Test you: The corrosion resistance principle of stainless steel

This post was last edited by mopeizhi on 2010-2-7 13:55. The corrosion resistance of stainless steel mainly comes from chromium. Experiments have shown that the corrosion resistance of steel will only improve when the chromium content exceeds 12%. * * Therefore, the chromium content in stainless steel is generally not less than 12%. Due to the increase in chromium content, it also has a great impact on the structure of steel. When the chromium content is high and the carbon content is small, chromium will balance the iron and carbon, and the Y phase area in the picture will shrink or even disappear. This kind of stainless steel has a ferrite structure and does not undergo phase change when heated. It is called ferritic stainless steel. When the chromium content is low (but higher than 12%) and the carbon content is high, the alloy can easily form martensite when cooled from high temperature, so this type of steel is called martensitic stainless steel. Nickel can expand the Y phase zone and make the steel have an austenitic structure. If the nickel content is sufficient to make the steel have an austenitic structure at room temperature, the steel is called austenitic stainless steel.

This post was last edited by mopeizhi on 2010-2-7 13:57 All metals react with oxygen in the atmosphere to form an oxide film on the surface. Unfortunately, the iron oxide formed on ordinary carbon steel continues to oxidize, allowing the rust to expand and eventually form holes. Carbon steel surfaces can be protected by electroplating with paint or oxidation-resistant metals (such as zinc, nickel, and chromium), but, as we all know, this protection is only a thin film. If the protective layer is damaged, the steel underneath begins to rust. The corrosion resistance of stainless steel depends on chromium, but because chromium is one of the components of steel, the protection methods are different. When the added amount of chromium reaches 10.5%, the atmospheric corrosion resistance of steel increases significantly. However, when the chromium content is higher, although the corrosion resistance can still be improved, it is not obvious. The reason is that alloying steel with chromium changes the type of surface oxide to one similar to that formed on pure chromium metal. This tightly adherent chromium-rich oxide protects the surface from further oxidation. This kind of extremely thin oxide layer, and the natural luster of the steel surface can be seen through it, giving the stainless steel a unique surface. Moreover, if the surface layer is damaged, the exposed steel surface will react with the atmosphere to repair itself, and re-form the oxide "passivation film" to continue to protect. Therefore, all stainless steel elements have a common characteristic, that is, the chromium content is above 10.5%.

This post was last edited by mopeizhi on 2010-2-7 13:58 The stainless property of stainless steel is due to the special passivation protective film on the surface of the steel plate. The so-called passivation film is a thin film mainly composed of Cr2O3 on the surface of stainless steel. Due to the existence of this film, the corrosion of the stainless steel matrix in various media is blocked. This phenomenon is called passivation. There are two situations for the formation of this passive film. One is that stainless steel itself has the ability to self-passivate, and this self-passivation ability accelerates with the increase of chromium content. Another common formation condition is that stainless steel forms a passivation film during the corrosion process in various aqueous solutions (electrolytes), which blocks corrosion.

This post was last edited by mopeizhi on 2010-2-7 14:02 The corrosion resistance of stainless steel mainly comes from chromium. Experiments have shown that the corrosion resistance of steel will only improve when the chromium content exceeds 12%. * * Therefore, the chromium content in stainless steel is generally not less than 12%. Due to the increase in chromium content, it also has a great impact on the structure of steel. When the chromium content is high and the carbon content is small, chromium will balance the iron and carbon, and the Y phase area in the picture will shrink or even disappear. This kind of stainless steel has a ferrite structure and does not undergo phase change when heated. It is called ferritic stainless steel. When the chromium content is low (but higher than 12%) and the carbon content is high, the alloy can easily form martensite when cooled from high temperature, so this type of steel is called martensitic stainless steel. Nickel can expand the Y phase zone and make the steel have an austenitic structure. If the nickel content is sufficient to make the steel have an austenitic structure at room temperature, the steel is called austenitic stainless steel.

