Chromium electroplating with

In 1979, a viable theory to explain the mechanism of chromium electroplating from chromic acid baths was developed (176). An initial layer of polychromates, mainly HCr3 0 Q, is formed contiguous to the outer boundary of the cathode s Helmholtz double layer. Electrons move across the Helmholtz layer by quantum mechanical tunneling to the end groups of the polychromate oriented in the direction of the double layer. Cr(VI) is reduced to Cr(III) in one-electron steps and a colloidal film of chromic dichromate is produced. Chromous dichromate is formed in the film by the same tunneling mechanism, and the Cr(II) forms a complex with sulfate. Bright chromium deposits are obtained from this complex. 

The non-ferrous alloys include the misleadingly named nickel silver (or German silver) which contains 10-30% Ni, 55-65% Cu and the rest Zn when electroplated with silver (electroplated nickel silver) it is familiar as EPNS tableware. Monel (68% Ni, 32% Cu, traces of Mn and Fe) is used in apparatus for handling corrosive materials such as F2 cupro-nickels (up to 80% Cu) are used for silver coinage Nichrome (60% Ni, 40% Cr), which has a very small temperature coefficient of electrical resistance, and Invar, which has a very small coefficient of expansion are other well-known Ni alloys. Electroplated nickel is an ideal undercoat for electroplated chromium, and smaller amounts of nickel are used as catalysts in the hydrogenation of unsaturated vegetable oils and in storage batteries such as the Ni/Fe batteries. 

Steel objects are often protected from corrosion by electroplating with chromium. The most straightforward process would be to electrolyze a solution of Cr cations. This fails because aqueous Cr ions are not reduced at a useful rate. Instead, solutions containing chromate anions are used ... 

The nickel-chromium plating process includes the steps in which a ferrous base material is electroplated with nickel and chromium. The electroplating operations for plating the two metals are basically oxidation-reduction reactions. Typically, the part to be plated is the cathode, and the plating metal is the anode. 

Hexavalent chromium-bearing wastewaters are produced in the metal finishing industry in chromium electroplating, in chromate conversion coatings, in etching with chromic acid, and in metal finishing operations carried out on chromium as a basis material. 

Another example of where ultrasound influences the discharge rate of chromium is in the chromium electroplating of steel plates. When cold-rolled steel plate is elecUo-lytically chromated from a silent aqueous solution containing chromates and dichromates, a chrome coating of 13 mg m is obtained after 1 s, whilst in the presence of ultrasound a coating of 45 mg m is obtained [25] in the same time. The product is also accompanied with an increase in the brightness of the metal. Similar results have been found by other workers [26]. 

The process of electroplating metals is another important application of electrolysis. Metals that easily corrode (Fe and Cu) can be plated by metals that are resistant to corrosion. Chromo - plating steel parts used in vehicles is a good example of electroplating. Steel bumpers become more corrosion-resistant when they are electroplated with nickel first, then with chromium. 

Electroplating is achieved by passing an electric current through a solution containing dissolved metal ions as well as the metal object to be plated. The metal object acts as a cathode in an electrochemical cell, attracting metal ions from the solution. Ferrous and nonferrous metal objects are typically electroplated with aluminum, brass, bronze, cadmium, chromium, copper, iron, lead, nickel, tin, and zinc, as well as precious metals such as gold, platinum, and silver. Common electroplating bath solutions are listed in Table 7-1. 

Some car bumpers, as well as bicycle handlebars, are electroplated with chromium to prevent rusting. 

Equipment for the catering trade, particularly for bars, also offers opportunities for electroplated plastics items such as display heads for beer pumps may be made by combining electroplating with painted surfaces drip trays for glasses are moulded from plastics and finished with nickel-chromium to enhance their resistance to abrasion and for convenience in cleaning. 

As discussed in Chapter 6, ionic liquids have great potential as media for electroplating and therefore researchers in the Green Chemistry Group at the University of Leicester formed a spin-off company called Scionix. Scionix with Whyte Chemicals have developed a chromium electroplating process, based on choline chloride-Cr(iii) derived ionic liquids, that is currently operational on a pilot plant scale. 

This process is suitable both for the production of metallic chromium and for the coating of objects with thin, decorative or thicker hard chromium layers (chromium electroplating). 

A major problem in automotive vehicles is corrosion control, particularly in the northern areas of the country where salts are used to melt road ice. Protection of exposed steel from corrosion is accomplished by electroplating with copper-nickel-chromium films, particularly on trim and finish moldings. Electrophoretic painting of body panels is used industry-wide, and electrogalvanizing to protect steel under paint is soon to become a standard practice for all vehicles. Electrochemical accelerated tests for the integrity of coatings and for the determination of the concentration of coolants are used extensively. 

You might have costume jewelry that is electroplated with gold. Or you might admire an automobile whose steel parts such as the bumper have been made more corrosion-resistant by being electroplated first with nickel and then with chromium. 

Another practical apphcation of electrolysis is electroplating—the coating of one metal, which is made the cathode in an electrolysis cell, with another metal, which is made the anode. For example, dull iron or steel surfaces are electroplated with chromium to provide a mirror-like finish and also to protect against corrosion. 

An important category of the surface finishing industry is plating. This includes the process of electroplating which provides metals with surface coatings (usually by electrodeposition) for corrosion protection, wear resistance, improved electrical and mechanical properties, etc. Ferrous and non-ferrous materials can be electroplated with copper, nickel, chromium, zinc, lead, cadmium, etc. They can also be electroplated with precious metals such as gold and silver. 

Skin contact with homogeneous nickel, other than with some coins, is not common, but many items that are electroplated with nickel come into contact with skin. Bright nickel plate contains sulfur, which facilitates the release of nickel ions. Nickel plate is often covered with a topcoat of chromium, silver, gold, tin/ nickel, or different lacquers which, although they reduce formation of nickel ions, may not be adequate to prevent contact dermatitis. Topcoats often have pores or cracks and they are subject to wear. Nickel used as an inter-liner under a plating of gold, silver or chromium causes dermatitis (Liden et al. 1996). 

Method for Measurement of Corrosion Sites in Nickel Plus Chromium or Copper Plus Nickel Plus Chromium Electroplated Surfaces with the Double-Beam Interference Microscope. 

Electroplating is used a great deal in industry. For example, car bumpers are electroplated with chromium, to make them shiny and protect them from rust. Steel is electroplated with tin, to make "tins for food. 

In industry, the process of electroplating uses electrolysis to coat an object with a thin layer of a metal such as silver, platinum, or gold. Car bumpers are electroplated with chromium to prevent rusting. Silver-plated utensils, bowls, and platters are made by electroplating objects with a layer of silver. 

---