For read series, please refer to 1, 2, 3.
Electroplating is a process that uses principle of electrolysis to plate a thin layer of other metals or alloys on the surface of certain metals. It is a process that uses electrolysis to attach a metal film to the surface of metal or other material parts to prevent metal oxidation (such as rust), improves wear resistance, conductivity, reflectivity, corrosion resistance (copper sulfate, etc.) and improves appearance. Outer layer of many coins is also electroplated.

 

A technology that uses principle of electrolytic cells to deposit metal coatings on mechanical products that adhere well but have different properties and base materials. Electroplated layer is more uniform than hot dip layer and is generally thinner, ranging from a few microns to dozens of microns. Through electroplating, decorative protective and various functional surface layers can be obtained on mechanical products, workpieces with wear and processing errors can also be repaired.
In addition, it has different functions according to various electroplating needs. Examples are as follows:
1. Copper plating: used as a primer to improve adhesion and corrosion resistance of electroplating layer. (Copper is easily oxidized. After oxidation, patina no longer conducts electricity, so copper-plated products must be protected by copper)
2. Nickel plating: used as base or appearance to improve corrosion resistance and wear resistance (chemical nickel has a higher wear resistance than chromium plating in modern processes). (Note that many electronic products, such as DIN connectors and N connectors, no longer use nickel base, mainly because nickel is magnetic and will affect passive intermodulation in electrical performance)
3. Gold plating: Improve conductive contact resistance and enhance signal transmission. (Gold is the most stable and the most expensive.)
4. Palladium-plated nickel: improves conductive contact resistance, enhances signal transmission, and has higher wear resistance than gold.
5. Tin-lead plating: improves soldering ability and is soon to be replaced by other substitutes (most lead-containing products are now plated with bright tin and matte tin).
6. Silver plating: Improve conductive contact resistance and enhance signal transmission. (Silver has the best performance, is easily oxidized, and also conducts electricity after oxidation)

Electroplating is divided into rack plating, barrel plating, continuous plating and brush plating, which are mainly related to size and batch size of parts to be plated. Rack plating is suitable for products of general size, such as car bumpers, bicycle handlebars, etc. Barrel plating is suitable for small parts such as fasteners, washers, pins, etc. Continuous plating is suitable for batch production of wire and strip. Brush plating is suitable for local plating or repair. Electroplating solutions include acidic, alkaline, acidic and neutral solutions with added chromium compounds. No matter what plating method is used, plating tank, hangers, etc. that come into contact with products to be plated and plating solution should have a certain degree of versatility.

According to composition of coating, it can be divided into three categories: single metal coating, alloy coating and composite coating.
If classified according to use, it can be divided into: ① protective coating; ② protective decorative coating; ③ decorative coating; ④ repair coating; ⑤ functional coating

Single metal electroplating has a history of more than 170 years, and 33 metals on periodic table of elements can be produced by electrodeposition from aqueous solutions. Commonly used ones include electroplating of zinc, nickel, chromium, copper, tin, iron, cobalt, cadmium, lead, gold, silver and more than 10 kinds. Coating formed by depositing two or more elements simultaneously on cathode is an alloy coating. Alloy coatings have structural structures and properties that single metal coatings do not have, such as amorphous Ni-P alloys, SN alloys that are not shown in phase diagram, alloy coatings with special decorative appearance, particularly high corrosion resistance, excellent weldability, and magnetic properties.

