Electroplating is a process that uses an electric current to reduce dissolved metal cations so that they form a thin coherent metal coating on an electrode. The term is also used for electrical oxidation of anions on to a solid substrate, as in the formation of silver chloride on silver wire to make silver/silver-chloride electrodes. Electroplating is primarily used to change the surface properties of an object (such as abrasion and wear resistance, corrosion protection, lubricity, aesthetic qualities), but may also be used to build up thickness on undersized parts or to form objects by electroforming.
Direct current (DC) is the unidirectional flow of electric charge. A battery is a good example of a DC power supply. Direct current may flow in a conductor such as a wire, but can also flow through semiconductors, insulators, or even through a vacuum as in electron or ion beams. The electric current flows in a constant direction, distinguishing it from alternating current (AC). A term formerly used for this type of current was galvanic current.
Redox is a chemical reaction in which the oxidation states of atoms are changed. Any such reaction involves both a reduction process and a complementary oxidation process, two key concepts involved with electron transfer processes. Redox reactions include all chemical reactions in which atoms have their oxidation state changed; in general, redox reactions involve the transfer of electrons between chemical species. The chemical species from which the electron is stripped is said to have been oxidized, while the chemical species to which the electron is added is said to have been reduced. It can be explained in simple terms:
An electrode is an electrical conductor used to make contact with a nonmetallic part of a circuit. The word was coined by William Whewell at the request of the scientist Michael Faraday from two Greek words: elektron, meaning amber, and hodos, a way.
The process used in electroplating is called electrodeposition. It is analogous to a concentration cell acting in reverse. The part to be plated is the cathode of the circuit. In one technique, the anode is made of the metal to be plated on the part. Both components are immersed in a solution called an electrolyte containing one or more dissolved metal salts as well as other ions that permit the flow of electricity. A power supply supplies a direct current to the anode, oxidizing the metal atoms that it comprises and allowing them to dissolve in the solution. At the cathode, the dissolved metal ions in the electrolyte solution are reduced at the interface between the solution and the cathode, such that they "plate out" onto the cathode. The rate at which the anode is dissolved is equal to the rate at which the cathode is plated and thus the ions in the electrolyte bath are continuously replenished by the anode.
Electrophoretic deposition (EPD), is a term for a broad range of industrial processes which includes electrocoating, cathodic electrodeposition, anodic electrodeposition, and electrophoretic coating, or electrophoretic painting. A characteristic feature of this process is that colloidal particles suspended in a liquid medium migrate under the influence of an electric field (electrophoresis) and are deposited onto an electrode. All colloidal particles that can be used to form stable suspensions and that can carry a charge can be used in electrophoretic deposition. This includes materials such as polymers, pigments, dyes, ceramics and metals.
In battery technology, a concentration cell is a limited form of a galvanic cell that has two equivalent half-cells of the same composition differing only in concentrations. One can calculate the potential developed by such a cell using the Nernst equation. A concentration cell produces a small voltage as it attempts to reach chemical equilibrium, which occurs when the concentration of reactant in both half-cells are equal. Because an order of magnitude concentration difference produces less than 60 millivolts at room temperature, concentration cells are not typically used for energy storage.
An electrolytic cell is an electrochemical cell that drives a non-spontaneous redox reaction through the application of electrical energy. They are often used to decompose chemical compounds, in a process called electrolysis—the Greek word lysis means to break up.
Other electroplating processes may use a non-consumable anode such as lead or carbon. In these techniques, ions of the metal to be plated must be periodically replenished in the bath as they are drawn out of the solution.The most common form of electroplating is used for creating coins, such as US pennies, which are made of zinc covered in a layer of copper.
Lead is a chemical element with symbol Pb and atomic number 82. It is a heavy metal that is denser than most common materials. Lead is soft and malleable, and also has a relatively low melting point. When freshly cut, lead is silvery with a hint of blue; it tarnishes to a dull gray color when exposed to air. Lead has the highest atomic number of any stable element and three of its isotopes each include a major decay chain of heavier elements.
Carbon is a chemical element with symbol C and atomic number 6. It is nonmetallic and tetravalent—making four electrons available to form covalent chemical bonds. It belongs to group 14 of the periodic table. Three isotopes occur naturally, 12C and 13C being stable, while 14C is a radionuclide, decaying with a half-life of about 5,730 years. Carbon is one of the few elements known since antiquity.