This post was last edited by mopeizhi on 2010-2-7 14:04 Metals react with oxygen in the atmosphere to form an oxide film on the surface. Unfortunately, the iron oxide formed on ordinary carbon steel continues to oxidize, allowing the rust to expand and eventually form holes. Carbon steel surfaces can be protected by electroplating with paint or oxidation-resistant metals (such as zinc, nickel, and chromium), but, as we all know, this protection is only a thin film. If the protective layer is damaged, the steel underneath begins to rust. 　　The corrosion resistance of stainless steel depends on chromium, but because chromium is one of the components of steel, the protection methods are different. 　　When the added amount of chromium reaches 10.5%, the atmospheric corrosion resistance of steel increases significantly. However, when the chromium content is higher, although the corrosion resistance can still be improved, it is not obvious. The reason is that alloying steel with chromium changes the type of surface oxide to one similar to that formed on pure chromium metal. This tightly adherent chromium-rich oxide protects the surface from further oxidation. This kind of extremely thin oxide layer, and the natural luster of the steel surface can be seen through it, giving the stainless steel a unique surface. Moreover, if the surface layer is damaged, the exposed steel surface will react with the atmosphere to repair itself, and re-form the oxide "passivation film" to continue to protect. 　　Therefore, all stainless steel elements have a common characteristic, that is, the chromium content is above 10.5%.

This post was last edited by mopeizhi on 2010-2-7 14:05 The corrosion resistance of stainless steel generally increases with the increase in chromium content. The basic principle is that when there is enough chromium in the steel, a very thin and dense oxide film is formed on the surface of the steel, which can prevent further oxidation or corrosion. An oxidizing environment can strengthen this film, while a reducing environment will inevitably destroy this film, causing corrosion of steel. (1) Corrosion resistance in various environments ① Atmospheric corrosion The atmospheric corrosion resistance of stainless steel basically changes with the chloride content in the atmosphere. Therefore, proximity to the ocean or other sources of chloride contamination is extremely important to the corrosion of stainless steel. A certain amount of rainwater is only important if it affects the chloride concentration on the steel surface. Rural environment 1Cr13, 1Cr17 and austenitic stainless steel can be adapted to various uses without significant changes in their appearance. Therefore, the stainless steel used in rural areas can be selected based on price, market supply, mechanical properties, manufacturing and processing performance and appearance. Industrial environment In an industrial environment without chloride pollution, 1Cr17 and austenitic stainless steel can work for a long time and basically remain rust-free. A foul film may form on the surface, but after the foul film is removed, it still maintains its original bright appearance. In industrial environments with chlorides, stainless steel will corrode. Marine environment 1Cr13 and 1Cr17 stainless steel will form a thin rust film in a short period of time, but will not cause obvious dimensional changes. Austenitic stainless steels such as 1 Cr 17Ni7, 1 Cr 18Ni9 and 0 Cr 18Ni9 may experience some rust when exposed to marine environments. Rust is usually superficial and can be easily removed. 0 Cr 17 Ni 12M 02 Molybdenum-containing stainless steel is basically corrosion-resistant in marine environments. In addition to atmospheric conditions, there are two other factors that affect the atmospheric corrosion resistance of stainless steel, namely surface condition and manufacturing process. The level of finishing affects the corrosion resistance of stainless steel in chloride environments. Matt surfaces (matte surfaces) are very sensitive to corrosion, i.e. normal industrially finished surfaces are less sensitive to rust. The level of surface finish also affects the removal of dirt and rust. It is easy to remove dirt and rust from highly finished surfaces, but difficult to remove from matte surfaces. For matte surfaces, more frequent cleaning is required if the original surface condition is to be maintained. ②Freshwater Freshwater can be defined as water, whether acidic, salty or brackish, originating from rivers, lakes, ponds or wells. The corrosiveness of fresh water is affected by the pH, oxygen content and scaling tendency of the water. The corrosiveness of scaled (hard) water is primarily determined by the amount and type of scale that forms on metal surfaces. The formation of this scale is a function of the minerals present and temperature. Non-scaling (soft) water, which is generally more corrosive than hard water. Its corrosiveness can be reduced by increasing the pH or reducing the oxygen content. 