Why does stainless steel rust?

People are often surprised to see brown rust spots on the surface of stainless steel pipes, believing that “Stainless steel doesn't rust; if it rusts, it's not stainless steel, and there must be a problem with the steel quality.” This is actually a one-sided and mistaken view stemming from a lack of understanding of stainless steel. Stainless steel can rust under certain conditions.

Stainless steel has the ability to resist atmospheric oxidation —that is, it is rust-resistant—and also has the ability to resist corrosion in acidic, alkaline, and saline media—that is, corrosion resistance. However, the extent of its corrosion resistance varies with the chemical composition of the steel itself, its heat treatment state, the conditions of use, and the type of environmental medium. For example, 304 steel pipes have excellent rust resistance in dry, clean air, but if they are moved to a coastal area and exposed to sea fog containing a large amount of salt, they will rust quickly; while 3 steel pipes perform well. Therefore, not all stainless steel is corrosion-resistant and rust-proof in any environment.

There are many ways in which the surface film can be damaged; common ones in daily life include:

Stainless steel's rust resistance comes from an extremely thin, strong, dense, and stable chromium-rich oxide film (protective film) formed on its surface, which prevents oxygen atoms from continuing to penetrate and oxidize, thus achieving rust resistance. Once this film is damaged for some reason, oxygen atoms in the air or liquid will continuously penetrate, or iron atoms in the metal will continuously separate, forming loose iron oxide, and the metal surface will be continuously corroded. There are many ways in which this surface film can be damaged; common ones in daily life include:

1. Dust containing other metallic elements or foreign metal particles accumulates on the stainless steel surface. In humid air, the condensate water between the deposit and the stainless steel forms a micro-battery, triggering an electrochemical reaction that damages the protective film, a phenomenon known as electrochemical corrosion.

2. Organic substances (such as juice from vegetables and fruits, soup, phlegm, etc.) adhere to the surface of the stainless steel. In the presence of water and oxygen, organic acids are formed, and prolonged exposure to these acids corrodes the metal surface.

3. Acidic, alkaline, or saline substances (such as alkaline water or limewater splashes from wall decoration) adhere to the surface of the stainless steel, causing localized corrosion.

4. In polluted air (such as air containing large amounts of sulfides, carbon monoxide, and nitrogen oxides), condensation water forms sulfuric acid, nitric acid, and acetic acid droplets, causing chemical corrosion.

All of the above situations can damage the protective film on the surface of the stainless steel, leading to rust.

Therefore, to ensure that the metal surface remains permanently bright and free from rust, we recommend:

1. Regularly clean and wipe the decorative stainless steel surface to remove deposits and eliminate external factors that cause rust.

2. Use 316 stainless steel in coastal areas; 316 stainless steel can resist seawater corrosion.

3. Some stainless steel pipes on the market do not meet the corresponding national standards in terms of chemical composition and do not meet the requirements of 304 material. This can also cause rust, so users need to carefully select products from reputable manufacturers.

Why does stainless steel sometimes become magnetic?

People often use magnets to test the quality and authenticity of stainless steel, believing that non-magnetic stainless steel is good and genuine, while magnetic stainless steel is fake. This is actually an extremely one-sided and inaccurate method of identification.

There are many types of stainless steel. At room temperature, they can be classified into several categories according to their microstructure:

1. Austenitic: such as 201, 202, 301, 304, 316, etc.;

2. Martensitic or Ferritic: such as 430, 420, 410, etc.;

Austenitic stainless steel is non-magnetic or weakly magnetic, while martensitic or ferritic stainless steel is magnetic.

Most stainless steel used for decorative pipes and plates is austenitic 304 material, which is generally non-magnetic or weakly magnetic. However, due to fluctuations in chemical composition during smelting or different processing states, magnetism may also appear. This does not mean that it is fake or substandard. What is the reason for this?

As mentioned above, austenitic stainless steel is non-magnetic or weakly magnetic, while martensitic or ferritic stainless steel is magnetic. Due to segregation during smelting or improper heat treatment, austenite 304 stainless steel may contain a small amount of martensite or ferrite. As a result, the 304 stainless steel will exhibit weak magnetism.

In addition, 304 stainless steel undergoes cold working, and its microstructure will transform into martensite. The greater the degree of cold working deformation, the more martensite transformation, and the greater the magnetism of the steel. For example, for the same batch of steel strip, producing Φ76 pipes shows no obvious magnetic induction, while producing Φ9.5 pipes shows more obvious magnetic induction due to greater cold bending deformation. Rectangular pipes, due to larger deformation than round pipes, especially at the corners where deformation is more intense, show even more obvious magnetism.

To completely eliminate the magnetism of 304 steel caused by the above reasons, high-temperature solution treatment can be used to restore the stable austenitic structure and thus eliminate the magnetism.

It is particularly important to note that the magnetism of 304 stainless steel caused by the above reasons is not of the same level as that of other stainless steels, such as 430 and carbon steel. In other words, the magnetism of 304 steel always shows weak magnetism.

This tells us that if stainless steel exhibits weak magnetism or is completely non-magnetic, it should be identified as 304 or 316 material; if it has the same magnetism as carbon steel, showing strong magnetism, it should be identified as not 304 material.