Studies On Corrosion Electrochemical Properties Of Bulk Fe-50Co Alloys With Different Sizes

Properties of materials are not only determined by their composition,but also closely related to their microstructures such as grain size,shape or interface.As the grain size becomes smaller,the number of atoms located on the surface increases,and the ratio of surface area to volume also becomes bigger,which will enhance the surface active sites and lead to some special physical and chemical properties.As a branch of metal nanomaterials,alloy nanomaterials play an important role in modern production and life,national defense and high-tech development.It is crucial to solve the corrosion question before the application of nanomaterials.The nanocrysralline MA Fe-50Co and conventional size PM Fe-50Co alloyed powders were prepared by mechanical alloying(MA)and powder metallurgy(PM)methods,respectively,and two corresponding bulk alloys were obtained by vacuum hot pressing technique.The corrosion electrochemical behavior of two Fe-50Co bulk alloys in NaCl,HCl and H₂SO₄solutions with different concentrations was studied using electrochemical workstation by testing the open circuit potential curves,electrochemical impedance microscopies and potentiodynamic polarization curves,and the relationship between the grain size and corrosion resistance was also studied.The following results were obtained:(1)The densities of nanocrysralline MA Fe-50Co and conventional size PM Fe-50Co bulk alloys are 98.27%and 99.05%,respectively.The grain sizes of nanocrystalline MA Fe-50Co alloyed powders and bulk alloy are 10.6 nm and17.3 nm,respectively.The grain size grows after hot pressing,but it is still in the nanometer range.Two Fe-50Co alloys are composed of Co-rich?phase and Fe-rich?phase.The alloy matrix is composed of Co-rich?phase,and Fe-rich?phase is distributed on the alloy matrix in the form of nets.The microstructure of the nanocrystalline Fe-50Co alloy is far more uniform than that of the conventional size PM Fe-50Co alloy.(2)NaCl,HCl and H₂SO₄solution concentrations are 0.05,0.10,0.30,0.50,0.70 and 0.90mol/L,respectively.It can be found that the corrosion current densities of nanocrysralline MA Fe-50Co and conventional size PM Fe-50Co alloys increase,and the charge transfer resistance values decrease with the increase of solution concentrations,indicating that the corrosion rates increase with the increase of solution concentrations.(3)Under the condition of the same NaCl or H₂SO₄solution concentration,the corrosion current density of MA Fe-50Co alloy is larger than that of PM Fe-50Co alloy,the charge transfer resistance and the average activation energy of MA Fe-50Co alloy are smaller than those of PM Fe-50Co alloy.Thus,the corrosion rate of Fe-50Co alloy increases and its corrosion resistance decreases after nanocrystalizations.Conversely,under the condition of the same HCl solution concentration,the corrosion current density of MA Fe-50Co alloy is smaller than that of PM Fe-50Co alloy,the charge transfer resistance and the average activation energy of MA Fe-50Co alloy are larger than those of PM Fe-50Co alloy.Thus,the corrosion rate of Fe-50Co alloy decreases and its corrosion resistance increases after nanocrystalizations.(4)With the increase of polarization potentials,both PM Fe-50Co and MA Fe-50Co alloys can produce passivation phenomena in NaCl solutions,and passive current density of MA Fe-50Co alloy is larger than that of PM Fe-50Co alloy.Furthermore,MA Fe-50Co alloy produce passivation phenomena in H₂SO₄or 0.90 mol/L HCl solutions.Moreover,the corrosion rates are relative to the corrosion solutions.The corrosion rates of two Fe-50Co alloys in NaCl solutions are much smaller than those in HCl and H₂SO₄ solutions.