Why do railway wagons use weathering steel?

Weathering steel has the ability to resist atmospheric corrosion, because alloy elements form a protective film on the surface of weathering steel substrate, which reduces the conductivity of rust layer and hinders the rapid growth of corrosion products.
The alloy elements that can improve the atmospheric corrosion resistance of weathering steel have the following conditions: first, the solubility in iron is greater than that in rust layer.
Secondly, it can form a solid solution with iron.
Thirdly, the electrode potential of weathering steel can be increased.

The influence of alloying elements on atmospheric corrosion resistance is as follows:

Carbon element is unfavorable to atmospheric corrosion resistance of weathering steel.
Reducing carbon content in weathering steel to reduce the precipitation of carbide can prevent the formation of galvanic cells.
Normally, the carbon content of weathering steel is controlled under 0.12%.

The corrosion resistance of weathering steel can be improved when copper content of 0.2% ~ 0.4% is added to weathering steel.

Phosphorus is one of the most effective elements to improve the corrosion resistance of steel.
Phosphorus can be uniformly dissolved in weathering steel, which helps to form a dense protective film.
Generally, the corrosion resistance is best when the phosphorus content in steel is 0.08% ~ 0.15%.

Chromium can form a dense oxide film on the surface of weathering steel, improve the passivation ability of weathering steel and slow down the growth rate of rust layer.
In general, the chromium content in weathering steel ranges from 0.4% to 1.0%.

Nickel is a relatively stable element. The addition of nickel can change the self-corrosion potential of weathering steel in the positive direction and increase the stability of weathering steel.

Rare earth elements (RE) can refine grain, change the state of inclusions in steel, reduce the number of harmful inclusions and reduce the corrosion source point, thus improving the anti-atmospheric corrosion performance of steel.