Zinc is a chemical element with symbol Zn and atomic number 30. It is the first element in group 12 of the periodic table. In some respects zinc is chemically similar to magnesium: both elements exhibit only one normal oxidation state (+2), and the Zn2+ and Mg2+ ions are of similar size. Zinc is the 24th most abundant element in Earth's crust and has five stable isotopes. The most common zinc ore is sphalerite (zinc blende), a zinc sulfide mineral. The largest workable lodes are in Australia, Asia, and the United States. Zinc is refined by froth flotation of the ore, roasting, and final extraction using electricity (electrowinning).
The cations associate with the anions in the solution. This cations are reduced at the cathode to deposit in the metallic, zero valence state. For example, for copper plating, in an acid solution, copper is oxidized at the anode to Cu2+ by losing two electrons. The Cu2+ associates with the anion SO2−
4 in the solution to form copper(II) sulfate. At the cathode, the Cu2+ is reduced to metallic copper by gaining two electrons. The result is the effective transfer of copper from the anode source to a plate covering the cathode.
Copper plating is the process of plating a layer of copper electrolytically on the surface of an item. It takes place in an electrolytic cell where electrolysis which uses direct electric current to dissolve a copper rod and transport the copper ions to the item. Into a container of water are placed a copper rod, and the item. The water contains an ionic solution which allows a direct electric current to flow from the copper rod to the item. The copper rod is the anode and the item is the cathode. This current flow causes the copper to ionize, become oxidized which means each atom becomes positively charged by losing an electron. As the copper ions dissolve into the water, they form a coordination complex with salts already present. The copper then physically flows to the item, where it is reduced to the metallic state by gaining electrons. This forms a thin, solid, metallic copper film on the surface of the item.
Copper(II) sulfate, also known as copper sulphate, are the inorganic compounds with the chemical formula CuSO4(H2O)x, where x can range from 0 to 5. The pentahydrate (x = 5) is the most common form. Older names for this compound include blue vitriol, bluestone, vitriol of copper, and Roman vitriol.
The plating is most commonly a single metallic element, not an alloy. However, some alloys can be electrodeposited, notably brass and solder. It should be noted that plated "alloys" are not true alloys, i.e. solid solutions, but rather discrete tiny crystals of the metals being plated. In the case of plated solder, it is sometimes deemed necessary to have a "true alloy", and the plated solder is melted to allow the Tin and Lead to combine to form a true alloy. The true alloy is more corrosion resistant than the as-plated alloy.
A chemical element is a species of atom having the same number of protons in their atomic nuclei. For example, the atomic number of oxygen is 8, so the element oxygen consists of all atoms which have exactly 8 protons.
An alloy is a combination of metals and of a metal or another element. Alloys are defined by a metallic bonding character. An alloy may be a solid solution of metal elements or a mixture of metallic phases. Intermetallic compounds are alloys with a defined stoichiometry and crystal structure. Zintl phases are also sometimes considered alloys depending on bond types.
Brass is an alloy of copper and zinc, in proportions which can be varied to achieve varying mechanical and electrical properties. It is a substitutional alloy: atoms of the two constituents may replace each other within the same crystal structure. In contrast, bronze is an alloy of copper and tin.
Many plating baths include cyanides of other metals (such as potassium cyanide) in addition to cyanides of the metal to be deposited. These free cyanides facilitate anode corrosion, help to maintain a constant metal ion level and contribute to conductivity. Additionally, non-metal chemicals such as carbonates and phosphates may be added to increase conductivity.
A cyanide is a chemical compound that contains the group C≡N. This group, known as the cyano group, consists of a carbon atom triple-bonded to a nitrogen atom.
Potassium cyanide is a compound with the formula KCN. This colorless crystalline salt, similar in appearance to sugar, is highly soluble in water. Most KCN is used in gold mining, organic synthesis, and electroplating. Smaller applications include jewelry for chemical gilding and buffing.
