Stainless steel: refers to air, steam, water and other weak corrosive medium and acid, alkali, salt and other chemical corrosive medium of steel, also known as stainless acid resistant steel. In practical application, steel resistant to weak corrosion medium is often called stainless steel, while steel resistant to chemical medium is called acid resistant steel. There are some differences in chemical composition between the two. The former is not necessarily resistant to chemical medium corrosion, while the latter is generally stainless. The corrosion resistance of stainless steel depends on the alloying elements contained in the steel. Corrosion resistance mechanism: chromium is the basic element for stainless steel to obtain corrosion resistance. When the chromium content in the steel reaches about 12%, chromium reacts with oxygen in the corrosive medium and forms a very thin oxide film (self passivation film Cr2O3) on the steel surface, which is very difficult to dissolve in water and can further prevent oxygen and iron corrosion. Similarly, the destruction of Cr2O3 passivation film means the destruction of its anti oxygen corrosion ability. In addition, the halogen elements in corrosive medium (such as the common chloride ions in water) can also replace CR under certain conditions, so stainless steel will rust under certain conditions, and will be corroded in the medium containing acid, alkali and salt,. Therefore, the corrosion resistance of stainless steel varies with its chemical composition, additive state, service condition and environmental medium type. The corrosion of stainless steel in water is mainly caused by chloride ion in water. The other elements in stainless steel also play different roles. For example, Mo can counteract the corrosion caused by chloride ion to a certain extent, but it also has a scope of application. Commonly used stainless steel: 304316 belongs to austenitic stainless steel, its brand is the abbreviation of foreign name (generally imported), 300 series. American brand is AISI standard of American Iron and Steel Association, Japan is sus standard of Japanese Industrial Association. China is mainly represented by composition, as shown in the table below. The composition difference of picture 304, 304L, 316 and 316L. L means low, which means less carbon. The welding performance of L-band is good. If the number is the same, there is no big difference in the composition except carbon. Because the price difference between 316 and 316L is not big, 316L is chosen directly. It can be seen from the composition table that the biggest difference between 304 and 316 is the content of Mo (molybdenum), so 316 has stronger resistance to chloride ion concentration than 304. It is also the main basis for us to choose 304 or 316. The chloride ion concentration of the two materials is shown in the table on the next page, and the supplementary description of the plate is: the application range of stainless steel in chlorine containing medium (mg / L) (equivalent to ppm in water medium) plate material / temperature 25 ℃ 50 ℃ 75 ℃ 100 ℃ 120 ℃ 304 / 304L 100 75 40 20 10 316 / 316L 400 180 120 50 25 If the chloride ion concentration exceeds, Titanium (TI) or other metals are selected. For example, the characteristics and service conditions of common materials for titanium (TI) plate used in seawater. The supplementary index to evaluate the corrosion resistance of materials is that the larger the "local corrosion resistance equivalent pre" value is, the better the corrosion resistance is. It is mainly determined by the content of Cr, Mo and Ni. 1) 304 stainless steel: used in organic and inorganic media, concentration < 30%, temperature < 100 / concentration > 30%, temperature < 50, nitric acid temperature < 100, various concentrations of carbonic acid, ammonia and alcohols. 304L material is basically the same as 304 material, which has better weldability and can be used as welded heat exchanger. 2) 316L natural cooling water, cooling tower water, softened water, carbonic acid, acetic acid and caustic soda solution with concentration less than 50%, alcohol, acetone and other solvents, dilute nitric acid (< 20%) and dilute phosphoric acid (< 30%) with temperature less than 100 ℃, but not suitable for sulfuric acid. 316 is basically the same as it. 3) 317 is suitable for more conditions than 316L. 4) Aisi904l and sus890l are more cost-effective than the above materials. Especially suitable for general sulfuric acid, phosphoric acid and halide. 5) Smo 254 is a super stainless steel which improves 316 by increasing Mo content. It has excellent resistance to chloride and crevice corrosion. Suitable for salt water, inorganic acid. 6) Smo654 is a better material than 254 and can be used in cold seawater. 7) RS-2 (0cr20ni26mo3cu3si2nb) stainless steel, which is equivalent to 316 made in China, has better stress corrosion resistance and can be used for concentrated sulfuric acid above 80 ℃（ Concentration 90% - 98%) remarks: see the following table for details: