Electroplating, also known as electrochemical deposition or electrodeposition, is a process for producing a metal coating on a solid substrate through the reduction of cations of that metal by means of a direct electric current. The part to be coated acts as the cathode (negative electrode) of an electrolytic cell; the electrolyte is a solution of a salt of the metal to be coated; and the anode (positive electrode) is usually either a block of that metal, or of some inert conductive material. The current is provided by an external power supply.
Electropolishing, a process that uses an electric current to remove metal cations from the surface of a metal object, may be thought of as the opposite of electroplating.[1]
Throwing power is an important parameter, which describes the uniformity of electroplating current (and of the electroplated thickness). It depends mostly on the composition and temperature of the electroplating solution. Those compositions/conditions, that favor secondary current distribution with a small dependence of current on voltage (i.e. with low transfer coefficients) have a higher (better) throwing power.[2]
Process
Simplified diagram for electroplating copper (orange) on a conductive object (the cathode, "Me", gray). The electrolyte is a solution of copper sulfate, CuSO 4. A copper anode is used to replenish the electrolyte with copper cations Cu as they are plated out at the cathode.
The electrolyte should contain positive ions (cations) of the metal to be deposited. These cations are reduced at the cathode to the metal in the zero valence state. For example, the electrolyte for copper plating can be a solution of copper(II) sulphate, which dissociates into Cu2+ cations and SO2− 4 anions. At the cathode, the Cu2+ is reduced to metallic copper by gaining two electrons.
When the anode is made of the coating metal, the opposite reaction may occur there, turning it into dissolved cations. For example, copper would be oxidized at the anode to Cu2+ by losing two electrons. In this case, the rate at which the anode is dissolved will be equal to the rate at which the cathode is plated and thus the ions in the electrolyte bath are continuously replenished by the anode. The net result is the effective transfer of metal from the anode to the cathode.[3]
The anode may instead be made of a material that resists electrochemical oxidation, such as lead or carbon. Oxygen, hydrogen peroxide, or some other byproducts are then produced at the anode instead. In this case, ions of the metal to be plated must be periodically replenished in the bath as they are drawn out of the solution.[4]
The plating is most commonly a single metallic element, not an alloy. However, some alloys can be electrodeposited, notably brass and solder. Plated "alloys" are not "true alloys" (solid solutions), but rather they are tiny crystals of the elemental 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 into a true alloy. The true alloy is more corrosion resistant than the as-plated mixture.
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.
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.[5]
The ability of a plating to cover uniformly is called throwing power; the better the throwing power the more uniform the coating.[6]
Strike
Initially, a special plating deposit called a strike or flash may be used to form a very thin (typically less than 0.1μm 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.[4]
The pulse electroplating or pulse electrodeposition (PED) process involves the swift alternating of the electrical 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.[7]
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 a certain electroplating period with the current or potential applied. The effective current/potential is calculated by multiplying the duty cycle and peak value of the current or potential. Pulse electroplating could help to improve the quality of electroplated film and release the internal stress built up during fast deposition. A combination of the short-duty cycle and high frequency could decrease surface cracks. However, in order to maintain the constant effective current or potential, a high-performance power supply may be required to provide high current/potential and a 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 such as platinum.
Other factors that could affect the pulse electroplating include temperature, anode-to-cathode gap and stirring. Sometimes 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.
Brush electroplating
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.
This technique of electroplating is one of the most common used in the industry for large numbers of small objects. The objects are placed in a barrel-shaped non-conductive cage, and then immersed in the chemical bath containing dissolved ions of the metal, that is to be plated onto them. The barrel is then rotated, and electrical currents are run through the various pieces in the barrel which complete circuits as they touch one another. The result is a very uniform and efficient plating process, though the finish on the end products will likely suffer from abrasion during the plating process. It is unsuitable for highly ornamental or precisely engineered items.[8]
Cleanliness
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.
Effects
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.[9] 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.
There are a number of alternative processes to produce metallic coatings on solid substrates that do not involve electrolytic reduction:
Electroless deposition uses a bath containing metal ions and chemicals that will reduce them to the metal by redox reactions. The reaction should be autocatalytic, so that new metal will be deposited over the growing coating, rather than precipitated as a powder through the whole bath at once. Electroless processes are widely used to deposit nickel-phosphorus or nickel-boron alloys for wear and corrosion resistance, silver for mirror-making, copper for printed circuit boards, and many more. A major advantage of these processes over electroplating is that they can produce coatings of uniform thickness over surfaces of arbitrary shape, even inside holes, and the substrate need not be electrically conducting. Another major benefit is that they don't need power sources or especially shaped anodes. Disadvantages include lower deposition speed, consumption of relatively expensive chemicals, and limited choice of coating metals.
