Plastic Plating-Electroplating Process
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The coating obtained by electroplating methods has the same mechanical, physical, chemical and other properties and compositions, but it is quite different from the surface protective layers obtained by other methods. The main reason is that the composition and organizational structure of the electroplating layer are unique., and this difference is determined by various factors such as the chemical reaction on the electrode surface during electroplating, the electrocrystallization process and the working conditions during electroplating.
Electroplating reaction is a typical electrolytic reaction, the most typical plating bath and the direction of movement of ions in the bath. The basic issues and theoretical explanations involved in electroplating belong to the category of electrochemistry, and the changes in the physical properties of the deposited layer need to be studied from the perspective of metallurgy. From the perspective of surface phenomena, electroplating is a process in which metal ions in solution gain electrons on the cathode surface under the action of applied current, are reduced to metal and deposited on its surface, but the actual situation is much more complicated. When metal ions undergo cathodic reduction at a certain current density, their deposition potential is equal to the sum of its equilibrium potential and overpotential.
In theory, as long as the potential of the cathode is negative enough, any metal ions may be reduced and deposited on it. However, when electrodeposition is carried out in an aqueous solution, there are competitive reduction reactions of various ions such as hydrogen ions and easily reduced anions on the cathode, so some metal ions with very negative reduction potential cannot be reduced and deposited on the electrode. In other words, whether metal ions can be reduced in aqueous solutions depends not only on their own electrochemical properties, but also on the reduction potential or deposition potential of hydrogen ions and the like on the electrode. It can be seen from the calculation formula of the deposition potential that the concentration of ions has a certain influence on whether they can be precipitated, but the key is the overpotential, and its magnitude directly affects the deposition of metal.
Generally, the metal electrodeposition process includes the following basic steps.
(1)Liquid-phase mass transfer step: During metal electrodeposition, metal ions near the cathode surface participate in the cathode reaction and are rapidly consumed, forming a concentration gradient with the concentration of metal ions gradually increasing from cathode to anode. Metal hydrated ions or complex ions transfer to the cathode surface in the solution through electromigration, diffusion and convection.
(2)Electrochemical reduction step: It includes pre-conversion and charge transfer. In most electroplating solutions, the form in which metal ions participate in electrode reactions is different from their main form in the solution. Before electrochemical reduction, its main existing form undergoes chemical transformation near or on the surface of the cathode and is transformed into a form that participates in electrode reactions. This process is called the pre-conversion step. Then, the metal ions gain electrons on the cathode surface in this form and are reduced to metal atoms. This process is called a charge transfer step. For most multivalent metal ions, charge transfer is not completed in one step.
(3)Electrocrystallization step: Metal atoms form a new phase on the cathode surface, including the formation and growth of crystal nuclei.
Different plating species or different solutions of the same plating species, under different conditions, the above steps are carried out at different speeds. The slowest step controls the plating speed and becomes the "control step". From the change of cathode potential during the electroplating process and the thickness of the coating crystal, it can be analyzed which step is the control step of the electroplating process: if the cathode potential is almost unchanged as the cathode current density increases; or the cathode potential quickly becomes negative, while the cathode current density is unchanged, approaching or reaching the limit current density. This phenomenon indicates that concentration polarization has occurred. The electrodeposition process is controlled by a liquid-phase mass transfer process, which can be improved through mechanical agitation; If at a relatively low current density, a large negative shift occurs in the cathode potential, which indicates that electrochemical polarization has occurred and the plating process is controlled by the electrochemical reduction step. Generally electrochemical polarization occurs at relatively low current densities. As the current density gradually increases, the electrochemical polarization changes to concentration polarization, and the process at this time shows mixed control.
The electrocrystallization process of metals is similar to the crystallization process of salts in salt solutions. The equilibrium potential state corresponds to the saturated state of the solution, while the overpotential of the cathode corresponds to the supersaturation of the solution. The greater the cathodic polarization of the solution when metal is precipitated, the higher the overpotential, the faster the crystal nuclei are formed, and the finer the grains of the coating will be; on the contrary, the formation rate of crystal nuclei is lower than the growth rate, and the grains of the coating will be thicker. The thickness of the coating grains can be changed by adjusting the composition and ratio of the solution, adding appropriate additives to the solution, and controlling the process parameters during electroplating.
During the electroplating process, the structure of the base metal often affects the precipitated metal in various ways. The electrodeposition process is also carried out by nucleation and growth. In fact, it is always the first to nucleate by adsorbed atoms at kink or steps on the surface of the base metal, and then gradually grow through diffusion, because this requires the smallest thermodynamic driving force. The diffusion step of the adsorbed atoms controls the growth rate of the crystal, and the diffusion rate of the adsorbed atoms is directly related to their atomic concentration, which in turn is determined by the magnitude of the overpotential.
If the lattice spacing of the plated metal is exactly the same as the lattice spacing of the crystal surface of the base metal, the crystal structure of the base metal will be continued into the plated metal. This phenomenon is called liquid-phase epitaxial growth. It generally occurs when coating formation and growth
Initial stage. Because the overpotentials of metal electrodeposition on different crystal surfaces are different, the metal deposition rates are different, resulting in different growth rates of crystal surfaces, which will change the original crystal structure and appear new crystal surfaces.
The crystal morphology of the coating is roughly of basic types such as layered, massive, and pyramidal. Under certain conditions, the electrocrystallization structure appears preferential orientation and forms a crystalline structure.
