Corrosion protection of electrodeposited multilayer nanocomposite Zn-Ni-SiO_2 coatings

摘要

Multilayer nanocomposite coatings of Zn-Ni-SiO_2 were deposited galvanostatically on mild steel (MS) from Zn-Ni bath, having Zn~(+2) and Ni~(+2 ions and uniformly dispersed nano-SiO_2 particles. The corrosion characteristics and properties of multilayered nanocomposite (MNC) coatings were evaluated by electrochemical polarization and impedance methods. Such deposition conditions as, bath composition, cyclic cathode current densities (CCCD's) and number of layers were optimized for peak performance of coatings against corrosion. A significant improvement in the corrosion performance of MNC coatings was observed when a coating was changed from a monolayer to multilayer type. Corrosion rate (CR) of MNC coating decreased progressively with number of layers up to an optimal level, and then started increasing. The increase of CR at a higher degree of layering is attributed to diffusion of layers due to a very short deposition time, failing to give the enhanced corrosion protection. The formation of layers, inclusion of silica particle in MNC coating matrix were confirmed by SEM and XRD study. At optimal current densities, i.e. at 3.0-5.0 A/cm~2, the Zn-Ni-SiO_2 coating having 300 layers, represented as (Zn-Ni-SiO_2)_(3.0/5.0/300) is found to be about 107 times more corrosion resistant than a monolayer Zn-Ni-SiO_2 coating, developed from the same bath for the same time. The reasons responsible for the extended corrosion protection of MNC Zn-Ni-SiO_2 coatings, compared to corresponding monolayer Zn-Ni and (Zn-Ni-SiO_2) coatings were analyzed, and results were discussed.