Effects of Organic Additives on Bright Zinc Plating from Ammonium-free Weak Acidic Chloride Bath

Bright zinc plating, from weak acidic (pH 5.5) chloride bath containing ZnCl2, KCl and H3BO3 as main components, were investgated by Hull cell testings, scanning electron microscopic observations and cathodic polarization measurements. It was found that the addition of nonionic surfactant (nonyl phenyl eicosa-ethylene glycol ether: NPEGE), benzal acetone (BA), cinnamic acid (CA), 1, 2, 3-benzotriazole (BT) and nicotinic acid (NA) to the bath produced bright deposits under wide current density range. The range of the current density giving bright deposits widened with increasing zinc ion concentration. In all cases, however, bright deposits could never be obtained at current density higher than the limiting current. BA was an excellent brightener, but water-insoluble alone. NPEGE mainly severed as solubilizer for BA and CA. CA improved the appearance of the deposits at low current density and BT did so at lower current density region. NA improved the appearance of the deposits at high current density near or a little above the limiting current. SEM observations showed that the grain size from additive-free bath decreased with increasing the current density, but at higher than the limiting current. some of the deposits grew into dendrites. The bright deposits from additive bath had fine round grains, of which size ranged from 200Å to 600Å, and semi-bright deposits were needle-like crystals. Among the additives, NA behaved as a deepolarizer, but the others acted as cathodic polarizer, especially BA polarized severely.