Electroplating is the process of plating a thin layer of other metals or alloys on some metal surfaces by electrolysis. It is a process of attaching a metal film to the surface of metal or other materials by electrolysis to prevent metal oxidation ( Such as rust), improve wear resistance, electrical conductivity, light reflectivity, corrosion resistance (copper sulfate, etc.) and enhance the appearance of beauty. The outer layers of many coins are also electroplated.
During electroplating, a plated metal or other insoluble material is used as an anode, and a workpiece to be plated is used as a cathode, and a cation of the plated metal is reduced on the surface of the workpiece to be plated to form a plating layer. In order to eliminate the interference of other cations and make the plating layer uniform and firm, it is necessary to use a solution containing a plating metal cation as a plating solution to keep the concentration of the metal cation of the plating layer unchanged. The purpose of electroplating is to plate a metal coating on the substrate to alter the surface properties or dimensions of the substrate. Electroplating enhances the corrosion resistance of metals (the corrosion-resistant metal is mostly used for plating metals), increases hardness, prevents wear, improves electrical conductivity, smoothness, heat resistance and surface aesthetics.
Related role
A technique of depositing a good metal coating with different properties and matrix material on a mechanical article by means of an electrolytic cell principle. The electroplated layer is more uniform than the hot dip layer and is generally thin, ranging from a few microns to tens of microns. Through electroplating, it is possible to obtain decorative protective and various functional surface layers on mechanical products, as well as to repair worn and machined workpieces.
In addition, there are different roles depending on various plating needs. Examples are as follows:
1. Copper plating: used as a primer to improve the adhesion of the plating layer and the ability to resist corrosion. (Copper is easily oxidized. After oxidation, patina is no longer conductive, so copper-plated products must be protected by copper.)
2. Nickel plating: base or appearance, improve corrosion resistance and wear resistance, (where chemical nickel is more resistant to chrome in modern processes). (Note that many electronic products, such as DIN heads and N heads, no longer use nickel as the base, mainly because nickel is magnetic, which affects the passive intermodulation in electrical performance)
3. Gold plating: Improve the conductive contact resistance and improve signal transmission. (Gold is the most stable and expensive.)
4. Palladium-plated nickel: Improve the conductive contact resistance, improve signal transmission, and wear resistance higher than gold.
5. Tin-plated lead: Improves the welding ability and is quickly replaced by other substitutes (because lead is mostly changed to bright tin and matte tin).
6. Silver plating: Improve the conductive contact resistance and improve signal transmission. (Silver has the best performance, is easy to oxidize, and is also conductive after oxidation)
Electroplating is a method of laying a layer of metal on a conductor using the principle of electrolysis.
In addition to electrical conductors, electroplating can also be used on specially treated plastics.
The process of electroplating is basically as follows:
Plating metal at the anode
The substance to be plated is at the cathode.
The anode and cathode are connected by an electrolyte solution composed of plated metal positive ions.
After passing through the DC power supply, the metal of the anode will oxidize (loss electrons), and the positive ions in the solution will be reduced (obtained electrons) at the cathode to form atoms and accumulate on the surface of the cathode.
The appearance of the plated object after electroplating is related to the magnitude of the current. The smaller the current, the more beautiful the object to be plated; otherwise, the uneven shape will appear.
The main uses of electroplating include preventing metal oxidation (such as rust) and decorating. The outer layers of many coins are also electroplated.
Sewage from electroplating (such as electrolytes that are ineffective) is an important source of water pollution. Electroplating processes have been widely used in semiconductor and microelectronic component leadframe processes.
VCP: Vertical continuous plating, the new machine used in the circuit board, better quality than traditional suspension plating.
Partial silver plating
Aluminum parts plating solution formulation process:
High-temperature weak alkali etching→cleaning→acid washing→cleaning→zinc-plating→cleaning→secondary zinc immersion→cleaning→pre-plating copper→cleaning→pre-silvering→cyanide bright silver plating→recycling washing→cleaning→silver protection→cleaning → Drying.
From the process flow, the selected protective material must be resistant to high temperatures (about 80 ° C), alkali resistance, acid resistance, and secondly, the protective material can be easily peeled off after silver plating.
Commercially available protective materials include peelable rubber, peelable paint, general adhesive tape, and tape. The protective materials were tested for acid resistance, alkali corrosion resistance, high temperature resistance (the maximum temperature of the alkali etching solution is about 80 ° C), and peelability.
