The Corrosion Resistance Of AlCrCoFe_xNi(x=1,1.5,2) High Entropy Alloy In NaCl Solution

In recent years,wide spread interest has been developed in multi-principal elements high-entropy alloy due to its new design idea of alloy composition.AlCrCoFeNi high-entropy alloy has potential application,which is ascribed to its outstanding comprehensive performance.However,it has a major limitation in term of the expensive cost of raw materials.In order to explore the possibility to reduce the cost of the alloy further,two high-iron content high entropy alloy system were proposed and prepared in the present study,viz.,AlCrCoFe1.5Ni and AlCrCoFe2Ni.AlCrCoFexNi(x=1,1.5,2)high entropy alloys were prepared by water cooled copper crucible vacuum induction levitation melting in this paper.The effects of Fe content on the microstructure of AlCrCoFexNi high entropy alloys were investigated by metallographic microscope,scanning electron microscope(SEM),X-ray diffraction(XRD).Furthermore,the electrochemical performance of the corrosion of AlCrCoFexNi high entropy alloy in NaCl solutions under various salinities(2,3.5,5,6.5wt.%),temperatures(0,20,40,60℃),pH values(pH=1,4,7,10,13)were studied via electrochemical impendence spectroscopy(EIS)and potentiodynamic polarization test in this paper.The main conclusions were formulated as follows:The phase structure of the as-prepared AlCrCoFeNi high entropy alloy is simple BCC phases with dendrite morphology.The as-prepared AlCrCoFe1.5Ni high-entropy alloy also shows simple BCC phase with equiaxed grain morphology.In contrast,the phase structure of as-prepared AlCrCoFe2Ni high-entropy alloy is single FCC coupled with morphology of equiaxed grain.With the increasing of the heat treatment temperature,the corrosion current density changes a little and the corrosion resistances of all alloys in lmol/L NaCl solution show better than that of 304 stainless steel,but slightly lower than that of 904 stainless steel.The corrosion current density of AlCrCoFe2Ni is the smallest and the corrosion resistance in 1mol/L NaCl solution is better than the others,which indicates that the corrosion resistantance of alloys increases by adding proper Fe.When T=20 ℃ and pH=7,the experimental results show that the corrosion process is similar in different concentrations,the corrosion current density increases at first then decreases with the increase of salinity,the salinity threshold are different in different alloys.In the electrochemical impedance spectra,there is only a capacitance reactance and the polarization resistance decreases with the increase of salinity firstly and then increases.It indicates that the corrosion resistance of alloys decreases at first then increases with the increase of salinity.When NaCl salinity is 3.5wt.%and pH=7,the corrosion current density increases with the increase of temperature and the corrosion resistance of the alloys decreases gradually.There is only a capacitance reatance in the electrochemical impedance spectra,and the polarization resistance decreases with the increase of temperature.It indicates that the corrosion resistance of alloys decreases with the increase of temperature.The activation energies are:20.056 kJ/mole for the AlCrCoFeNi and 18.051 kJ/mole for AlCrCoFe1.5Ni and 18.051 kJ/mole for the AlCrCoFe2Ni in 3.5wt.%NaCl.It is found that AlCrCoFe2Ni alloy has the most optimal corrosion properties in the alloy series.When NaCl salinity is 3.5wt.%and T=20 ℃,the corrosion resistance of alloy exists a great difference at different pH values,the corrosion current density ranges one or two orders of magnitude under the condition of acidic and alkaline.The corrosion density decreases and the polarization resistance increases with the increase of pH value.It indicates that the corrosion resistantance of alloy increases with the increase of pH value.The corrosion resistance of alloy under the condition of alkaline is superior to the acidic conditions.