The behaviour of coumarin in the electrodeposition of nickel has been investigated at -1125 and -1275 mV (sce) and compared with its behaviour at -960 mV . A rotating disk electrode was used to control the rate of transport of additive to the electrode surface .
The variation of deposit structure and rate of deposition with coumarin concentration and potential shows that inhibition of deposition and a change from a columnar to a structureless deposit both occur at a critical coverage of certain surface sites . These sites are probably those at which coumarin is reduced to o-hydroxyphenyl propanol and o-propyl phenol . The appearance of the deposits formed at -1125 and -1275 mV was markedly different from those at -960 mV . Many disks had bright structureless central areas and clearly defined annuli . The relative widths of the two regions varied with additive concentration, rotation speed and potential. These effects have been shown to arise from the variation of nickel-deposition current density, iar, across the disk . Their occurrence does not invalidate the use of the rotating disk electrode system .
Coumarin inhibits deposition on a surface on which nickel is depositing less than on a polished nickel surface : it is similar in this respect to other unsaturated additives though the effect is less marked . The reactivity of the surface for coumarin reduction increases with far . The results suggest that this is due to an increase in surface density of B sites.