Differentiating Stainless Steel from Other Materials
Differentiating Stainless Steel from Other Materials
1. Magnetic Test
A straightforward method for distinguishing annealed austenitic stainless steel from ferritic stainless steel is the magnetic test. Austenitic stainless steel is non-magnetic initially but may acquire slight magnetism after cold working under pressure. In contrast, pure chromium steel and low alloy steel exhibit high magnetism.
2. Nitric Acid Point Test
Stainless steel’s inherent corrosion resistance to concentrated and dilute nitric acid is a key characteristic. This resistance sets it apart from most other metals or alloys. However, high-carbon 420 and 440 steels may exhibit slight corrosion during nitric acid point tests, while non-ferrous metals corrode immediately upon contact with concentrated nitric acid. Dilute nitric acid poses a significant corrosion risk to carbon steel.
3. Copper Sulfate Point Test
The copper sulfate spot test offers a quick means of differentiation between ordinary carbon steel and various types of stainless steel. A 5~10% concentration of copper sulfate solution is typically used. Before conducting the test, ensure the test area is thoroughly cleaned of grease and impurities. Polish a small area with a soft grinding cloth, then apply copper sulfate solution using a dropper bottle. Ordinary carbon steel or iron will develop a surface layer of metallic copper within seconds, while stainless steel will not produce copper precipitation or exhibit copper coloration.
4. Sulfuric Acid Test
The sulfuric acid immersion test aids in distinguishing between 302/304 and 316/317 stainless steel. Begin by finely grinding the cut edge of the sample, followed by cleaning and passivation in nitric acid (2030% concentration, 6066°C temperature) for half an hour. The sulfuric acid test solution, heated to 71°C with a 10% concentration, is then used. When 302 and 304 steel are immersed, rapid corrosion occurs, generating bubbles and turning the sample black within minutes. In contrast, 316 and 317 steel exhibit minimal corrosion or slow reaction (no bubble production) and maintain their color for 10 to 15 minutes. For increased accuracy, conduct tests on samples with known compositions simultaneously for comparison.
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