Chromic acid electroplating bath

Anodes. There are two types of anodes soluble and insoluble. Most electroplating baths use one or the other specifically however, a few baths use either or both. Chromic acid plating baths use insoluble anodes alkaline zinc cyanide baths use both noncyanide alkaline zincs may use either. Soluble anodes are designed to dissolve efficiendy with current flow and preferably, not to dissolve when the system is idle. A plating solution having the anode efficiency close to the cathode efficiency provides a balanced process that has fewer control problems and is less cosdy. If the anode efficiency is much greater than the cathode efficiency and there are only small solution losses, the dissolved metal concentration rises until at some time the bath has to be diluted back or the excess metal has to be reduced by some other means. If the anode efficiency is less than the cathode efficiency, the dissolved metal decreases, pH decreases, and eventually metal salt additions and other solution corrections are required. Based on the cost of metal, it is usually considerably more economical to plate from the anode rather than add metal salt. Copper cyanide, for example, costs about twice as much to add than to dissolve a comparable amount of copper anode. Additionally, the anion added with the metal salt may build up in the plating solution. 

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. 

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

Electroplating Application of a coating by use of electric current. An anode (-I-) and cathode —) is immersed in an aqueous medium (electrolyte) through which current is passed. For instance, in chromium plating the electrolyte contains chromic acid, sulfuric acid, sodium fluoride and sodium fluosilicate. The bath is operated at 90° C at current densities up to 30A/cm. ... 

The excellent chemical stability permits fluorinated surfactants to be used in electroplating baths where hydrocarbon-type surfactants would not survive [1,8,9,87-93]. Fluorinated surfactants are remarkably stable in a solution of chromic oxide in sulfuric acid at 50°C [94,95]. (See Section 3.1.)... 

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