Causes of stainless steel rusting
Table of Contents
Generally speaking, stainless steel is not easy to rust steel, in fact, part of stainless steel, both rust resistance, and acid resistance (corrosion resistance).
Stainless steel’s rust resistance and corrosion resistance are due to the formation of chromium rich oxide film (passive film) on its surface. This film isolates the metal from the external medium, prevents the metal from being further corroded, and has the ability of self-healing. If it is damaged, the chromium in the steel and the oxygen in the medium will re form a passive film, which will continue to play a protective role.
The rust resistance and corrosion resistance are relative. The results show that the corrosion resistance of steel increases with the increase of chromium content in weak medium such as atmosphere and water, and in oxidizing medium such as nitric acid. When the chromium content reaches a certain percentage, the corrosion resistance of steel changes abruptly, that is, from easy to rust, from non corrosion to corrosion resistance. Stainless steel rust, but also related to the use of the environment, different environments, to use different chromium content of stainless steel.
The chromium content is the fundamental factor to determine the properties of stainless steel. It is reported that European and American standards stipulate that the minimum chromium content should not be less than 10.5%, Japan 11% and China 12%.
Classification of stainless steel
There are five basic types of stainless steel: austenite, ferrite, martensite, duplex stainless steel, precipitation hardening stainless steel.
- (1) Austenite stainless steel has no magnetism, and the representative steel is to add 18% chromium and contain certain nickel to increase the corrosion resistance. They are widely used steel.
- (2) Ferrite has magnetic properties, chromium is its main content, the proportion is 17%, this material has a good oxidation resistance.
- (3) Martensitic stainless steel also has magnetism, with a chromium content of 13% and a proper proportion of carbon, which can be hardened by quenching and tempering.
- (4) Duplex stainless steel has the mixed structure of ferrite and austenite. The content of chromium is between 18% and 28%, and the content of nickel is 4.5% – 8%. They have good effect on the corrosion of chloride.
- (5) The conventional content of precipitation type stainless steel chromium is 17, and a certain amount of nickel, copper and niobium are added. They can be hardened by precipitation and aging.
According to the metallographic structure, it can be divided into:
- (1) Ferritic stainless steel (400 Series), is chromium stainless steel, mainly represented by Gr13, G17, gr27-30;
- (2) Austenitic stainless steel (300 Series), chromium nickel stainless steel, mainly represented by 304316321, etc; (3) martensitic stainless steel (200 Series), chromium manganese stainless steel, high carbon content, mainly representative of 1gr13, etc.
Rust cause: chromium is the key
The rust phenomenon of stainless steel materials may be caused by the following reasons:
(1) Chlorine ions in the service environment
Chloride ions are widely present, such as salt / sweat / sea water / wind / soil, etc. Stainless steel in the presence of chloride ion environment, corrosion is very fast, even more than ordinary low carbon steel. Therefore, the use environment of stainless steel has requirements, and it needs to be wiped frequently to remove dust and keep it clean and dry. (this will allow him to make a “improper use.”
There is an example in the United States: an enterprise uses an oak container to contain a solution containing chloride ions. The container has been used for more than 100 years. It is planned to be replaced in the 1990s. Because the oak materials are not modern enough, the container is leaked due to corrosion 16 days after the stainless steel is replaced.
(2) No solution treatment
The alloy element did not dissolve into the matrix, which resulted in the low content of the alloy and poor corrosion resistance.
(3) Intergranular corrosion
The materials without titanium and niobium tend to intergranular corrosion. The intergranular corrosion can be reduced by adding titanium and niobium and then treated stably. Stainless steel is a kind of high alloy steel which can resist corrosion in air or chemical corrosion medium. Stainless steel has good surface and corrosion resistance. It does not need to be treated by plating color. It is often called stainless steel, which is used in many aspects of steel.
