Investigations Of Nucleation Mechanism In Initial Period For Direct Electroless Nickel-boron Plating On Magnesium Alloy And Electrochemical Properties Of Plating Solution

Magnesium alloy has excellent capabilities,but its widespread applications are restricted by its poor corrosion resistance.In this work,we had investigated the variation of magnesium alloy surface with pretreatment.Series techniques,such as scan electron microscope(SEM),X-ray diffraction(XRD),scratch test,and electrochemical methods,are employed to determine the properties of coating obtained under optimal technology condition.Nucleation mechanism in initial period is discussed by using energy diffraction spectroscopy(EDS) for elemental analysis and SEM to abserve surface morphologies.The influences of bath compositions and different electrodes on electrochemical properties of plating solution are studied by using electrochemical method,cyclicvoltammetry.Result shows a silvery white oxide film with a net structure got on magnesium alloy surface after acid etched.It can be seen obviously that the coating surface, obtained under optimal technology conditions,presents a cystiform structure,very compact,and no evident surface crack.Whereas,cross-section morphology of Ni-B coating indicates coating combines closely with substrate,what appears a mechanical occlude status.The scratch test also shows an excellent bonding.XRD patterns indicate the coating structure is a mixture of crystal and non-crystal.The results of electrochemical corrosion resistance indicate Ni-B coating has an excellent corrosion resistance in 3.5%sodium chloride solution.Compared with the self-corrosion current density of substrate,28.548μA/cm²,what of Ni-B coating is less than 0.7μA/cm² and decreases almost two degrees.The investigation of nucleation mechanism in initial period shows two phases, with different chemical properties and should cause different deposition processes, present on substrate surface.The first one is exposed magnesium,second is magnesia remained on surface after acid etched.The reactions of nickel electrochemical nucleation and substrate corrosion happen on the first position.The initial nickel grains appear on the second phase of substrate,βphase of Mg₁7Al₁2.The main reaction is nickel nucleation.On the second one,magnesia film dissolves firstly,a fresh substrate surface exposes and then nucleation reaction on the first position happened.The reaction is mainly for nickel grains growth.Three oxidation peaks,corresponding with oxidations of NaBH₄,nickel and B-rich Ni-B phase respectively,could be seen on CV patterns.But the oxidation peak of B-rich Ni-B phase is unobvious,what may be ascribed to the less of B content in coating.The initial concentration for NaBH₄ oxidation is 0.015M on glassy carbon, 0.01M on Ni.The results also indicate the currents at upper cathodic limit and NaBH₄ oxidation increase with NaBH₄ and Ni²⁺ concentrtions.Results under different electrodes show Ni has catalysis for NaBH₄ oxidation.However,the interactions between anodic and cathodic processes are probably generated through the deprotonation reaction of the BH₄^- anion.It represents both the oxidation and the discharge currents are larger on glassy carbon electrode,where H* produced by the deprotonation reaction of the BH₄^- anion,cannot penetrate into the substrate,than on nickel.