Stable operation of chromium plating solutions based on trivalent-chromium compounds needs continuous or frequent replenishment: additions of chromium salts and alkalies. It leads to excessive built-up of salts (e.g. sodium or ammonium sulfate). A combination of two anodes placed in two half-cells with appropriate ion-exchange membranes enables easy and economic automatic control of the concentrations of chromium ions, sulfates and pH-value in the plating solution. Any losses of organic components, such as formic acid, and formation of chromate due to anodic oxidation are also eliminated. Higher cathodic current efficiency is reached due to the prevention of the incomplete reduction of Cr{sup}(3+) to Cr{sup}(2+) followed by the reverse anodic oxidation of Cr{sup}(2+) to Cr{sup}(3+).
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