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Why does stainless steel also rust?

December 11, 2023

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Why does stainless steel also rust?


What is stainless steel?

A type of steel that has the ability to resist atmospheric oxidation and also has the ability to resist corrosion in media containing acids, alkalis, and salts!

Introduction to stainless steel

In layman's terms, stainless steel is steel that does not rust easily. In fact, some stainless steel has both rust resistance and acid resistance (corrosion resistance). The rustlessness and corrosion resistance of stainless steel are due to the formation of a chromium-rich oxide film (passivation film) on its surface. This film isolates the metal from the external medium, prevents the metal from being further corroded, and has the ability to repair itself. , if once damaged, the chromium in the steel will regenerate a passivation film with the oxygen in the medium to continue to play a protective role.

This kind of stainlessness and corrosion resistance are relative. Tests have shown that the corrosion resistance of steel in weak media such as the atmosphere, water, and oxidizing media such as nitric acid increases with the increase in the chromium water content in the steel. When the chromium content reaches a certain percentage, the corrosion resistance of the steel deteriorates. Mutation, that is, from easy to rust to not easy to rust, from not resistant to corrosion to corrosion resistant. The rustlessness of stainless steel is also related to the use environment. Different environments require the use of stainless steel with different chromium content. The level of chromium content is the fundamental factor that determines the performance of stainless steel. It is reported that the standards of European and American countries stipulate that the minimum chromium content cannot be less than 10.5%, Japan's stipulation is 11%, and our country's is 12%.


Stainless steel classification

There are five basic types of stainless steel: austenitic, ferritic, martensite, duplex stainless steel, and precipitation hardening stainless steel.

(1) Austenitic stainless steel is non-magnetic. The typical steel type is added with 18% chromium and contains a certain amount of nickel to increase corrosion resistance. They are widely used steel types.

(2) Ferrite is magnetic and its main content is chromium, with a proportion of 17%. This material has good oxidation resistance.

(3) Martensitic stainless steel is also magnetic. The chromium content is usually 13% and contains an appropriate proportion of carbon. They can be hardened through quenching and tempering.

(4) Duplex stainless steel has a mixed structure of ferrite and austenite. The chromium content is between 18% and 28%, and the nickel content is between 4.5% and 8%. They have great resistance to chloride corrosion. Good results.

(5) The conventional chromium content of precipitated stainless steel is 17, with a certain amount of nickel, copper and niobium added, which can be hardened through precipitation and aging.


According to the metallographic structure, it can be divided into:

(1) Ferritic stainless steel (400 series), which is chromium stainless steel, the main representatives are Gr13, G17, Gr27-30; (2) Austenitic stainless steel (300 series), chromium-nickel stainless steel, the main representatives are 304, 316, 321, etc.; ( 3) Martensitic stainless steel (200 series), chromium-manganese stainless steel, high carbon content, the main representatives are 1Gr13, etc.

Why stainless steel resists rust

Elements such as nickel and nickel are added to the surface of stainless steel to form an extremely thin, strong, fine, and stable chromium-rich oxide film (protective film), which prevents oxygen atoms from continuing to penetrate and oxidize, thereby gaining the ability to resist rust.

Why does stainless steel also rust?

Stainless steel will rust when the dense oxide protective layer is destroyed.

Under what conditions will the protective film be damaged?

1. Bleaching powder 2. Mechanical wear 3. Water

Stainless steel materials may rust due to the following reasons:

(1) Chloride ions exist in the use environment

Chloride ions exist widely, such as table salt/sweat stains/sea water/sea breeze/soil, etc. Stainless steel corrodes very quickly in the presence of chloride ions, even more than ordinary low carbon steel. Therefore, there are requirements for the environment in which stainless steel is used, and it needs to be wiped frequently.

Wipe to remove dust and keep clean and dry. (In this way, he can be classified as "improper use".) There is an example in the United States: a company uses an oak container to hold a solution containing chloride ions. The container has been used for nearly a hundred years and was planned to be replaced in the 1990s. , because the oak material was not modern enough, the container leaked due to corrosion 16 days after it was replaced with stainless steel.

(2) Without solid solution treatment

The alloy elements are not dissolved into the matrix, resulting in a low alloy content in the matrix structure and poor corrosion resistance.

