The effects of coumarin on the electrodeposition of nickel

Abstract

Systematic variations in the thickness of the diffusion boundary layer over a rough surface are an essential factor in producing the levelling action of metals electrodeposited in the presence of levelling additives. Information on the behaviour of these additives at an electrode at which metal is being deposited and their effects on metal deposition under conditions of known diffusion-layer thickness is therefore valuable for understanding the mechanism of levelling. Experiments are described which show that the behaviour of additives in the electrodeposition of metals can be studied under conditions of known rates of transport of additive to the electrode using a rotating disk electrode.In the deposition of nickel from a Watts-type solution, coumarin reduces the current efficiency for nickel deposition. The current efficiency for hydrogen evolution is increased, the coumarin encouraging evolution from a few points on the electrode.The analysis of products has confirmed that reduction to melilotic acid is the principal cathodic reaction of coumarin but there is a small amount of a second product. Coulomb balances show that this is a product involving the addition of many more than two electrons to each coumarin molecule.The diffusion coefficient for coumarin in the Watts nickel solution has been measured and used to calculate rates of transport of coumarin to the cathode under various conditions. These have been compared with measured rates of coumarin consumption. The comparison has shown conditions where the rate of consumption is transport controlled and where it is electrochemically controlled.The levelling power of coumarin on a rough surface has been derived from the results. This shows maxima in levelling power as a function of coumarin concentration resulting from the change over from transport to electrochemical control of the rate of coumarin consumption.