Study On Protection Of High Corrosion-resistant Electroless Nickel Coating On Magnesium Alloy

Magnesium alloys have excellent properties such as low density,high strength,high rigidity,strong shock resistance and toughness.Based on these properties,magnesium alloys are mainly used in the fields of national defense,aerospace,transportation,electronics and so on.In the manufacture of mechanical parts,it will be expected to replace aluminum and steel.However,magnesium alloy has poor corrosion resistance and high chemical reactivity,which limits its wide application in industrial production.The corrosion resistance of the electroless nickel plating(ENP)coating was preliminary evaluated through immersion in 3.5%(wt.%)NaCl solution.Adhesion of ENP coating was tested by scribe and grid measurement according to GB/T5270-2005 and ISO 2819.The surface and cross-sectional morphologies as well as the elemental composition of the coatings were observed and inspected,respectively,by scanning electron microscopy(SEM,Hitachi S-4800,Japan)and energy-dispersive X-ray(EDX,HORIBA EX-350).X-ray diffractometer(XRD,D8 Advance,Germany)was used to characterize the crystal microstructure of the coating.The as-prepared coating was immersed in a certain concentration of NaCl solution.The corrsion potential(Ecorr),corrosion current density(ic)of the coating were estimated from the electrochemical data.The experimental results are shown as follows:(1)The coatings deposited by adding nano silica oxide particles(NSPs)directly into the fluoride-containing ENP bath were shown to display poor adhesion and corrosion resistance,which results from the formation of byproduct due to the reaction between fluoride and NSPs.In contrast,using a two-step deposition method,a functionalized duplex coating with smoother and more uniform surface was shown to exhibit enhanced corrosion resistance on AZ31 Mg alloy because of the introduction of NSPs in the outer layer of the coating.The incorporation of NSPs does not influence on the crystalline state structure of the NNNS(Ni-P/Ni-P-nano-SiO2)coatings,all of which remain amorphous structures.The corrosion inhibition effectiveness of the optimum coating in NaCl solution was higher than that in HCl solution.The enhancement of the as-prepared coatings is attributed to the mismatch of coating defects and the “barrier effect” of NSPs during corrosion.(2)We have demonstrated the preparation of a self-healing metallic Ni coating containing uniformly dispersed F@MSNs(mesoporous Si nanocontainers)on Mg alloy towards enhanced corrosion resistance.The as-prepared F@MSNs have been shown to enhance corrosion-resistant capability of bare Mg alloy after exposure in corrosive media for a long period of time.This is ascribed to the formation of a protective film,MgF2,arising from the reaction between the Mg ions and the fluoride ions released from F@MSNs.The release of fluoride ions and formation of MgF2 film are important for repairing defects in the natural oxide film on Mg alloy surface,thereby changing the corrosion mechanism and decreasing the corrosion rate of Mg alloy.The self-healing functionality of the nanocomposite Ni coating has been further substantiated by the observation of stable Ecorr and the decrease of ic and increase of Rct with increasing immersion time in the corrosive media.