Corrosion Resistance Of Stainless Steel

　　The corrosion resistance of stainless steel is generally improved with the increase of chromium content, the basic principle is that when there is enough chromium in the steel, a very thin dense oxide film is formed on the surface of the steel, which can prevent further oxidation or corrosion. Oxidizing environment can strengthen this film, and reducing environment will inevitably destroy this film, resulting in corrosion of steel.

　　Corrosion resistance in a variety of environments

　　① Atmospheric corrosion

　　The atmospheric corrosion resistance of stainless steel is basically changed with the content of chloride in the atmosphere. Therefore, proximity to the ocean or other chloride sources is extremely important for corrosion of stainless steel. A certain amount of rain is important only if it has an effect on the chloride concentration on the steel surface.

　　Rural environment 1Cr13. 1Cr 17 and Austenitic stainless steels can be adapted to a variety of uses without significant changes in appearance. Therefore, the stainless steel used in rural exposure can be selected according to price, market supply, mechanical properties, production processing properties and appearance.

　　Industrial environment In an industrial environment without chloride pollution, 1Cr17 and Austenitic stainless steel can work for a long time, basically maintain no rust, may form a dirty film on the surface, but when the dirty film is removed, it still maintains the original bright appearance. In the industrial environment with chloride, it will cause corrosion of stainless steel.

　　Marine environment 1Cr13 and 1Cr 17 stainless steel will form a thin rust film in a short period of time, but will not cause significant changes in size. Austenitic stainless steels such as 1 Cr 17Ni7. 1 Cr 18Ni9 and 0 Cr 18Ni9 May show some rust when exposed to the Marine environment. Rust is usually shallow and can be easily removed. 0 Cr 17 Ni 12M 02 Molybdenum containing stainless steel is 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 state and the production process. The finish grade affects the corrosion resistance of stainless steel in chlorinated environments. Dull surfaces (rough surfaces) are very sensitive to corrosion, that is, normal industrial finishing 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.

　　② Fresh Water

　　Fresh water can be defined as water that is not acidic, salty or brackish and comes from rivers, lakes, ponds or Wells.

　　The corrosiveness of fresh water is affected by the pH value, oxygen content and scaling tendency of the water. Scaling (hard) water, whose corrosiveness is mainly determined by the amount and type of scaling that forms on the metal surface. The scale forms as a result of the minerals present in it and the temperature. Non-scaling (soft) water, which is generally more corrosive than hard water. It can be reduced by increasing the pH or reducing the oxygen content.

　　1Cr13 stainless steel is significantly more resistant to fresh water corrosion than carbon steel, and has excellent characteristics for use in fresh water. This steel is widely used in applications such as shipyards 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 in fresh water. However, pitting corrosion can be avoided by cathodic protection. 1Cr17 and Austenitic stainless steels are almost completely resistant to fresh water corrosion at room temperature (ambient temperature).

　　③ Acidic water

　　Acid water refers to the contaminated natural water leaching from ore and coal, which is much more corrosive than natural fresh water due to its strong acidity. Due to the leaching effect of water on sulfide contained in ores 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 very large effect on the corrosion of carbon steel.

　　Carbon steel equipment subjected to acidic water usually corrodes quickly. The test results of various materials subjected to 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 obstruction to heat conduction. Therefore, stainless steel pipes are widely used in heat exchange purposes.

　　④ Salt water

　　The corrosion of salt water is often in the form of pitting corrosion. For stainless steel, it is largely due to the salt water that causes the local destruction of the passivation film that acts as a corrosion resistance. Other causes of pitting corrosion in these steels are the concentration batteries of oxygen that can be formed by the tea and other seawater organics attached to the stainless steel equipment. Once formed, these cells are very active and cause a lot of corrosion and pitting. In the case of high speed flow of salt water, such as the impeller of a pump, the corrosion of austenitic stainless steel is usually very small.

　　For condensers using stainless steel tubes, it is necessary to keep the water flow rate greater than 1.5m/s to minimize the accumulation of organic matter and other solids in the sea water in the tube. For the structure of stainless steel equipment that treats saline water, it is best to reduce gaps and use thick-walled parts in the design.

　　⑤ Soil

　　The metals embedded in the soil are in a complex state that changes from time to time, depending on the weather and other factors. Practice has proved that Austenitic stainless steel generally has excellent resistance to most soil corrosion, and 1Cr13 and 1Cr17 will produce pitting corrosion in many soils. 0 Cr 17Ni12M Dare2 stainless steel can fully resist pitting corrosion in all soil tests.

　　⑥ nitric acid

　　Ferritic stainless steel and austenitic stainless steel containing not less than 14% chromium have excellent resistance to nitric acid corrosion. 1Cr17 stainless steel has been widely used in processing equipment in nitric acid factories. However, because 0 Cr 18 Ni 9 usually has better formability and weldability, it has largely replaced 1Cr17 stainless steel in the above uses.

　　The nitric acid corrosion resistance of other Austenitic stainless steels is similar to 0 Cr 18 Ni 9. The corrosion rate of 1Cr17 stainless steel is usually slightly higher than that of 0 Cr 18 Ni 9. and the higher temperature and concentration have a greater harmful effect on it.

　　If the heat treatment of the steel is not appropriate, 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 stainless steel resistant to this type of corrosion can be used.

　　⑦ sulfuric acid

　　Standard stainless steel grades are rarely used for sulfuric acid solutions because of their narrow range of use. 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%. However, in the higher concentration range, carbon steel is usually used. Martensitic and ferritic stainless steels are generally not resistant to sulfuric acid solution corrosion.

　　As in the case of 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 grade 00Cr19Ni10 or 00Cr17Ni14Mo2. or stabilized grade 0Cr18Ni11Ti or 0Cr18Ni11Nb stainless steel should be used.