Corrosion Electrochemical Properties Of Nanocrystalline Bulk Co-50Ni/Co-45Ni-10Cr Alloys

People pay very great attention to the nanocrystalline materials because they have unique physical and chemical properties as compared with the convention size materials.The corrosion property of the nanocrystalline materials is one of the important factors affecting the application of the nanocrystalline materials.Thus,it is very important in theoretical and applied aspects to study the corrosion properties of the nanocrystalline materials in various corrosive media,and explore their corrosion mechanisms,and find corrosion rate-contralling factors in order to improve their corrosion resistances,and provide theoretical and technical supports for the practical application of the nanocrystalline materials.In this paper,four nanocrystalline bulk Co-50Ni and Co-45Ni-10Cr alloys were obtained at first by hot pressing alloyed powders prepared using mechanical alloying(MA)and liquid phase reduction(LPR)methods.In the following,two conventional bulk size Co-50Ni and Co-45Ni-10Cr alloys were obtained by powder metallurgy(PM)method for the comparison.The electrochemical tests were carried out using electrochemical workstation for six Co-50Ni/Co-45Ni-10Cr alloys in H₂SO₄and NaCl media.Furthermore,the electrochemical corrosion properties as well as the effect of the grain refinement and Cr addition on them were also studied for six Co-50Ni/Co-45Ni-10Cr alloys in H₂SO₄and NaCl media.Some important results were obtained as below.The bulk Co-50Ni/Co-45Ni-10Cr alloys prepared by three different methods have very high densities,and the grain sizes of MA and LPR Co-50Ni/Co-45Ni-10Cr alloys are in the nanometer range.The Co-50Ni/Co-45Ni-10Cr alloys are composed of two phases,and the distribution of two phases in the nanocrystalline MA and LPR Co-50Ni/Co-45Ni-10Cr alloys is more uniform than that in the conventional size PM Co-50Ni/Co-45Ni-10Cr alloys.The distribution of two phases in the nanocrystalline LPR Co-50Ni/Co-45Ni-10Cr alloys is more uniform than that in the nanocrystalline MA Co-50Ni/Co-45Ni-10Cr alloys.In H₂SO₄and NaCl media,Co-50Ni and Co-45Ni-10Cr alloys have similar changing trends of the corrosion properties.With the increase of the solution concentrations,the self-corrosion potentials of PM,MA and LPR Co-50Ni/Co-45Ni-10Cr alloys move negatively,the corrosion current densities increase,and the charge transfer resistances decrease.Thus,the corrosion resistances decrease.In the same H₂SO₄or NaCl concentration,the corrosion current density of MA Co-50Ni/Co-45Ni-10Cr alloy is smallest,the charge transfer resistance and average activation energy are largest.On the contrary,the corrosion current density of LPR Co-50Ni/Co-45Ni-10Cr alloys is largest,the charge transfer resistance and average activation energy are smallest.These indicate that the corrosion resistances of the nanocrystalline MA Co-50Ni/Co-45Ni-10Cr alloys increase,while those of the nanocrystalline LPR Co-50Ni/Co-45Ni-10Cr alloys decreases compared with the conventional size PM Co-50Ni/Co-45Ni-10Cr alloys.It can be concluded that whether the corrosion resistance increases or decreases after the nanocrystallization depends on the preparation method of the nanocrystalline alloys.The corrosion rates of both Co-50Ni and Co-45Ni-10Cr alloys are different.In the same H₂SO₄or NaCl media,the corrosion rate of PM,MA or LPR Co-45Ni-10Cr alloy is significantly higher than that of PM,MA or LPR Co-50Ni alloy.The addition of 10 at.%Cr element can improve the corrosion resistance of the Co-50Ni alloys,and the nanocrystalline MA Co-45Ni-10Cr alloy has best corrosion resistance.The corrosion properties of both Co-50Ni and Co-45Ni-10Cr alloys are related to the corrosive media.The corrosion rate of Co-50Ni/Co-45Ni-10Cr alloys in H₂SO₄media is higher than that in NaCl media.Thus,the corrosion resistance of Co-45Ni-10Cr alloy in NaCl media is obviously better than that in H₂SO₄media,and the nanocrystalline MA Co-45Ni-10Cr alloy has best corrosion resistance in NaCl media.