In chemistry, a carbonate is a salt of carbonic acid (H2CO3), characterized by the presence of the carbonate ion, a polyatomic ion with the formula of CO2−
3. The name may also refer to a carbonate ester, an organic compound containing the carbonate group C(=O)(O–)2.
When plating is not desired on certain areas of the substrate, stop-offs are applied to prevent the bath from coming in contact with the substrate. Typical stop-offs include tape, foil, lacquers, and waxes.
The ability of a plating to cover uniformly is called throwing power; the better the throwing power the more uniform the coating.
Initially, a special plating deposit called a strike or flash may be used to form a very thin (typically less than 0.1 μm thick) plating with high quality and good adherence to the substrate. This serves as a foundation for subsequent plating processes. A strike uses a high current density and a bath with a low ion concentration. The process is slow, so more efficient plating processes are used once the desired strike thickness is obtained.
The striking method is also used in combination with the plating of different metals. If it is desirable to plate one type of deposit onto a metal to improve corrosion resistance but this metal has inherently poor adhesion to the substrate, a strike can be first deposited that is compatible with both. One example of this situation is the poor adhesion of electrolytic nickel on zinc alloys, in which case a copper strike is used, which has good adherence to both.
Electrochemical deposition is generally used for the growth of metals and conducting metal oxides because of the following advantages: the thickness and morphology of the nanostructure can be precisely controlled by adjusting the electrochemical parameters; relatively uniform and compact deposits can be synthesized in template-based structures; higher deposition rates are obtained; and the equipment is inexpensive due to the non-requirements of either a high vacuum or a high reaction temperature.
A simple modification in electroplating is pulse electroplating. This process involves the swift alternating of the potential or current between two different values resulting in a series of pulses of equal amplitude, duration and polarity, separated by zero current. By changing the pulse amplitude and width, it is possible to change the deposited film's composition and thickness.
The experimental parameters of pulse electroplating usually consist of peak current/potential, duty cycle, frequency and effective current/potential. Peak current/potential is the maximum setting of electroplating current or potential. Duty cycle is the effective portion of time in certain electroplating period with the current or potential applied. The effective current/potential is calculated by multiplying the duty cycle and peak value of current or potential. Pulse electroplating could help to improve the quality of electroplated film and release the internal stress built up during fast deposition. Combination of the short duty cycle and high frequency could decrease the surface cracks. However, in order to maintain the constant effective current or potential, a high performance power supply may be required to provide high peak current/potential and fast switch. Another common problem of pulse electroplating is that the anode material could get plated and contaminated during the reverse electroplating, especially for the high cost, inert electrode like platinum.
Other factors that could affect the pulse electroplating include temperature, anode-to-cathode gap and stirring. Sometimes the pulse electroplating can be performed in heated electroplating bath to increase the depositing rate since the rate of almost all the chemical reaction increases exponentially with temperature per Arrhenius law. The anode-to-cathode gap is related to the current distribution between anode and cathode. Small gap to sample area ratio may cause uneven distribution of current and affect the surface topology of plated sample. Stirring may increase the transfer/diffusion rate of metal ions from bulk solution to the electrode surface. Stirring setting varies for different metal electroplating processes.
A closely related process is brush electroplating, in which localized areas or entire items are plated using a brush saturated with plating solution. The brush, typically a stainless steel body wrapped with an absorbent cloth material that both holds the plating solution and prevents direct contact with the item being plated, is connected to the anode of a low voltage direct current power source, and the item to be plated connected to the cathode. The operator dips the brush in plating solution then applies it to the item, moving the brush continually to get an even distribution of the plating material.
Brush electroplating has several advantages over tank plating, including portability, ability to plate items that for some reason cannot be tank plated (one application was the plating of portions of very large decorative support columns in a building restoration), low or no masking requirements, and comparatively low plating solution volume requirements. Disadvantages compared to tank plating can include greater operator involvement (tank plating can frequently be done with minimal attention), and inability to achieve as great a plate thickness.
Usually an electrolytic cell (consisting of two electrodes, electrolyte, and external source of current) is used for electrodeposition. In contrast, electroless deposition uses only one electrode and no external source of electric current. However, the solution for electroless deposition needs to contain a reducing agent so that the electrode reaction has the form:
In principle any hydrogen-based reducing agent can be used although the redox potential of the reducing half-cell must be high enough to overcome the energy barriers inherent in liquid chemistry.[ clarification needed ] Electroless nickel plating uses hypophosphite as the reducer while plating of other metals like silver, gold and copper, typically use low-molecular-weight aldehydes.
