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What is Surface Finishing?
Surface finishing is a broad range of industrial processes that alter the surface of a manufactured item to achieve a certain property. Finishing processes may be employed to: improve appearance, adhesion or wettability, solderability, corrosion resistance, tarnish resistance, chemical resistance, wear resistance, hardness, modify electrical conductivity, remove burrs and other surface flaws, and control the surface friction. In limited cases some of these techniques can be used to restore original dimensions to salvage or repair an item.
Surface finishing processes can be categorized by how they affect the workpiece:
- Removing or reshaping finishing
- Adding or altering finishing
Mechanical processes may also be categorized together because of similarities the final surface finish.
What is Electroplating/Electrodeposition?
Electroplating is a plating process in which metal ions in a solution are moved by an electric field to coat an electrode. The process uses electrical current to reduce cations of a desired material from a solution and coat a conductive object with a thin layer of the material, such as a metal. Electroplating is primarily used for depositing a layer of material to bestow a desired property (e.g., abrasion and wear resistance, corrosion protection, lubricity, aesthetic qualities, etc.) to a surface that otherwise lacks that property. Another application uses electroplating to build up thickness on undersized parts.
The process used in electroplating is called electrodeposition. It is analogous to a galvanic 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 comprise it 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, vis-a-vis the current flowing through the circuit. In this manner, the ions in the electrolyte bath are continuously replenished by the anode.
Other electroplating processes may use a non-consumable anode such as lead. 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.
Electroplating of a metal (Me) with copper in a copper sulfate bath
The anode and cathode in the electroplating cell are both connected to an external supply of direct current — a battery or, more commonly, a rectifier. The anode is connected to the positive terminal of the supply, and the cathode (article to be plated) is connected to the negative terminal. When the external power supply is switched on, the metal at the anode is oxidized from the zero valence state to form cations with a positive charge. These cations associate with the anions in the solution. The cations are reduced at the cathode to deposit in the metallic, zero valence state. For example, in an acid solution, copper is oxidized at the anode to Cu2+ by losing two electrons. The Cu2+ associates with the anion SO42- in the solution to form copper 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.
The plating is most commonly a single metallic element, not an alloy. However, some alloys can be electrodeposited, notably brass and solder.
Many plating baths include cyanides of other metals (e.g., 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.
What Is an Anodized Finish?
An anodized finish is a clear or colored finish that is processed through a chemical bath and chemically applied to the surface of ‘Aluminum’. Because of its strength and durability, it was used for a variety of purposes. Many of the satellites circulating the Earth are protected from space debris by layers of anodized aluminum. The automobile industry relies heavily on anodized aluminum for trims and protective housings on exposed parts. Furniture designers often use anodized aluminum as the framework for outdoor pieces as well as the base metal for other decorative items. Modern home appliances and computer systems may utilize anodizing for its protective housing. From reducing thread galling on some fasteners to preventing corrosion on others, an anodized finish adds not only protection to the surface of the metal, it adds strength as well. The color of the anodized finish is not always created by colors or dyes, rather the colors are often directly related to the amount of time the piece was left exposed to the anodizing chemicals. There are, however, a large number of dyes that make it possible to create nearly any color on an anodized product. Aluminum is considerably a softer metal, and can often benefit from the protective properties of an anodized finish. On the surface that will be removed and replaced in excess, the anodized finish allows lubricants to be infused into the surface of the threads, adding to the ease of installation.
By placing the aluminum into a bath of liquid chemicals that are often nothing more than acetone and applying an electrical current through the aluminum component, a chemical reaction occurs that is similar to a type of rust forming on the aluminum. This rust-like material is the anodized finish and actually works to increase the strength of the aluminum’s surface. Similar to gold or silver plating, the color of the anodized finish is dependent on the types of metal sheets immersed into the liquid chemicals.
While anodizing might not necessary be appropriate for all applications because of its non-conductive nature. Unlike other metals, the oxidation process doesn’t seem to weaken aluminum. The layer of ‘aluminum rust’ is still part of the original aluminum and will not transfer to consumable products or easily flake off under certain stress. This makes it rather popular for food-industry applications and industrial applications where durability is highly recommended.
Although it is not confirmed, the Parthian Battery may have been the first system used for electroplating.
Modern electrochemistry was invented by Italian chemist Luigi V. 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.
Galvanoplastic sculpture on St. Isaac’s Cathedral in Saint Petersburg.
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 Norddeutsche Affinerie in Hamburg was the first modern electroplating plant starting its production in 1876.
As the science of electrochemistry grew, its relationship to the electroplating process became understood and other types of non-decorative metal electroplating processes 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 from 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.
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