electroplating process
Electroplating process is a method of laying a metal layer on the conductive body using the principle of electrolysis. Electroplating refers to a surface processing method in which the cations of the preplated genus are deposited on the surface of the substrate metal through electrolysis in the salt solution containing the preplated genus, taking the substrate metal as the cathode.
The properties of the coating are different from those of the base metal. According to the function of the coating is divided into protective coating, decorative coating and other functional coating.
Electroplating, plating metal or other insoluble materials as the anode, the workpiece to be plated as the cathode, plating metal cations on the surface of the workpiece to be plated is reduced to form a coating. It can enhance the corrosion resistance of the metal (coated metal is mostly corrosion-resistant metal), increase hardness, prevent wear, improve electrical conductivity, smooth, heat resistance and surface appearance.
Fundamentals editing
Electroplating is an electrochemical process as well as a REDOX process. The basic process of electroplating is to immerse the parts in a solution of metal salt as the cathode, the metal plate as the anode, after connecting to the DC power supply, the required coating is deposited on the parts. For example, in nickel plating, the cathode is the part to be plated, and the anode is the pure nickel plate. The following reactions occur respectively in the anode and cathode:
Cathode (plating) : Ni2++2e→Ni (main reaction)
2H++2e→H2↑ (side reaction)
Anode (nickel plate) : Ni-2e→Ni2+ (main reaction)
4OH--4e→2H2O+O2 (side reaction)
Not all metal ions can be deposited from aqueous solution, and it is difficult to precipitate metal ions on the cathode if the side reaction of reducing hydrogen ions to hydrogen on the cathode is dominant. According to the experiment, the possibility of electrodeposition of metal ions from aqueous solution can be obtained from the periodic table of elements.
Anodes are divided into soluble anodes and insoluble anodes, and most anodes are soluble anodes corresponding to the coating, such as: Zinc plating for zinc anode, silver plating for silver anode, tin-lead alloy using tin-lead alloy anode, but a few electroplating due to the anode dissolution difficulties, the use of insoluble anode, such as acidic gold plating is more than platinum or titanium anode, plating solution by adding salt ions prepared by the standard gold solution to supplement, chromium plating anode using pure lead, lead-tin alloy, lead-antimony alloy and other insoluble anode. [1]
Electroplating classification editing
Classification by coating composition:
(1) Chrome plating. Chromium is a silver-white metal with a slight sky blue tinge. Although the electrode potential is very negative, it has a strong passivation performance, quickly passivation in the atmosphere, 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 acid and other corrosive media is very stable, but soluble in hydrochloric acid and other halohalic acid and hot concentrated sulfuric acid.
The chromium layer has high hardness, good wear resistance, strong reflective ability and good heat resistance. Under 500°C, there is no obvious change in luster and hardness. The temperature is greater than 500°C begins to oxidize and change color; It doesn't start to soften until 700 degrees. Because of its excellent properties, chrome coating is widely used as the outer layer of protective decorative coating system and functional coating.
(2) copper plating. The copper plating layer is pink, soft, has good ductility, electrical conductivity and thermal conductivity, easy to polish, after appropriate chemical treatment can obtain bronze, copper green, black and natural color decoration. Copper plating is easy to lose luster in the air, and carbon dioxide or chloride action, the surface of a layer of basic copper carbonate or copper chloride film, by the action of sulfide will produce brown or black copper sulfide, therefore, as a decorative copper plating layer needs to be coated with organic coating on the surface.
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Copper plating
(3) cadmium plating. Cadmium is a silvery white glossy soft metal, its hardness is harder than tin, softer than zinc, plasticity, easy to forge and roll. Cadmium is chemically similar to zinc, but does not dissolve in lye, dissolves in nitric acid and ammonium nitrate, and dissolves very slowly in dilute sulfuric acid and hydrochloric acid. Cadmium vapor and soluble cadmium salt are toxic, so cadmium pollution must be strictly prevented. Because cadmium pollution is very harmful and expensive, zinc or alloy coating is usually used to replace cadmium plating. The types of cadmium plating solutions widely used in domestic production 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.
(4) tin plating. Tin has a silver-white appearance, an atomic weight of 118.7, a density of 7.3g/cm^3, a melting point of 231.89℃, and a valence of two and four valence, so the electrochemical equivalent of 2.12g/A.h and 1.107g/A.h, respectively. Tin has the advantages of corrosion resistance, non-toxic, easy iron welding, softness and good ductility. Tin coating has the following characteristics and uses:
1. High chemical stability;
2. The standard potential of tin in the electrochemical sequence is that the iron is positive, which is negative polarity coating for steel. Only when the coating has no pores can it effectively protect the substrate;
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 fever", at this time has completely lost the nature of tin;
5. Tin, like zinc and cadmium coating, can grow into whiskers under high temperature, humidity and closed conditions, known as long hair; 6, after tin plating in 231.89℃ above hot oil resolution treatment, can obtain the glossy pattern tin layer, can be used as a decorative coating of daily use.
tinning
tinning
(5) Galvanized. Zinc dissolves easily in acids and also in alkalis, so it is called an amphoteric metal. Zinc changes little in dry air. In moist air, a basic zinc carbonate film forms on the surface of zinc. In sulfur dioxide, hydrogen sulfide and Marine atmosphere, zinc corrosion resistance is poor, especially in high temperature and high humidity atmosphere containing organic acid, zinc coating is easy to be corroded. The standard electrode potential of zinc is -0.76V, for the steel substrate, zinc coating belongs to the anodic coating, it is mainly used to prevent the corrosion of steel, its protective performance and the thickness of the coating is very important.
galvanizing
galvanizing
The protective and decorative properties of zinc coating can be improved significantly after passivation treatment, dyeing or coating with light protection agent. With the development of galvanizing process and the use of high performance galvanizing brightener, galvanizing has entered the protective and decorative application from the simple purpose of protection.