A major benefit of this approach over electroplating is that the power sources and plating baths are not needed, reducing the cost of production. This technique can also plate diverse shapes and types of surface. The downside is that plating is usually slower and cannot create thick plates of metal. As a consequence of these characteristics, electroless deposition is quite common in the decorative arts.
Cleanliness is essential to successful electroplating, since molecular layers of oil can prevent adhesion of the coating. ASTM B322 is a standard guide for cleaning metals prior to electroplating. Cleaning includes solvent cleaning, hot alkaline detergent cleaning, electrocleaning, and acid treatment etc. The most common industrial test for cleanliness is the waterbreak test, in which the surface is thoroughly rinsed and held vertical. Hydrophobic contaminants such as oils cause the water to bead and break up, allowing the water to drain rapidly. Perfectly clean metal surfaces are hydrophilic and will retain an unbroken sheet of water that does not bead up or drain off. ASTM F22 describes a version of this test. This test does not detect hydrophilic contaminants, but electroplating can displace these easily since the solutions are water-based. Surfactants such as soap reduce the sensitivity of the test and must be thoroughly rinsed off.
Electroplating changes the chemical, physical, and mechanical properties of the workpiece. An example of a chemical change is when nickel plating improves corrosion resistance. An example of a physical change is a change in the outward appearance. An example of a mechanical change is a change in tensile strength or surface hardness which is a required attribute in tooling industry.Electroplating of acid gold on underlying copper- or nickel-plated circuits reduces contact resistance as well as surface hardness. Copper-plated areas of mild steel act as a mask if case hardening of such areas are not desired. Tin-plated steel is chromium-plated to prevent dulling of the surface due to oxidation of tin.
Electroplating, or electroless plating may be used as a way to render a metal part radioactive, by using an aqueous solution prepared from nickel–phosphorus concentrates which contain radioactive hypophosphite 32P ions.
Modern electrochemistry was invented by Italian chemist Luigi Valentino Brugnatelli in 1805. Brugnatelli used his colleague Alessandro Volta's invention of five years earlier, the voltaic pile, to facilitate the first electrodeposition. Brugnatelli's inventions were suppressed by the French Academy of Sciences and did not become used in general industry for the following thirty years. By 1839, scientists in Britain and Russia had independently devised metal-deposition processes similar to Brugnatelli's for the copper electroplating of printing press plates.
Boris Jacobi in Russia not only rediscovered galvanoplastics, but developed electrotyping and galvanoplastic sculpture. Galvanoplastics quickly came into fashion in Russia, with such people as inventor Peter Bagration, scientist Heinrich Lenz and science fiction author Vladimir Odoyevsky all contributing to further development of the technology. Among the most notorious cases of electroplating usage in mid-19th century Russia were gigantic galvanoplastic sculptures of St. Isaac's Cathedral in Saint Petersburg and gold-electroplated dome of the Cathedral of Christ the Saviour in Moscow, the tallest Orthodox church in the world.
Soon after, John Wright of Birmingham, England discovered that potassium cyanide was a suitable electrolyte for gold and silver electroplating. Wright's associates, George Elkington and Henry Elkington were awarded the first patents for electroplating in 1840. These two then founded the electroplating industry in Birmingham from where it spread around the world. The Woolrich Electrical Generator of 1844, now in Thinktank, Birmingham Science Museum, is the earliest electrical generator used in industry.It was used by Elkingtons.
The Norddeutsche Affinerie in Hamburg was the first modern electroplating plant starting its production in 1876.
As the science of electrochemistry grew, its relationship to electroplating became understood and other types of non-decorative metal electroplating were developed. Commercial electroplating of nickel, brass, tin, and zinc were developed by the 1850s. Electroplating baths and equipment based on the patents of the Elkingtons were scaled up to accommodate the plating of numerous large scale objects and for specific manufacturing and engineering applications.
The plating industry received a big boost with the advent of the development of electric generators in the late 19th century. With the higher currents available, metal machine components, hardware, and automotive parts requiring corrosion protection and enhanced wear properties, along with better appearance, could be processed in bulk.
