When the surface of the stainless steel pipe appears brown rust spots (spots), people are surprised: that "stainless steel is not rusty, rust is not stainless steel, there may be a problem with the steel."
In fact, this is a one-sided wrong view of the lack of understanding of stainless steel. Stainless steel will rust under certain conditions.
Stainless steel has the ability to resist atmospheric oxidation - that is, rust, but also has the ability to 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 protection 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 gen 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.
This surface film can be damaged in many ways.
There are several common ones in daily life:
1. The stainless steel surface contains dust or attachments containing other metal elements or exotic metal particles, in the humid air, the condensate between the attachments and stainless steel, the two are connected into a microbattery, causing an electrochemical reaction, the protective film is damaged, called electrochemical corrosion.
2. The surface of stainless steel adheres to organic juice (such as melons and vegetables, noodle soup, sputum, etc.), which constitutes organic acid in the case of water oxygen, and the organic acid corrods the metal surface for a long time.
3. Stainless steel surface adhesion containing acid, alkali, salt substances (such as alkaline water, lime water splash on the decoration wall), causing local corrosion.
4. In polluted air (such as the atmosphere containing a large number of sulfides, carbon oxide, nitrogen oxide), in case of condensation, sulfuric acid, nitric acid, acetic acid liquid point is formed, causing chemical corrosion. Tips To ensure that the metal surface is permanently bright and not corroded, we recommend:
1) The decorative stainless steel surface must be cleaned and scrubbed frequently to remove attachments and eliminate external factors that cause modification.
2) The seaside area should use 316 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.
Stainless Steel (Stainless Steel) is the abbreviation of stainless acid-resistant steel, resistant to air, steam, water and other weak corrosive media or stainless steel; The steel that is resistant to chemical corrosion media (acid, alkali, salt and other chemical etching) is called acid-resistant steel.
Common categories:
Usually divided by metallographic organization:
Generally, according to the metallographic organization, the ordinary stainless steel is divided into three categories: Austenitic stainless steel, ferritic stainless steel, martensitic stainless steel. On the basis of these three basic metallographic structures, for specific needs and purposes, and derived from the duplex steel, precipitation hardening stainless steel and iron content of less than 50% of the high alloy steel.
1, Austenitic stainless steel.
The matrix is mainly stainless steel with a face-centered cubic crystal structure of austenitic structure (CY phase), non-magnetic, mainly through cold working to strengthen it (and may result in a certain magnetic). The American Iron and Steel Institute uses 200 and 300 series numbers, such as 304.
2, iron type stainless steel.
The matrix is dominated by a ferrite structure (phase A) of body-centered cubic crystal structure, magnetic, generally can not be hardened by heat treatment, but cold working can make it slightly strengthened stainless steel. The American Iron and Steel Institute is marked 430 and 446.
3. Martensitic stainless steel
The matrix is martensitic structure (body-centered cubic or cubic), magnetic stainless steel, its mechanical properties can be adjusted by heat treatment. The American Iron and Steel Institute uses numbers 410, 420, and 440. Martensite has an austenitic structure at high temperatures, and when cooled to room temperature at an appropriate rate, the austenitic structure is able to convert to martensite (i.e., hardened).
4, austenitic ferritic (duplex) stainless steel.
The matrix has both austenitic and ferritic two-phase structure, of which the content of the less phase matrix is generally greater than 15%, is magnetic, and can be strengthened by cold working stainless steel, 329 is a typical duplex stainless steel. Compared with austenitic stainless steel, duplex steel has higher strength, higher resistance to intergranular corrosion, chloride stress corrosion and point corrosion.
5, precipitation hardening stainless steel.
Stainless steel whose matrix is austenitic or martensitic and can be hardened by precipitation hardening. The American Iron and Steel Institute uses 600 series numbers, such as 630, which is 17-4PH.
In general, in addition to alloys, the corrosion resistance of austenitic stainless steel is relatively excellent, in a less corrosive environment, ferritic stainless steel can be used, in a mildly corrosive environment, if the material is required to have high strength or high hardness, martensitic stainless steel and precipitation hardened stainless steel can be used.
Features and uses
Surface technology
What kind of stainless steel is not easy to rust?
There are three main factors affecting stainless steel corrosion:
1, the content of alloying elements.
Generally speaking, the content of chromium in 10.5% steel is not easy to rust. The higher the content of chromium nickel, the better the corrosion resistance, such as 304 material nickel content in 8-10%, chromium content reached 18-20%, such stainless steel in general circumstances will not rust.
2, the smelting process of the production enterprise will also affect the corrosion resistance of stainless steel.
Large stainless steel plants with good smelting technology, advanced equipment and advanced technology can be guaranteed whether in the control of alloying elements, the removal of impurities, and the control of billet cooling temperature, so the product quality is stable and reliable, the internal quality is good, and it is not easy to rust. On the contrary, some small steel mills are backward in equipment and technology, and impurities cannot be removed in the smelting process, and the products produced will inevitably rust.
3, the external environment, the climate is dry and ventilated environment is not easy to rust.
The air humidity is large, continuous rainy weather, or environmental areas with high pH in the air are easy to rust. 304 stainless steel, if the surrounding environment is too poor will rust.
Stainless steel appears rust spot how to deal with?
1. Chemical methods
Use pickling paste or spray to assist its rust parts to re-passivate to form chromium oxide film to restore its corrosion resistance. After pickling, in order to remove all contaminants and acid residues, it is very important to properly rinse with water. After all treatment, re-polish with polishing equipment, and seal with polishing wax. For local slight rust spots can also be used 1:1 gasoline, oil mixture with a clean cloth to wipe rust spots.
