Working principle of electroplating

Electroplating requires a low voltage and high current power supply to the electroplating tank, as well as an electrolysis device composed of electroplating solution, parts to be plated (cathode), and anode. The composition of the electroplating solution varies depending on the coating, but all contain a main salt that provides metal ions, a complexing agent that can form complexes with metal ions in the main salt, a buffer used to stabilize the pH of the solution, an anode activator, and special additives such as brighteners, grain refiners, leveling agents, wetting agents, stress relievers, and fog suppressants. The electroplating process is the process in which metal ions in the plating solution are reduced to metal atoms through electrode reactions under the action of an external electric field, and metal deposition is carried out on the cathode. Therefore, this is a metal electrodeposition process that includes steps such as liquid phase mass transfer, electrochemical reaction, and electrocrystallization.
In a plating bath containing electroplating solution, the cleaned and specially pretreated parts to be plated serve as the cathode, and the anode is made of coated metal. The two poles are respectively connected to the positive and negative poles of the DC power supply. The electroplating solution is composed of an aqueous solution containing compounds containing coated metals, conductive salts, buffering agents, pH regulators, and additives. After being electrified, the metal ions in the electroplating solution move to the cathode under the action of potential difference to form a coating. The metal ions formed by the anode enter the electroplating solution to maintain the concentration of the metal ions being plated. In some cases, such as chrome plating, it is an insoluble anode made of lead or lead antimony alloy, which only serves to transfer electrons and conduct current. The concentration of chromium ions in the electrolyte needs to be maintained by regularly adding chromium compounds to the plating solution. During electroplating, the quality of the anode material, the composition of the electroplating solution, temperature, current density, electrification time, stirring strength, precipitated impurities, power waveform, and other factors can all affect the quality of the coating, and timely control is necessary.
Firstly, the electroplating solution has six elements: main salt, additional salt, complexing agent, buffer, anode activator, and additive.
The principle of electroplating includes four aspects: electroplating solution, electroplating reaction, electrode and reaction principle, and the process of metal electrodeposition.
Electrochemical reaction in electroplating reaction: The schematic diagram of the electroplating device in the "Electroplating Principle Diagram" below shows that the plated part is the cathode, connected to the negative pole of the DC power supply, the metal anode is connected to the positive pole of the DC power supply, and both the anode and cathode are immersed in the plating solution. When a certain potential is applied between the anode and cathode, the following reaction occurs at the cathode: the metal ion Mn+that diffuses 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, a completely opposite reaction occurs at the anode, where metal M dissolves at the anode interface, releasing n electrons to generate metal ions Mn+.
reaction mechanism
A. Electrode potential
When a metal electrode is immersed in a solution containing the metal ion, there exists an equilibrium between the reaction of the metal losing electrons and dissolving in the solution, and the reverse reaction of the metal ion gaining electrons and precipitating the metal: Mn++ne=M
The equilibrium potential is related to the nature of the metal and the temperature and concentration of the solution. In order to accurately compare the influence of material nature on equilibrium potential, it is stipulated that when the solution temperature is 25 ℃ and the concentration of metal ions is 1mol/L, the measured potential is called the standard electrode potential. Metals with higher negative standard electrode potentials are prone to electron loss and oxidation, while metals with higher positive standard electrode potentials are prone to electron reduction.
B. Polarization
The so-called polarization refers to the phenomenon where the electrode potential deviates from the equilibrium electrode potential when a current passes through the electrode. Therefore, the current potential curve is also referred to as the polarization curve. The main reasons for polarization are electrochemical polarization and concentration polarization.
1. Electrochemical polarization
The polarization effect caused by the electrode potential moving in the negative direction due to the electrochemical reaction speed on the cathode being less than the speed of the external power supply electrons.
2. Concentration polarization
The polarization caused by the difference between the concentration of the adjacent electrode surface liquid layer and the concentration of the solution body is called concentration polarization, which is caused by the ion diffusion speed in the solution being less than the electron motion.
The electroplating process is the process in which metal ions in the plating solution are reduced to metal atoms through electrode reactions under the action of an external electric field, and metal deposition is carried out on the cathode.
In simple terms, the principle of electroplating is to use the substrate metal as the cathode in a salt solution containing the metal to be plated. Through electrolysis, the cations of the metal to be plated in the plating solution are deposited on the surface of the substrate metal, forming a coating.