November 29, 2023
1.Color Differentiation: After acid washing, the surface of stainless steel appears silver-white and clean:
The surface color of untreated stainless steel:
Cold-rolled, non-annealed chromium-nickel stainless steel has a reflective silver-white surface.
- Chromium-nickel stainless steel exhibits a silver-white color with a jade-like hue.
- Chromium stainless steel is slightly white with a subtle gray gloss.
- Chromium-manganese-nitrogen stainless steel has a color similar to chromium-nickel stainless steel but slightly lighter.
- Chromium-nickel steel appears beige-white.
- Chromium steel appears brown-black.
- Chromium-manganese-nitrogen steel appears black (these colorations refer to heavier oxidation).
2.Identification Using a Magnet: A magnet can help distinguish between two types of stainless steel. Chromium stainless steel is magnetic in any state, while annealed chromium-nickel stainless steel is generally non-magnetic. However, cold-worked chromium-nickel stainless steel may exhibit magnetism. High manganese steel with high manganese content is non-magnetic. The magnetic properties of chromium-nickel-nitrogen stainless steel are more complex, varying among different samples.
3.Identification Using Copper Sulfate: Removing the oxide layer from the steel, adding a drop of water, and rubbing with copper sulfate can aid in identification. No color change indicates stainless steel. A purple-red color indicates non-magnetic high manganese steel, while a magnetic reaction suggests ordinary or low-alloy steel.
4.Additional Identification Methods for Special Steels:
- Spark testing: Grinding stainless steel on a grinding wheel and observing the sparks. Streamlined sparks with numerous dense flowers indicate higher manganese or manganese-nitrogen steel, while the absence of flowers suggests chromium or chromium-nickel stainless steel.
- Annealing test: Heating a small piece of cold-worked chromium-nickel stainless steel in a fire and allowing it to cool naturally or placing it in water (annealing). Generally, magnetism significantly weakens or disappears after annealing.
- Chemical qualitative analysis: Dissolving a small piece of stainless steel in aqua regia, diluting with water, adding ammonia, and then gently injecting nickel reagent. The presence of red velvety material on the liquid surface indicates the presence of nickel, while the absence suggests the absence of nickel (note: due to the low nickel content in stainless steel, usually a few percent, determining the specific amount may require multiple experiments with standard samples).
In summary, the identification of stainless steel using sensory methods requires a combination of several techniques. The results can determine a specific type of stainless steel but cannot specify which alloy elements are present or their exact concentrations. Therefore, sensory identification methods are currently imperfect and may be prone to errors. Further exploration is needed to understand the underlying physical phenomena."