2. Mechanical method
Sand blasting, shot blasting with glass or ceramic particles, annihilation, brushing and polishing. With mechanical methods it is possible to erase contamination caused by previously removed, polished or annihilated materials. All kinds of pollution, especially foreign iron particles, can be a source of corrosion, especially in humid environments. Therefore, the mechanical cleaning surface should preferably be cleaned under dry conditions. The use of mechanical methods can only clean the surface, can not change the corrosion resistance of the material itself. Therefore, it is recommended to re-polish with polishing equipment after mechanical cleaning and seal with polishing wax.
Instrument commonly used stainless steel grade and performance
1, 304 stainless steel. It is one of the most widely used austenitic stainless steel, suitable for manufacturing deep drawing parts and acid pipelines, containers, structural parts, all kinds of instrument bodies, etc., and can also manufacture non-magnetic, low-temperature equipment and components.
2, 304L stainless steel. In order to solve the serious intergranular corrosion tendency of 304 stainless steel caused by Cr23C6 precipitation under some conditions, the sensitized state of the ultra-low carbon austenitic stainless steel has significantly better intergranular corrosion resistance than 304 stainless steel. In addition to slightly lower strength, other properties with 321 stainless steel, mainly used for corrosion resistant equipment and components that need to be welded and can not be treated with solid solution, can be used to manufacture all kinds of instrument bodies.
3, 304H stainless steel. The internal branch of 304 stainless steel, carbon mass fraction of 0.04%-0.10%, high temperature performance is better than 304 stainless steel.
4, 316 stainless steel. The addition of molybdenum on the basis of 10Cr18Ni12 steel makes the steel have good resistance to reducing media and point corrosion. In seawater and other media, corrosion resistance is better than 304 stainless steel, mainly used for pitting resistance materials.
5, 316L stainless steel. Ultra-low carbon steel, with good resistance to sensitized intergranular corrosion, suitable for the manufacture of welded parts and equipment with thick cross-section sizes, such as corrosion resistant materials in petrochemical equipment.
6, 316H stainless steel. 316 stainless steel internal branch, carbon mass fraction of 0.04%-0.10%, high temperature performance is better than 316 stainless steel.
7, 317 stainless steel. Pitting and creep resistance is better than 316L stainless steel, used in the manufacture of petrochemical and organic acid corrosion resistant equipment.
8, 321 stainless steel. Titanium stabilized austenitic stainless steel, adding titanium to improve intergranular corrosion resistance, and has good high temperature mechanical properties, can be replaced by ultra-low carbon austenitic stainless steel. It is not recommended in general except for special occasions such as high temperature or hydrogen corrosion resistance.
9, 347 stainless steel. Niobium stabilized austenitic stainless steel, adding niobium to improve intergranular corrosion resistance, corrosion resistance in acid, alkali, salt and other corrosive media with 321 stainless steel, good welding performance, can be used as corrosion resistant materials can be used as heat resistant steel, mainly used in thermal power, petrochemical fields, such as the production of containers, pipelines, heat exchangers, shafts, furnace tubes in industrial furnaces and furnace tube thermometers.
10, 904L stainless steel. Super complete austenitic stainless steel is a super austenitic stainless steel invented by OUTOKUMPU (Finland) company, whose nickel mass fraction is 24% to 26%, carbon mass fraction is less than 0.02%, excellent corrosion resistance, and has good corrosion resistance in non-oxidizing acids such as sulfuric acid, acetic acid, formic acid and phosphoric acid. At the same time, it has good resistance to crevice corrosion and stress corrosion. It is suitable for sulfuric acid of various concentrations below 70℃, and has good corrosion resistance to acetic acid and mixed acid of formic acid and acetic acid at any concentration and temperature under normal pressure. The original standard ASMESB-625 classified it as a nickel-based alloy, and the new standard classified it as stainless steel. China only similar grade 015Cr19Ni26Mo5Cu2 steel, a few European instrument manufacturers of the key material is 904L stainless steel, such as E+ H mass flow meter measurement tube is 904L stainless steel, Rolex watch case also uses 904L stainless steel.
11, 440C stainless steel. Martensitic stainless steel, the highest hardness in the hardenable stainless steel, stainless steel, hardness is HRC57. Mainly used to make nozzles, bearings, valve spool, seat, sleeve, stem and so on.
12, 17-4PH stainless steel. Martensitic precipitation-hardened stainless steel, HRC44, with high strength, hardness and corrosion resistance, cannot be used at temperatures higher than 300 ° C. It has good corrosion resistance to atmosphere and diluted acid or salt, and its corrosion resistance is the same as 304 stainless steel and 430 stainless steel, which is used to manufacture offshore platforms, turbine blades, valve spool, seat, sleeve, valve stem and so on.
In the instrument professional, combined with versatility and cost problems, the conventional austenitic stainless steel selection sequence is 304-304L-316-316L-317-321-347-904L stainless steel, of which 317 is less used, 321 is not recommended, 347 is used for high temperature corrosion resistance, 904L is only the default material for some components of individual manufacturers. The 904L is generally not actively selected in the design.
In the design and selection of the instrument, there will usually be occasions when the instrument material is different from the pipe material, especially in high temperature conditions, special attention should be paid to whether the choice of the instrument material meets the design temperature and design pressure of the process equipment or pipeline, such as the pipeline is high temperature chromium molybdenum steel, and the instrument chooses stainless steel, then there is likely to be a problem, you must go to the temperature and pressure meter of the relevant material.
In the instrument design and selection, often encounter a variety of different systems, series, grades of stainless steel, selection should be based on the specific process media, temperature, pressure, stress parts, corrosion, cost and other aspects of consideration.