Researches On The Structure And Corrosion Resistance Of Ni-W Alloy For Connector Applications

An electrical connector is typically used to connect two active devices to transfer current and signal.It's an essential part of electronic devices.The most critical position in the connector is the interface which greatly influence the performance of connector.Commonly used metal structure at the connector interface is a Cu/Ni/Au structure.However,as the price of gold rises year by year,it's necessary to reduce the use of gold in consideration of cost.However,the leading problem caused by the thinner gold film is the rising porosity which exposes more Ni layer to the air.Ni is more susceptible to corrosion than Au,so the corrosion resistance of the connector interlayer decreased.The demand for better corrosion resistance of connecter interface proposes a higher performance requirement on the interlayer metal.By virtue of its low-cost and high-performance,Ni-W alloy has gradually became a strong candidate of the replacement of Ni.This paper studied the effect of structure and W content on the corrosion resistance of Ni-W alloys,and proposed a corrosion mechanism of Ni-W alloys based on the"colony"structure,which provided a new way to further improve the corrosion resistance of Ni-W alloys.The specific research contents are as follows:Firstly,this paper investigated the relationship between thickness,structure and corrosion resistance of the Ni-W alloy.The samples were prepared by direct current plating and the thickness of the samples was manipulated by controlling the plating time.The corrosion resistance of the Ni-W samples was characterized by potentio-dynamic polarization method and electrochemical impedance spectroscopy.SEM was employed to characterize the surface morphologies and the cross-section morphologies.The results show that the surface of the as-deposited sample presented a mesoscale structure called"colony".“Colony”were consisted of many grains and the boundaries of“colony”were orders of magnitude larger than the grain boundaries.The density of intercolony boundaries was measured by using software Image J,and we found that with the increase of thickness of Ni-W alloy,the density of intercolony boundaries first decreased and then increased which can be contributed to the variation of residual stress.At the same time,the corrosion resistance of Ni-W alloy presented a reverse trend.Since W segregated at the boundaries of“colony”during the electrodeposition,and W was more susceptible to corrosion than Ni,the boundaries of“colony”were preferential dissolution.Moreover,the groove structure of“colony”was favorable for stress concentration and crack propagation.Thus,the increase of intercolony boundary would result in the deteriorating of the corrosion resistance of Ni-W alloy.Secondly,this paper studied the relationship between W content and corrosion resistance of the Ni-W coating in both acidic(3.5 wt.%Na Cl,p H=10)and alkaline environment(1 mol/L H₂SO₄,p H=3).The samples were also prepared by direct current plating,and the W content of samples was controlled by changing the content of Ni²⁺in the plating baths.Surface morphologies and composition of the surface were characterized by SEM and XPS respectively,while the crystal structure and grain size were determined by XRD.The results show that as the W content increased,the density of intercolony boundary increased and the grain size decreased.Besides,when the grain size decreased to a definite value called“amorphous limit”,the Ni-W alloy started to transform from nanocrystalline to amorphous.These factors have different effects in non-passivating systems and passivation systems.In alkaline non-passivating systems,the increase of W content,grain boundary and"colony"boundary density all contributed to the sharply decrease of the corrosion resistance of Ni-W alloy.However,when the grain size decreased to the"amorphous limit",the decrease rate of the corrosion resistance was slowed down due to the reduction of grain boundary.But the overall variation trend remained unchanged.In the acidic passivation system,the increase of W content and the decrease of grain size resulted in the increase of the corrosion resistance while the increase of"colony"boundary density led to the decrease of corrosion resistance.When the Ni-W alloy existed in the form of nanocrystalline,the increase of W content played a dominant role due to the strong passivation of Ni-W alloy.The corrosion resistance of Ni-W alloy increased with the increase of W content.When the Ni-W alloy coating began to transform from nanocrystalline to amorphous,the boundary density of"colony"became the decisive factor due to the weakening of the passivation,and the corrosion resistance of Ni-W alloy decreased with the increase of the"colony"boundary density.