Electroplating, also known aselectrochemical deposition orelectrodeposition, is a process for producing ametal coating on a solid substrate through thereduction ofcations of that metal by means of adirect electric current. The part to be coated acts as thecathode (negativeelectrode) of anelectrolytic cell; theelectrolyte is asolution of asalt whose cation is the metal to be coated, and theanode (positive electrode) is usually either a block of that metal, or of some inertconductive material. The current is provided by an externalpower supply.
Electroplating is widely used in industry anddecorative arts to improve the surface qualities of objects—such as resistance toabrasion andcorrosion,lubricity,reflectivity,electrical conductivity, or appearance. It is used to build up thickness on undersized or worn-out parts and to manufacture metal plates with complex shape, a process calledelectroforming. It is used to deposit copper and other conductors in formingprinted circuit boards andcopper interconnects in integrated circuits. It is also used to purify metals such ascopper.
The aforementioned electroplating of metals uses an electroreduction process (that is, a negative orcathodic current is on the working electrode). The term "electroplating" is also used occasionally for processes that occur under electro-oxidation (i.e positive oranodic current on the working electrode), although such processes are more commonly referred to asanodizing rather than electroplating. One such example is the formation ofsilver chloride on silver wire inchloride solutions to makesilver/silver-chloride (AgCl) electrodes.
Electropolishing, a process that uses an electric current to selectively remove the outermost layer from the surface of a metal object, is the reverse of the process of electroplating.[1]
Throwing power is an important parameter that provides a measure of the uniformity of electroplating current, and consequently the uniformity of the electroplated metal thickness, on regions of the part that are near to the anode compared to regions that are far from it. It depends mostly on the composition and temperature of the electroplating solution, as well as on the operatingcurrent density.[2] A higherthrowing power of the plating bath results in a more uniform coating.[3]
Electroplating was invented in Europe by Italian chemistLuigi Valentino Brugnatelli in 1805. Brugnatelli used his colleagueAlessandro Volta's invention of five years earlier, thevoltaic pile, to facilitate the first electrodeposition. Brugnatelli's inventions were suppressed by theFrench Academy of Sciences and did not become used in general industry for the following thirty years. By 1839, scientists inBritain andRussia had independently devised metal-deposition processes similar to Brugnatelli's for the copper electroplating ofprinting press plates.
Research from the 1930s had theorized that electroplating might have been performed in theParthian Empire using a device resembling aBaghdad Battery, but this has since been refuted; the items were fire-gilded using mercury.[4]
Boris Jacobi in Russia not only rediscovered galvanoplastics, but developedelectrotyping andgalvanoplastic sculpture. Galvanoplastics quickly came into fashion in Russia, with such people as inventorPeter Bagration, scientistHeinrich Lenz, and science-fiction authorVladimir Odoyevsky all contributing to further development of the technology. Among the most notorious cases of electroplating usage in mid-19th century Russia were the gigantic galvanoplastic sculptures ofSt. Isaac's Cathedral inSaint Petersburg and gold-electroplateddome of theCathedral of Christ the Saviour inMoscow, the third tallest Orthodox church in the world.[5]
Soon after,John Wright ofBirmingham, England discovered thatpotassium cyanide was a suitableelectrolyte 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 inBirmingham from where it spread around the world. TheWoolrich Electrical Generator of 1844, now inThinktank, Birmingham Science Museum, is the earliest electrical generator used in industry.[6] It was used byElkingtons.[7][8][9]
TheNorddeutsche Affinerie inHamburg was the first modern electroplating plant starting its production in 1876.[10]
As the science ofelectrochemistry grew, its relationship to electroplating became understood and other types of non-decorative metal electroplating were developed. Commercial electroplating ofnickel,brass,tin, andzinc 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 ofelectric generators in the late 19th century. With the higher currents available, metal machine components, hardware, andautomotive parts requiring corrosion protection and enhanced wear properties, along with better appearance, could be processed in bulk.
The two World Wars and the growingaviation industry gave impetus to further developments and refinements, including such processes as hardchromium plating,bronze alloy plating, sulfamate nickel plating, and numerous other plating processes. Plating equipment evolved from manually operatedtar-lined wooden tanks to automated equipment capable of processing thousands of kilograms per hour of parts.
One of the American physicistRichard Feynman's first projects was to develop technology for electroplating metal ontoplastic. 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.[11]
.svg)
The electrolyte in the electrolytic plating cell 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 forcopper electroplating can be a solution ofcopper(II) sulfate, which dissociates into Cu2+ cations andSO2−
4 anions. At the cathode, the Cu2+ is reduced to metallic copper by gaining two electrons.
