The role of chromium II catalysed olation reactions in the sustained deposition of chromium and its alloys from environmentally acceptable chromium III electrolytes

SummaryIn recent publications the present authors have demonstrated sustained deposition of chromium, and its alloys from chromium III electrolytes through control of transient chromium II. This original work used model electrolytes based on amides which are rather toxic. The ability to sustain deposition through control of chromium II has now been extended to a number of aqueous electrolytes of much greater environmental and economic acceptability, It has now been shown that the transient chromium II catalyses interfering olation reactions in the diffusion layer which at the pH values, about pH4, found in the diffusion layer would otherwise occur too slowly to be significant. Sustained deposition has been achieved by potentio-static deposition in an agitated/circulated electrolyte with anodic generation of a dissolved halogen required for thicknesses greater than 50–100μm. In most cases a significantly higher concentration of ligand, for the chromium III, is required for sustained deposition (as competition to olation) than is necessary for thin decorative coatings. The undesired catalytic role of the chromium II can be minimised by altering the chemical form of chromium II. The role of reagents which promote formation of dinuclear chromium II Cr24+ complexes will be discussed. Use of such reagents allows lower ligand concentration for the complexation of the chromium III.The electrolytes investigated have been shown to have a common ageing pattern due to chromium II catalysis of slow ligand exchange and of slow olation reactions. This will be discussed in depth with the possibilities of electrolyte rejuvenation and of improved electrolyte stability.