Chromium complexes electroplating

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 entire electroplating process generates complex wastewater streams. For the purpose of treatment these can be considered to occur in four segments — acid, alkali, chromium and cyanide streams [29]. The acid and alkali wastes are combined and the pH adjusted. The chromium is reduced from Cr to Cr " with sulphur dioxide at pH 2-3. The cyanides are destroyed with chlorine or hypochlorite. Destruction of cyanides is essential for effective precipitation of the heavy metals, as they form strong complexes with the metals and thereby increase... 

There are presently very few electroplating processes which utilize a divided cell, primarily due to the increased cell voltage (and, hence, power costs), constructional complexity and increased maintenance requirements. One example is the "Envirochrome process for decorative chromium plating which has been trial-manufactured during the last Bve years. This utilizes a Cr(iii) electrolyte which avoids problems associate with the use and disposal of Cr(vi) solutions. 

Electrochemical behavior of Cr(III) complexes have been investigated in a number of publications [20-28], This is due to the fact that electroreduction of Cr(III) ions has practical significance in respect of an important problem of trivalent chromium electroplating [28-30] as well as flow battery applications [31], Additionally, electrochemical couple Cr(III)/Cr(II) is a convenient object for experimental testing various theoretical models. 

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