Ultrasound-assisted electrodeposition of Ni and Ni-Mo coatings from a citrate-ammonia electrolyte solution

The effect of ultrasound (US) agitation (at different intensities of US from 0 to 40 Wcm−2) on the electrodeposition of Ni-Mo and Ni (as reference) coatings from an aqueous complex citrate-ammonia electrolyte was investigated. Increased US intensity has a meaningful influence on Mo content in Ni-Mo alloys, which vary from about 14.5 wt% to 19.5 wt%. Enrichment of alloys in Mo resulted in the refinement of the microstructure (crystallite size below 10 nm), and the enhancement of coatings' microhardness (from about 720 to 820 HV) and corrosion properties in chloride media (3.5% NaCl). The opposite effect was observed for Ni coatings, where US agitation did not significantly change the efficiency of Ni electrodeposition, phase composition and crystallite size (about 14 nm) of coatings, which led to the same microhardness values (about 700 HV) of the deposits in the whole range of the US intensities. Obtained results confirmed that US agitation applied during electrodeposition mainly acts as an additional factor affecting the hydrodynamic condition of the process, reducing the diffusion layer thickness and influences kinetics of the Mo reduction controlled by diffusion. Thus, increased US intensities in the analysed range (20-40 Wcm−2), resulting in increased Mo content in Ni-Mo alloys, directly influences their functional properties.