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Causes and preventive measures for stainless steel rust

In layman’s terms, stainless steel is a steel that is not easily rusted. In fact, some stainless steels have both rust and acid resistance (corrosion resistance). The stainless steel’s rust and corrosion resistance is due to the formation of a chromium-rich oxide film (passivation film) on the surface that isolates the metal from the outside medium, prevents the metal from being further corroded, and has the ability to repair itself. If it is destroyed, the chromium in the steel will re-generate the passivation film with the oxygen in the medium and continue to protect.
This rust and corrosion resistance are relative. The test shows that the corrosion resistance of steel in the weak medium such as atmosphere and water and the oxidizing medium such as nitric acid increases with the increase of the water content of chromium in the steel. When the chromium content reaches a certain percentage, the corrosion resistance of the steel occurs. Mutations, from rust to rust, from corrosion to corrosion. The stainless steel is also related to the environment in which it is used. In different environments, stainless steel with different chromium content is used. The level of chromium is the fundamental factor determining the performance of stainless steel. It is reported that the standards of Europe and the United States and other countries stipulate that the minimum chromium content should not be less than 10.5%, the Japanese regulations are 11%, and the country is 12%.

Stainless steel classification

There are five basic types of stainless steel: austenite, ferrite, martensite, duplex stainless steel, precipitation hardened stainless steel.

  • (1) Austenitic stainless steels are not magnetic. Representative steel grades are 18% chromium and contain a certain amount of nickel to increase corrosion resistance. They are widely used steel grades.
  • (2) Ferrite is magnetic, and its main content is chrome, with a ratio of 17%. This material has good oxidation resistance.
  • (3) Martensitic stainless steels are also magnetic, typically having a chromium content of 13% and containing 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 are very resistant to chloride attack. Good results.
  • (5) Precipitated stainless steel chromium has a conventional content of 17, and a certain amount of nickel, copper and bismuth are added, which can be hardened by precipitation and aging.

According to the metallographic organization can be divided into:

Ferritic stainless steel (400 series), which is chrome stainless steel, mainly represented by Gr13, G17, Gr27-30; (2) austenitic stainless steel (300 series), chrome-nickel stainless steel, mainly representing 304, 316, 321, etc.; 3) Martensitic stainless steel (200 series), chrome-manganese stainless steel, high carbon content, mainly representing 1Gr13, etc.

Why does stainless steel rust?
Causes of rust: Chrome is a key stainless steel material that rusts, and may have the following reasons:

  • (1) The presence of chloride ions in the use environment is widespread, such as salt/sweat/seawater/sea breeze/soil. Stainless steel in the presence of chloride ions, corrosion is very fast, even more than ordinary low carbon steel. Therefore, there is a requirement for the use environment of stainless steel, and it is necessary to wipe frequently to remove dust and keep it clean and dry. (This would give him a “useless”.) There is an example in the United States: a company uses a oak container to hold a solution containing chloride ions that has been used for nearly a hundred years and is scheduled to be replaced in the 1990s. Because the oak material is not modern enough, the container leaks due to corrosion after 16 days of replacement with stainless steel.
  • (2) The alloying elements which have not been subjected to solution treatment are not dissolved in the matrix, resulting in a low content of the matrix structure alloy and poor corrosion resistance.
  • (3) Natural intergranular corrosion This material containing no titanium or antimony has a tendency to intergranular corrosion. The addition of titanium and niobium, together with a stable treatment, can reduce intergranular corrosion. A high-alloy steel that resists corrosion in air or chemically corrosive media. Stainless steel has an aesthetically pleasing surface and good corrosion resistance. It does not require surface treatment such as plating to give full surface properties to stainless steel. One of the aspects of steel, commonly referred to as stainless steel. Representative properties include high-alloy steels such as 13 chrome steel and 18-8 chrome nickel steel. From the perspective of metallography, because stainless steel contains chromium and the surface forms a very thin chromium film, this film is isolated from the intrusion of oxygen in the steel to resist corrosion. In order to maintain the corrosion resistance inherent in stainless steel, steel must contain more than 12% chromium. Used in applications where soldering is required. The lower carbon content minimizes the precipitation of carbides in the heat affected zone near the weld, which may result in intergranular corrosion (weld erosion) in certain environments. It causes damage to the stainless steel surface and attaches iron powder to cause rust.

In daily life, we sometimes find that the stainless steel of some flagpoles, bus shelters, light boxes and other facilities on the street have obvious rusting pickling phenomenon. Since it is stainless steel passivation, why is it rusting? There are two reasons for these situations. One is that the chromium content of the material is low and it belongs to inferior stainless steel. The second is not stainless steel at all, but the use of electroplating to deceive users. It is understood that many decorative materials are now treated with this plating process. Since the material is general steel, when the plating layer is peeled off, it will naturally rust.
Stainless steel has the ability to resist atmospheric oxidation – that is, rust, and also has the ability to resist corrosion in medium containing acid, alkali, and salt – that is, corrosion resistance. However, the magnitude of its corrosion resistance varies with the chemical composition of the steel itself, the state of addition, the conditions of use, and the type of environmental medium. For example, 304 steel pipe has absolutely excellent rust resistance in a dry and clean atmosphere, but it is moved to the coastal area, and it will soon rust in sea fog containing a lot of salt; while 316 steel pipe will perform. good. Therefore, it is not any kind of stainless steel, it can resist corrosion and rust in any environment.

Stainless steel is a very thin and strong and stable chromium-rich oxide film (protective film) formed on the surface to prevent the oxygen atoms from continuing to infiltrate and continue to oxidize, thereby obtaining the ability to resist rust. Once for some reason, the film is continually destroyed, oxygen atoms in the air or liquid will continue to infiltrate or iron atoms in the metal will be continuously separated, forming loose iron oxide, and the metal surface will be continuously rusted. 

There are many forms of such surface film damage, and the following are common in daily life:

  1. The surface of stainless steel contains dust or other metal particles attached to other metal elements. In humid air, the condensed water between the attached material and the stainless steel connects the two into a micro battery, triggering an electrochemical reaction. The protective film is damaged, which is called electrochemical corrosion.
  2. The surface of the stainless steel adheres to the organic juice (such as melon, noodle soup, wolfberry, etc.), in the case of water and oxygen, constitutes an organic acid, and the organic acid corrodes the metal surface for a long time.
  3. The surface of the stainless steel adheres to acid, alkali and salt substances (such as alkali water and lime water splashing on the wall), causing local corrosion.

In polluted air (such as the atmosphere containing a large amount of sulfide, carbon oxide, nitrogen oxide), in the case of condensed water, sulfuric acid, nitric acid, acetic acid liquid point is formed, causing chemical corrosion.
All of the above can cause the corrosion of the stainless steel surface protective film to cause rust. Therefore, in order to ensure that the metal surface is permanently bright and not rusted, the following suggestions:

  1. The surface of the decorative stainless steel must be cleaned and scrubbed frequently to remove the deposits and eliminate external factors that cause modification.
  2. The coastal area should use 316 stainless steel, 316 material can resist seawater corrosion.
  3. Some chemical components of stainless steel pipes on the market cannot meet the corresponding national standards and cannot meet the requirements of 304 materials. Therefore, it will also cause rust, which requires the user to carefully select the products of reputable manufacturers.

Source: China Stainless Steel Fitting Manufacturer – Yaang Pipe Industry Co., Limited (www.yaang.com)

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