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1. The color of the air oxide film: the color should be even after bluish, and no stains and part of the air oxide film are not allowed. Because of the different surface smoothness of the same part, or parts with partial heat treatment or electric welding position, the color difference is allowed. After the carbon steel and high-alloy steel parts turn blue, there should be a symmetrical black air oxide film. Castings and silicon-containing carbon steel parts are allowed to be yellowish brown or dark brown after turning blue.

2. Compressive strength of air oxide film: The air oxide film and the main fusion compressive strength, rub hard with a dry cloth, and do not show the metal texture.

3. The tightness of the air oxide film: When the parts become blue, use 3% sodium thiosulfate solution to corrode for about 30 seconds before applying oil, and no copper electroplating is allowed on the surface of the parts. However, a small amount of copper plating stains can occur on sharp edges, corners and welding positions.

4. Anti-corrosion of air oxide film: corroded by 3% edible salt solution for 3 hours without corrosion.

5. The smoothness of the parts and it: After bluing cleaning, drop 1-2 drops of phenolphthalein alcohol solution on the product piece. For example, the phenolphthalein alcohol solution is pale pink, indicating that the cleaning is not clean.