The codeposition behavior of Zn
2+, Ni
2+ and SiO
2 was investigated by a galvanostatic method using colloidal SiO
2 (20nm). The SiO
2 content of the electodeposits increased with the increase of Ni
2+ content in the bath, and the amount of Ni
2+ adsorbed on SiO
2 particles was greater than the amount of Zn
2+ adsorbed on SiO
2 in the bath. Ni
2+ played an important role on the deposition of SiO
2. The relation between the amount of SiO
2 in the bath and in the electrodeposit changed in accordance with Guglielmi's theory. Adsorption of the SiO
2 on the cathode seems to occur in the first stage of the Zn-Ni-SiO
2 electrodeposition process. A part of the SiO
2 in the electrodeposit coagulated into larger particles during electrolysis because the pH on the cathode became higher than that of the electrolyte. The surface pH on the cathode during Zn-Ni-SiO
2 electrodeposition measured by micro-Sb electrode was about 5-7 at a bath pH of 2, and a current density of 10-20A/dm
2. This value coincided with the pH of the coagulation of SiO
2. These results indicate that the factors controlling the codeposition of SiO
2, Ni
2+, and Zn
2+ were: 1) adsorption of Ni
2+ on the SiO
2 in the bath; 2) adsorption of SiO
2 on the cathode according to Guglielmi's theory; and 3) coagulation of SiO
2 by the increase of pH on the cathode during electrolysis.
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