The high alloy steel, such as 13 chromium steel and 18-8 chromium nickel steel, is representative of the performance. From the perspective of metallography, the surface of stainless steel has chromium, which is very thin, which can resist corrosion by separating from the oxygen which is invaded in the steel. In order to maintain the inherent corrosion resistance of stainless steel, the steel must contain more than 12% chromium.
For welding. The lower carbon content reduces the amount of carbide precipitated in the heat affected zone near the weld to the minimum, and the precipitation of carbide may lead to intergranular corrosion (welding erosion) of stainless steel in some environments. It can damage the stainless steel surface and attach iron powder to form rust.
In our daily life, we sometimes find that stainless steel in some street flagpoles, waiting stands, light boxes and other facilities has obvious rust pickling phenomenon. Since stainless steel is passivated, why will it rust? There are two reasons for these situations. One is that the chromium content in the material is low, which belongs to inferior stainless steel. Second, it is not stainless steel at all, but with electroplating deceive users.
It is understood that many decorative materials are used to treat the appearance with this electroplating process. Because the material is general steel, the coating will rust naturally after the coating is peeled off.
Stainless steel has the ability to resist atmospheric oxidation – that is, rust free, and also corrosion resistance in acid, alkali and salt containing medium – namely corrosion resistance. But the corrosion resistance of the steel is changed with the chemical composition, the interaction state, the service conditions and the type of environmental medium.
For example, 304 steel pipe has absolutely excellent corrosion resistance in dry and clean atmosphere, but it will soon rust in sea fog with a large amount of salt, while 316 steel pipe will be in good performance. Therefore, it is not any kind of stainless steel, and can be corrosion resistant and rust free under any environment.
Stainless steel is a thin and solid and fine stable chromium rich oxide film (protective film) formed on its surface, which can prevent the oxygen atoms from penetrating and oxidation, and obtain the ability of anti rust. Once there is a certain reason, the film is constantly destroyed, and oxygen atoms in air or liquid will continue to penetrate or iron atoms will be separated from each other, forming loose iron oxide, and the metal surface will be constantly rusted. There are many forms of destruction of the surface film.
- 1. There are dust or other metal particles on the surface of stainless steel. In humid air, the condensate between the attachment and stainless steel connects them into a micro cell, which leads to electrochemical reaction and the protective film is damaged, which is called electrochemical corrosion.
- 2. The surface of stainless steel adheres to the organic juice (such as melon, vegetable, soup, phlegm, etc.), which forms organic acid when there is water and oxygen, and the corrosion of organic acid on the metal surface for a long time.
- 3. The surface adhesion of stainless steel contains acid, alkali and salt (such as alkali water and lime water splashing on the decoration wall), which causes local corrosion.
- 4. In polluted air (such as the atmosphere containing a large amount of sulfide, carbon oxide and nitrogen oxide), the condensate will form sulfuric acid, nitric acid and acetic acid liquid point, which will cause chemical corrosion.
The above conditions can cause the corrosion caused by the damage of the protective film on the stainless steel surface. Therefore, in order to ensure permanent bright metal surface, not rust, the following suggestions:
- 1. The surface of decorative stainless steel must be cleaned and cleaned frequently to remove the attachment and eliminate the external factors that cause the modification.
- 2. The coastal area should use 316 stainless steel, 316 material can resist sea water corrosion.
- 3. Some stainless steel tubes in the market cannot meet the corresponding national standards and can not meet the 304 material requirements. Therefore, it will also cause rust, which requires users to choose products from reputable manufacturers.
Source: China Stainless Steel Flange Manufacturer – Yaang Pipe Industry (www.epowermetals.com)
(Yaang Pipe Industry is a leading manufacturer and supplier of nickel alloy and stainless steel products, including Super Duplex Stainless Steel Flanges, Stainless Steel Flanges, Stainless Steel Pipe Fittings, Stainless Steel Pipe. Yaang products are widely used in Shipbuilding, Nuclear power, Marine engineering, Petroleum, Chemical, Mining, Sewage treatment, Natural gas and Pressure vessels and other industries.)
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