 

Composite plating is a process in which solid particles are added to plating solution and co-deposited with metal or alloy to form a metal-based surface composite material to meet special application requirements. According to electrochemical properties between coating and base metal, electroplating can be divided into two categories: anodic coating and cathodic coating. When potential of coating metal relative to base metal is negative, coating acts as an anode when forming a corrosion microbattery, so it is called an anodic coating, such as zinc coating on steel parts; when potential of coating metal relative to base metal is positive, coating acts as cathode when forming a corrosion microbattery, so it is called cathodic coating, such as nickel plating and tin plating on steel parts.
It can be divided into:
① Protective coatings: coatings such as Zn, Ni, Cd, Sn and Cd-Sn are used as anti-corrosion coatings that are resistant to the atmosphere and various corrosive environments;
② Protection. Decorative coatings: such as Cu-Ni-Cr, Ni-Fe-Cr composite coatings, etc., which are both decorative and protective;
③ Decorative coating: such as Au, Ag and Cu. Sun imitation gold plating, black chrome, black nickel plating, etc.;
④ Reparative coating: such as electroplating Ni, Cr, Fe layer to repair some expensive wear parts or processing out-of-tolerance parts;
⑤ Functional coatings: conductive coatings such as Ag and Au; magnetic coatings such as Ni-Fe, Fe-Co and Ni-Co; high-temperature anti-oxidation coatings such as Cr and Pt-Ru; reflective coatings such as Ag and Cr; anti-reflective coatings such as black chromium and black nickel; wear-resistant coatings such as hard chromium, Ni and SiC; anti-wear coatings such as Ni, VIEE, Ni, C (graphite); weldable coatings such as Pb, Cu, Sn and Ag; Pb, Cu, Sn, Ag; anti-carburization Cu plating, etc.

Electroplating requires a low-voltage, high-current power supply that supplies power to electroplating tank and an electrolytic device consisting of a plating solution, parts to be plated (cathode) and anode. Composition of electroplating solution varies depending on coating, but they all contain main salts that provide metal ions and can complex
It is a complexing agent that combines metal ions in main salt to form a complex, a buffer used to stabilize pH of solution, an anode activator and special additives (such as brighteners, grain refiners, levelers, wetting agents, stress relievers and fog suppressors, etc.). Electroplating process is a process in which metal ions in plating solution are reduced to metal atoms through electrode reactions under action of an external electric field, and metal deposition is performed on cathode. Therefore, this is a metal electrodeposition process that includes steps such as liquid phase mass transfer, electrochemical reaction, and electrocrystallization.
In plating tank containing electroplating solution, cleaned and specially pretreated parts to be plated serve as cathode, and anode is made of plated metal. The two poles are connected to negative and positive poles of DC power supply respectively. Electroplating solution consists of an aqueous solution containing metal plating compounds, conductive salts, buffers, pH adjusters, additives, etc. After energization, metal ions in electroplating solution move to cathode under action of potential difference to form a plating layer. Metal of anode forms metal ions into plating solution to maintain concentration of metal ions being plated. In some cases, such as chromium plating, an insoluble anode made of lead or lead-antimony alloy is used, which only serves to transfer electrons and conduct current. Concentration of chromium ions in electrolyte needs to be maintained by regularly adding chromium compounds to plating solution. During electroplating, quality of anode material, composition of plating solution, temperature, current density, power-on time, stirring intensity, precipitated impurities, power supply waveform, etc. will all affect quality of coating and need to be controlled in a timely manner.
First of all, electroplating solution has six elements: main salt, additional salt, complexing agent, buffer, anode activator and additives.
Principle of electroplating includes four aspects: electroplating solution, electroplating reaction, electrode and reaction principle, and metal electrodeposition process.
Electrochemical reaction in electroplating reaction: Figure below is a schematic diagram of electroplating device. Part to be plated is cathode, which is connected to negative electrode of DC power supply. Metal anode is connected to positive electrode of DC power supply. Both anode and cathode are immersed in plating solution. When a certain potential is applied between cathode and anode, following reaction occurs at cathode: metal ions Mn+ that diffuse from inside plating solution to interface between electrode and plating solution obtain n electrons from cathode and are reduced to metal M. On the other hand, completely opposite reaction to that at cathode occurs at anode, that is, dissolution of metal M occurs at anode interface, releasing n electrons to generate metal ions Mn+.

 

For more read, pleast refer to Surface treatment 2: aluminum oxidation.