Why does stainless steel rust?

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Why does stainless steel rust?

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

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Why does stainless steel rust?

[ Data:2024-05-17 View:2Times ]

Why does iron rust?


Iron exposed in the air, with oxygen and water chemical reaction, produce a variety of iron oxide, the specific performance is "rust"!


What is stainless steel?


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


Introduction to stainless steel


Generally speaking, stainless steel is not easy to rust steel, in fact, a part of stainless steel, both stainless, and acid resistance (corrosion resistance). Stainless steel rust and corrosion resistance is due to the formation of chromium-rich oxide film (passivation film) on its surface, this film makes the metal isolated from the external medium, prevent the metal from being further corroded, and has the ability to self-repair, if once damaged, the chromium in the steel and the oxygen in the medium re-passivation film, continue to play a protective role.


This rustlessness is relative to corrosion resistance. The test shows that the corrosion resistance of steel in the atmosphere, water and other weak medium and nitric acid and other oxidizing medium increases with the increase of chromium water content in steel, when the chromium content reaches a certain percentage, the corrosion resistance of steel mutates, that is, from easy to rust to not easy to rust, from corrosion resistance to corrosion resistance. Stainless steel stainless steel, 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 that determines the performance of stainless steel. It is reported that the European and American standards stipulate that the minimum chromium content cannot be less than 10.5%, Japan stipulates that 11%, and China is 12%.


Stainless steel classification


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


(1) Austenitic stainless steel is not magnetic, the representative steel grade is added 18% chromium and contains a certain amount of nickel to increase corrosion resistance, they are widely used steel grades.


(2) Ferrite is magnetic, chromium element is its main content, the proportion is 17%, this material has good oxidation resistance.


(3) Martensitic stainless steel is also magnetic, the content of chromium is usually 13%, and contains 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%, which has a good effect against the erosion of chloride.


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


According to the metallographic structure can be divided into:


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


Why does stainless steel resist rust


The stainless steel surface is added with elements such as chromium and nickel to form an extremely thin and solid stable chromium-rich oxide film (protective film) to prevent the continued infiltration and oxidation of oxygen atoms, and obtain the ability to resist corrosion.


Why does stainless steel rust?


Stainless steel rusts when the dense protective oxide layer is destroyed.


Under what conditions can the protective film be destroyed?


1.Bleach powder 2.mechanical wear 3.Water


Types of stainless steel


There are many kinds of stainless steel, different properties, and different use environments.


For example: 304 steel pipe, in a dry and clean atmosphere, has absolutely excellent corrosion resistance, but it is moved to the seaside area, in the sea fog containing a lot of salt, it will soon rust; The 316 steel pipe performed well.


According to the composition can be divided into Cr system (SUS400), Cr-Ni system (SUS300), CR-Mn-Ni (SUS200) and precipitation 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 quickly hardened by machining. Good weldability. The wear resistance and fatigue strength are better than 304 stainless steel.


302 - Corrosion resistance is the same as 304, and the strength is better because the carbon content is relatively high.


303 - By adding a small amount of sulfur, phosphorus to make it easier to cut than 304.


304 - i.e. 18/8 stainless steel. The GB brand is 0Cr18Ni9.


309 - has better temperature resistance than 304.


316 - After 304, the second most widely used steel grade, mainly used in the food industry and surgical equipment, adding molybdenum to make it obtain a special structure that is resistant to corrosion. Because it has better resistance to chloride corrosion than 304, it is also used as "Marine steel". SS316 is commonly used in nuclear fuel recovery units. Grade 18/10 stainless steel is also usually suitable for this application level.


321 - Other properties similar to 304 except that the addition of titanium reduces the risk of corrosion in the weld of the material.


400 Series - Ferritic and martensitic stainless steel.


408 - Good heat resistance, weak corrosion resistance, 11% Cr, 8% Ni.


409 - Cheapest model (Anglo-American), commonly used as an automobile exhaust pipe, ferritic stainless steel (chromium steel).


410 - Martensite (high strength chromium steel), good wear resistance, poor corrosion resistance.


416 - The addition of sulfur improves the processing properties of the material.


420 - "cutting tool grade" martensitic steel, similar to Brinell high chromium steel this earliest stainless steel. Also used in surgical knives, can be made very bright.


430 - Ferritic stainless steel, for decorative purposes, e.g. for car accessories. Good formability, but poor temperature resistance and corrosion resistance.


440 - high strength cutting tool steel, slightly higher carbon content, after appropriate heat treatment can obtain high yield strength, hardness can reach 58HRC, is one of the hardest stainless steel. The most common example is the "razor blade". There are three common models: 440A, 440B, 440C, and 440F (easy to process).


