- Mar 15, 2018 -
A. Electrode potential
When the metal electrode is immersed in a solution containing the metal ion, there is a balance that the reaction in which the metal loses electrons and dissolves in the solution and the reverse reaction of the metal ion and electrons to precipitate the metal should exist simultaneously: 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 effect of the nature of the material on the equilibrium potential, it is stated that when the solution temperature is 250° C. and the metal ion concentration is 1 mol/L, the measured potential is called the standard electrode potential. Metals with large negative electrode potentials of the standard electrode potential are likely to lose the electrons to be oxidized, while metals with large positive electrode potentials of standard electrode potentials are easily reduced by electrons.
Polarization refers to the phenomenon that when the current passes through the electrode, the potential of the electrode deviates from the potential of the balanced electrode. Therefore, the current-potential curve is also called a polarization curve. The main causes of polarization are electrochemical polarization and concentration polarization.
1. Electrochemical polarization
Because the speed of the electrochemical reaction at the cathode is less than the speed at which electrons are supplied from the external power source, the polarization effect caused by the movement of the electrode potential in the negative direction is caused.
2. Concentration polarization
The polarization caused by the difference between the concentration of the liquid layer adjacent to the surface of the electrode and the concentration of the solution body is called concentration polarization because the ion diffusion rate in the solution is smaller than the electron movement.
The electroplating process is a process in which a metal ion in a plating solution is reduced to a metal atom by an electrode reaction under the action of an external electric field and a metal deposition process is performed on the cathode.
The principle of electroplating is simple, that is, in the salt solution containing the metal to be plated, the plated metal is used as the cathode, and the cations of the metal to be plated are deposited on the surface of the substrate metal to form a plating layer through electrolysis.
Electroplating key elements:
1. Cathode: Plating, refers to various connector terminals.
2. Anode: If it is a soluble anode, it is intended to be plated with metal. For insoluble anodes, most of them are precious metals (platinum, antimony oxide).
3. Electroplating liquid: plating bath containing metal ions to be plated.
4. Plating tank: Can withstand, store plating pot, generally consider the strength, corrosion resistance, temperature and other factors.
5. Rectifier: Equipment that provides DC power.
Polishing→ Polishing→ Hanging→ Degreasing and degreasing→ Washing→ Electropolishing or Chemical polishing→ Pickling activation→ Prepreg→ Plating→ Washing→ Post-treatment→ Washing→ Drying→ Hanging down→ Inspection and packaging.
Electroplating working conditions refer to the operating change factors during electroplating, including current density, temperature, stirring, and power waveforms.
Cathode current density
Any bath has a current density range for good plating. The minimum current density for good plating is called the lower current density, and the maximum current density for good plating is called the upper current density. In general, when the cathode current density is too low, the cathode polarization effect is small and the crystal grain of the plating layer is coarse, and an excessively low cathode current density is rarely used in production. As the cathode current density increases, the polarization of the cathode also increases (the amount of increase in polarization depends on various plating solutions), and the crystals of the coating also become finer and denser; however, on the cathode The current density should not be too large and should not exceed the allowable upper limit (different plating solutions have different cathode current density limits under different process conditions). After exceeding the permissible upper limit, there is a serious lack of metal ions near the cathode. For the sake of reason, a metal plating layer shaped like a branch is generated at the tip and projection of the cathode, or a loose plating such as a sponge is formed on the entire surface of the cathode. Frequently encountered in production is the phenomenon of “burning” at sharp corners and edges of parts, and dendritic crystals or spongy coatings are formed in severe cases.
The temperature of the plating solution
When other conditions (that is, the voltage is constant, the ion diffusion rate is accelerated, the current will increase), the temperature of the solution will increase, which will usually speed up the cathode reaction rate and ion diffusion rate, and reduce the polarization of the cathode. Crystallized the coating. However, it cannot be considered that increasing the temperature of the solution is unfavorable. If the temperature is properly matched with other process conditions, raising the temperature of the solution will achieve good results. For example, increasing the temperature can increase the upper limit of the allowable cathode current density. The increase of the cathode current density increases the cathode polarization to compensate for the lack of temperature increase. This not only does not make the plating crystal thicker but also accelerates the deposition rate. Increase productivity. In addition, it can increase the conductivity of the solution, promote anodic dissolution, increase cathode current efficiency (except chrome plating), reduce pinholes, and reduce the effect of plating stress.
Stirring accelerates the convection of the solution, allowing the metal ions consumed near the cathode to be replenished in time and reducing the concentration polarization of the cathode. Therefore, under the same conditions, the stirring will cause the coating to crystallize.
The stirring plating solution must be filtered periodically or continuously to remove various solid impurities and scum from the solution, otherwise it will reduce the binding force of the coating and make the coating rough, loose and porous.
The power supplies commonly used in electroplating production include rectifiers and DC generators. Different current waveforms are available depending on the number of phases of the AC power supply and the rectifier circuit. For example, single-phase half-wave, single-phase full-wave, three-phase half-wave and three-phase full-wave. Practice has proved that the current waveform affects the crystallographic structure of the coating, the brightness, the dispersion ability and coverage of the bath, the alloy composition, and the consumption of additives. Therefore, the selection of the current waveform should be emphasized. In addition to the general DC power, periodic commutation currents and pulsed currents can also be used according to actual needs.