Zinc-nickel electrodeposits have been widely adopted for surface treatment of automobile body steel sheet. Cadmium additive enhances nucleation of hexagonal columnar crystals of the zinc-nickel electrodeposit. These hexagonal columnar crystals are made up of lateral growth of hexagonal plates along the (00-1)eta. With 30 ppm of cadmium, cadmium absorbs at active sites on the (00-1)eta and interferes with surface diffusion of metal adatoms. This enhances nucleation and forms many hexagonal plates on the (00-1)eta. With 100 ppm of cadmium, not only do many hexagonal plates on the (00-1)eta but also fine hexagonal columnar crystals form on the {10-0}eta. With further increase of cadmium to 200 ppm, the entire deposit becomes fine hexagonal crystals.