1Cr13 stainless steel is obviously more resistant to fresh water corrosion than carbon steel, and has excellent characteristics when used in fresh water. This steel is widely used for purposes such as docks and dams where high strength and corrosion resistance are required. However, it should be taken into account that in some cases 1Cr13 may be susceptible to moderate pitting corrosion in fresh water. However, pitting corrosion can be completely avoided using cathodic anti-corrosion methods. 1Cr17 and austenitic stainless steel are almost completely resistant to freshwater corrosion at room temperature (ambient temperature). ③Acid water Acid water refers to polluted natural water leached from ore and coal. Because it is highly acidic, it is much more corrosive than natural fresh water. Acidic water usually contains large amounts of free sulfuric acid due to the leaching effect of water on sulfides contained in ores and coals. In addition, this water contains a large amount of iron sulfate, which has a great effect on the corrosion of carbon steel. Carbon steel equipment exposed to acidic water often corrodes quickly. The results of tests using various materials affected by acidic river water show that austenitic stainless steel has higher corrosion resistance in this environment. Austenitic stainless steel has excellent corrosion resistance in fresh water and acidic river water, especially its corrosion film has less hindrance to heat conduction. Therefore, stainless steel tubes are widely used in heat exchange applications. ④Salt water The corrosion characteristics of salt water often appear in the form of pitting corrosion. For stainless steel, it is largely due to the local destruction of the passive film that plays a corrosion-resistant role due to salt water. Other causes of pitting corrosion in these steels are concentration cells of oxygen that can be formed by chlorine and other seawater organic matter attached to the stainless steel equipment. Once formed, these cells are very active and cause a lot of corrosion and pitting. In situations where salt water flows at high speeds, such as pump impellers, corrosion of austenitic stainless steels is usually very minimal. For condensers using stainless steel tubes, the water flow rate must be maintained greater than 1.5m/s to minimize the accumulation of seawater organic matter and other solids in the tubes. The structure of stainless steel equipment handling saline water is best designed to reduce gaps and use thick-walled components. ⑤Soil Metals embedded in the soil are in a complex state that changes at any time, depending on weather and other factors. Practice has proven that austenitic stainless steel generally has excellent resistance to corrosion in most soils, while 1Cr13 and 1Cr17 will cause pitting corrosion in many soils. 0 Cr 17Ni12Mо2 stainless steel is fully resistant to pitting corrosion in all soil tests. ⑥Ferritic stainless steel and austenitic stainless steel with nitric acid containing no less than 14% chromium have excellent resistance to nitric acid corrosion. 1Cr17 stainless steel has been widely used in processing equipment in nitric acid plants. However, since 0 Cr 18 Ni 9 generally has better formability and welding properties, it has largely replaced 1Cr17 stainless steel in the above uses. The nitric acid corrosion resistance of other austenitic stainless steels is similar to that of 0 Cr 18 Ni 9. 1Cr17 stainless steel generally has a slightly higher corrosion rate than 0 Cr 18 Ni 9, and higher temperatures and concentrations have a greater detrimental effect on it. Hot nitric acid will cause intergranular corrosion in austenitic and ferritic stainless steels if the steel is not heat treated properly. Therefore, this type of corrosion can be prevented by appropriate heat treatment, or by using stainless steel that is resistant to this type of corrosion. ⑦Sulfuric acid standard stainless steel grades are rarely used in sulfuric acid solutions because their usable range is very narrow. At room temperature, 0Cr17Ni12Mo2 stainless steel (the standard grade most resistant to sulfuric acid corrosion) is corrosion resistant when the sulfuric acid concentration is less than 15% or greater than 85%. In the higher concentration ranges however, carbon steel is usually used. Martensitic and ferritic stainless steels are generally not resistant to corrosion by sulfuric acid solutions. As is the case with nitric acid, sulfuric acid can cause intergranular corrosion if stainless steel is not properly heat treated. For welded structures that cannot be heat treated after welding, low carbon grades 00Cr19Ni10 or 00Cr17Ni14Mo2, or stabilized grades 0Cr18Ni11Ti or 0Cr18Ni11Nb stainless steel should be used. 1#inlegend

