Abstract
The effect of a brightener on the electrodeposition of Sn-Ni alloy plated from acidic chloride baths has been studied by microscopic observation, cathodic polarization measurements and direct determination of the brightener adsorbed. The brightener used was N-(1-naphthyl) ethylenediamine dihydrochloride (NEDA). The amount of HCl-free NEDA adsorbed in the alloy deposits was at most about 2% and varied with current density and bath composition, whereas the metal composition of the deposits remained almost unaltered despite variations in the metal ion concentration in the bath. Even if the NEDA content in the alloy deposits was very low, hydrogen evolution potential on the deposits shifted to a far nobler potential than on NEDA-free deposits, which resulted in the depolarization of the bright alloy surface. It is considered that the codeposited hydrogen atoms on the surface inhibited the crystal growth of the metals, thus facilitating the brightening effect of NEDA. SEM observations showed that the bright deposits from additive baths had fine uniform grains of about 0.2-0.3μm, whereas the dull deposits from additive-free baths had coarser grans of about 3-4μm.
