibits the permeation of corrosive anions.

(The left is weathering steel, the right is rust, you can clearly see the difference between the two)

It is this dense rust layer that makes the weathering steel rust on the surface, but the interior will not continue to be rusted. In fact, as long as you distinguish carefully, you can see that the surface of weathering steel is not the same as ordinary rust: the rust of weathering steel is uniform and dense, and it is close to the surface of the steel to protect the steel; while the rust is mottled, loose and porous, It will fall off when touched. Instead of protecting the steel, such rust "brings wolves into the room" and draws water and oxygen to the surface of the steel.