This post was last edited by mopeizhi on 2010-2-7 14:06 1) Add alloying elements to increase the electrode potential of the steel matrix, thereby improving the steel's resistance to electrochemical corrosion. Generally, adding Cr, Ni, and Si elements to steel can increase its electrode potential. Due to the lack of Ni, the addition of a large amount of Si will make the steel brittle. Therefore, only Cr is the element commonly used to significantly increase the electrode potential of the steel matrix. (2) Add alloying elements to form a stable, complete purification film on the surface of the steel that is firmly combined with the steel matrix. Thereby improving the chemical corrosion resistance of steel. For example, adding Cr, Si.Al and other alloying elements to steel can form a dense Cr2O3, SiO2, Al2O3 and other oxide films on the surface of the steel, which can improve the corrosion resistance of the steel. (3) Adding alloying elements enables steel to exist in a single-phase state at room temperature, reducing the number of micro-batteries and thereby improving the corrosion resistance of steel. If a sufficient amount of Cr or Cr-Ni is added, the steel will obtain single-phase ferrite or single-phase austenite at room temperature. (4) Add elements such as Mo and Cu to improve corrosion resistance (5) Add elements such as Ti and Nb to eliminate intergranular segregation of Cr, thereby reducing the tendency of intergranular corrosion (6) Add elements such as Mn and N to replace part of Ni to obtain a single-phase austenite structure, and at the same time * * Improve the corrosion resistance of chromium stainless steel in organic acids.

This post was last edited by mopeizhi on 2010-2-7 14:09. The corrosion resistance of stainless steel mainly comes from chromium. Experiments have shown that the corrosion resistance of steel will only improve when the chromium content exceeds 12%. * * Therefore, the chromium content in stainless steel is generally not less than 12%. Due to the increase in chromium content, it also has a great impact on the structure of steel. When the chromium content is high and the carbon content is small, chromium will balance the iron and carbon, and its Y phase area will shrink or even disappear. This kind of stainless steel has a ferrite structure and does not undergo phase change when heated. It is called ferritic stainless steel. When the chromium content is low (but higher than 12%) and the carbon content is high, the alloy can easily form martensite when cooled from high temperature, so this type of steel is called martensitic stainless steel. Nickel can expand the Y phase zone and make the steel have an austenitic structure. If the nickel content is sufficient to make the steel have an austenitic structure at room temperature, the steel is called austenitic stainless steel. Stainless acid-resistant steel is referred to as stainless steel. It is composed of two parts: stainless steel and acid-resistant steel. In short, steel that can resist atmospheric corrosion is called stainless steel, while steel that can resist chemical medium corrosion is called acid-resistant steel. Generally speaking, steel with a Wcr content greater than 12% has the characteristics of stainless steel. Stainless steel can be divided into five categories according to its microstructure after heat treatment.: That is, ferritic stainless steel, martensitic stainless steel, austenitic stainless steel, austenitic-ferritic stainless steel and precipitation carbonized stainless steel. All metals react with oxygen in the atmosphere to form an oxide film on the surface. Unfortunately, the iron oxide formed on ordinary carbon steel continues to oxidize, allowing the rust to expand and eventually form holes. Carbon steel surfaces can be protected by electroplating with paint or oxidation-resistant metals (such as zinc, nickel, and chromium), but, as we all know, this protection is only a thin film. If the protective layer is damaged, the steel underneath begins to rust. 　　The corrosion resistance of stainless steel depends on chromium, but because chromium is one of the components of steel, the protection methods are different. 　　When the added amount of chromium reaches 10.5%, the atmospheric corrosion resistance of steel increases significantly. However, when the chromium content is higher, although the corrosion resistance can still be improved, it is not obvious. The reason is that alloying steel with chromium changes the type of surface oxide to one similar to that formed on pure chromium metal. This tightly adherent chromium-rich oxide protects the surface from further oxidation. This kind of extremely thin oxide layer, and the natural luster of the steel surface can be seen through it, giving the stainless steel a unique surface. Moreover, if the surface layer is damaged, the exposed steel surface will react with the atmosphere to repair itself, and re-form the oxide "passivation film" to continue to protect. 　　Therefore, all stainless steel elements have a common characteristic, that is, the chromium content is above 10.5%.