Material requirements
The coating is mostly a single metal or alloy, such as titanium palladium, zinc, cadmium, gold or brass, bronze, etc.; also has a dispersion layer, such as nickel-silicon carbide, nickel-fluorinated graphite, etc.; and a laminate layer, such as steel Copper-nickel-chromium layer, silver-indium layer on steel, and the like. In addition to iron-based cast iron, steel and stainless steel, electroplated base materials are non-ferrous metals, or ABS plastics, polypropylene, polysulfone and phenolic plastics, but must be specially activated and sensitized prior to plating.
Working principle
Electroplating requires a low-voltage, high-current power supply that supplies power to the plating bath and an electrolysis device consisting of a plating solution, a part to be plated (cathode), and an anode. The composition of the plating solution varies depending on the plating layer, but both contain a main salt providing a metal ion, a complexing agent capable of complexing a metal ion in the main salt to form a complex, a buffer for stabilizing the pH of the solution, an anode activator and Special additives (such as brighteners, grain refiners, levelers, wetting agents, stress relieving agents, and anti-fogging agents). The electroplating process is a process in which metal ions in the plating solution are reduced to metal atoms by electrode reaction under the action of an external electric field, and metal deposition is performed on the cathode. Therefore, this is a metal electrodeposition process including steps of liquid phase mass transfer, electrochemical reaction, and electrocrystallization.
In the plating tank containing the plating solution, the parts to be plated which have been cleaned and specially pretreated are used as the cathode, and the anode is made of the plated metal, and the two poles are respectively connected with the positive electrode and the negative electrode of the direct current power source. The plating solution is composed of an aqueous solution containing a plating metal compound, a conductive salt, a buffer, a pH adjuster, and an additive. After energization, the metal ions in the plating solution move to the cathode under the action of a potential difference to form a plating layer. The metal of the anode forms metal ions into the plating solution to maintain the concentration of the plated metal ions. In some cases, such as chrome plating, it is an insoluble anode made of lead, lead-bismuth alloy, which only acts to transmit electrons and conduct current. The concentration of chromium ions in the electrolyte is maintained by periodically adding a chromium compound to the plating solution. During plating, the quality of the anode material, the composition of the plating solution, the temperature, the current density, the energization time, the stirring strength, the precipitated impurities, the power supply waveform, etc. all affect the quality of the coating and need to be controlled at the right time.
First, the plating solution has six elements: a main salt, an additional salt, a complexing agent, a buffer, an anodic activator, and an additive.
The electroplating principle consists of four aspects: electroplating solution, electroplating reaction, electrode and reaction principle, and electrodeposition process of metal.
Electrochemical reaction in the electroplating reaction: The following figure is a schematic diagram of the electroplating apparatus. The plated part is a cathode, connected to the negative electrode of the DC power source, the metal anode is coupled with the positive electrode of the DC power source, and the anode and the cathode are both immersed in the plating solution. When a certain potential is applied between the anode and the cathode, the following reaction occurs at the cathode: the metal ions Mn+ diffused from the inside of the plating solution to the interface between the electrode and the plating solution obtain n electrons from the cathode, and are reduced to the metal M. On the other hand, at the anode, a reaction completely opposite to that of the cathode occurs, that is, dissolution of the metal M occurs at the anode interface, and n electrons are generated to generate metal ions Mn+.
Reaction mechanism
A, electrode potential
When the metal electrode is immersed in a solution containing the metal ion, there is a balance that the metal is deelectronized and the reaction dissolved in the solution and the reverse reaction of the metal ion 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 influence of the nature of the substance on the equilibrium potential, it is stipulated that when the solution temperature is 250 ° C and the concentration of the metal ion is 1 mol / L, the measured potential is called the standard electrode potential. Metals with a large negative electrode potential are prone to loss of electrons, while metals with a large positive electrode potential are susceptible to electron reduction.
B, polarization
Polarization refers to the phenomenon that the potential of the electrode deviates from the potential of the balanced electrode when a current passes through the electrode. Therefore, the current-potential curve is called a polarization curve. The main reasons for the polarization are electrochemical polarization and concentration polarization.
1. Electrochemical polarization
Since the electrochemical reaction speed on the cathode is smaller than the speed at which the external power source supplies electrons, the polarization of the electrode potential is shifted in the negative direction.
2. Concentration polarization
The polarization due to the difference between the concentration of the surface layer of the adjacent electrode and the concentration of the solution body is called polarization polarization, which is caused by the ion diffusion speed in the solution being smaller than the electron motion.
