Study On The Dealloying Mechanism Of Ag(Al)-Au Alloy In Acid Solutions

Nowadays,nano-porous metals are widely used in catalysis,sensing,fuel cells and other fields.As an important method to prepare nano-porous metals,dealloying has received extensive attentions.In this paper,Ag-Au and Al-Au alloys are selected to study their dealloying mechanism in acidic environment.(1)Study on critical dealloying potential and passivation mechanism of Ag-Au alloyWe take Ag70Au30 alloy as the sample and characterize the influence of passivating potentiostatic treatment by atomic force microscope(AFM),linear sweep voltammetric test(LSV)and electrochemical impedance spectroscopy(EIS)methods.The characterization results show that the passivating potential applied results in humps with specific wavelength forming on the surface.We perform continuous passivating potentiostatic treatments with increasing and decreasing potential and the relevant results demonstrate that the passivating effect on surface is potential-irreversible.Based on the theory developed by Sieradzki,we give a qualitative and quantitative explanation for the effect of passivating potential by "passivating-trimming" model:the potential applied will "trim" the surface and leave humps on surface with a specific wavelength;the wavelength decreases with the increase of potential;as the potential applied increases to a high degree,where the corresponding wavelength is lower than the average scale of Ag percolation channels,the dealloying occurs.(2)Study on surface state evolution of Al2Au ribbon in HCl solutionBy ex-situ X-ray photoelectron spectroscopy(XPS)and scanning electron microscope(SEM),the dealloying process of Al2Au ribbon in HCl solution is divided into two stages,induction stage and pore-formed stage.In induction stage,the chemical state of Au atoms undergoes a transformation from initial state to intermediate state,where no nano-porous structure is formed;in pore-formed stage,the chemical state of Au atoms undergoes a transformation from intermediate state to pure Au state.We define the surface containing highest ratio of intermediate state Au as critical surface.It is found that the higher concentration of HCl solution can result in a higher ratio of intermediate state Au in critical surface.We measured the open-circuit potential(OCP)evolution during dealloying and find that the OCP noise exhibits different modes in induction stage and pore-formed stage.The direct link between OCP noise mode and dealloying stage is meaningful to realize in-situ monitoring on dealloying process.(3)Study on structural adjustment and dealloying behavior of Al50Au50 ribbonBy adjusting melt-spin parameter and applying annealing treatment,we prepared monophasic and multiphasic Al50Au50 ribbon.The multiphase Al50Au50 has different phase distribution on the free side and roller surface.The free side is of a multiphase structure composed of Al2Au,AlAu and AlAu2 phases,while the roller side is of a single-phase AlAu structure.We found that only the free side can be dealloyed into nano-porous structure with uniform pore-ligament size through high temperature chemical corrosion.The preferential corrosion of Al2Au phase in free side not only bring extra reaction area for AlAu and corrosive solution,the nano-porous structure left also relived the Au accumulation on AlAu.Single-phase Al50Au50 ribbons are obtained by annealing as-spun ribbons at 550℃ for 10 h.We found that the single-phase AlAu ribbon exhibits good passivation ability in HCl solution and has a fluctuating opencircuit potential,which is not commensurate with its passivation ability.We carry out a series of electrochemical studies on the abnormal fluctuating OCP(including MottSchottky tests,potentiostatic treatment,EIS tests et al.)and finally proved that the essence of OCP fluctuation is the spontaneous charge and discharge behavior of space charge region in the oxide film.(3)Study on the influence of oxide layer on Al2Au’s dealloying morphologiesThe as-spun Al2Au ribbons were respectively annealed at 550℃ and 800℃ for 10 h to produce oxide layer on surface.The structural characterization results show that the Al2Au crystal structure and orientation of the as-spun ribbon almost keep unchanged after 550℃ annealing.While under 800℃ annealing condition,the as-spun ribbon undergoes serious recrystallization with AlAu phase precipitated at the position of original grain boundary.By exploring the chemical state and microstructure of the oxide layers of the ribbon annealed at 550℃ and 800℃,we find that the Au element in oxide layer of 800℃ annealed ribbon exist in the form of solute atoms and has a binding energy about 84.7 eV,while in the case of 550℃ annealed ribbon,the Au element exist in the form of clusters,which has a binding energy about 85.4 eV.The Au clusters exhibit pinning effect on oxide film,greatly enhance its HCI resistance and consequently result in a different dealloying morphology.The solute Au atoms enhance little on HCl resistance and tend to diffuse away from the corrosion interface and accumulate into cotton-like corrosion product.