Immersion coating processes exploit displacement reactions in which the subtrate metal is oxidized to soluble ions while ions of the coating metal get reduced and deposited in its place. This process is limited to very thin coatings, since the reaction stops after the subtrate has been completely covered. Nevertheless, it has some important applications, such as the electroless nickel immersion gold (ENIG) process used to obtain gold-plated electrical contacts on printed circuit boards.
Sputtering uses an electron beam or a plasma to eject microscopic particles of the metal onto the substrate in a vacuum.
Chemical vapor deposition uses a gas containing a volatile compound of the metal, which gets deposited onto the substrate as a result of a chemical reaction.
Gilding is a traditional way to attach a gold layer onto metals by applying a very thin sheet of gold held in place by an adhesive
It has been theorized that the first electroplating was done in Parthian Empire era. Wilhelm König was an assistant at the National Museum of Iraq in the 1930s had observed a number of very fine silver objects from ancient Iraq, plated with very thin layers of gold, and speculated that they were electroplated[11][circular reference]. He corroborated his idea by referring to a possible Parthian battery discovered in 1938 near the metropolis of Ctesiphon, the capital of the Parthian (150 BC – 223 AD) and Sasanian (224–650 AD) empires of Persia. However this has been widely debunked by researchers. Modern archaeologists, however, now generally agree that the objects seen by König were not, in fact, electroplated at all, but rather fire-gilded using mercury. There are therefore no known examples of objects from ancient Mesopotamia that can be reliably described as showing signs of electroplating.[12]
Electroplating 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.
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.[19]
A zinc solution tested in a Hull cell
Hull cell
Haring–Blum cell
The Hull cell is a type of test cell used to semi-quantitatively check the condition of an electroplating bath. It measures useable current density range, optimization of additive concentration, recognition of impurity effects and indication of macro-throwing power capability.[20] 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 milliliters of a plating bath solution. This shape allows one to place the test panel on an angle to the anode. As a result, the deposit is plated at a range current densities along its length, which can be measured with a hull cell ruler. The solution volume allows for a semi-quantitative measurement of additive concentration: 1gram addition to 267mL is equivalent to 0.5oz/gal in the plating tank.[21] The equation for calculating quantitative measurement of liquid additives is:
milliliters added to hull cell divided by 267 multiplied by production bath volume (gallons) equals addition of additive needed in gallons to production bath you could break it down further and multiply the gallons needed by 3785 to convert to milliliters
milliliters added to hull cell/267 • (production bath volume gallons) = addition to production bath in gallons
milliliters added to hull cell/267 • (production bath volume gallons) • 3785 = addition to production bath in milliliters
Haring–Blum cell
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.[22][23]
Electrochemistry is the branch of physical chemistry concerned with the relationship between electrical potential difference, as a measurable and quantitative phenomenon, and identifiable chemical change, with the potential difference as an outcome of a particular chemical change, or vice versa. These reactions involve electrons moving via an electronically-conducting phase between electrodes separated by an ionically conducting and electronically insulating electrolyte.
Corrosion is a natural process that converts a refined metal into a more chemically stable oxide. It is the gradual deterioration of materials by chemical or electrochemical reaction with their environment. Corrosion engineering is the field dedicated to controlling and preventing corrosion.
Chrome plating is a technique of electroplating a thin layer of chromium onto a metal object. A chrome-plated item is called chrome. The chromed layer can be decorative, provide corrosion resistance, ease of cleaning, or increase surface hardness. Sometimes, a less expensive imitator of chrome may be used for aesthetic purposes.
Electropolishing, also known as electrochemical polishing, anodic polishing, or electrolytic polishing, is an electrochemical process that removes material from a metallic workpiece, reducing the surface roughness by levelling micro-peaks and valleys, improving the surface finish. Electropolishing is often compared to, but distinctly different from, electrochemical machining. It is used to polish, passivate, and deburr metal parts. It is often described as the reverse of electroplating. It may be used in lieu of abrasive fine polishing in microstructural preparation.
