Corrosion Behavior Of FCC-Structure Ni₄₀Fe₃₀Co₂₀Al₁₀ High-Entropy Alloys

Metal corrosion has always been a critical issue in engineering applications,threatening the durability and even the loss of life of structural materials Therefore,high-level corrosion resistance of metals is essential for their structural applications,and the exploration and development of new high-performance corrosion-resistant materials have always been a hot topic of concern for scholars in the field of materials.High-entropy alloys（HEAs）,with their unique composition design concept and phase structures,hold significant promise for the development of high-performance corrosion-resistant materials.Based on the excellent structural properties such as good strength-ductility combination and outstanding high-temperature ductility of Ni-rich HEAs,we have explored their corrosion behavior to provide experimental basis and theoretical support for practical applications in complex environments such as aerospace and deep-sea exploration.In this study,a face-centered cubic（FCC）type Ni₄₀Fe₃₀Co₂₀Al₁₀ HEA with good strength-plasticity synergy was selected as the main research object.The as-cast HEA was prepared by copper-mold suction casting process.Using annealing and the cryogen treatments,the phase structures and corrosion behavior in three solutions of 3.5 wt.%NaCl,0.5 M HCl and 0.5 M H₂SO₄,have been systematically studied for the HEA in three treatment states.Moreover the corresponding corrosion resistance mechanism has been explored.Our work will provide a theoretical basis and experimental support for the development of high-performance High-entropy alloys in complex environments.The main conclusions were as follows:1.The as-cast HEA had a single FCC phase with a uniform composition distribution.According to the results of the potentiodynamic polarization curves,the corrosion behaviors of this HEA in the three solutions were similar,showing cathodic activation,anodic activation and passivation behaviors.HEA-H₂SO₄ had the most positive corrosion potential(E_corr)and the largest passive region（ΔE）,displaying an excellent general corrosion resistance.The electochemical impedance spectroscopy（EIS）results showed that the charge-transfer resistance(R_ct)of HEA-NaCl was about 2.5 times higher than that of HEA-HCl and the largest total resistance（R_t）,indicating that the passive film had a high charge-transfer hindrance and the slowest dissolution rate.Combined with the existence of relatively uniform and compact passivation film,it confirmed that HEA-NaCl had the most excellent pitting resistance.In addition,the H⁺and Cl^-complexation in the HCl solution destabilized the as-cast HEA passive film.And the R_ct and film resistance（R_f）of HEA-HCl were only 39%and 0.18%of those of HEA-NaCl,respectively.In addition,the grain boundary corrosion and pitting corrosion occurred for HEA-HCl.The uniform dense passive film of the HEA-NaCl was composed of Ni O/Ni（OH）₂ and Fe₂O₃/Fe（OH）₃ and small amounts of Co O/Co（OH）₂ and Al₂O₃/Al（OH）₃,showing the high resistance to single Cl^-attack.The corrosion of the sample by H⁺occurred mainly at the grain boundaries,while Cl^-damaged the sample surface mainly through pitting behavior.The results of inductive coupled plasma（ICP）emission spectrometer showed that the concentration of metal ions in the NaCl solution was lowest,indicating that the passive film was resistant to solutions containing only Cl^-.Moreover,the content of Al ions in all three solutions was higher than that of other metal ions,indicating that the HEA passive film could not effectively prevent the selective dissolution of Al.2.The 400 and 800℃-annealed HEAs maintained a single FCC structure,and phase separation occurred after the 1200℃ annealing treatment.In NaCl solution,the general corrosion resistance of the annealed samples were better than that of the as-cast sample.Although the narrowerΔE the as-annealed HEAs had lower passive current density(i_pass)possessing the characteristics of the formed passive film.In addition,the annealing treatment enhanced the stability of the passive film of the samples,resulting in a large R_t value.Among them,the A-800 sample had the best corrosion resistance,which was attributed to its minimal lattice strain（1.35%）and high degree of composition homogenization.The corrosion behavior of the tested samples in HCl solution was similar to that in NaCl solution,and the general corrosion resistance of the A-400 and A-800 samples was better than that of the as-cast one.The EIS analysis showed that the annealing treatment improved the resistance of the sample passive film to the complexation of H⁺and Cl^-,being attributed to the weakened composition segregates and the formation of dense passive films.The tested samples showed a complex reaction type of grain boundary corrosion and pitting in HCl solution,but the pitting behavior of the annealed samples was weakened.The comprehensive corrosion resistance of annealed HEAs in H₂SO₄ was inferior to the as-cast sample.indicated by smaller R_ct value.Furthermore,the grain boundary corrosion and pitting reaction occurring in the annealed HEA were more severe than those in the as-cast samples.Due to the grain refinement of A-400 and A-800,the more contents of H⁺was easy to enter the metastable grain boundaries and accelerate corrosion.3.HEA showed less lattice strain and grain size than the as-cast sample after cryogenic treatment at-20 and-100℃,with the lowest lattice strain（0.984%）for C-20.Unlike the corrosion behavior in H₂SO₄ solution,the cryogenic HEA had an excellent general corrosion resistance in NaCl and HCl solutions,and the i_pass of C-20 sample was lower,showing easy form passive film.The R_ct and R_t values of the cryogenic HEAs were larger than as-cast,indicating a strong ability to hinder charge transfer and a small dissolution rate of the passive film.The R_t values of the C-20 and C-100 samples in NaCl solution were 3 and 2.6 times higher than as-cast sample,respectively,while they were 1.2 and 1.04 times higher in HCl solution,respectively,cryogenic treatment substantially improved the corrosion resistance of HEA to a single Cl^-solution.Similar to the as-cast sample,the corrosion morphologies of the cryogenic HEA in NaCl solution presented the smooth and compact passive film,and the small number of pits.Unlike the as-cast HEA the cryogenic samples in HCl bear only one pitting behavior.The cryogenic treatment promoted uniform distribution of elements and reduced lattice strain,which makes it is easier to form a stable passive film enhancing the resistance to single Cl^-and H⁺and Cl^-complexation.4.The results of the total immersed experiments in 3.5 wt.%NaCl solution for 150 days showed that the C-20 and A-400 samples have lower corrosion rates of 22.92 and 21.14mg·m^-2·d^-1,respectively.Combined with the smooth and compact surface characteristics,and the smaller number of pits,it confirmed the excellent corrosion resistance of A-400 HEA.