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Austenitic stainless steel and ferritic stainless steel can be easily differentiated using a magnetic test. Austenitic grades are typically non-magnetic in their annealed state, offering excellent corrosion resistance, good formability, toughness, and weldability. However, after significant cold working, they may develop slight magnetism. In contrast, ferritic and martensitic stainless steels, along with carbon steel and low-alloy steels, are generally magnetic. This makes the magnetic test one of the simplest and most effective methods for identifying the type of stainless steel.
The corrosion resistance of stainless steel tubes can often be identified through a nitric acid spot test. This method works well for most stainless steel grades, as they resist corrosion in nitric acid. However, some high-carbon grades like 420 and 440 stainless steel may show slight etching during the test. It's important to note that concentrated nitric acid is highly corrosive to non-ferrous metals, while even diluted nitric acid can rapidly corrode carbon steel. Therefore, this test helps distinguish stainless steel from other materials.
Another common identification method is the copper sulfate spot test, which can differentiate between ordinary carbon steel and various types of stainless steel. To perform the test, clean the surface of the sample thoroughly, remove any grease or impurities, and polish a small area with a soft cloth. Then apply a drop of copper sulfate solution. If the material is carbon steel or iron, a layer of metallic copper will appear on the surface. In contrast, stainless steel tubes will not show any copper deposition or color change, making it easy to identify them.
To distinguish between different stainless steel grades such as 302 and 304, or 316 and 317, a sulfuric acid test can be used. Prepare a nitric acid solution with a volume concentration of 20–30% and heat it to 60–66°C. Grind the sample finely, then clean it in the nitric acid and let it passivate for about an hour. Next, immerse the samples in a 10% sulfuric acid solution heated to 71°C. The 302 and 304 grades will corrode quickly, producing many bubbles and turning black, while 316 and 317 will remain largely unaffected or react very slowly. This test is useful for identifying specific stainless steel alloys based on their chemical resistance.