(3) Natural intergranular corrosion

This titanium- and niobium-free material is prone to intergranular corrosion. Adding titanium and niobium, coupled with stabilization treatment, can reduce intergranular corrosion. A kind of high-alloy steel that can resist corrosion in the air or chemically corrosive media. Stainless steel has a beautiful surface and good corrosion resistance. It does not need to undergo surface treatment such as color plating, but can utilize the inherent surface properties of stainless steel and is used in many applications. A type of steel, often called stainless steel. Representative performance include 13 chromium steel, 18-8 chromium-nickel steel and other high alloy steels. From a metallographic point of view, because stainless steel contains chromium, a very thin chromium film is formed on the surface. This film isolates the oxygen that intrudes into the steel and plays a role in corrosion resistance. In order to maintain the inherent corrosion resistance of stainless steel, the steel must contain more than 12% chromium. Used where welding is required. The lower carbon content minimizes the precipitation of carbides in the heat-affected zone close to the weld, and the precipitation of carbides may cause intergranular corrosion (welding erosion) of stainless steel in certain environments. Because it will damage the surface of stainless steel and adhere to iron powder, causing rust.

In daily life, we sometimes find that the stainless steel in some flagpoles, bus shelters, light boxes and other facilities on the street has obvious rust and pickling. Since the stainless steel is passivated, why does it still rust? There are two reasons for these situations. First, the content of chromium in the material is low and it is inferior stainless steel. Second, it is not stainless steel at all, but uses electroplating to deceive users. It is understood that many decorative materials now use this electroplating process to treat their appearance. Since the material is ordinary steel, when the electroplated layer peels off, it will naturally rust.

Stainless steel has the ability to resist atmospheric oxidation---that is, stainless steel, and also has the ability to resist corrosion in media containing acids, alkali, and salt---that is, corrosion resistance. However, its corrosion resistance changes with the chemical composition of the steel itself, its additive state, usage conditions and environmental medium types. Like 304 steel pipes, in the atmosphere of dry cleansing, absolute good resistant to tarnishing ability is arranged. But if it is moved to riviera, in containing the sea fog of a large amount of salts, will soon get rusty; 316 steel pipes show excellent corrosion resistance. good. Therefore, not any kind of stainless steel can resist corrosion and rust in any environment.

Stainless steel relies on the formation of an extremely thin, strong, fine, and stable chromium-rich oxide film (protective film) on its surface to prevent the continued penetration and oxidation of oxygen atoms, thereby obtaining the ability to resist rust. Once this film is continuously damaged for some reason, oxygen atoms in the air or liquid will continue to penetrate or iron atoms in the metal will continue to separate out, forming loose iron oxide, and the metal surface will be continuously corroded. There are many ways in which this surface film is damaged, and the most common ones in daily life are as follows:

1. Dust containing other metal elements or attachments of heterogeneous metal particles accumulate on the surface of stainless steel. In humid air, the condensed water between the attachments and stainless steel connects the two into a micro-battery, triggering an electrochemical reaction. , the protective film is damaged, which is called electrochemical corrosion.

2. Organic juices (such as vegetables, noodle soup, phlegm, etc.) adhere to the surface of stainless steel. In the presence of water and oxygen, they form organic acids. For a long time, the organic acids will corrode the metal surface.

3. The surface of stainless steel contains acid, alkali, and salt substances (such as alkaline water and lime water splash on the wall decoration), causing local corrosion.

4. In polluted air (such as an atmosphere containing a large amount of sulfides, carbon oxides, and nitrogen oxides), when encountering condensed water, sulfuric acid, nitric acid, and acetic acid liquid points will form, causing chemical corrosion.

All of the above situations can cause damage to the protective film on the stainless steel surface and cause corrosion. Therefore, in order to ensure that the metal surface is permanently bright and will not be rusted, the following suggestions are recommended:

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

2. 316 stainless steel should be used in seaside areas. 316 material can resist seawater corrosion.

3. The chemical composition of some stainless steel pipes on the market cannot meet the corresponding national standards and cannot meet the 304 material requirements. Therefore, it will also cause rust, which requires users to carefully choose products from reputable manufacturers.

Why does stainless steel also rust?

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