The electroplating process is a process in which metal ions in the plating solution are reduced to metal atoms by electrode reaction and metal deposition is performed on the cathode under the action of an external electric field.
The electroplating principle is simply that in the salt solution containing the metal to be plated, the plated metal is used as a cathode, and the cation to be plated in the plating solution is deposited on the surface of the base metal by electrolysis to form a plating layer.
Elements of electroplating:
1. Cathode: The object to be plated refers to various connector terminals.
2. Anode: If it is a soluble anode, it is intended to be metallized. In the case of insoluble anodes, most of them are precious metals (platinum, ruthenium oxide).
3. Electroplating solution: Contains electroplating solution to be plated with metal ions.
4. Electroplating tank: It can withstand and store the tank of electroplating syrup. Generally consider the factors of strength, corrosion resistance and temperature resistance.
5. Rectifier: A device that provides DC power.
(polishing→polishing)→hanging→degreasing degreasing→water washing→(electropolishing or chemical polishing)→acid washing activation→(pre-dip)→electroplating→water washing→(post-treatment)→washing→drying→downing→inspection package
Plating operating conditions refer to operational variations in plating, including: current density, temperature, agitation, and power supply waveforms.
Plating method
Electroplating is divided into hanging plating, barrel plating, continuous plating and brush plating, etc., mainly related to the size and batch of the parts to be plated. Hang plating is suitable for general-sized products such as bumpers for automobiles, handlebars for bicycles, etc. Barrel plating is suitable for small parts, fasteners, washers, pins, etc. Continuous plating is suitable for wire and strip production in batches. Brush plating is suitable for partial plating or repair. The electroplating solution is acidic, alkaline and acidic and neutral solution with chrome mixture. Regardless of the plating method, the plating tank and hanging hanger which are in contact with the product to be plated and the plating solution should have a certain degree. Versatility.
Plating classification
According to the composition of the coating, it can be divided into three types: single metal plating, alloy plating and composite plating.
If classified by purpose, it can be divided into:
1. protective coating;
2. protective decorative coating;
3. decorative coatings;
4. repair coating;
5. functional coating
Single metal plating：
Single metal plating has been in existence for more than 170 years, and 33 metals have been electrodeposited from aqueous solutions on the periodic table. Commonly used are electroplated zinc, nickel, chromium, copper, tin, iron, cobalt, cadmium, lead, gold, silver and other more than 10 kinds. A plating layer formed by simultaneously depositing two or more elements on the cathode is an alloy plating layer. The alloy coating has the structure and properties not possessed by a single metal plating, such as an amorphous Ni-P alloy, each of the Sn alloys not present on the phase diagram, and has a special decorative appearance, particularly high corrosion resistance and excellent weldability. , magnetic alloy plating, etc.
Composite plating：
Composite plating is a process in which solid particles are added to a bath to co-deposit with a metal or alloy to form a metal-based surface composite to meet specific application requirements. According to the electrochemical properties between the coating and the base metal, the plating layer can be divided into two types: anodized coating and cathodic coating. Where the potential of the plated metal relative to the base metal is negative, the plating layer is an anode when the micro cell is formed, so it is called an anodized layer, such as a galvanized layer on the steel piece; and the potential of the plated metal relative to the base metal is positive, When the corrosive microbattery is formed, the plating layer is a cathode, so it is called an cathodic coating layer, such as a nickel plating layer and a tin plating layer on steel parts.
Classified by purpose can be divided into:
1. protective coating: such as Zn, Ni, Cd, Sn and Cd-Sn coating, as an anti-corrosion coating resistant to atmospheric and various corrosive environments;
2. protection. Decorative coating: such as Cu-Ni-Cr, Ni-Fe-Cr composite coating, etc., both decorative and protective;
3. decorative coating: such as Au, Ag and Cu. Sun imitation gold plating, black chrome, black nickel plating, etc.;
4. remediation coating: such as electroplating Ni, Cr, Fe layer to repair some expensive wearing parts or processing over-difference parts;
5. functional coating: such as Ag, Au and other conductive coating; Ni-Fe, Fe-Co, Ni-Co and other magnetically conductive coating; Cr, Pt-Ru and other high temperature oxidation resistant coating; Ag, Cr and other reflective coating; black chrome, Anti-reflective coating such as black nickel; wear-resistant coating such as hard chrome and Ni.SiC; Ni.VIEE, Ni.C (graphite) anti-friction coating; Pb, Cu, Sn, Ag and other weldable coatings.