There are two types of galvanizing solution: cyanide plating solution and cyanide - free plating solution. Cyanide plating solution is divided into mild cyanide, low cyanide, medium cyanide, and high cyanide. Cyanide-free plating solution includes alkaline zincate plating solution, ammonium plating solution, sulfate plating solution and ammonia-free chloride plating solution. Cyanide galvanizing solution has good plating ability, and the coating obtained is smooth and fine, which has been used for a long time in production. However, because cyanide is highly toxic and causes serious pollution to the environment, it tends to use low cyanide, small cyanide, cyanide-free galvanizing solution. [2]
Process editing
Generally including electroplating pretreatment, electroplating and plating after treatment three stages.
The complete process:
1. Acid leaching → whole plate copper plating → pattern transfer → acid oil removal → two-stage countercurrent rinsing → micro-etching → two-stage acid leaching → tinning → two-stage countercurrent rinsing
2. Counter-current rinsing → acid leaching → graphic copper plating → two-stage counter-current rinsing → nickel plating → two-stage washing → citric acid leaching → gold plating → Recovery →2-3 stage pure water washing → drying
Electroplating process of plastic shell:
Chemical degreasing → water washing → acetone leaching → water washing → chemical coarser water sensitization → water washing → activation → reduction → electroless copper plating → water washing bright sulfate copper plating → water washing → bright sulfate nickel plating → water washing → bright chromium plating → water drying and testing.
In the above process, bright sulfate copper plating is the most prone to failure, and its phenomena are poor deep plating ability, roughness, etc., poor deep plating ability should be treated differently. If the low current area is not bright, while the high current area is very bright and tends to be white, then whether N(ethylthiourea) is too much can be considered. Adjust the way to go, adding suitable amount of M (2 - mercapto narrowing Cuo truxene) and SP (poly two propane sulfonic acid sodium disulfide), if still no, 10 minutes to stir in 50 ~ 100 ml hydrogen peroxide test plating, if low current area is not bright, high current area is very bright and be bright orange, can consider to whether M too much, adjustment method is to add a moderate amount of N and P (polyethylene glycol), if still not, 50~100ml dioxy can be added. Water stirring for 10 minutes test plating, if the low current area is not bright, and the brightness of high current K is poor, you can consider whether N or M is too little, the adjustment method is to add the right amount of M or N(can also add the right amount of SP), if the high current area long burr, usually add appropriate SP can be eliminated. If there is fine hemp sand on the surface of beryllium layer either on or down, then M is too much, and appropriate amount of SP can be added to eliminate, but if there is hemp sand on the surface of the coating, then whether there is copper powder should be considered, and 50ml hydrogen peroxide can be added to eliminate.
Generally speaking, bright copper plating solution should be added at the end of every day 50ml hydrogen peroxide. In addition to bright copper plating is prone to the above faults, coarsening is also prone to faults, usually the surface of the home appliance shell after coarsening yellow or white powder. At this time, we should consider the following three aspects: first, whether the coarsening temperature is too high, too long time; Second, whether the content of sulfuric acid in the roughing liquid is excessive; 3. Whether the concentration and temperature of the organic solvent used before coarsening are too high and the time is too long. In addition, if the rework of household appliance shell coarsening again, easy coarsening excessive, resulting in plating is not bright, this should be appropriate to reduce the coarsening temperature and shorten the coarsening time, before coarsening, generally no longer give acetone. When electroless plating steel, the plating solution should be prevented from muddy (that is, a large amount of copper powder is produced). If the solution is muddy, the plastic shell of the tank must be completely reworked, and the solution of electroless plating mens should be filtered immediately. Generally, the electroless plating of household appliances should be plated immediately. Storage period is 2 days, hanging problem is also very important, with bandage method insulation hanging device is not suitable for plastic plating, because we electroplating ABS plastic and ring, from coarse to electroless copper plating do not use hanging device, but only in bright copper - bright nickel - bright chrome - bright copper - bright nickel - bright chrome -...... Is used during the cycle of the electrical cycle, and after the beryllium chromium (which is then entered into the copper plating solution), the chromium plating solution carried with the pendant could cause the deterioration of the bright copper plating solution. Similarly, the alcohol and chromium plating solution were contaminated by such a wrapped and isolated pendant. If limited by conditions, can only use the dressing method, the hanging device must be cleaned and then soaked in water for more than ten minutes, so that the penetration of the chrome plating solution into the dressing all ooze out, bright copper plating solution once slightly polluted with small current treatment 1 ~ 2 days.