How to identify the authenticity of stainless steel elbow

　　Stainless steel elbow is applicable to weak corrosive media such as air, steam and water, and chemical corrosive media such as acid, alkali and salt. In practical applications, steel that is not afraid of weak corrosion medium corrosion is often called stainless steel, while steel that is not afraid of chemical medium corrosion is called acid resistant steel. Due to the difference in chemical composition between the two, the former is not correspondingly afraid of chemical medium corrosion, while the latter is generally rust-free. The method to identify the stainless steel elbow is as follows:

　　1. Magnetic test: magnetic test is a simple method to distinguish annealed austenitic stainless steel from ferritic stainless steel. Austenitic stainless steel is non-magnetic steel, but it will have slight magnetism after cold working under high pressure; The pure chromium steel and low alloy steel are both strong magnetic steel.

　　2. Nitric acid point test: an obvious feature of stainless steel elbow is its inherent resistance to corrosion of concentrated nitric acid and dilute nitric acid. This property makes it easy to distinguish it from most other metals or alloys. However, the high-carbon 420 and 440 steels are slightly corroded during the nitric acid point test, and the non-ferrous metals will be corroded immediately when they encounter concentrated nitric acid. However, dilute nitric acid has strong corrosivity to carbon steel.

　　3. Sulfuric acid test: the sulfuric acid immersion stainless steel elbow test can distinguish 302 and 304 from 316 and 317. The cut edge of the sample shall be finely ground, and then cleaned and passivated in nitric acid (specific gravity 1. 42) with a volume concentration of 20~30% and a temperature of 60~66 ℃ for half an hour. The volume concentration of sulfuric acid test solution is 10%, and it is heated to 71 ℃. When 302 and 304 steel are immersed in this hot solution, they are corroded and produce a large number of bubbles, and the sample turns black within a few minutes; The samples of 316 and 317 steel are not corroded or react slowly (no bubbles), and the samples do not change color within 10-15 minutes. If the sample with known composition is tested at the same time for approximate comparison, the test can be made more accurate.

　　4. Copper sulfate spot test: copper sulfate spot test is a simple method to quickly distinguish ordinary carbon steel and all types of stainless steel. The concentration of copper sulfate solution used is 5-10%. Before the spot test, the test area shall be greased or various impurities, and a small area shall be polished with a soft abrasive cloth, and then the copper sulfate solution shall be dripped into the cleaned area with a drip bottle. Ordinary carbon steel or iron will form a layer of surface metal copper within a few seconds, while the surface of stainless steel will not produce copper precipitation or display the color of copper.