Why does stainless steel rust?

When the surface of the stainless steel tube appears brown rust spot (point), people are surprised: think that "stainless steel is not rust, rust is not stainless steel, may be the steel has a problem".

In fact, this is the lack of understanding of stainless steel a one-sided wrong view. Stainless steel will rust under certain conditions.

Stainless steel has the ability to resist atmospheric oxidation - that is, rust, but also has the ability to corrosion in the medium containing acid, alkali, salt - that is, corrosion resistance. However, its anti-corrosion ability varies with its chemical composition, protective status, service conditions and environmental media types.

Such as 304 steel pipe, in the dry and clean atmosphere, there is absolutely excellent corrosion resistance, but it will be moved to the coastal area, in the sea fog containing a lot of salt, will soon rust; And 316 steel pipe is good performance. Therefore, not any kind of stainless steel, in any environment can withstand corrosion, rust.

Stainless steel is formed by its surface of a layer of very thin and solid fine stable chromium oxide film (protective film), to prevent oxygen atoms continue to penetrate, continue to oxidize, and obtain the ability to resist corrosion. Once for some reason, this film is continuously damaged, oxygen atoms in the air or liquid will continue to infiltrate or iron atoms in the metal is constantly removed, forming loose iron oxide, metal surface is also subjected to continuous corrosion.

This surface film can be damaged in many ways.

Common in daily life are as follows:

1. Stainless steel surface deposited with dust or different metal particles containing other metal elements of the attachment, in the humid air, the condensation between the attachment and stainless steel, the two connected into a micro battery, triggered an electrochemical reaction, the protective film is damaged, known as electrochemical corrosion.

2. Stainless steel surface adhesion organic juice (such as vegetables, soup, sputum, etc.), in the case of water and oxygen, organic acid, organic acid corrosion of metal surface for a long time.

3. Stainless steel surface adhesion contains acid, alkali, salt substances (such as decoration of the wall of alkali water, lime water splash), cause local corrosion.

4. In the polluted air (such as the atmosphere containing a large number of sulfide, carbon oxide, nitrogen oxide), when condensed water is encountered, sulfuric acid, nitric acid, acetic acid liquid point is formed, causing chemical corrosion. Tips To ensure that the metal surface is permanently bright and not corroded, we recommend:

1) The surface of decorative stainless steel must be cleaned and scrubbed frequently to remove attachments and eliminate external factors causing modification.

2) Stainless steel 316 shall be used in the coastal area, which can resist corrosion of sea water.

3) Some stainless steel tube chemical composition on the market can not meet the corresponding national standards, can not meet the requirements of 304 material. Therefore, it will also cause rust, which requires users to carefully choose the products of reputable manufacturers.

And give you the most complete information about stainless steel

Stainless Steel (Stainless Steel) is short for Stainless acid-resistant Steel, air, steam, water and other weak corrosion medium or Stainless Steel called Stainless Steel; And the chemical corrosion resistance medium (acid, alkali, salt and other chemical etching) corrosion of steel called acid steel.

Common categories:

Usually divided into:

Usually, according to the metallographic organization, the ordinary stainless steel is divided into three categories: austenitic stainless steel, iron type stainless steel, Martensite type stainless steel. On the basis of these three basic metallographic structures, duplex steels, precipitate hardened stainless steels and high alloy steels with less than 50% iron content are derived for specific needs and purposes.

1. Austenitic stainless steel.

The matrix is stainless steel with face-centered cubic crystal structure austenite structure (CY phase), non-magnetic, mainly through cold processing to strengthen (and may lead to a certain degree of magnetism). The American Iron and Steel Institute uses 200 and 300 series numbers, such as 304.

2, iron type stainless steel.

The matrix is a stainless steel with ferrite structure (a phase) dominated by body centered cubic crystal structure. It is magnetic and generally cannot be hardened by heat treatment, but it can be slightly strengthened by cold working. The American Iron and Steel Institute is 430 and 446.

3, Martensite stainless steel.

The matrix of martensitic structure (body centered cubic or cubic), magnetic, through heat treatment can adjust its mechanical properties of stainless steel. The American iron and steel institute uses numbers 410,420 and 440. Martensite has austenitic structure at high temperature, which is able to change to martensite (i.e. hardened) when cooled to room temperature at an appropriate rate.

4, austenitic ferrite (duplex) stainless steel.

The matrix has both austenite and ferrite phase structure, and the content of the less phase matrix is generally more than 15%, which is magnetic and can be strengthened by cold working. 329 is a typical duplex stainless steel. Compared with austenitic stainless steel, duplex steel has higher strength, higher resistance to intergranular corrosion, chloride stress corrosion and spot corrosion.

5, precipitation hardening stainless steel.

Stainless steel whose substrate is austenitic or martensitic and can be hardened by precipitation hardening. The American Iron and Steel Institute uses 600 series numbers such as 630, or 17-4pH.

Generally speaking, in addition to alloy, the corrosion resistance of austenitic stainless steel is relatively excellent, in the environment of low corrosion, ferritic stainless steel can be used, in mildly corrosive environment, if the material is required to have high strength or high hardness, martensitic stainless steel and precipitated hardened stainless steel can be used.

What kind of stainless steel is not easy to rust?

There are three main factors affecting the corrosion of stainless steel:

1. The content of alloying elements.

Generally speaking steel does not rust easily with chromium content of 10.5%. The higher the content of chromium and nickel, the better the corrosion resistance, such as 304 material nickel content in 8-10%, chromium content of 18-20%, such stainless steel in general is not rust.

2. The smelting process of the production enterprise will also affect the corrosion resistance of stainless steel.

Smelting technology is good, advanced equipment, advanced process of large stainless steel plant whether in the control of alloy elements, impurities removal, billet cooling temperature control can be guaranteed, so the product quality is stable and reliable, good internal quality, not easy to rust. On the contrary, some small steel equipment is backward, backward process, smelting process, impurities can not be removed, the production of products will inevitably rust.

3. The external environment, the climate is dry and ventilated environment is not easy to rust.

And the air humidity is large, continuous rainy weather, or the environment with large acidity in the air is easy to rust. 304 stainless steel material, if the surrounding environment is too poor will rust.

How should stainless steel appear rust spot to handle?

1. Chemical methods

After pickling, it is very important to wash properly with clean water in order to remove all contaminants and acid residues. After all processing with polishing equipment to polish, with polishing wax closed. For local slight rust spots can also be used 1:1 gasoline, oil mixture with clean cloth to wipe the rust spots.

2. Mechanical methods

Sandblasting, shot blasting with glass or ceramic particles, annihilation, scrub and polish. It is possible to wipe out contamination caused by previously removed material, polishing material or annihilating material by mechanical means. All kinds of pollution, especially foreign iron particles, can be a source of corrosion, especially in humid environments. Therefore, mechanical cleaning surfaces should be properly cleaned under dry conditions. The use of mechanical method can only clean its surface, can not change the material itself corrosion resistance. Therefore, it is recommended to re-polish with polishing equipment after mechanical cleaning and seal with polishing wax.