500 Series - Heat-resistant chrome alloy steel.


600 Series - Martensitic precipitation hardening stainless steel.


630 - The most commonly used precipitation hardened stainless steel model, usually called 17-4; 17%Cr, 4%Ni.


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


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


Chloride ions exist widely, such as salt/sweat/sea water/sea breeze/soil and so on. Stainless steel in the presence of chloride ions in the environment, corrosion quickly, even more than ordinary mild steel. Therefore, there are requirements for the use of stainless steel environment, and it needs to be wiped frequently


Wipe, remove dust, keep clean and dry. (That would give him a "misuse" designation.) There is an example in the United States: a company with an oak container containing a chloride ion solution, the container has been used for more than 100 years, the last century in the 1990s to replace, because the oak material is not modern enough, the use of stainless steel replacement 16 days after the container due to corrosion leakage.


(2) No solid solution treatment


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


(3) Innate intergranular corrosion


This material, which does not contain titanium and niobium, has a tendency to intergranular corrosion. The intergranular corrosion can be reduced by adding titanium and niobium and then stabilizing them. In the air or chemical corrosive medium can resist corrosion of a kind of high alloy steel, stainless steel is a beautiful surface and good corrosion resistance, do not have to go through plating and other surface treatment, and play the inherent surface properties of stainless steel, used in many aspects of steel, usually called stainless steel. Representative properties are 13 chromium steel, 18-8 chromium nickel steel and other high alloy steel. From the metallographic point of view, because stainless steel contains chromium and the surface of the formation of a very thin chromium film, this film is separated from the oxygen intrusion in the steel has a role in corrosion resistance. In order to maintain the corrosion resistance inherent in stainless steel, the steel must contain more than 12% chromium. For applications where welding is required. The lower carbon content minimizes the carbides precipitated in the heat-affected zone near the weld, which can lead to intergranular corrosion (welding erosion) in stainless steel in some environments. Because it will damage the surface of stainless steel, and attach iron powder and form rust.


In daily life, we sometimes find that the stainless steel of some flagpoles, bus shelters, light boxes and other facilities on the street has obvious rust pickling phenomenon, since it is stainless steel passivation, why will it rust? There are two reasons for these situations, one is the low content of chromium in the material, which is inferior stainless steel. The second is that it is not stainless steel at all, but uses electroplating to deceive users. It is understood that many decorative materials are treated with this plating process, because the material is general steel, so when the plating layer is peeled off, it will rust naturally.


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. However, the size of its corrosion resistance is changed with the chemical composition of the steel itself, the mixing state, the use conditions and the type of environmental media. Such as 304 steel pipe, in a dry and clean atmosphere, has absolutely excellent corrosion resistance, but it is moved to the seaside area, in the sea fog containing a lot of salt, it will soon rust; The 316 steel pipe performed well. Therefore, not any kind of stainless steel, in any environment can resist corrosion, does not rust.


Stainless steel is formed on its surface by a layer of extremely thin and solid fine stable chromium-rich oxide film (protective film), to prevent the continued infiltration of oxygen atoms, continue to oxidize, and obtain the ability to resist corrosion. Once for some reason, this film is constantly destroyed, the oxygen atoms in the air or liquid will continue to penetrate or the iron atoms in the metal will continue to be resolved out, forming loose iron oxide, and the metal surface will be constantly corroded. There are many forms of damage to this surface film, which are commonly seen in daily life as follows:


1, the surface of stainless steel contains other metal elements of dust or exotic metal particles of the attachment, in the humid air, the attachment and stainless steel condensate between the two into a microbattery, triggered an electrochemical reaction, the protective film is damaged, called electrochemical corrosion.


2, the surface of stainless steel adhesion organic juice (such as melons and vegetables, noodle soup, sputum, etc.), in the case of water oxygen, constitute organic acids, long-term corrosion of organic acids on the metal surface.


3, stainless steel surface adhesion containing acid, alkali, salt substances (such as the decoration of the wall of soda, lime water splash), causing local corrosion.


4, in the polluted air (such as containing a large number of sulfide, carbon oxide, nitrogen oxide atmosphere), in the case of condensate, the formation of sulfuric acid, nitric acid, acetic acid liquid point, causing chemical corrosion.


The above conditions can cause the damage of the stainless steel surface protective film and lead to corrosion. Therefore, in order to ensure that the metal surface is permanently bright and not corroded, the following recommendations are made:


1, it is necessary to clean and scrub the decorative stainless steel surface frequently, remove attachments, and eliminate external factors that cause modification.


2, the seaside area to use 316 material stainless steel, 316 material can resist seawater 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 304 material requirements. 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!


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