What are the specific requirements for the treatment of high strength blackened bolts?
Degreasing, derusting and cleaning shall be carried out for high strength blackened bolts before treatment. Incomplete cleaning will affect the blackening effect. It is recommended to use hydrochloric acid of more than 15% concentration for pickling.

The treatment of high strength blackened bolt has the following requirements:
1. The appearance of blackening treatment is blue black or dark black.
2. The film crystallizes compactly and evenly, and the product has no bottom exposure, obvious color difference, whitening and other abnormal phenomena. If the blackening process is not normal, the following situation will occur.
3. White spots and flowers. This phenomenon is mainly manifested as the oxide film of different colors, white spots, flowers.
4. The color is not right. The product turns blue or red after blackening.
5. The oxide film is not firm. Due to incomplete degreasing and cleaning of the product before blackening, too long blackening treatment time, too thick blackening layer, forming a blackening floating layer and poor adhesion, the clean water after pickling of the product is not clean, and the residual acid is brought into the blackening liquid, resulting in local excessive reaction and floating rust on the surface of the workpiece.

ASTM A490 Hex Bolts High Strength Structural Bolt

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