Why do copper bolts and fittings turn black?

Why do copper bolts and fittings turn black?

In the fastener industry, copper bolts and copper accessories can be seen everywhere. But have you ever seen a surface that turns black, red, purple, or green? What is the cause? Why are some copper products not easily discolored in the same environment? Today, the editor will give you a detailed analysis:
Reason for discoloration:

1.There are dust containing other metal elements or attachments of heterospecific metal particles accumulated on the surface of copper bolts. In humid air, connecting the two into a micro battery triggers an electrochemical reaction that destroys the protective film, which is called electrochemical corrosion.”.

  1. Organic juices adhere to the surface of copper bolts, forming organic acids in the presence of water and oxygen. Organic acids can corrode the metal surface over time.
  2. The surface of copper bolts is adhered with acid, alkali, and salt substances, causing local corrosion.
  3. When encountering condensed water in polluted air, liquid points of sulfuric acid, nitric acid, and acetic acid can form, causing chemical corrosion. If your copper products are placed above, they will rust over time. There will be discolored rust spots on its surface.
Why do copper bolts and fittings turn black?

How to prevent oxidation and discoloration of copper? After the processing of copper screws, passivation and oxidation prevention treatment should be carried out to prevent the occurrence of undesirable phenomena such as oxidation, discoloration, and blackening of the surface of copper screws. The treatment process is as follows:

  1. Degreasing: The processed product contains a lot of oil, which must be cleaned before the copper screw can fully react with the passivation liquid during passivation to obtain a complete passivation film.
    During water washing, the degreasing agent must be cleaned to prevent it from contaminating the passivation liquid.
  2. Passivation treatment: use environmentally friendly passivation liquid and passivate at room temperature for more than 5 minutes.
  3. Wash and dry with water, and the drying temperature should be 80-100 ℃. It is recommended to tumble and dry for the best effect

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Why do copper bolts and fittings turn black?