The Corrosion Resistance Of Electrodeposited And Annealed Nanocrystal Nickel-Molybdenum Alloy Coatings For Electronic Connector Application

The reliability of electronic connectors depends largely on the anti-corrosion performance of the surface coatings.To cut down the use of high-cost gold,Ni and Ni-based alloy stands out for their low cost,easy preparation and excellent corrosion resistance in both acidic conditions and corrosion media containing Cl^-ions.Electrodeposition is an easy route for synthesizing nanocrystal Ni and Ni-based alloy coatings with adjustable grain sizes and structures.However,under the harsh environment,the mechanical properties and corrosion performances of nanocrystal Ni-based materials may be deteriorated due to the diffusion of atoms and the coarsening of grain size.In addition,when grain sizes are smaller than 30nm,the instability of grain boundary may result in a degenerated corrosion resistance as it depends greatly on its grain boundary.It was proposed that the Ni-Mo alloy annealed at proper temperature can in turn stabilize the grain boundary.To this end,nanocrystal Ni-Mo alloy coatings were prepared by direct current electrodeposition and further annealed at high temperature in the aim of anti-corrosion for connectors.The surface morphologies,chemical compositions,crystal structures and thickness of the Ni-Mo coatings were characterized by SEM,EDS,XRD and XRF,respectively.The electrochemical measurements were further employed to investigate the anti-corrosion performance of the coatings.The main results are as follow:?a?The Mo content was tuned by altering the composition of plating bath,and the results show that the Mo content of as-deposited Ni-Mo coatingsexert a predominant impact on the surface morphologies and corrosionbehavior.The grain size decreases with the increase of Mo content,yetthe“colony”size increases.In this case,the anti-corrosion resistancedecreases as Mo content increases.In addition,the passivation behaviorwas observed for samples in 1M H₂SO₄ while no obvious passivationhappened in 3.5 wt%NaCl.?b?The impact of the current density on corrosion resistance wasinvestigated.With the increase of current density,the“colony”sizeincreased due to the high polarization effects,resulting in the densityof colony boundaries decrease,deteriorating the anti-corrosionperformance of Ni-Mo coatings.?c?The effect of heat treatment on the corrosion resistance of Ni-Mo alloywas studied.The samples annealed at 600 ^oC exhibited the highestcorrosion resistance due to the smallest grain size and stabilized grainboundaries which prevent the surface coating from being penetrated bycorrosive substances.