Studies On Corrosion Properties Of Binary Two-Phase Bulk Nanocrystalline Ag-25Ni Alloy

Some researches on the stability of nanocrystalline materials in various solutions have attracted more and more attention from some researchers.Up to date,some researches on the corrosion behavior of nanocrystalline materials is mainly focused on coatings,thin films and surface nanocrystallizations.On the contrary,some researches on the corrosion behavior of bulk nanocrystalline materials,especially,binary two-phase bulk nanocrystalline alloys are very rarely reported.In this paper,the nanocrystalline Ag-25 Ni alloyed powders were prepared through the optimization for the reactive conditions by means of liquid phase reduction and mechanical alloying methods,respectively.The bulk nanocrystalline Ag-25 Ni alloys were also obtained through the control of hot pressing process parameters by hot pressing the nanocrystalline alloyed powders synthesized by above methods.The corrosion properties of two bulk nanocrystalline Ag-25 Ni alloys as well as the effect of microstructures on them were investigated in neutral NaCl,H₂SO₄ and NaCl acidic solutions as compared with the coarse grained Ag-25 Ni alloy prepared by powders metallurgy methods by means of PARM273 A and M5210 electrochemical apparatus through potentiodynamic polarization,electrochemical impedance spectroscopy,average activation energy and Mott-Schottky curves,and so on.The results showed that three Ag-25 Ni alloys prepared by the different processes have some differences in the microstructures such as the grain size as well as the composition and distribution of phases.The microstructure of bulk coarse grained Ag-25 Ni alloy prepared by powders metallurgy method is very inhomogeneous with the very low solubility between two components.The microstructure of bulk nanocrystalline Ag-25 Ni alloy prepared by mechanical alloying method though hot pressing are relatively homogeneous with some solubility between two components.The microstructure of bulk nanocrystalline Ag-25 Ni alloy prepared by liquid phase reduction method though hot pressing is extremely homogeneous with the relatively low solubility between two components.The corrosion current densities of PM Ag-25 Ni,LPR Ag-25 Ni and MA Ag-25 Ni alloys increase when NaCl neutral,H₂SO₄ and Na Cl acidic solutions are added to Na₂SO₄ neutral solutions and at the same time they also increase with the increment of solution concentrations.The corrosion rates of nanocrystalline MA Ag-25 Ni alloy decrease,while the corrosion rates of nanocrystalline LPR Ag-25 Ni alloy increase when the grain size is refined.In NaCl neutral solutions,electrochemical impedance spectroscopies of three Ag-25 Ni alloys are composed of a single capacitive loop without diffusion tails These indicate that corrosionprocesses are controlled by the electrochemical reactions.In H₂SO₄ solutions,Electrochemical impedance spectroscopies of PM Ag-25 Ni and MA Ag-25 Ni alloys are composed of a single capacitive loop,while those of LPRAg-25 Ni alloy occur diffusion tails,which indicate that corrosion processes are controlled by the diffusion.In NaCl acidic solutions,electrochemical impedance spectroscopies of PM Ag-25 Ni alloys are composed of a single capacitive loop,while those of MA Ag-25 Ni and LPR Ag-25 Ni alloy occur diffusion tails.The change trend of charge transfer resistances of three Ag-25 Ni alloys is adverse to that of corrosion current densities.All three Ag-25 Ni alloys prepared by the different methods can produce passive phenomena in NaCl neutral,H₂SO₄ and NaCl acidic solutions.The passive films formed on three Ag-25 Ni alloy surfaces are n-type semiconductors and the types of passive films cannot produce the change when the grain size is decreased to the nanometre range.In NaCl neutral and acidic solutions,the carrier densities of passive films decrease in the range of LPR Ag-25 Ni,PM Ag-25 Ni and MA Ag-25 Ni alloys.In H₂SO₄ solutions,the carrier densities of passive films decrease in the range of LPR Ag-25 Ni,MA Ag-25 Ni and PM Ag-25 Ni alloys.In acidic NaCl solutions,the carrier densities of passive films decrease in the range of MA Ag-25 Ni,PM Ag-25 Ni and LPR Ag-25 Ni alloys.These correspond to passive current densities.