Electroplating is the process of plating a thin layer of other metals or alloys on some metal surfaces using the principle of electrolysis. Electroplating can enhance the corrosion resistance of metals (corrosion resistant metals are used for coating metals), increase hardness, prevent wear, improve electrical conductivity, lubricity, heat resistance, and beautiful surface. According to the composition of the coating, the electroplating process can be divided into chromium plating, copper plating, cadmium plating, tin plating, zinc plating, nickel plating, gold plating, silver plating a total of 8 categories. Next, xiaobian will introduce the principles and characteristics of 8 classifications of electroplating process in detail.
Chromium is a silver-white metal tinged with azure. It has strong passivation performance, atmospheric passivation quickly, showing the nature of precious metals, so the chromium plating layer of iron parts is the cathode coating. Chromium layer is very stable in the atmosphere, can maintain its luster for a long time, in alkali, nitric acid, sulfide, carbonate and organic acids and other corrosive media is very stable, but soluble in hydrochloric acid and other hydrohalogenic acid and hot concentrated sulfuric acid.
Chromium layer has high hardness, good wear resistance, strong reflective ability and good heat resistance. No obvious change in gloss and hardness was observed below 500°C. Oxidation discoloration begins when the temperature is greater than 500°C; It starts to soften above 700°C. Because of the excellent performance of chromium plating layer, it is widely used as the outer layer and functional coating of protective decorative coating system.
Copper plating layer is pink, soft, with good ductility, electrical conductivity and thermal conductivity, easy to polish, after appropriate chemical treatment can be bronze, copper green, black and natural color and other decorative colors. Copper plating is easy to lose luster in the air, and carbon dioxide or chloride action, the surface to generate a layer of basic copper carbonate or copper chloride film, by the action of sulfide will generate brown or black copper sulfide, therefore, as a decorative copper plating layer on the surface coated with organic coating.
Cadmium is a silvery white shiny soft metal, harder than tin, softer than zinc, malleable, easy to forge and roll. Cadmium is chemically similar to zinc, but does not dissolve in alkaline solution, is soluble in nitric acid and ammonium nitrate, and dissolves slowly in dilute sulfuric acid and dilute hydrochloric acid. Cadmium vapors and soluble cadmium salts are toxic, so cadmium pollution must be strictly prevented. Because cadmium pollution after the harm is very high, the price is usually used to replace the cadmium plating layer galvanized layer or alloy coating. In domestic production, more cadmium plating solution types are: ammonia carboxylic complex cadmium plating, acid sulfate cadmium plating and cyanide cadmium plating. In addition, there are pyrophosphate cadmium plating, alkaline triethanolamine cadmium plating and HEDP cadmium plating.
Tin has the appearance of silver white, atomic weight of 118.7, density of 7.3g/cm^3, melting point of 231.89℃, valence of bivalent and tetravalent, so the electrochemical equivalent of 2.12g/A.h and 1.107g/A.h respectively. Tin has the advantages of corrosion resistance, easy iron welding, softness and good ductility. Tin coating has the following characteristics and uses:
(1) High chemical stability;
(2) in electrochemical sequence tin standard potential PI iron is, for iron and steel is negative polarity coating, only when the coating without pores can effectively protect the matrix;
(3) tin conductivity is good, easy to weld;
(4) Tin from -130℃ crystallization began to mutate, to -300℃ will be completely transformed into a crystalline isomer, commonly known as "tin blast", at this time has completely lost the nature of tin;
(5) Tin and zinc, cadmium coating, in high temperature, wet and closed conditions can grow whiskers, known as long hair;
(6) After tin plating, it can be redissolved in hot oil above 231.89℃ to obtain a shiny pattern tin layer, which can be used as decorative coating for daily necessities.
Zinc is soluble in both acids and bases, so it is called a amphoteric metal. Zinc hardly changes in dry air. In humid air, a basic zinc carbonate film forms on the zinc surface. In the atmosphere of sulfur dioxide, hydrogen sulfide and Marine, zinc corrosion resistance is poor, especially in the atmosphere of high temperature and high humidity containing organic acids, zinc coating is easy to be corroded. The standard electrode potential of zinc is -0.76V, for the steel matrix, zinc coating belongs to the anode coating, it is mainly used to prevent the corrosion of steel, its protective performance and the thickness of the coating has a great relationship.
