Electroplating is the process of plating a thin layer of other metals or alloys on some metal surfaces by using electrolysis principle, to prevent metal oxidation (such as rust), improve wear resistance, conductivity, reflectivity and corrosion resistance and improve the aesthetic effect.
In electroplating, the coating metal or other insoluble materials are used as anodes, and the workpiece to be plated is used as cathodes. The cations of the coating metal are reduced to form a coating on the surface of the workpiece to be plated. To eliminate the interference of impurity cations and make the coating layer uniform and firm, the solution containing the coated metal cations should be used as the electroplating solution to keep the concentration of the coated metal cations unchanged.
Besides the basic anti-corrosion, electroplating also has other effects.
Examples are as follows:
1. Copper plating
Copper plating used as a base coating before another metal coating to improve the adhesion and corrosion resistance of electroplating. (Copper is easy to oxidize, after oxidation, copper green no longer conducts electricity, so copper-plated products must be Passivated.)
2. Nickel plating
Nickel plating used as both base coating and appearance coating to improve corrosion resistance and wearing resistance.
Same as Chromium, Nickel can form a very thin passivation layer in the air which can resist the corrosion of atmosphere, alkali, and some acids.
Electroplated nickel has very fine crystals and excellent polishing properties. The polished nickel coating can obtain a long-lasting mirror-like gloss. Therefore, electroplating is often used for decoration. The hardness of nickel coating is relatively high, which can improve the wear resistance of the product surface.
The thick nickel plating layer has good wear resistance and can be used as a wear-resistant coating. Especially in recent years, composite electroplating has been developed, which can deposit composite nickel coating with wear-resistant particles. Its hardness and wear resistance are higher than those of pure nickel coating. If graphite or graphite fluoride is used as dispersed particles, the obtained Ni-graphite or Ni-graphite fluoride composite coatings have good self-lubrication and can be used as lubricating coatings.
Black nickel coating is also widely used as a coating or decorative coating for optical instruments. The black nickel coating layer is harder but wears resistance and corrosion resistance is not as good as nickel coating, and directly plating black nickel on steel parts, the adhesion between the coating and inner metal is rather poor, and the color will become lighter after polishing. Normally, a thin black nickel layer (only about 2 microns) is plated on the top of a bright nickel layer and then coated with paint or oil, to obtain an elegant and unique gun black nickel bright coating.
Black nickel coating can be obtained from Ni-Zn alloy plating bath and Ni-Sn alloy plating bath. In Ni-Zn black nickel plating, there are higher non-metallic phases, such as nickel sulfide, zinc sulfide, and organic compounds, etc. The formation of sulfide is due to the reduction of sulfur ion by thiocyanate radical on the cathode, and the formation of nickel ion and zinc ion in the plating bath, which is co-deposited with nickel on the cathode, and black is the plating layer. The presence of black nickel sulfide, or the color of the coating structure itself. Black nickel plating from black nickel plating bath contains 40-60% nickel, 20-30% zinc, 10-15% sulfur and about 10% organic matter. In electroplating, nickel electroplating has many excellent properties, and its popularity is second only to zinc electroplating.
3. Gold plating
Improve conductive contact impedance and enhance signal transmission. (Gold is a very chemical stable metal but expensive. )
4. Palladium-nickel plating
Improve the conductive contact impedance, enhance signal transmission, wearing resistance is better than gold plating.
5. Tin-plated lead
Improve welding ability and be replaced by other substitutes (because most of the lead is now replaced by bright tin and fog tin).
6. Silver plating
Improving conductive contact impedance and enhancing signal transmission. (Silver has the best properties, is easy to oxidize and conducts electricity after oxidation.)
The most common electroplatings are nickel plating, zinc plating, and chrome plating.