Anodizing is an electrolytic passivation process used to increase the thickness of the natural oxide layer on the surface of metal parts.
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.
Copper electroplating is the process of electroplating a layer of copper onto the surface of a metal object. Copper is used both as a standalone coating and as an undercoat onto which other metals are subsequently plated. The copper layer can be decorative, provide corrosion resistance, increase electrical and thermal conductivity, or improve the adhesion of additional deposits to the substrate.
Gold plating is a method of depositing a thin layer of gold onto the surface of another metal, most often copper or silver, by chemical or electrochemical plating. This article covers plating methods used in the modern electronics industry; for more traditional methods, often used for much larger objects, see gilding.
Metallizing is the general name for the technique of coating metal on the surface of objects. Metallic coatings may be decorative, protective or functional.
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.
Plasma electrolytic oxidation (PEO), also known as electrolytic plasma oxidation (EPO) or microarc oxidation (MAO), is an electrochemical surface treatment process for generating oxide coatings on metals. It is similar to anodizing, but it employs higher potentials, so that discharges occur and the resulting plasma modifies the structure of the oxide layer. This process can be used to grow thick, largely crystalline, oxide coatings on metals such as aluminium, magnesium and titanium. Because they can present high hardness and a continuous barrier, these coatings can offer protection against wear, corrosion or heat as well as electrical insulation.
Electroless deposition (ED) or electroless plating is defined as the autocatalytic process through which metals and metal alloys are deposited onto nonconductive surfaces. These nonconductive surfaces include plastics, ceramics, and glass etc., which can then become decorative, anti-corrosive, and conductive depending on their final functions. Electroless deposition is a chemical processes that create metal coatings on various materials by autocatalytic chemical reduction of metal cations in a liquid bath. Electroless plating in contrast with electroplating processes, where the reduction is achieved by an externally generated electric current. Electroless deposition deposits metals onto 2D and 3D structures such as screws, nanofibers, and carbon nanotubes ; unlike PVD, CVD, and electroplating which are limited to 2D surfaces. Commonly the surface of the substrate is characterized via pXRD, SEM-EDS, and XPS. Key qualifying parameters of these methods are crucial for a researcher’ or company’s purpose for the conductive surfaces. Electroless deposition continues to rise in importance within the microelectronic industry, oil and gas, and aerospace industry.
Electroless nickel-phosphorus plating is a chemical process that deposits an even layer of nickel-phosphorus alloy on the surface of a solid substrate, like metal or plastic. The process involves dipping the substrate in a water solution containing nickel salt and a phosphorus-containing reducing agent, usually a hypophosphite salt. It is the most common version of electroless nickel plating and is often referred by that name. A similar process uses a borohydride reducing agent, yielding a nickel-boron coating instead.
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. Such Zinc electroplating or Zinc alloy 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, acronym of "Steel, Electrogalvanized, Cold-rolled, Commercial quality". Compared to hot dip galvanizing, electroplated zinc offers these significant advantages:
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.
Electroless nickel-boron coating is a metal plating process that can create a layer of a nickel-boron alloy on the surface of a solid substrate, like metal or plastic. The process involves dipping the substrate in a water solution containing nickel salt and a boron-containing reducing agent, such as an alkylamineborane or sodium borohydride. It is a type of electroless nickel plating. A similar process, that uses a hypophosphite as a reducing agent, yields a nickel-phosphorus coating instead.
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. This phenomenon is named after Italian physician Luigi Galvani (1737-1798).
Electroless copper plating is a chemical process that deposits an even layer of copper on the surface of a solid substrate, like metal or plastic. The process involves dipping the substrate in a water solution containing copper salts and a reducing agent such as formaldehyde.
Immersion zinc plating is an electroless (non-electrolytic) coating process that deposits a thin layer of zinc on a less electronegative metal, by immersion in a solution containing a zinc or zincate ions, Zn(OH)2− 4. A typical use is plating aluminum with zinc prior to electrolytic or electroless nickel plating.
↑ Wendt, Hartmut; Gerhard, Kreyse (1999). Electrochemical Engineering: Science and Technology in Chemical and Other Industries. Springer. p.122. ISBN3540643869.
Bibliography
Dufour, Jim (2006). An Introduction to Metallurgy (5thed.). Cameron.[ISBNmissing]
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