Effect of small amount of inorganic additives on the lightness and the gloss of electrogalvanized steel sheets

Zn was electrodeposited on a commercial steel sheet from the sulfate baths containing a small amount of inorganic additives. The lightness and the gloss of deposited Zn were evaluated by the conventional techniques and the morphology was observed using an atomic force microscope. On the basis of their effect on the appearance of deposited Zn, the additives used were classified into following three groups: Fe, Ni, Co, Al, Mg and Mn (Group I), Cr, W, Mo and Zr (Group II) and Cu, Pb, Cd, In, Sn, Ge, Sb and Ag (Group III). The additives of group I hardly affected the morphology, crystal orientation and surface roughness of deposited Zn, and therefore they caused no change in the lightness and the gloss compared with the deposit from additive-free bath. Since the additives of group II enhanced the (0002) orientation index of deposited Zn, the surface roughness decreased significantly. As a result, the lightness and the gloss of deposited Zn increased although the morphology was scarcely affected by these additives. The additives of group III caused a significant change in the morphology of deposited Zn and therefore the surface roughness changed considerably depending on the additive concentration in the baths. With an increase in the additive concentration in the baths, the lightness and the gloss were generally decreased, although the addition of extremely small amount of Cd, Ge, Sb and Ag compounds to the baths increased the lightness and the gloss of deposited Zn.