Why Does Mild Steel Pipe Rust?

Rusting principle of mild steel pipe

The main reason for rusting of mild steel pipes is due to the oxidation reaction between their surface and oxygen and moisture in the air. The carbon steel pipe generally contains iron elements, which are easily oxidized to form iron oxide (Fe2O3) and iron hydroxide (Fe(OH)2), which causes red rust on the surface of mild steel pipes.

What causes mild steel pipe to rust

1. Oxygen and moisture in the air

After the surface of the mild steel pipe comes into contact with oxygen and moisture in the air, an oxidation reaction occurs, resulting in rust. Like common seamless steel pipe, in underground pipe corridors with poor ventilation and high humidity, oxidation and rust are very likely to occur due to long-term contact with humid air.

2. Environmental factors

If there are certain acidic and alkaline substances in the environment, it may also cause rusting of mild steel pipes. In addition, if exposed to a humid environment for a long time, it is also easy to cause rusting of mild steel pipes. For example, in the production area of chemical enterprises, some carbon steel pipes that transport acidic or alkaline media are affected by the volatilization or leakage of chemicals, which accelerates the rusting process of the pipes.

3. Improper surface treatment

Improper surface treatment of mild steel pipes, such as unclean cleaning, grease pollution, surface indentation, etc., are easy to cause rusting of ms pipes. In some factories, after welding with pipe flange, the welding residues are not cleaned up, which becomes the starting point of rust.

Why does mild steel rust easily

The main component of rust on mild steel is iron oxide. In humid environments, the iron on the surface of mild steel reacts with oxygen and water in the air to form hydrated iron(III) oxides, which is what we commonly call "rust."

The main chemical component of rust is iron oxide (Fe2O3). It is reddish-brown, porous, and easily peels off. Compared to the original steel material, its strength and toughness are significantly reduced.

Mild steel has a low carbon content, only 0.05%-0.25%, making it more susceptible to corrosion from moisture, fog, acidic and alkaline environments compared to other steels, thus leading to rusting. Simultaneously, the iron in mild steel also reacts with oxides in the air to form red rust, further exacerbating its susceptibility to rust.

The rust formation process of mild steel

The rusting process of mild steel is a typical electrochemical corrosion process.

In a humid environment, a thin film of water forms on the surface of steel. Dissolved oxygen in the water reacts with the iron in a redox reaction. Iron, acting as the anode, is oxidized to ferric ions (Fe²⁺), while oxygen is reduced to hydroxide ions (OH⁻) at the cathode. The two combine to form ferrous hydroxide (Fe(OH)₂).

Subsequently, ferrous hydroxide is further oxidized to ferric hydroxide (Fe(OH)₃), and finally dehydrates to form iron oxide (Fe₂O₃), i.e., rust.

Differences in rusting between mild steel and high-carbon steel

1. Differences in Chemical Composition

The biggest difference between mild steel and high-carbon steel lies in their carbon content. Mild steel has a carbon content below 0.05%, while high-carbon steel has a carbon content above 0.6%. Besides this, there are other differences in the chemical composition of the two types of steel.

2. Rusting Mechanism

Due to its lower carbon content, mild steel readily reacts with oxygen when exposed to air, moisture, and other organic matter. When the surface of mild steel is moist, the iron on the surface is oxidized and converted into iron oxides, i.e., rust.

High-carbon steel has a higher carbon content, resulting in a higher chromium content. Chromium is a natural oxide that forms a dense chromium oxide layer on the steel surface, effectively preventing rust.

3. Rust Prevention Methods

For mild steel, rust can be prevented or treated by applying rust inhibitors such as paint, coatings, or iron oxide to the steel surface; increasing the steel's rust resistance level, such as by zinc plating after surface treatment or adding other corrosion-resistant elements like copper, nickel, or molybdenum.

For high-carbon steel, a simpler method is sufficient: keep the surface clean and dry. If slight rust appears, gently wipe it with sandpaper or a steel brush.

How to avoid rusting of mild steel pipes

1. Control environmental conditions

Try to store mild steel pipes in a dry, ventilated, and weathering-free environment to avoid the invasion of mild steel pipes by moisture, acidic and alkaline substances.

2. Surface treatment

For the treatment of the surface of mild steel pipes, it is necessary to ensure that the surface is clean, remove all impurities such as grease, steel slag, etc., and take anti-corrosion measures, such as galvanizing and chrome plating.

3. Packaging and storage

During production, transportation and storage, attention should be paid to the packaging of mild steel pipes to prevent mild steel pipes from being affected by the external environment. At the same time, when storing, it should be stored in a dry, ventilated and temperature-appropriate place to avoid the mild steel pipe from being attacked by a humid environment.

Conclusion

The above is a detailed introduction to the causes of rusting of mild steel pipes and preventive measures. In the process of using mild steel pipes, it is necessary to pay attention to the surface treatment of mild steel pipes, control the storage environment, and avoid the mild steel pipes from being attacked by moisture, acidic and alkaline substances. As long as it is stored and used in the correct way, the problem of rusting of mild steel pipes can be effectively avoided.

Read more: Difference between mild steel pipe and carbon steel pipe or How long can mild steel pipes last?