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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. The oxidizing environment can strengthen the film, and the reducing environment will inevitably destroy the film, resulting in corrosion of steel.

Corrosion resistance in various environments

① Atmospheric corrosion

The atmospheric corrosion resistance of stainless steel is basically changed with the chloride content in the atmosphere. Therefore, proximity to marine or other sources of chloride pollution is extremely important for corrosion of stainless steel. A certain amount of rainwater is important only when it acts on the chloride concentration of the steel surface.

Rural environment 1Cr13, 1Cr17 and austenitic stainless steel can be adapted to a variety of uses, its appearance will not have a significant change. Therefore, stainless steel exposed in rural areas can be selected according to price, market supply, mechanical properties, production processing performance and appearance.

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

Marine environment 1Cr13 and 1Cr17 stainless steel in a short period of time will form a thin rust film, 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 marine environments. The rust is usually shallow 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 the surface state and the production process. The finish grade affects the corrosion resistance of stainless steel in chloride environments. The matte surface (rough surface) is very sensitive to corrosion, that is, the normal industrial finishing surface is 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 high-finish 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

Freshwater can be defined as water derived from rivers, lakes, ponds or wells that is not acidic, salty or brackish.

The corrosivity of fresh water is affected by the pH value, oxygen content and scale forming tendency of the water. The corrosiveness of scale (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 temperature. Non-scaling (soft) water, which is generally more corrosive than hard water. Its corrosivity 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 for use in fresh water. This steel is widely used in applications such as docks and dams where high strength and corrosion resistance are required. 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 corrosion protection. 1Cr17 and austenitic stainless steels are almost completely resistant to fresh water corrosion at room temperature (ambient temperature).

③ Acidic water

Acidic water refers to contaminated natural water leached from ore and coal, which is much more corrosive than natural fresh water due to its strong acidity. Acidic water usually contains a large amount of free sulfuric acid due to the leaching effect of water on the sulfides contained in ore and coal. 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 affected by acidic water is usually quickly corroded. The results of tests with various materials subjected to acidic river water show that austenitic stainless steel has high 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 resistance to heat conduction. Therefore, stainless steel pipes are widely used in heat exchange applications.

④ Salt water

The corrosion characteristic of salt water is often in the form of pitting. For stainless steel, it is largely due to the local destruction of the passive film, which acts as a corrosion resistance, due to the salt water. Other causes of pitting corrosion of these steels are the formation of oxygen concentration batteries by Minghe Jie and other seawater organics attached to 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 saline water, such as the impeller of a pump, the corrosion of austenitic stainless steel is usually very small.

For condensers using stainless steel tubing, it is necessary to maintain a water velocity greater than 1.5 m/s to minimize the accumulation of seawater organics and other solids in the tubing. For the structure of stainless steel equipment for treating salt water, it is best to reduce gaps and use thick-walled parts when designing.

⑤ Soil

The metals embedded in the soil, depending on the weather and other factors, are in a complex state that changes at all times. Practice has proved that austenitic stainless steel generally has excellent resistance to most soil corrosion, while 1Cr13 and 1Cr17 have pitting corrosion in many soils. 0 Cr 17Ni12M о 2 stainless steel is completely resistant to pitting corrosion in all soil tests.

⑥ Nitric acid

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

The nitric acid corrosion resistance of other austenitic stainless steels is similar to that of 0 Cr 18 Ni 9. 1Cr17 stainless steel usually 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, this type of corrosion can be prevented by an appropriate heat treatment, or a 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 most standard grade 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 range, however, carbon steel is typically used. Martensitic and ferritic stainless steels are generally not resistant to corrosion in sulfuric acid solutions.

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 grades 00Cr19Ni10 or 00 Cr17Ni14Mo2, or stabilized grades 0Cr18Ni11Ti or 0Cr18Ni11Nb stainless steel should be used.