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Basic knowledge of electroplating - electroplating reaction

August 2, 2023

1. Electrochemical reaction
The figure below is a schematic diagram of the electroplating device. The part to be plated is the cathode, which is connected to the negative pole of the DC power supply. The metal anode is connected to the positive pole of the DC power supply. Both the anode and the cathode are immersed in the plating solution. When a certain potential is applied between the cathode and the anode, the following reaction occurs at the cathode: the metal ion Mn+ diffused from the inside of the plating solution to the interface between the electrode and the plating solution obtains n electrons from the cathode and is reduced to metal M On the other hand, at the anode Then the completely opposite reaction to the cathode occurs, that is, the dissolution of metal M occurs on the anode interface, and n electrons are released to generate metal ions Mn+.




2. Faraday's law
When the current passes through the plating solution, electrolytic reaction occurs in the electrolyte solution, the metal on the cathode is continuously precipitated, and the anode metal is continuously dissolved. Therefore, the amount of precipitation (or dissolution) of metal must be related to the charge passing through. Based on a large number of experimental results, Faraday established the law of the relationship between the precipitated (or dissolved) substance and the electric charge.

Farah's first law: The weight of the precipitated (or dissolved) substance on the electrode is proportional to the charge passed through during the electrolytic reaction, that is: m=kQ=kIt(m is the mass of the precipitated or dissolved substance on the electrode; Q is the passed charge When; K is the proportional constant; I is the current; t is the electrification time.

Faraday's second law: In different electrolytes, when the same amount of charge is passed, the amount of substance that precipitates (or dissolves) on the electrode is equal, and the amount of charge required to precipitate (or dissolve) 1mol of any substance is equal to It is 9.65X104C. This constant is called Faraday's constant, represented by F, K=M/F.


3. Current efficiency
During electroplating, the mass of the substance actually precipitated on the cathode is not equal to the calculation result obtained according to Farah's law, and the actual value is always smaller than the calculated value. This is because there is more than one reaction on the electrode. In addition to the main reaction, side reactions also occur.

4. Dispersion ability of plating solution
The dispersion ability of the plating solution refers to the ability of the electroplating solution to evenly distribute the thickness of the metal layer, also known as throwing ability. The better the dispersion ability of the electroplating solution, the more uniform the thickness of the metal layer deposited on different cathode parts.

5. Coverage of plating solution
In electroplating production, another concept commonly used is coverage ability, also known as deep plating ability, which refers to the ability of electroplating solution to deposit metal coating on deep recesses of plated parts. Dispersion ability and covering ability are different. The former is a question of how uniform the metal is distributed on the surface of the cathode. Its premise is that there is a coating on the surface of the cathode; while the latter refers to the problem of whether the metal is deposited on the deep recesses of the cathode surface.