Abstract: This article aims to explain four commonly used methods for detecting the material of stainless steel elbows to achieve the purpose of identifying the material of stainless steel elbows.
1. Nitric acid point test
The significant feature of stainless steel elbows: they have inherent corrosion resistance to concentrated nitric acid and dilute nitric acid. This property can be used to quickly distinguish stainless steel elbows from most other metal elbows or alloy elbows. Non-ferrous metal elbows will be corroded immediately when they encounter concentrated nitric acid. Dilute nitric acid is highly corrosive to carbon steel elbows, but high-carbon 420 and 440 steel elbows are slightly corroded during nitric acid point tests, which requires dialectical confirmation.
2. Sulfuric acid test
The sulfuric acid immersion stainless steel elbow test can distinguish different types of stainless steel elbows. For example, the samples of 302 and 304 stainless steel elbows are finely ground after trimming, and then placed in nitric acid (specific gravity 1.42) with a volume concentration of 20~30% and a temperature of 60~66℃ for cleaning and passivation for half an hour. Finally, it is placed in a sulfuric acid test solution with a volume concentration of 10% and heated to 71℃. When 302 and 304 stainless steel elbow samples are immersed in this hot solution, they are corroded and produce a lot of bubbles, turning black within a few minutes; while 316 and 317 stainless steel elbow samples are not corroded or react very slowly (no bubbles are produced), and the samples do not change color within 10-15 minutes.
3. Copper sulfate spot test
The copper sulfate spot test is a simple method to distinguish between ordinary carbon steel elbows and all types of stainless steel elbows. The test area needs to be free of grease or various impurities and polished with a soft abrasive cloth, and then a 5-10% copper sulfate solution is dripped into the cleaned area with a dropper. Ordinary carbon steel or iron elbows will form a layer of surface metallic copper within a few seconds, while the surface of stainless steel will not produce copper precipitation or show copper color.
4. Magnetic test
The magnetic test is a simple method to distinguish annealed austenitic stainless steel elbows from ferritic stainless steel elbows. Austenitic stainless steel is non-magnetic steel (when encountering austenitic stainless steel elbows with weak magnetism, it is necessary to consider that they will have slight magnetism after cold working under high pressure, and generally the magnetism will be eliminated after annealing and solution treatment); while pure chromium steel and low alloy steel elbows are both strongly magnetic steels.
