What is the reason of stainless steel rusting?
Table of Contents
Generally speaking, stainless steel is not easy to rust steel, in fact, some stainless steel, both stainless steel and acid resistance (corrosion resistance).
The rust resistance and corrosion resistance of stainless steel are due to the formation of chromium rich oxide film (passivation film) on the surface of stainless steel. This film can isolate the metal from the external medium, prevent the metal from further corrosion, and have the ability of self-healing. Once damaged, the chromium in the steel and the oxygen in the medium will regenerate the passive film and continue to play a protective role.
The relative corrosion resistance and rust resistance. The results show that the corrosion resistance of steel increases with the increase of chromium water content in the weak medium such as atmosphere, water and nitric acid. When the chromium content reaches a certain percentage, the corrosion resistance of steel changes suddenly, that is, from easy to rust to not easy to rust, from non corrosion to corrosion resistance. Stainless steel stainless steel, but also related to the use of the environment, different environments, to use different chromium content of stainless steel.
The content of chromium is the basic factor to determine the properties of stainless steel. It is reported that the minimum chromium content in European and American standards should not be less than 10.5%, Japan’s 11%, and China’s 12%.
Classification of stainless steel
There are five basic types of stainless steel: austenite, ferrite, martensite, duplex stainless steel, precipitation hardening stainless steel.
- (1) Austenitic stainless steel has no magnetism. The representative steel is added with 18% chromium and some nickel to increase its corrosion resistance. They are widely used.
- (2) The ferrite is magnetic, and chromium is the main content of it, the proportion is 17%. This material has good oxidation resistance.
- (3) Martensitic stainless steels are also magnetic, usually containing 13% chromium and an appropriate proportion of carbon, which can be hardened by quenching and tempering.
- (4) Duplex stainless steel has a mixed structure of ferrite and austenite. The content of chromium is between 18% and 28%, and the content of nickel is between 4.5% and 8%. They have a good effect on resisting chloride corrosion.
- (5) The conventional content of chromium in precipitated stainless steel 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 and 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 represented by 1gr13.
Why does stainless steel rust?
- Cause of rust: chromium is the key element.
There may be several reasons for stainless steel to rust:
There are chloride ions in the environment
Chloride ions are widespread, such as salt / sweat / sea water / sea breeze / soil and so on. Stainless steel in the presence of chloride ion environment, corrosion is very fast, even more than ordinary low carbon steel. Therefore, there are requirements for the use of stainless steel environment, and need to often wipe, remove dust, keep clean and dry. (in this way, he can be given a “improper use.”)
There is an example in the United States: an enterprise used an oak container to hold a solution containing chloride ions. The container has been used for nearly 100 years. It was planned to replace it in the 1990s. Due to the lack of modern oak materials, the stainless steel was used to replace the container, and the container leaked due to corrosion 16 days later.
No solution treatment
The alloy elements do not dissolve into the matrix, resulting in low alloy content and poor corrosion resistance.
Inherent intergranular corrosion
The material without titanium and niobium has the tendency of intergranular corrosion. The intergranular corrosion can be reduced by adding titanium and niobium together with stabilizing treatment. A kind of high alloy steel that can resist corrosion in air or chemical corrosion medium. Stainless steel has beautiful surface and good corrosion resistance. It doesn’t need to be treated by color plating and other surface treatment. It plays the inherent surface properties of stainless steel and is used in many kinds of steel. It is usually called stainless steel.
The representative properties are 13 chromium steel, 18-8 chromium nickel steel and other high alloy steel. From the metallographic point of view, because the stainless steel contains chromium, a very thin chromium film is formed on the surface, which is separated from the oxygen invading into the steel and plays a role of corrosion resistance. In order to maintain the inherent corrosion resistance of stainless steel, the steel must contain more than 12% chromium.
For welding applications. The lower carbon content makes the carbides precipitated in the heat affected zone near the weld to a minimum, and the precipitation of carbides may cause intergranular corrosion (welding corrosion) of stainless steel in some environments. Because it will damage the surface of stainless steel and adhere to iron powder and form rust.
In our daily life, we sometimes find that the stainless steel of some flagpoles, waiting booths, light boxes and other facilities in the street has obvious rust pickling phenomenon. Since the stainless steel is passivated, why will it rust? There are two reasons for this. One is the low content of chromium in the material, which belongs to inferior stainless steel. Second, it is not stainless steel at all, but to deceive users with electroplating.
As far as the surface of steel is generally rusted, it is known that the plating process is natural.
Corrosion resistance of stainless steel in acid and salt medium is also corrosion resistant. However, the corrosion resistance of the steel varies with its chemical composition, additive state, service condition and environmental medium type.
For example, 304 steel pipe has absolutely excellent corrosion resistance in dry and clean atmosphere, but if it is moved to coastal area, it will rust quickly in sea fog containing a lot of salt; while 316 steel pipe performs well. Therefore, not any kind of stainless steel, in any environment can be resistant to corrosion, rust.
Stainless steel is formed on its surface by a layer of thin and firm, stable chromium rich oxide film (protective film), to prevent oxygen atoms from continuing to penetrate, continue to oxidize, and obtain the ability to resist corrosion. Once there is some reason, the film is constantly damaged, oxygen atoms in air or liquid will continue to penetrate or metal iron atoms will continue to separate out, forming loose iron oxide, the metal surface will be constantly corroded. There are many forms of damage to the surface film, such as the following in daily life:
- 1. Dust containing other metal elements or attachments of dissimilar metal particles are deposited on the surface of stainless steel. In humid air, the condensed water between the attachment and stainless steel connects them to form a micro cell, causing electrochemical reaction and damaging the protective film, which is called electrochemical corrosion.
- 2. The surface of stainless steel adheres organic juice (such as melon and vegetable, noodle soup, phlegm, etc.) to form organic acid in the presence of water and oxygen, which will corrode the metal surface for a long time.
- 3. The adhesion of stainless steel surface contains acid, alkali and salt substances (such as alkali water and lime water splashing on the decoration wall), causing local corrosion.
- 4. In the polluted air (such as the atmosphere containing a large amount of sulfide, carbon oxide and nitrogen oxide), sulfuric acid, nitric acid and acetic acid liquid spots will be formed when meeting condensate water, causing chemical corrosion.
The above conditions can cause the damage of the protective film on the surface of stainless steel and cause corrosion. Therefore, in order to ensure that the metal surface is permanently bright and not corroded, the following suggestions are made:
- 1. It is necessary to clean and scrub the surface of decorative stainless steel frequently to remove the attachment and eliminate the external factors causing the modification.
- 2. 316 stainless steel should be used in coastal area, which can resist seawater corrosion.
- 3. The chemical composition of some stainless steel tubes in the market can not meet the corresponding national standards, and can not meet the 304 material requirements. Therefore, it will also cause rust, which requires users to carefully select 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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