The protective and decorative properties of zinc coating can be significantly improved after passivation treatment, dyeing or coating with gloss agent. With the development of galvanizing technology and the use of high performance galvanizing brightener, galvanizing has entered the protection - decorative application from the pure purpose of protection.
Zinc plating solution has cyanide plating solution and non cyanide plating solution. Cyanide bath is divided into micro cyanide, low cyanide, medium cyanide, and high cyanide several categories. Cyanide-free plating solution has alkaline zinc acid plating solution, ammonium salt plating solution, sulfate plating solution and ammonia chloride plating solution. Cyaniding zinc plating solution has good plating ability, the obtained coating is smooth and delicate, and has been used in production for a long time. But because cyanide is highly toxic, serious pollution to the environment, has tended to use low cyanide, micro cyanide, cyanide-free zinc plating solution.
1, electroplating nickel layer in the air stability is very high, because the metal nickel has a strong passivation ability, on the surface can quickly generate a layer of very thin passivation film, can resist atmospheric, alkali and some acid corrosion.
2, electroplating nickel crystal is very small, and has excellent polishing performance. The polished nickel coating gives a mirror-like luster and maintains its luster in the atmosphere for a long time. Therefore, electroplating is often used for decoration.
3. The hardness of nickel coating is relatively high, which can improve the wear resistance of the surface of products. In the printing industry, nickel plating layer is commonly used to improve the hardness of lead surface. Because metal nickel has higher chemical stability, some chemical equipment is also commonly used thicker nickel coating, in order to prevent corrosion by the medium. Nickel plating is also widely used in functional aspects, such as repairing worn and corroded parts, and local electroplating by brush plating technology. The electroforming process is used to manufacture electroforming plates, record molds and other molds for the printing industry. The thick nickel coating has good wear resistance and can be used as a wear - resistant coating. Especially in recent years, the development of composite electroplating, can deposit a composite nickel coating with wear-resistant particles, its hardness and wear resistance is higher than the nickel plating layer. If graphite or fossil fluoride ink is used as dispersed particles, the ni-graphite or ni-fossil fluoride ink composite coating has good self-lubricity and can be used as lubrication coating. Black nickel coating is also widely used as optical instrument coating or decorative coating.
4, nickel plating application is very wide, can be used as a protective decorative coating, in steel, zinc die casting, aluminum alloy and copper alloy surface, protect the matrix material from corrosion or play a bright decorative role; It is also often used as the middle coating of other coatings, and then a thin layer of chromium or a layer of imitation gold layer is plated on it, which has better corrosion resistance and more beautiful appearance. In terms of functional applications, nickel plating on parts of special industries is about 1~3mm thick, which can achieve the purpose of repair. Especially in the continuous casting mold, electronic component surface mold, alloy die casting mold, complex shape aerospace engine parts and the manufacture of micro electronic components and so on more and more widely used.
Electroplated coating has strong corrosion resistance, good electrical conductivity, easy welding, high temperature resistance, and has a certain wear resistance (such as hard gold mixed with a small amount of other elements), has good resistance to discoloration, and gold alloy coating has a variety of colors, gold plating on silver can prevent discoloration. And the coating has good ductility and easy polishing, so it is often used as a decorative coating, such as plating jewelry, clock parts, art, etc.
It is also widely used in precision instruments, printing boards, integrated circuits, electronic tube shells, electrical contacts and other parts that require long-term stability of electrical parameter performance electroplating. However, due to the high price of gold, its application is limited.
The coating is used to prevent corrosion, increase conductivity, reflectance and aesthetics. Widely used in electrical appliances, instruments, meters and lighting appliances and other manufacturing industries. For example, when copper or copper alloy parts are plated with silver, they must be de-oiled and de-rusted; Then pre-plating thin silver or immersed in the solution made of mercury chloride, mercury treatment, so that a layer of mercury film is plated on the surface of the workpiece; The product is then used as a cathode and pure silver plate as an anode, immersed in a silver cyanide potassium electrolyte made of silver nitrate and potassium cyanide, for electroplating. Electrical appliances, instruments and other industries also use cyanide-free silver plating. Electroplating solution for thiosulfate, sulfite, thiocyanate, ferrocyanide, etc. In order to prevent silver coating discoloration, usually after treatment, mainly dip bright, chemical and electrochemical passivation, precious metal or rare metal plating or coating coating.