The two World Wars and the growing aviation industry gave impetus to further developments and refinements including such processes as hard chromium plating, bronze alloy plating, sulfamate nickel plating, along with numerous other plating processes. Plating equipment evolved from manually operated tar-lined wooden tanks to automated equipment, capable of processing thousands of kilograms per hour of parts.
One of the American physicist Richard Feynman's first projects was to develop technology for electroplating metal onto plastic. Feynman developed the original idea of his friend into a successful invention, allowing his employer (and friend) to keep commercial promises he had made but could not have fulfilled otherwise.
The Hull cell is a type of test cell used to qualitatively check the condition of an electroplating bath. It allows for optimization for current density range, optimization of additive concentration, recognition of impurity effects and indication of macro-throwing power capability.The Hull cell replicates the plating bath on a lab scale. It is filled with a sample of the plating solution, an appropriate anode which is connected to a rectifier. The "work" is replaced with a Hull cell test panel that will be plated to show the "health" of the bath.
The Hull cell is a trapezoidal container that holds 267 mL of solution. This shape allows one to place the test panel on an angle to the anode. As a result, the deposit is plated at different current densities which can be measured with a hull cell ruler. The solution volume allows for a quantitative optimization of additive concentration: 1 gram addition to 267 mL is equivalent to 0.5 oz/gal in the plating tank.
The Haring–Blum cell is used to determine the macro throwing power of a plating bath. The cell consists of two parallel cathodes with a fixed anode in the middle. The cathodes are at distances from the anode in the ratio of 1:5. The macro throwing power is calculated from the thickness of plating at the two cathodes when a direct current is passed for a specific period of time. The cell is fabricated out of perspex or glass.
Electrochemistry is the branch of physical chemistry that studies the relationship between electricity, as a measurable and quantitative phenomenon, and identifiable chemical change, with either electricity considered an outcome of a particular chemical change or vice versa. These reactions involve electric charges moving between electrodes and an electrolyte. Thus electrochemistry deals with the interaction between electrical energy and chemical change.
In chemistry and manufacturing, electrolysis is a technique that uses a direct electric current (DC) to drive an otherwise non-spontaneous chemical reaction. Electrolysis is commercially important as a stage in the separation of elements from naturally occurring sources such as ores using an electrolytic cell. The voltage that is needed for electrolysis to occur is called the decomposition potential.
A galvanic cell or voltaic cell, named after Luigi Galvani or Alessandro Volta, respectively, is an electrochemical cell that derives electrical energy from spontaneous redox reactions taking place within the cell. It generally consists of two different metals immersed in an electrolyte, or of individual half-cells with different metals and their ions in solution connected by a salt bridge or separated by a porous membrane.
Chrome plating, often referred to simply as chrome, is a technique of electroplating a thin layer of chromium onto a metal object. The chromed layer can be decorative, provide corrosion resistance, ease cleaning procedures, or increase surface hardness. Sometimes, a less expensive imitator of chrome may be used for aesthetic purposes.
Plating is a surface covering in which a metal is deposited on a conductive surface. Plating has been done for hundreds of years; it is also critical for modern technology. Plating is used to decorate objects, for corrosion inhibition, to improve solderability, to harden, to improve wearability, to reduce friction, to improve paint adhesion, to alter conductivity, to improve IR reflectivity, for radiation shielding, and for other purposes. Jewelry typically uses plating to give a silver or gold finish.
Electrowinning, also called electroextraction, is the electrodeposition of metals from their ores that have been put in solution via a process commonly referred to as leaching. Electrorefining uses a similar process to remove impurities from a metal. Both processes use electroplating on a large scale and are important techniques for the economical and straightforward purification of non-ferrous metals. The resulting metals are said to be electrowon.
The Daniell cell is a type of electrochemical cell invented in 1836 by John Frederic Daniell, a British chemist and meteorologist, and consisted of a copper pot filled with a copper (II) sulfate solution, in which was immersed an unglazed earthenware container filled with sulfuric acid and a zinc electrode. He was searching for a way to eliminate the hydrogen bubble problem found in the voltaic pile, and his solution was to use a second electrolyte to consume the hydrogen produced by the first. Zinc sulfate may be substituted for the sulfuric acid. The Daniell cell was a great improvement over the existing technology used in the early days of battery development. A later variant of the Daniell cell called the gravity cell or crowfoot cell was invented in the 1860s by a Frenchman named Callaud and became a popular choice for electrical telegraphy.
