Abstract
The deposition behavior and the coating characteristics of an electrodeposited Zn-SiO2 composite coating have been studied. SiO2 particles were maintained in good suspension in the plating bath without any special agitation. The SiO2 deposition rate was increased when sodium nitrate was added to the plating bath. The amount of sodium nitrate required for SiO2 deposition was mainly determined by the pH of the plating bath. When the pH was low, sodium nitrate was required in relatively large amounts for SiO2 deposition.
SiO2 particles were distributed uniformly in the coating except for the extreme surface. The extreme surface layer consists of SiO2-rich phase. The grain size is fine and the orientation of the deposit is random for the Zn-SiO2 composite coating. The coating contains small amounts of an oxidized zinc, probably zinc hydroxide.
The Zn-SiO2 deposition mechanism has been proposed as follows:
(1) Nitrate is electrochemically reduced at the cathode and raises the pH of the electrolyte at the cathode-solution interface.
(2) The increase in pH makes the SiO2 particles coagulate and, at the same time, zinc hydroxide precipitates on the surface of the SiO2 particles.
(3) The cathode is covered with those co-precipitates and the deposition occurs through them. We have speculated that, according to "the Adsorption mechanism" proposed by GUGLIELMI, the SiO2 deposition rate is high for the nitrate-added bath because the cathode is adsorbed by the SiO2-rich precipitate.
