The effect of alumina-silica sols on electrodeposited zinc coatings for sintered NdFeB

Alumina-silica particles contained zinc coatings on NdFeB are prepared by an in-situ electro-deposition method from alumina-silica sols contained plating bath. Alumina-silica sols with varied Si:Al ratio are prepared through the two step hydrolysis and condensation of aluminium isopropoxide (AIP) and tetraethyl orthosilicate(TEOS) with acid catalyst. As negative potential applied to NdFeB substrates promotes the deposition of zinc, it also causes the increase of hydroxyl ions concentration, which promotes the particle condensation and gel of alumina-silica sols. The effect of alumina-silica sols on the surface morphology, composition and performance of the electro deposited zinc coatings are studied for NdFeB substrate. The results show that the adsorption and gel of alumina-silica particles are along with the electro-deposition of zinc coating. As the Si:Al ratio increases in added sols, there are more alumina particles get embedded during electro-deposition, which affect the surface morphology and uniformity of zinc coating. Electrochemical and mechanical tests demonstrate that the embedded alumina-silica particles in zinc coatings may serve as inhibitors and help to improve the corrosion resistance and mechanical performance of the zinc coatings on NdFeB magnets.