When the anode is made of the metal that is intended for coating onto the cathode, the opposite reaction may occur at the anode, 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 equal 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.[12]
The anode may instead be made of a material that resists electrochemical oxidation, such aslead orcarbon.Oxygen,hydrogen peroxide, and some other byproducts are then produced at the anode instead. In this case, ions of the metal to be plated must be replenished (continuously or periodically) in the bath as they are drawn out of the solution.[13]
The plating is most commonly a single metallicelement, not analloy. However, some alloys can be electrodeposited, notablybrass andsolder. 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 includecyanides of other metals (such aspotassium 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 ascarbonates andphosphates 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, andwaxes.[14]
Initially, a special plating deposit called astrike orflash 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, then a strike can be first deposited that is compatible with both. One example of this situation is the poor adhesion of electrolyticnickel onzinc alloys, in which case a copper strike is used, which has good adherence to both.[13]
The pulse electroplating or pulse electrodeposition (PED) process involves the swift alternating of theelectrical potential orcurrent 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.[15]
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 a high-cost, inert electrode such asplatinum.
Other factors that affect the pulse electroplating include temperature, anode-to-cathode gap, and stirring. Sometimes, pulse electroplating can be performed in a heated electroplating bath to increase the deposition rate, since the rate of most chemical reactions increases exponentially with temperature per theArrhenius law. The anode-to-cathode gap is related to the current distribution between anode and cathode. A small gap-to-sample-area ratio may cause uneven distribution of current and affect the surface topology of the plated sample. Stirring may increase the transfer/diffusion rate of metal ions from the bulk solution to the electrode surface. The ideal 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 agraphite body wrapped with an absorbentcloth material that both holds the plating solution and prevents direct contact with the item being plated, is connected to theanode of a low-voltage and 3-4 amperedirect-current power source, and the item to be plated (thecathode) is grounded. The operator dips the brush in plating solution and 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, the 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. It is mainly used industrially in part repair, with worn bearing surfaces getting a nickel or silver deposit. With technological advancement deposits up to .025" have been achieved and retained uniformity. Disadvantages compared to tank plating can include greater operator involvement (tank plating can frequently be done with minimal attention and the solutions used are often toxic), and the inconsistency in achieving 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 a 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.[16]
Cleanliness is essential to successful electroplating, since molecular layers ofoil 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,ultrasonic cleaning and acid treatment. 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 arehydrophilic 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 assoap reduce the sensitivity of the test and must be thoroughly rinsed off.
Throwing power (ormacro throwing power) is an important parameter that provides a measure of the uniformity of electroplating current, and consequently the uniformity of the electroplated metal thickness, on regions of the part that are near the anode compared to regions that are far from it. It depends mostly on the composition and temperature of the electroplating solution.[2]Micro throwing power refers to the extent to which a process can fill or coat small recesses such asthrough-holes.[17] Throwing power can be characterized by the dimensionlessWagner number:
whereR is theuniversal gas constant,T is the operatingtemperature,κ is the ionic conductivity of the plating solution,F is theFaraday constant,L is the equivalent size of the plated object,α is thetransfer coefficient, andi the surface-averaged total (includinghydrogen evolution) current density. The Wagner number quantifies the ratio of kinetic to ohmic resistances. A higher Wagner number produces a more uniform deposition. This can be achieved in practice by decreasing the size (L) of the plated object, reducing the current density|i|, adding chemicals that lowerα (make the electric current less sensitive to voltage), and raising the solution conductivity (e.g. by addingacid). Concurrenthydrogen evolution usually improves the uniformity of electroplating by increasing|i|; however, this effect can be offset by blockage due to hydrogen bubbles and hydroxide deposits.[18]
The Wagner number is rather difficult to measure accurately; therefore, other related parameters, that are easier to obtain experimentally with standard cells, are usually used instead. These parameters are derived from two ratios: the ratioM =m1 /m2 of the plating thickness of a specified region of the cathode "close" to the anode to the thickness of a region "far" from the cathode and the ratioL =x2 /x1 of the distances of these regions through the electrolyte to the anode. In a Haring-Blum cell, for example,L = 5 for its two independent cathodes, and a cell yielding plating thickness ratio ofM = 6 has Harring-Blum throwing power100% × (L −M) /L = −20%.[17] Other conventions include the Heatley throwing power100% × (L −M) / (L − 1), Field throwing power100% × (L −M) / (L +M − 2),[19] and Luke throwing power100% ×L / (L +M − 1). A more uniform thickness is obtained by making the throwing power larger (less negative), except for Luke's throwing power, which has the advantage of having a minimum of 0 and a maximum of 100, in terms of the less negative value, according to any of these definitions.
Parameters that describe cell performance such as throwing power are measured in small test cells of various designs that aim to reproduce conditions similar to those found in the production plating bath.[17]
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 adirect current is passed for a specific period of time. The cell is fabricated out ofperspex or glass.[20][21]
TheHull 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.[22] The Hull cell replicates the plating bath on a lab scale. It is filled with a sample of the plating solution and an appropriate anode which is connected to arectifier. 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: 1 gram addition to 267 mL is equivalent to 0.5 oz/gal in the plating tank.[23]
Electroplating changes the chemical, physical, and mechanical properties of the workpiece. An example of a chemical change is whennickel 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 intensile strength or surfacehardness, which is a required attribute in the tooling industry.[24] 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 ifcase-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:
