Electroplating of Chromium

The deposition and dissolution of metals is a multi-billion dollar industry that is almost totally based on aqueous acids and alkalis. The use of water limits the metals 

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Electroplatings of Chromium. Until the middle to late 1970s, all of the commercially electroplated chromium was produced from plating... 

Electroplating of chromium from chromic acid leads to spent or contaminated baths and rinsewaters. 

Most chromium is used in three basic industries chemical, metallurgical and refractory (heat-resistant) (Barceloux 1999), with the metal industry using the majority. Ferrochrome, an iron/chromium alloy, is used as an additive to produce chromium-steels which are stainless and hard. Chromic acid solutions are employed in electroplating of chromium on other metals to imPart corrosion resistance and for decorative purposes. The refractory industry has... 

Aqueous Electrodeposition. The theory of electro deposition is well known (see Electroplating). Of the numerous metals used in electro deposition, only 10 have been reduced to large-scale commercial practice. The most commonly plated metals are chromium, nickel, copper, zinc, rhodium, silver, cadmium, tin, and gold, followed by the less frequendy plated metals iron, cesium, platinum, and palladium, and the infrequendy plated metals indium, mthenium, and rhenium. Of these, only platinum, rhodium, iddium, and rhenium are refractory. 

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. 

Practices for preparation of and electroplating on Chromium (Electrodeposits) on Chromium Copper and Copper-Base Mllojs Iron Castings Eead and EeadMllojs Magnesium and Magnesium Mllojs Molybdenum and Molybdenum Mlloys MickelMlloys... 

BS4641 Electroplated Coatings of Chromium for Engineering Purposes. ... 

Other alloy additions in commercial use include iron (often a two-layer electroplated coating with less iron—typically 20% —in the under-layer to assist formability and more iron—often 80% —in the outer layer to assist paintability) cobalt (0.15-0.35%) similar amounts of chromium (the zinc/ chromium/chromium-oxide coating known as Zincrox) and a range of ternary alloys and of composite coatings. 

The ideal disposal method is a chemical treatment that can convert hazardous waste into environmentally benign materials. For example, trichloroethylene (CI2 C I CHCl) is highly toxic to aquatic life, but this compound can be made nontoxic by chemical treatment that converts its chlorine atoms into chloride anions. Similarly, the chromium-containing waste from electroplating operations contains highly toxic CrOq anions, but a chemical treatment that converts CrOq into Cr causes the chromium to precipitate from the solution as insoluble Cr (OH). This removal of chromium detoxifies the water. 

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 ... 

Treatment for the removal of chromium and nickel from electroplating wastewater involves neutralization, hexavalent chromium reduction, pH adjustment, hydroxide precipitation, and final solid-liquid separation.15 37 48... 

Having identified the main features of electrochemistry, the remainder of this chapter will focus on the use of electrolytic cells and will use as examples the electrodeposition (or electroplating) of metals such as copper, zinc, iron, chromium, nickel and silver. The chapter will also consider the electrochemistry of some organic molecules. Electroanalysis will not be considered since a full description is not within the scope of this chapter. Eor those interested readers, there is a review on the topic [2],... 

Let us now discuss examples of the use of ultrasound in the electroplating of such metals as chromium, zinc, iron, copper, silver and nickel (electro and electroless). 

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