This post was last edited by mopeizhi on 2010-2-7 14:10 Stainless steel is an alloy steel with chromium as the main element and other elements added. The higher the chromium content, the better the corrosion resistance of the steel. In addition to chromium, stainless steel also contains nickel, manganese, titanium, silicon and other elements, which can affect the strength, plasticity, toughness and corrosion of stainless steel. The principle of corrosion resistance of stainless steel is that because chromium is more active than iron, in stainless steel, chromium first reacts with oxygen to form an oxide passivation film that is firmly bonded to the steel substrate. It protects alloy steel. Will not cause rust. According to different corrosion characteristics, it is divided into two categories: ordinary stainless steel and acid-resistant stainless steel.

The corrosion resistance of stainless steel generally increases with the increase of chromium content. The basic principle is that when there is enough chromium in the steel, a very thin and dense oxide film is formed on the surface of the steel, which can prevent further oxidation or corrosion. An oxidizing environment can strengthen this film, while a reducing environment will inevitably destroy this film, causing corrosion of steel. (1) Corrosion resistance in various environments ① Atmospheric corrosion The atmospheric corrosion resistance of stainless steel basically changes with the chloride content in the atmosphere. Therefore, proximity to the ocean or other sources of chloride contamination is extremely important to the corrosion of stainless steel. A certain amount of rainwater is only important if it affects the chloride concentration on the steel surface. Rural environment 1Cr13, 1Cr17 and austenitic stainless steel can be adapted to various uses without significant changes in their appearance. Therefore, the stainless steel used in rural areas can be selected based on price, market supply, mechanical properties, manufacturing and processing performance and appearance. Industrial environment In an industrial environment without chloride pollution, 1Cr17 and austenitic stainless steel can work for a long time and basically remain rust-free. A foul film may form on the surface, but after the foul film is removed, it still maintains its original bright appearance. In industrial environments with chlorides, stainless steel will corrode. Marine environment 1Cr13 and 1Cr17 stainless steel will form a thin rust film in a short period of time, but will not cause obvious dimensional changes. Austenitic stainless steels such as 1 Cr 17Ni7, 1 Cr 18Ni9 and 0 Cr 18Ni9 may experience some rust when exposed to marine environments. Rust is usually superficial and can be easily removed. 0 Cr 17 Ni 12M 02 Molybdenum-containing stainless steel is basically corrosion-resistant in marine environments. In addition to atmospheric conditions, there are two other factors that affect the atmospheric corrosion resistance of stainless steel, namely surface condition and manufacturing process. The level of finishing affects the corrosion resistance of stainless steel in chloride environments. Matt surfaces (matte surfaces) are very sensitive to corrosion, i.e. normal industrially finished surfaces are less sensitive to rust. The level of surface finish also affects the removal of dirt and rust. It is easy to remove dirt and rust from highly finished surfaces, but difficult to remove from matte surfaces. For matte surfaces, more frequent cleaning is required if the original surface condition is to be maintained. ②Freshwater Freshwater can be defined as water, whether acidic, salty or brackish, originating from rivers, lakes, ponds or wells. The corrosiveness of fresh water is affected by the pH, oxygen content and scaling tendency of the water. The corrosiveness of scaled (hard) water is primarily determined by the amount and type of scale that forms on metal surfaces. The formation of this scale is a function of the minerals present and temperature. Non-scaling (soft) water, which is generally more corrosive than hard water. Its corrosiveness can be reduced by increasing the pH or reducing the oxygen content. 