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Does stainless steel rust?

Does stainless steel rust?

At first glance it seems like a false proposition. But life stainless steel rust everywhere, such as the picture below!

20210220024109 28091 - Does stainless steel rust?

Why does iron rust?

Iron exposed in the air, and oxygen and water chemical reaction, produce a variety of iron oxide, the specific performance is “rust”!
20210220024133 39717 - Does stainless steel rust?

What is stainless steel?

Has the ability to resist atmospheric oxidation, but also has the ability to resist corrosion in the medium containing acid, alkali, salt!

Introduction to stainless steel

Generally speaking, stainless steel is not easy to rust steel, in fact, a part of stainless steel, both rust, and acid resistance (corrosion resistance). Stainless steel does not rust and corrosion resistance is due to its rich chromium oxide film on the surface (passivation film), the formation of the membrane make metal dielectric isolation with the outside world, prevent the metal from being further corrosion, and the ability to repair itself, if once destroyed, the chromium in steel can with the oxygen in medium to regenerate the passivation membrane, continue to protection.
This rust resistance and corrosion resistance are opposite. The results show that the corrosion resistance of steel increases with the increase of chromium water content in weak media such as atmosphere, water and oxidizing media such as nitric acid. When the chromium content reaches a certain percentage, the corrosion resistance of steel changes from easy to rust, and from no corrosion resistance 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 level of chromium content is the fundamental factor to determine the performance of stainless steel. It is reported that European and American standards stipulate chromium content minimum cannot be less than 10.5%, Japan is 11%, China is 12%.

Classification of stainless steel

There are five basic types of stainless steels: austenite, ferrite, martensite, dual phase stainless steels, precipitated hardened stainless steels.

  • (1) austenitic stainless steel is not magnetic, the representative steel is added 18% chromium and contains a certain amount of nickel, to increase corrosion resistance, they are widely used steel.
  • (2) ferrite has magnetic, chromium element is the main content, the proportion of 17%, this material has a good oxidation resistance.
  • (3) martensitic stainless steel is also magnetic, the content of chromium is usually 13%, and contains a proper proportion of carbon, they 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%, the content of nickel is between 4.5% and 8%, they have a good effect against chloride erosion.
  • (5) the conventional content of precipitated stainless steel chromium is 17, and add a certain amount of nickel, copper and niobium, they can be hardened through precipitation and aging.

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

Why is stainless steel resistant to rust?

The surface of stainless steel added cro and nickel and other elements, the formation of a layer of very thin and solid fine stable chromium oxide film (protective film), to prevent the oxygen atom to continue to penetrate, continue to oxidation, and obtain the ability to resist corrosion.

Why does stainless steel rust?

When the surface of the stainless steel tube appears brown rust (point), people are surprised: “stainless steel is not rust, rust is not stainless steel, may be a problem with the steel”. 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 is liable to rust.

It also forms an oxide on the surface. At present, all stainless steel on the market is because of the rust mechanism of Cr element. The fundamental reason (mechanism) of corrosion resistance of stainless steel is passivation film theory. The so-called passivation film is in the surface of stainless steel with a layer of Cr2O3 based film. Because the existence of this film makes the stainless steel matrix in a variety of media corrosion is blocked, this phenomenon is called passivation. The formation of this passivation film has two kinds of situations, one is that stainless steel itself has the ability of self-passivation, the self-passivation ability with the increase of chromium content and speed up, so it has the rust resistance; Another widely formed condition is that stainless steel in a variety of aqueous solutions (electrolytes), in the process of corrosion formed passivation film and corrosion blocked. When the passivation film is damaged, a new passivation film can be formed immediately.
Stainless steel passivation film has the ability to resist corrosion, there are three characteristics: one is the passivation film thickness is very thin, under the condition of chromium content >10.5%, generally only a few microns; Second, the specific gravity of the passivation film is greater than the specific gravity of the matrix; These two characteristics indicate that the passivation film is thin and dense, so it is difficult for the passivation film to be broken down by the corrosive medium to rapidly corrode the substrate. The third characteristic is that the chromium concentration of the passivation film is more than three times higher than that of the substrate; Therefore, the passivation film has high corrosion resistance.

Under what conditions will the protective film be destroyed?

  • 1. Bleach
  • 2. Mechanical wear
  • 3. Water

Stainless steel under certain conditions, but also will be corroded.

