Nitric acid is a strong oxidizing acid, strong oxidative corrosion, a lot of chemical fiber used in chemical fertilizers, dyes and other chemical industries. Stainless steel acid-resistant steel is developed from the application of nitric acid industry. Cr13 stainless steel has a certain corrosion resistance in dilute nitric acid at room temperature. Cr18 stainless steel has a very good corrosion resistance to dilute nitric acid at various temperatures. However, nitric acid industry is relatively large amount of application or better overall performance of Cr18Ni9 type stainless steel. Since molybdenum has a negative effect on the corrosion resistance of acid-resistant steels in nitric acid, it is not usual to select molybdenum-containing stainless steels in the nitric acid industry. Also in the production of concentrated nitric acid, but also the need to use a specific stainless steel.
When nitric acid with chlorine, chromium plasma, its corrosion will be strengthened. Chlorine ion in nitric acid easily causes pitting corrosion of stainless steel acid-resistant steel. Intergranular corrosion of nitric acid is also very strong, easy to weld heat affected zone knife-like corrosion. Boiling nitric acid at a concentration of 65% is not only sensitive to the tendency of intergranular corrosion caused by precipitation of chromium carbide, but also to the tendency of intergranular corrosion caused by precipitation of d phase.
Industrial nitric acid concentrated nitric acid (96% ~ 98%) and dilute nitric acid (<65%) two. Next, a brief introduction of stainless steel materials in dilute nitric acid, concentrated nitric acid production equipment.
Application of stainless steel in dilute nitric acid production
18-8 type alone – nickel austenitic stainless steel is excellent resistance to dilute nitric acid, it is widely used in dilute nitric acid concentrations below 65%, the same is true of duplex stainless steel 022Cr2lNi5Ti, 022Cr25Ni6M02N. The mechanical properties of the castings are usually less demanding, even less welding, can be replaced by more economical Cr17 ferritic stainless steel. When required corrosion resistance and high strength, but also the use of martensitic stainless steel 12Cr13 (403), 14Cr17Ni2 steel, used in the quenched and tempered state.
Application of stainless steel in concentrated nitric acid production
When the nitric acid concentration is greater than 68%, ordinary 18-8 chrome-nickel stainless steel has been unable to meet the requirements. Concentration ≤ 85%, the general use of Cr25Ni20-type stainless steel. If the nitric acid concentration is too high, the corrosion resistance of Cr25Ni20 stainless steel will drop sharply due to nitric acid peroxidation and will be severely corroded. Therefore, it is necessary to use stainless steel with 4% to 6% silicon content, such as 14Cr18NiIISi4AITi, 06Cr18Ni13Si4 and 022Cr20N. When the nitric acid concentration ≥ 97%, you can use the chromium content of 5% to 6% of chromium-nickel stainless steel.
Stainless steel in sulfuric acid equipment applications
Sulfuric acid is also a very important basic chemical raw material, used to produce and put sulfuric acid equipment or containers generally use stainless acid steel.
Although sulfuric acid is an oxyacid, dilute sulfuric acid and moderate concentrations of sulfuric acid are not very oxidizing and are reducing acids. The concentrated sulfuric acid, especially hot concentrated sulfuric acid oxidation is very strong, is an oxidizing acid. Because of this characteristic of sulfuric acid, the choice of sulfuric acid-resistant stainless steel, with sulfuric acid concentration, temperature of the different grades selected.
Dilute sulfuric acid, medium concentration of sulfuric acid can choose 06Cr17Ni12M02 (316), 022Cr17 Ni12 M02 (316 1), 06Cr17Ni12M02Ti (316Ti) and other molybdenum content is 2% -3% stainless steel. Molybdenum-free stainless steel 18-8 chrome-nickel austenitic stainless steel, due to intolerance of dilute sulfuric acid, medium-grade sulfuric acid corrosion, it can not be used in the case of sulfuric acid corrosion. When high molybdenum content, this stainless steel corrosion will be enhanced again. Stainless steel containing molybdenum, Cu, Si (3% to 4%) of Cr-Ni austenitic stainless steel and duplex stainless steel better sulfuric acid resistance.
In the concentration of 90% -98% of the high-temperature concentrated sulfuric acid, according to the use of temperature and concentration, the use of silicon content of about 6% of stainless steel works well.
Source: China Stainless Steel Pipes Manufacturer – Yaang Pipe Industry Co., Limited (www.yaang.com)