Studies On Microstructures And Corrosion Properties Of Bulk Cu-20Ni-20Co Alloys

Nanomaterials show many unique physical and chemical properties due to their unique small size effect,surface and interface effect,and quantum size effect.Thus,nanomaterials have become one of the hot spots in the field of materials science.As an important branch of nanomaterials,metal nanomaterials have been widely concerned in recent years.In fact,the chemical stability of metal nanomaterials should be solved firstly before their applications.At present,some researches on the corrosion properties of metal nano materials mainly focus on nanocoatings,while there are few studies on the corrosion properties of bulk alloys.Thus,it has important theoretical and practical significance to investigate the corrosion properties of bulk alloys with different microstructures prepared by different methods at high and room temperatures.Nanostructured Cu-20Ni-20Co(MA)alloyed powders were prepared by mechanical alloying(MA)methods and corresponding Cu-20Ni-20Co(MA)bulk alloy was obtained by hot pressing technique.Moreover,the microstructures of Cu-20Ni-20Co alloy were characterized by XRD and SEM,and the corrosion properties as well as the effect of grain refinement on them were studied in comparison with the conventional size Cu-20Ni-20Co alloy prepared by powder metallurgy(PM).The results are gotten as below(1)The densities of conventional size Cu-20Ni-20Co(PM)and nanocrystalline Cu-20Ni-20Co(MA)bulk alloys are 99.2%and 98.6%,respectively.The grain sizes of nanocrystalline Cu-20Ni-20Co(MA)alloyed powders and bulk alloy are 16.8 and 25.5 nm,respectively.The grain size grows after hot pressing,but it is still in the nanometer range.Two Cu-20Ni-20Co alloys are composed of Cu-rich?phase and Co-rich?phase.The alloy matrix is composed of Cu-rich?phase,and Co-rich?phase is distributed on the alloy matrix in the form of islands or nets.The microstructure of the nanocrystalline Cu-20Ni-20Co alloy is far more uniform than that of the conventional size Cu-20Ni-20Co(PM)alloy.(2)The oxidation kinetics of Cu-20Ni-20Co(PM)alloy and Cu-20Ni-20Co(MA)alloy deviates from the parabolic rate law at 600-700?in 0.1MPa pure O₂,and the oxidation rates of Cu-20Ni-20Co(PM)and Cu-20Ni-20Co(MA)alloys increase with increment of temperatures.For Cu-20Ni-20Co(PM)alloy,the outer oxide layer is mainly bright Cu oxides,but contains a small amount of dark Co and Ni oxides.The outer oxide layer is stripped from the inner oxide layer.The inner layer is mainly composed of Co oxides,which contains a small amount of Ni oxides,and is accompanied by the internal oxidation of the reactive component Co.for Cu-20Ni-20Co(MA)alloy,the outer oxide layer is mainly composed of bright Cu oxides and dark Co and Ni oxides,no internal oxidation of the reactive component Co occurs.The nanocrystallization finishes the transformation of the reactive component Co from the internal oxidation to the external oxidation.(3)In NaCl solutions,the corrosion current densities of Cu-20Ni-20Co(PM)and Cu-20Ni-20Co(MA)alloys increases gradually,but the charge transfer resistances and activation energy decrease with the increase of solution concentrations.Thus,the corrosion rates become fast.Furthermore,under the condition of NaCl solution concentrations,the corrosion rate of Cu-20Ni-20Co(MA)alloy is larger than that of Cu-20Ni-20Co(PM)alloy.This indicates that the corrosion resistances of Cu-20Ni-20Co alloy decrease after nanocrystallizations.;Namely,nanocrystallization reduces the corrosion resistance of Cu-20Ni-20Co alloy.Moreover,under polarization conditions,both Cu-20Ni-20Co(PM)and Cu-20Ni-20Co(MA)alloys produce the passivation phenomena.The electrochemical impedance spectroscopy is composed of a single capacitive impedance arc and has no diffusion tail.Thus,the electrochemical reactions control the corrosion processes.