The application environment of stainless steel is extremely complex, and simple chromium oxide passivation film can not meet the requirements of high corrosion resistance. Therefore, according to the different use conditions also need to add molybdenum (Mo), copper (Cu), nitrogen (N) and other elements in the steel, in order to improve the composition of the passivation film, further improve the corrosion resistance of stainless steel. The addition of Mo strongly promotes the collective passivation and prevents the corrosion of the matrix due to the proximity of the corroded product MoO2 to the matrix. Adding Cu makes the passivation film on the surface of stainless steel contain CuCl, because it does not interact with the corrosive medium, and the corrosion resistance is improved. When N is added, the passivation film enriches Cr2N, and the Cr concentration in the passivation film increases, thus improving the corrosion resistance of stainless steel.
The corrosion resistance of stainless steel is conditional, a grade of stainless steel in a certain medium is corrosion resistance, but in another medium may be destroyed. At the same time, the corrosion resistance of stainless steel is also relative, so far, there is no stainless steel in all environments are absolutely corrosion-free.
Stainless steel has the ability to resist atmospheric oxidation, that is, rust, but also has the ability to corrode in the medium containing acid, alkali and salt, that is, corrosion resistance. But the size of its corrosion resistance is with the steel itself chemical composition, the state of protection, the use of conditions and the type of environmental media and change. Such as 304 steel pipe, in the dry and clean atmosphere, there is absolutely excellent anti-rust ability, but it will be moved to the coastal areas, in the sea fog containing a lot of salt, will soon rust; The 316 steel tube performed well. Therefore, not any kind of stainless steel, in any environment can be corrosion resistant, not rust.

Type of stainless steel

Stainless steel is divided into many kinds, different attributes, different use environment.
For example: 304 steel pipe, in the dry and clean atmosphere, has absolutely excellent anti-rust ability, but it will be moved to the coastal area, in the sea fog containing a lot of salt, will soon rust; The 316 steel tube performed well.
According to the composition, it can be divided into Cr (SUS400), Cr-Ni (SUS300), Cr-Mn-Ni (SUS200) and precipitated hardening system (SUS600).

  • 200 series – chromium – nickel – manganese austenitic stainless steel.
  • 300 series – chromium – nickel austenitic stainless steel.
  • 301 — Good ductility for molding products. It can also be rapidly hardened by machining. Good weldability. The wear resistance and fatigue strength are better than 304 stainless steel.
  • 302 – Corrosion resistance is similar to 304, with better strength due to higher carbon content.
  • 303 — It is easier to cut than 304 by adding a small amount of sulfur and phosphorus.
  • 304 — 18/8 stainless steel. GB brand is 0CR18NI9.
  • 309 — Better temperature resistance than 304.
  • 316 — The second most widely used steel after 304, mainly used in the food industry and in surgical equipment, is added molybdenum to give it a special corrosion resistant structure. It is also used as “Marine steel” because it has better resistance to chloride corrosion than 304. SS316 is commonly used in nuclear fuel recovery plants. Class 18/10 stainless steels are usually qualified for this application.
  • 321 — Similar to 304 except that the risk of weld corrosion is reduced due to the addition of titanium.
  • 400 series – ferritic and martensitic stainless steels.
  • 408 — Good heat resistance, weak corrosion resistance, 11% Cr, 8% Ni.
  • 409 — The cheapest model (UK and US), commonly used as a car exhaust pipe, of ferritic stainless steel (chrome steel).
  • 410 — martensite (high strength chrome steel), good wear resistance, poor corrosion resistance.
  • 416 — The addition of sulfur improves the processability of the material.
  • 420 — “tool grade” martensitic steel, similar to the earliest stainless steels of Brinell high chromium steel. It’s also used for surgical cutlery, which can be made very bright.
  • 430 — Ferritic stainless steel, decorative, e.g. for automotive accessories. Good formability, but poor temperature resistance and corrosion resistance.
  • 440 — high strength cutting steel, slightly higher carbon content, after proper heat treatment can obtain a high yield strength, hardness up to 58HRC, is among the hardest stainless steels. The most common example of an application is the “razor blade”. There are three commonly used models: 440A, 440B, 440C, and 440F (easy to process).
  • 500 series – heat resistant chromium alloy steel.
  • 600 series – martensite precipitated hardened stainless steel.
  • 630 — The most commonly used precipitation hardened stainless steel model, also commonly known as 17-4; 4% 17% Cr, Ni.

Stainless steel material rust phenomenon, there may be the following reasons:

(1) The presence of chloride ions in the use environment

Chloride ions are found in a wide range of things, such as salt/sweat/sea water/sea breeze/soil etc. Stainless steel in the presence of chloride ions in the environment, corrosion quickly, even more than ordinary low carbon steel. Therefore, there are requirements for the use of stainless steel environment, and often need to wipe, remove dust, keep clean and dry. (That would give it a label of “misuse.”) There is an example in the United States: an enterprise uses an oak container to hold a solution containing chloride ions, the container has been used for more than 100 years, the 1990s plan to replace, because the oak material is not modern enough, after using stainless steel to replace the container 16 days because of corrosion leakage.

