In the pre-treatment stage of the plating process, conventional pickling processes (hydrochloric acid, etc.) are usually used to remove oxidized layers and rust from the surface of the workpiece. However, this process can lead to hydrogen embrittlement of the plated layer, which significantly affects the mechanical properties and service life of the plated product. Hydrogen embrittlement is a phenomenon in which the ductility and toughness of a material are significantly reduced in the presence of hydrogen, leading to an increase in its brittleness and susceptibility to fracture during use.
Mechanism of hydrogen embrittlement
1. Adsorption and penetration of hydrogen atoms:
During the pickling process, the workpiece is immersed in a solution containing a strong acid (e.g. sulfuric acid, hydrochloric acid). A large number of hydrogen ions generated during pickling are partially reduced to atomic hydrogen (H) on the surface of the workpiece. These atomic hydrogens are very active and are easily adsorbed by the surface of the workpiece and penetrate into the metal matrix by diffusion.
2. Hydrogen atom aggregation and pressure increase:
Hydrogen atoms infiltrating into the metal matrix usually accumulate at grain boundaries, dislocations or other microscopic defects and gradually form hydrogen molecules.
These hydrogen molecules develop internal pressure during their aggregation, causing stress concentrations in the metal matrix in localized areas, leading to increased material brittleness. This stress concentration often triggers cracking and fracture of the metal.
Influencing factors and preventive measures
1. Pickling time:
Excessive pickling time increases the amount of hydrogen penetration and thus increases the risk of hydrogen embrittlement. In order to minimize hydrogen embrittlement, the pickling time should be kept as short as possible to achieve removal of the oxide layer and rust.
2. Pickling temperature:
Higher pickling temperatures accelerate the process of hydrogen reduction and penetration. Therefore, controlling the pickling temperature within a reasonable range can help reduce the occurrence of hydrogen embrittlement.
3. Use of inhibitors:
The addition of appropriate inhibitors to the pickling solution can effectively reduce the generation and penetration of hydrogen atoms and reduce the risk of hydrogen embrittlement. These inhibitors are usually organic compounds that prevent the diffusion of hydrogen by forming a protective film on the metal surface. However, compounding such pickling agents often requires a certain amount of technical experience, as well as trial and error (a lot of time and effort) to adjust the formulation.
For plating plants, it is better to use Bigely acid salt activator, which is a mild formula on the neutral side that achieves the same results as hydrochloric acid pickling without causing hydrogen embrittlement.
4. Post-treatment dehydrogenation:
A dehydrogenation treatment (e.g. heat dehydrogenation) after pickling can accelerate the precipitation of hydrogen that has already penetrated and greatly reduce the risk of hydrogen embrittlement.
In conclusion, while cleaning the metal surface, the traditional pickling process inevitably introduces hydrogen atoms, causing hydrogen embrittlement in the plated layer. By using Bigely acid salt activator, hydrogen embrittlement can be avoided and the mechanical properties and service life of plated products can be guaranteed.
If you have any demands for acid salt activator, please feel free to contact us.
