The corrosion resistance of stainless steel corrosion-resistant centrifugal pumps 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 and cause corrosion of steel. The corrosion resistance of stainless steel corrosion-resistant centrifugal pumps in various environments is as follows:
①Atmospheric corrosion
The atmospheric corrosion resistance of stainless steel basically changes with the chloride content in the atmosphere. Therefore, the corrosion of stainless steel near the ocean or other chloride pollution sources is extremely important. A certain amount of rainwater is important only when it has an effect on 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 appearance. Therefore, stainless steel exposed for use in rural areas can be selected according to price, market supply, mechanical properties, manufacturing performance and appearance. Industrial environment In an industrial environment without chloride pollution, 1Cr17 and austenitic stainless steel can work for a long time, basically maintaining no rust, and may form a dirty film on the surface, but when the dirty film is removed, it still maintains the original Bright appearance. In an industrial environment with chlorides, it will cause stainless steel to rust. Marine environment 1Cr13 and 1 Cr 17 stainless steel will form a thin rust film in a short period of time, but will not cause significant dimensional changes. Austenitic stainless steels such as 1 Cr 17Ni7, 1 Cr 18Ni9 and 0 Cr 18Ni9 may show some corrosion when exposed to the marine environment. Rust is usually shallow and can be easily removed. 0 Cr 17 Ni 12M 02 Stainless steel chemical centrifugal pumps containing molybdenum are basically corrosion-resistant in the marine environment. In addition to atmospheric conditions, there are two other factors that affect the atmospheric corrosion resistance of stainless steel, namely the surface condition and manufacturing process. The finishing grade affects the corrosion resistance of stainless steel in a chloride environment. The matte surface (matte surface) is very sensitive to corrosion, that is, the normal industrial finishing surface is less sensitive to rust. The level of surface finishing also affects the removal of dirt and rust. It is easy to remove dirt and rust from a high-precision surface, but it is difficult to remove it from a matte surface. For matt surfaces, more frequent cleaning is required if the original surface condition is to be maintained.
②Fresh water
Fresh water can be defined as water that is sourced from rivers, lakes, ponds or wells regardless of acidity, saline or brackishness. The corrosivity of fresh water is affected by the pH value, oxygen content and fouling tendency of the water. The corrosiveness of scaled (hard) water is mainly determined by the amount and type of scale formed on the metal surface. The formation of this scale is a function of the minerals present in it and the temperature. Non-scaling (soft) water, which is generally more corrosive than hard water. The corrosiveness can be reduced by increasing the pH value or reducing the oxygen content. 1Cr13 stainless steel is obviously more resistant to fresh water corrosion than carbon steel, and it has excellent characteristics when used in fresh water. This steel is widely used for purposes such as docks and dams that require high strength and corrosion resistance. However, it should be considered that in some cases, 1Cr13 may be sensitive to moderate pitting corrosion in fresh water. However, pitting corrosion can be avoided by cathodic corrosion protection. 1Cr17 and austenitic stainless steels are almost completely resistant to fresh water corrosion at room temperature (ambient temperature).
③Acid water
Acidic water refers to the contaminated natural water leached from ore and coal. Because of its strong acidity, it is much more corrosive than natural fresh water. Due to the leaching effect of water on sulfide contained in ore and coal, acidic water usually contains a large amount of free sulfuric acid. 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 subjected to acidic water usually corrodes quickly. The results of tests with various materials affected by acid 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.
④Saline water
The corrosion characteristic of saline water is often in the form of pitting corrosion. For stainless steel, it is largely due to the partial destruction of the corrosion-resistant passive film caused by salty water. The other reason for the pitting corrosion of these steels is that Minghejie and other seawater organic substances attached to the stainless steel equipment can form oxygen concentration batteries. Once formed, these batteries are very active and cause a lot of corrosion and pitting. In the case of high-speed flow of saline water, such as the impeller of a pump, the corrosion of austenitic stainless steel is usually very small. For condensers using stainless steel tubes, the water flow rate must be kept above 1.5m/s to minimize the accumulation of seawater organic matter and other solids in the tube. For the structure of stainless steel equipment that handles saline water, it is best to reduce gaps and use thick-walled parts in the design.
⑤ Soil
The metal buried in the soil depends on the weather and other factors, and is in a complex state that changes at any time. Practice has proved that austenitic stainless steel generally has excellent resistance to corrosion in most soils, while 1Cr13 and 1Cr17 will produce pitting corrosion in many soils. 0 Cr 17Ni12Mо2 stainless steel is completely resistant to pitting corrosion in all soil tests.
⑥Nitric acid
Ferritic stainless steel and austenitic stainless steel with a chromium content of not less than 14% 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 performance, it has largely replaced 1Cr17 stainless steel in the above-mentioned applications. 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 temperature and concentration have a greater harmful effect on it. If the steel is not properly heat treated, hot nitric acid will cause intergranular corrosion of austenitic and ferritic stainless steels. Therefore, appropriate heat treatment can be used to prevent this type of corrosion, or use stainless steel resistant to this type of corrosion.
⑦Sulfuric acid
Standard stainless steel grades are rarely used in sulfuric acid solutions because of their narrow range of use. At room temperature, 0Cr17Ni12Mo2 stainless steel (the most standard grade resistant to sulfuric acid corrosion) is corrosion resistant when the sulfuric acid concentration is less than 15% or greater than 85%. However, in the higher concentration range, carbon steel is usually used. Martensitic and ferritic stainless steels are generally not resistant to corrosion by sulfuric acid solutions. As in the case of nitric acid, if the stainless steel is not properly heat-treated, sulfuric acid can cause intergranular corrosion. For welded structures that cannot be heat treated after welding, low-carbon grades of 00Cr19Ni10 or 00Cr17Ni14Mo2, or stabilized grades of 0Cr18Ni11Ti or 0Cr18Ni11Nb stainless steel should be used.