(2) without solid solution treatment

Alloying elements do not dissolve into the matrix, resulting in low alloy content in the matrix and poor corrosion resistance.

(3) Innate intergranular corrosion

This titanium – and niobium – free material is prone to intergranular corrosion. The addition of titanium and niobium, combined with stabilization treatment, can reduce intergranular corrosion. In the air or chemical corrosion medium can resist a kind of high alloy steel, stainless steel is a beautiful surface and good corrosion resistance, without plating color and other surface treatment, and play the inherent surface performance of stainless steel, used in a variety of steel, usually known as stainless steel. Represents the performance of 13 chromium steel, 18-8 chromium nickel steel and other high alloy steel. From the perspective of metallography analysis, because the stainless steel contains chromium and make the surface of the formation of a very thin chromium film, the film separated from the invasion of oxygen in the steel plays a role in corrosion resistance. In order to maintain the inherent corrosion resistance of stainless steel, the steel must contain at least 12% chromium. Used for welding applications. The lower carbon content minimizes the precipitation of carbides in the heat-affected zone near the weld, which may cause intergranular corrosion (welding erosion) of stainless steel in some environments. Because it will damage the surface of stainless steel, and attached to iron powder and the formation of rust.
In daily life, we sometimes find some street flagpoles, shelters, light boxes and other facilities of stainless steel rust pickling phenomenon, since it is stainless steel passivation, why also rust? These circumstances, there are two reasons, 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 with electroplating to deceive users. As we have learned, now a lot of decorative materials are processed with this kind of electroplating process appearance, because the material is general iron and steel, so when the electroplating peeling off, natural rust.
Stainless steel has the ability to resist atmospheric oxidation, that is, rust, but also has the ability to resist corrosion in the medium containing acid, alkali, salt, that is, corrosion resistance. But the size of its corrosion resistance is with the steel itself chemical composition, the state of interaction, the use of conditions and the type of environmental media and change. Such as 304 steel pipe, in the dry and clean atmosphere, there is absolutely excellent anti-rust ability, but it will be moved to the coastal areas, in the sea fog containing a lot of salt, will soon rust; The 316 steel tube performed well. Therefore, not any kind of stainless steel, in any environment can resist corrosion, not rust.
Stainless steel is formed on the surface of a layer of very thin and strong fine stable chromium oxide film (protective film), to prevent the oxygen atom to continue to penetrate, continue to oxidation, and obtain the ability to resist corrosion. Once for some reason, this film has been constantly destroyed, 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, metal surface will be constantly corroded. This kind of surface film is damaged in many forms, which are commonly seen in daily life as follows:

  • 1. Stainless steel surface deposited with dust containing other metal elements or similar metal particles of the attachment, in the humid air, the attachment and stainless steel condensate between the two together into a micro battery, triggered an electrochemical reaction, the protective film is damaged, called electrochemical corrosion.
  • 2. Stainless steel surface adhesion organic juice (such as vegetables, noodle soup, sputum, etc.), in the case of water and oxygen, constitute organic acids, long time is the corrosion of organic acids on the metal surface.
  • 3. Stainless steel surface adhesion contains acid, alkali, salt substances (such as decoration walls of alkali, limestone splashing), causing local corrosion.
  • 4. In the polluted air (such as containing a lot of sulfide, carbon oxide, nitrogen oxide atmosphere), condensate water, the formation of sulfuric acid, nitric acid, acetic acid liquid point, cause chemical corrosion.

The above circumstances can cause the destruction of the protective film on the surface of stainless steel corrosion. Therefore, in order to ensure that the metal surface is permanently bright and not corroded, the following suggestions are made:

  • 1. Must often clean and scrub the surface of decorative stainless steel, remove attachments, eliminate the external factors that cause modification.
  • 2. The seaside area to use 316 material stainless steel, 316 material can resist sea water corrosion.
  • 3. The chemical composition of some stainless steel pipes on the market can not meet the corresponding national standards, and can not meet the material requirements of 304. Therefore, it will also cause rust, which requires users to carefully choose the products of reputable manufacturers.

So there is no best stainless steel, only more suitable stainless steel.

Source: Network Arrangement – China Flanges Supplier: 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.)

If you want to have more information about the article or you want to share your opinion with us, contact us at sales@epowermetals.com

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