Why are stainless steel materials also rusty?

When brown rust spots appear on the surface of stainless steel tubes, people are amazed: " stainless steel is not rusted, rust is not stainless steel, it may be a problem with steel. "In fact, this is a lack of understanding of stainless steel one - sided wrong view. Stainless steel will rust under certain conditions. 

 

Stainless steel has the ability to resist atmospheric oxidation - that is, stainless steel, and also has the ability to resist corrosion in the medium containing acid, alkali and salt, that is, corrosion resistance. However, the corrosion resistance of the steel is changed with its chemical composition, additive state, use condition and environmental medium type. Such as 304 steel pipe, in the dry and clean atmosphere, there is absolutely excellent corrosion resistance, but moving it to the coastal area, in the sea of sea fog containing a lot of salt, will soon rust; And the 316 steel pipe shows good performance. Therefore, not any kind of stainless steel, in any environment can be corrosion - resistant, not rust.

 

The surface film is damaged in many forms, and several of the following are common:

 

Stainless steel is a very thin and stable chromium - rich oxide film ( protective film ) formed on its surface, which prevents the continuous infiltration and oxidation of oxygen atoms, and the ability to resist corrosion. Once there is some reason, the film is constantly destroyed, the oxygen atoms in the air or liquid will continue to seep into or iron atoms in the metal, forming a loose iron oxide, the metal surface is constantly corroded. The surface film is damaged in many forms, and several of the following are common:

 

1. the stainless steel surface is coated with dust or other metal particles containing other metal elements, in the moist air, the condensation water between the attached and stainless steel, connecting the two together into a micro cell, initiate the electrochemical reaction, and the protective film is damaged, which is called electrochemical corrosion. 

2. the stainless steel surface adhering to the organic juice ( such as vegetables, noodle soup, sputum, etc. ), in the presence of water oxygen, constitutes an organic acid, and long time the corrosion of the metal surface of the organic acid. 

3. the surface adhesion of stainless steel contains acid, alkali and salt substances ( such as alkali water of decoration wall and whitewash spray ), causing local corrosion. 

4. in the polluted air ( such as the atmosphere containing large amounts of sulfide, carbon oxide and nitrogen oxides ), in case of condensed water, sulfuric acid, nitric acid and acetic acid solution are formed, resulting in chemical corrosion.

 

How to ensure that the metal surface is permanently bright, not corroded? 

All above can cause the corrosion of the stainless steel surface protective film. Therefore, to ensure that the metal surface is permanently bright and not corroded, it is recommended that:

 

1. the decorative stainless steel surface must be cleaned and scrubbed frequently, and the attachment shall be removed to eliminate the external factors causing the modification. 

2. the seaside area shall use 316 stainless steel, and 316 materials can resist seawater corrosion. 

3. some stainless steel tubes in the market cannot meet the corresponding national standards, and can not meet the requirements of 304 materials. Therefore, it will also cause rust, which requires users to carefully select reputable manufacturers. 

Why is stainless steel also magnetic? 

 

People often think that the magnet adsorption of stainless steel, verify its advantages and disadvantages and authenticity, do not absorb magnetic, think is good, the real price; The suction are magnetic, and they think it is a fake. In fact, this is an extremely one-sided and practical wrong identification method. 

There are many kinds of stainless steel, and can be divided into several categories at room temperature according to the organization structure:

 

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

2. Martensite or ferrite type: such as 430, 420, 410, etc.; 

3. Austenitic type is non-magnetic or weak magnetic, martensite or ferrite is magnetic. 

 

Generally, the stainless steel used as the decorative tube sheet is the austenitic type 304 material, generally speaking is magnetic or weak magnetism, but the chemical composition fluctuation or processing state may appear magnetic, but this can not be considered as fake or unqualified, what reason is this? 

 

The above mentioned austenite is non-magnetic or weak magnetic, and martensite or ferrite is magnetic, because of the composition segregation in smelting or improper heat treatment, it will cause a small amount of martensite or ferrite in austenitic 304 stainless steel. Thus, 304 stainless steel will have weak magnetic. 

In addition, 304 stainless steel after cold working, the structure of the structure will also be converted to martensite, the greater the deformation degree of cold working, the more martensite transformation, the greater the magnetic properties of steel. Like a batch of steel strips, the production of φ 76 tubes, no apparent magnetic sense, production of φ 9.5 tubes. Because the deformation of the cold bending is more obvious, the deformation of the rectangular tube is larger than that of the circular tube, especially the angular part, the deformation is more intense and the magnetic is more obvious.

 

In order to eliminate the magnetic properties of 304 steel caused by the above reasons, the stable austenitic microstructure can be recovered by high temperature solution treatment, thus eliminating the magnetic properties. In particular, the magnetic properties of 304 stainless steel caused by the above reasons, and other materials such as stainless steel, such as 430, carbon steel is not at the same level, that is, the magnetic properties of 304 steel always show weak magnetic. This tells us that if the stainless steel band is weak magnetic or completely non - magnetic, it should be identified as 304 or 316 materials; If the magnetic properties of carbon steel, show strong magnetic, because the discriminant is not 304 material.