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Investigations of Nucleation Mechanism in Initial Period for Direct Electroless Nickel-boron Plating on Magnesium Alloy and Electrochemical Properties of Plating Solution

Author: PengShuHe
Tutor: WangZhouCheng
School: Xiamen University
Course: Applied Chemistry
Keywords: Magnesium alloy Nucleation mechanism in initial period Electrochemical properties
CLC: TG174.41
Type: Master's thesis
Year: 2009
Downloads: 151
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Abstract


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.

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CLC: > Industrial Technology > Metallurgy and Metal Craft > Metallurgy and Heat Treatment > Metal corrosion protection,metal surface treatment > Corrosion control and protection > Metal surface protection technology > Electrochemical protection
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