7. Zinc electroplating or electro-galvanized
Like many other surface treatment methods, Zinc electroplating forms a compact oxides layer （zinc oxides）protecting the inner metal from corrosion. Furthermore, in the case of electrochemical corrosion, zinc with high activity is the first to be corroded, not iron. In the zinc plating process, passivation has to be introduced at the final stage, different passivation creates results different color patterns on workpieces surface including rainbow (yellow), blue, black, white and deep green zinc coating. Deep green (some called ‘military green or army green‘) coating tends to provide the best protection but the hexavalent chrome used in the process is very environmental harmful. The black zinc coating is the most expensive.
Chromium metal is very easy to passivate in air, and a very thin passivation film is formed on the surface, which shows the properties of inert metals.
The hardness of the chromium plating layer is very high. The hardness of the chromium plating layer can vary from 400 to 1200 HV in a wide range according to the composition of plating solution and technological conditions. The chromium plating layer has good heat resistance. When heated below 500 C, the gloss and hardness of the chromium plating layer have no significant change. The chromium plating layer begins to oxidize and discolor when the temperature is higher than 500 C, and the hardness decreases when the temperature is higher than 700 C. The friction coefficient of the chromium plating layer is low, especially the dry friction coefficient, which is the lowest among all metals. And hence the chromium plating layer shows very good wearing resistance.
Chromium plating on iron-based alloys, zinc alloys, and aluminum alloys requires first copper plating and nickel plating as a base layer to increase the bonding strength of the chromium layer. Chromium plating is usually divided into decorative chromium and hard chromium plating (engineering chromium plating). Literally the thickness of decorative chromium plating layer is thinner and the hardness is lower, the hardness of hard chromium plating layer is higher (58 ~ 61 HRC), the thickness is thicker (more than 20 um), and the chromium layer thickness of parts with higher wearing resistance requirements can even reach several hundred um.
9. Electroplating for plastic
Plastic is a bad conductor, unlike metal materials, which can be directly electroplated. First, the plastic products are chemically roughened, so that it can absorb a layer of oxidizable substances, and then through redox reaction, the surface of the plastic products gains a layer of precious metal film (works as Catalyzer), and then through "copper precipitation", so that the surface of the plastic products deposits copper. Now the plastic can be electroplated like metal parts. The most common electroplating plastic is ABS. Teflon, Phenolic, Polycarbonate, Polysulfone, and Polyoxymethylene can also be electroplated.
About the Tolerances of Electroplating
The electroplating metal layer's thickness various from a few um to hundreds of um. Most electroplating also involves acid or/and alkali washing as a pre-plating treatment to remove oil and other unwanted substance. Both the plated metal layer and the pretreatment can affect the parts' size. If a tight tolerance is required on the workpiece, a carefully planned over cut has to be applied to leave enough room before electroplating is conducted. Another option is to over plated workpiece and then apply mechanical cutting or grinding after electroplating.
rack and barrel
In rack plating, workpieces are hanged on hangers then conducted with electricity while in barrel plating, workpieces are rolled in a barrel when electroplating is running. Rank plating could result in a more uneven metal layer even in a single piece, while barrel plating does not suffer such an issue. To gain a good tolerance, an after-plating mechanical process is often applied.
As a comparison, chemical plating also known as electroless plating can result in an easily controllable thickness of the coated metal layer.
A part of the hydrogen atoms generated during the electroplating or acid pickling of the metal parts escapes as a gas, and the other part diffuses into the internal stress concentration portion of the metal. This increases metal brittleness, makes metal parts easy to break or crack. Therefore, dehydrogenation treatment is often required after the electroplating process. Dehydrogenation usually involves heating the workpieces to 200-250 degrees Celsius for 2 hours.
Can Stainless Steel be electroplated?
The answer is yes but not as easy as other metals.
The stainless steel is stainless because it can form a dense oxidation layer that isolates the inner metal from the air. And once this layer is destroyed it can easily recover. That layer is not ‘active' for electroplating. To electroplate stainless steel, the first job is to destroy the layer and keep it from recovering. This is done in the pre-coating process, normally by an acid bath. And then nickel is coated on the stainless steel after that standard electroplating method can be applied.
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