Voltammetry is a category of electroanalytical methods used in analytical chemistry and various industrial processes. In voltammetry, information about an analyte is obtained by measuring the current as the potential is varied. The analytical data for a voltammetric experiment comes in the form of a voltammagram which plots the current produced by the analyte versus the potential of the working electrode.
Electrogravimetry is a method used to separate and quantify ions of a substance, usually a metal. In this process, the analyte solution is electrolyzed. Electrochemical reduction causes the analyte to be deposited on the cathode. The cathode is weighed before and after the experiment, and weighing by difference is used to calculate the amount of analyte in the original solution. Controlling the potential of the electrode is important to ensure that only the metal being analyzed will be deposited on the electrode.
Electroless nickel plating (EN) is an auto-catalytic reaction that deposits an even layer of nickel-phosphorus or nickel-boron alloy on the surface of a solid material, or substrate, like metal or plastic. The process involves dipping the substrate in a bath of plating solution, where a reducing agent, like hydrated sodium hypophosphite (NaPO2H2 · H2O), reacts with the material's ions to deposit the nickel alloy. The metallurgical properties of the alloy depend on the percentage of phosphorus, which can range from 2–5% (low phosphorus) to 11–14% (high phosphorus). Unlike electroplating, it is not necessary to pass an electric current through the plating solution to form a deposit. Electroless plating prevents corrosion and wear, and can be used to manufacture composite coatings by suspending powder in the bath. EN plating creates an even layer regardless of the geometry of the surface – in contrast to electroplating which suffers from flux-density issues as an electromagnetic field will vary due to the surface profile and result in uneven depositions. Depending on the catalyst, EN plating can be applied to non-conductive surfaces.
A voltameter or coulometer is a scientific instrument used for measuring quantity of electricity through electrolytic action. The SI unit of quantity of electricity is the coulomb.
Electrogalvanizing is a process in which a layer of zinc is bonded to steel in order to protect against corrosion. The process involves electroplating, running a current of electricity through a saline/zinc solution with a zinc anode and steel conductor. Zinc electroplating maintains a dominant position among other electroplating process options, based upon electroplated tonnage per annum. According to the International Zinc Association, more than 5 million tons are used yearly for both hot dip galvanizing and electroplating. The plating of zinc was developed at the beginning of the 20th century. At that time, the electrolyte was cyanide based. A significant innovation occurred in the 1960s, with the introduction of the first acid chloride based electrolyte. The 1980s saw a return to alkaline electrolytes, only this time, without the use of cyanide. The most commonly used electrogalvanized cold rolled steel is SECC steel. Compared to hot dip galvanizing, electroplated zinc offers these significant advantages:
Zinc alloy electroplating is an electrogalvanization process for corrosion protection of metal surfaces and increasing their wear resistance.
Nickel electroplating is a technique of electroplating a thin layer of nickel onto a metal object. The nickel layer can be decorative, provide corrosion resistance, wear resistance, or used to build up worn or undersized parts for salvage purposes.
The IsaKidd Technology is a copper electrorefining and electrowinning technology that was developed independently by Copper Refineries Proprietary Limited (“CRL”), a Townsville, Queensland subsidiary of MIM Holdings Limited, and at the Falconbridge Limited (“Falconbridge”) now-dismantled Kidd Creek refinery that was at Timmins, Ontario. It is based around the use of reusable cathode starter sheets for copper electrorefining and the automated stripping of the deposited “cathode copper” from them.
Galvanic corrosion is an electrochemical process in which one metal corrodes preferentially when it is in electrical contact with another, in the presence of an electrolyte. A similar galvanic reaction is exploited in primary cells to generate a useful electrical voltage to power portable devices.
Electrochemical stripping analysis is a set of analytical chemistry methods based on voltammetry or potentiometry that are used for quantitative determination of ions in solution. Stripping voltammetry have been employed for analysis of organic molecules as well as metal ions. Carbon paste, glassy carbon paste, and glassy carbon electrodes when modified are termed as chemically modified electrodes and have been employed for the analysis of organic and inorganic compounds.