Description of the corrosion resistance of stainless steel in different environments
The corrosion resistance of stainless steel decreases with the increase of carbon content. Therefore, most stainless steels have low carbon content, and the maximum carbon content does not exceed 1.2%. Some steels have carbon content even less than 0.03%.
The main alloying element in stainless steel is chromium, and only when the chromium content reaches a certain value, the steel has corrosion resistance. Therefore, stainless steel generally contains at least 10.5% chromium. Stainless steel also contains elements such as Ni, Ti, Mn, N, Nb, Mo, Si, and Cu.
Stainless steel refers to steel that is resistant to weak corrosive media such as air, steam, and water, and chemically corrosive media such as acid, alkali, and salt. In practical applications, steel that is resistant to corrosion by weak corrosive media is often called stainless steel, and steel that is resistant to chemical media is called acid-resistant 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 one-step oxidation or corrosion. An oxidizing environment can strengthen this film, while a reducing environment will inevitably destroy this film and cause corrosion of steel.
Corrosion resistance in various environments
Atmospheric corrosion: The resistance of stainless steel to atmospheric corrosion 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 affects the chloride concentration on the steel surface.
Freshwater: Freshwater can be defined as water that is sourced from rivers, lakes, ponds or wells regardless of acidity, saline or brackishness. The corrosiveness of freshwater is affected by the pH, 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.
Acidic 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 freshwater. 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.
Saline 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 passivation film caused by salty water. The other reason for the pitting corrosion of these steels is that the 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.
Soil: The metal buried in the soil is in a complex state that changes at any time depending on the weather and other factors. The 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 17Ni12M§à2 stainless steel is completely resistant to pitting corrosion in all soil tests. Nitric acid:
Ferritic stainless steel and austenitic stainless steel containing not less than 14% chromium have excellent corrosion resistance to nitric acid. 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 steel. Therefore, appropriate heat treatment can be used to prevent this type of corrosion, or use stainless steel resistant to this type of corrosion.
Sulfuric 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 resistant standard grade for 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.