1Cr13 stainless steel is obviously more resistant to fresh water corrosion than carbon steel, and has excellent characteristics when used in fresh water. This steel is widely used for purposes such as docks and dams where high strength and corrosion resistance are required. However, it should be taken into account that in some cases 1Cr13 may be susceptible to moderate pitting corrosion in fresh water. However, pitting corrosion can be completely avoided using cathodic anti-corrosion methods. 1Cr17 and austenitic stainless steel are almost completely resistant to freshwater corrosion at room temperature (ambient temperature). ③Acid water Acid water refers to polluted natural water leached from ore and coal. Because it is highly acidic, it is much more corrosive than natural fresh water. Acidic water usually contains large amounts of free sulfuric acid due to the leaching effect of water on sulfides contained in ores and coals. In addition, this water contains a large amount of iron sulfate, which has a great effect on the corrosion of carbon steel. Carbon steel equipment exposed to acidic water often corrodes quickly. The results of tests using various materials affected by acidic river water show that austenitic stainless steel has higher corrosion resistance in this environment. Austenitic stainless steel has excellent corrosion resistance in fresh water and acidic river water, especially its corrosion film has less hindrance to heat conduction. Therefore, stainless steel tubes are widely used in heat exchange applications. ④Salt water The corrosion characteristics of salt water often appear in the form of pitting corrosion. For stainless steel, it is largely due to the local destruction of the passive film that plays a corrosion-resistant role due to salt water. Other causes of pitting corrosion in these steels are concentration cells of oxygen that can be formed by chlorine and other seawater organic matter attached to the stainless steel equipment. Once formed, these cells are very active and cause a lot of corrosion and pitting. In situations where salt water flows at high speeds, such as pump impellers, corrosion of austenitic stainless steels is usually very minimal. For condensers using stainless steel tubes, the water flow rate must be maintained greater than 1.5m/s to minimize the accumulation of seawater organic matter and other solids in the tubes. The structure of stainless steel equipment handling saline water is best designed to reduce gaps and use thick-walled components. ⑤Soil Metals embedded in the soil are in a complex state that changes at any time, depending on weather and other factors. Practice has proven that austenitic stainless steel generally has excellent resistance to corrosion in most soils, while 1Cr13 and 1Cr17 will cause pitting corrosion in many soils. 0 Cr 17Ni12Mо2 stainless steel is fully resistant to pitting corrosion in all soil tests. ⑥Ferritic stainless steel and austenitic stainless steel with nitric acid containing no less than 14% chromium have excellent resistance to nitric acid corrosion. 1Cr17 stainless steel has been widely used in processing equipment in nitric acid plants. However, since 0 Cr 18 Ni 9 generally has better formability and welding properties, it has largely replaced 1Cr17 stainless steel in the above uses. The nitric acid corrosion resistance of other austenitic stainless steels is similar to that of 0 Cr 18 Ni 9. 1Cr17 stainless steel generally has a slightly higher corrosion rate than 0 Cr 18 Ni 9, and higher temperatures and concentrations have a greater detrimental effect on it. Hot nitric acid will cause intergranular corrosion in austenitic and ferritic stainless steels if the steel is not heat treated properly. Therefore, this type of corrosion can be prevented by appropriate heat treatment, or by using stainless steel that is resistant to this type of corrosion. ⑦Sulfuric acid standard stainless steel grades are rarely used in sulfuric acid solutions because their usable range is very narrow. At room temperature, 0Cr17Ni12Mo2 stainless steel (the standard grade most resistant to sulfuric acid corrosion) is corrosion resistant when the sulfuric acid concentration is less than 15% or greater than 85%. In the higher concentration ranges however, carbon steel is usually used. Martensitic and ferritic stainless steels are generally not resistant to corrosion by sulfuric acid solutions. As is the case with nitric acid, sulfuric acid can cause intergranular corrosion if stainless steel is not properly heat treated.