Why does stainless steel rust?
When brown rust stains appear on the surface of stainless steel pipes, people are often surprised: “Stainless steel does not rust. If that happens, it will no longer be stainless steel; There may be a problem with the quality of the steel.”
In fact, this is a one-sided misunderstanding due to lack of knowledge about stainless steel. Stainless steel can rust under certain conditions.
Stainless steel has the ability to resist atmospheric oxidation – stainless – and also has the ability to resist corrosion in media containing acid, alkali and salt – corrosion resistance.
However, the degree of its corrosion resistance varies according to the chemical composition of the steel itself, their mutual state, conditions of use and type of environmental environment.
For example, 304 stainless steel has excellent corrosion resistance in dry and clean environments, but if it is moved to a seaside area in a sea fog containing a large amount of salt, it will rust quickly; 316 stainless steel, on the other hand, performs well.
Therefore, not all types of stainless steel resist corrosion and rust in any environment.
Stainless steel relies on a very thin, firm, dense and stable chromium-rich oxide film on its surface (protective film) to prevent further oxygen infiltration and oxidation, thus gaining corrosion resistance.
If for some reason this thin film is continually damaged, the oxygen atoms in the air or liquid will continually infiltrate or the iron atoms in the metal will continually segregate, forming loose iron oxide, and the metal surface will then be constantly corroded.
Ways of damage to the surface film of stainless steel
There are many ways of damage to the surface film, and the most common in daily life are:
1. Stainless steel surfaces accumulate dust containing other metallic elements or foreign metallic particles. In humid air, water condenses between the fixture and the stainless steel connects the two into a microbattery, starting an electrochemical reaction. The protective film is damaged, this is called electrochemical corrosion.
2. Organic juice (such as melon and vegetable juice, noodle soup, phlegm, etc.) sticks to the surface of stainless steel. Under the condition of water and oxygen, it forms organic acid. Over time, organic acid corrodes the metal surface.
3. Substances containing acid, alkali and salt (such as alkaline water and lime water splashed from decorated walls) can adhere to the surface of stainless steel, causing localized corrosion.
4. In a polluted atmosphere (for example, air containing a significant amount of sulfides, carbon oxides and nitrogen oxides), condensed water forms droplets of sulfuric acid, nitric acid and acetic acid, resulting in chemical corrosion.
How to ensure that the stainless steel surface is not corroded?
To ensure that the metal surface remains permanently shiny and corrosion-free, we recommend:
All of the situations mentioned can result in damage to the protective film on the stainless steel surface, causing rust.
Therefore, to maintain a permanently shiny and rust-free metal surface, we suggest:
1. Regularly clean and clean the surface of decorative stainless steel to remove accessories and eliminate external factors that cause corrosion.
2. Some stainless steel pipes on the market do not meet the chemical composition standards of their respective countries and cannot meet the requirements of 304 materials. This may cause rust, so users should carefully select products from reliable manufacturers.
Why can stainless steel be magnetic?
People often think that if a magnet attracts stainless steel, it is a way to verify its quality and authenticity. If there is no magnetic attraction, they believe it is good and genuine. If there is magnetic attraction, they think it is a fake. However, this is a highly simplistic and inaccurate method of discernment.
There are many types of stainless steel, which can be classified into several categories at room temperature according to their organizational structure:
1. Austenitic type: such as 201, 202, 301, 304, 316, etc.
2. Martensitic or ferritic type: 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 pipes and decorative plates is austenitic type 304, which is generally non-magnetic or weakly magnetic. However, fluctuations in chemical composition due to casting or differences in processing conditions can also result in magnetism. However, this does not mean that the material is counterfeit or non-conforming. Why is that?
As mentioned above, austenitic stainless steel is non-magnetic or weakly magnetic, while martensitic or ferritic stainless steel is magnetic. During casting, component segregation or improper heat treatment may result in a small amount of martensitic or ferritic structure in austenitic 304 stainless steel. As such, 304 stainless steel will exhibit weak magnetism.
Furthermore, when 304 stainless steel is subjected to cold working, its organizational structure will also transform towards martensite. The greater the degree of deformation due to cold work, the greater the transformation of the martensite and the greater the magnetism of the steel.
For example, for the same batch of steel strips, there is no noticeable magnetic sensation when producing Φ76 tubes, but when producing Φ9.5 tubes, which undergo greater cold bending deformation, the magnetism becomes more noticeable. For square tubes, which undergo greater deformation than round tubes, especially at the corners, the magnetism is even more pronounced due to the more intense deformation.
To completely eliminate the magnetism of 304 steel caused by the above reasons, you can restore the stable austenitic structure through high temperature solution treatment to eliminate magnetism.
It should be specifically noted that the magnetism of 304 stainless steel caused by the above reasons is not at the same level as that of other types of stainless steel such as 430 carbon steel. That is, the magnetism of 304 steel is always weakly magnetic.
This tells us that if stainless steel is weakly magnetic or completely non-magnetic, it should be considered as material 304 or 316; If it shows strong magnetism like carbon steel, it should be considered as not 304 material.
When does stainless steel rust?
Stainless steel refers to steel that has the ability to resist atmospheric oxidation and corrosion. It will not rust in dry, clean environments, but under certain conditions it may rust, for example:
① When organic juice (such as melon, noodle soup, phlegm, etc.) adheres to the surface and forms organic acid under water and oxygen conditions, it can corrode the metal surface for a long period of time.
② Substances containing acids, alkalis and salt (such as alkaline water and lime water splashed on decorated walls) may adhere to the surface, causing localized corrosion.
③ In a polluted atmosphere (for example, air containing a significant amount of sulfides, carbon oxides and nitrogen oxides), condensed water forms droplets of sulfuric acid, nitric acid and acetic acid, causing chemical corrosion.
Therefore, it is essential to maintain cleanliness. Additionally, there are many types of stainless steel and several factors can potentially lead to corrosion.
