Study On Dealloying Of Au-Ni Alloys

Sponge-like nanoporous gold materials which can be fabricated by dealloying havelarge surface areas, chemical stability and high yield strength. They are of potential use innumerous applications such as catalysts, sensors, actuators, heat-exchanger and so on.The electrochemical behaviors and dealloyed morphologies of Au_25.3Ni_74.7 andAu_39.4Ni_60.6 alloys in the specimen of as-cast, solid solution and aging have been investigatedby means of X-ray diffraction analysis, scanning electron microscopy (SEM) and energydispersive X-ray (EDX) analysis. The results indicate that:As-cast Au_25.3Ni_74.7 and Au_39.4Ni_60.6 alloys are consist of Au-rich and Ni-rich binaryphases by discontinuous precipitation. Au-Ni alloys have single-phase microstructures withFCC lattice after solid solution treatment at 900℃. Further aging at 150℃for 21 h does notchange their microstructure.In 1 mol/L HNO₃ solution, the current density of as-cast Au_25.3Ni_74.7 alloy is lower thanthat of pure Ni, but much higher than that of the alloy with solid solution treatment and agingtreatment within 500 mV. The current density of as-cast Au_39.4Ni_60.6 alloy is much higher thanthat of alloy with solid solution treatment and aging treatment within 800 mV. The alloys withsolid solution treatment and aging treatment show similar anodic polarization behavior: thecurrent density is low below the critical potential, and it increases rapidly above the criticalpotential. The critical potentials of Au_25.3Ni_74.7 and Au_39.4Ni_60.6 alloys with solid solutiontreatment are 640 mV and 840 mV, respectively, and those of the alloys with aging treatmentare 600 mV and 890 mV, respectively. Compared with Au_39.4Ni_60.6 alloy, Au_25.3Ni_74.7 alloyshows lower critical potential and higher corrosion current density.The sponge-like nanoporous morphology could be obtained through the dealloying ofAu_25.3Ni_74.7 and Au_39.4Ni_60.6 alloys in HNO₃ solution. Only the Ni-rich phase of as-cast Au-Nialloys could be dealloyed, and the Au-rich phase doesn't undergo selective corrosion. Itresults in the formation of compound microstructure in micro scale, one is dense Au-richphase, and another is pure Au phase with nanoporous structure. Au-Ni alloys with solidsolution treatment undergo selective dissolution of Ni on the whole surface and show uniformporous structure. Pore size increases with increasing Ni content. The aging treatment does notchange the nanoporous morphology.