Effect Of Cr Addition On The Corrosion Behaviors Of High-entropy Alloy Coatings Fabricated By Laser Cladding

Laser cladding as a new promising surface modification technology can used toimprove corrosion resistance of metallic materials.AlCoCr_xFeNi and AlCoCr_xFeNiTi high-entropy alloys（HEAs）coatings were prepared on the surface of 45#steel and Ti6Al4V by laser cladding.The evolution in microstructure and phase compositions of the coatings with the change in x was investigated comprehensively using an X-ray diffractometer（XRD）,an optical microscope（OM）and a scanning electron microscope（SEM）coupled with an energy dispersive spectrometer（EDS）.The change in corrosion behavior with x was revealed in detail by potentiodynamic anodic polarization,electrochemical impedance spectroscopy and immersion tests along with X-ray photoelectron spectroscope analyses.1 AlCoCr_xFeNi HEAs coatings were prepared on the surface of 45#steel by lasercladding with different contents of Cr（x=0.50,0.75,1.00,1.50,2.00）.Results indicated that the dilution rate of the coatings was gradually increased with the increase in x.A model was established to estimate the content of each component in the coatings,based on which three parameters including mixing entropy(△S_mix),net driving factor（?）(?=T_m?△S_mix/|△H_mix|),and atomic size difference（δ）were calculated to precisely predict phase constituents in the coatings.The coatings with x=1.00,1.50 were composed of a single solid solution with face-centered cubic（FCC）structure.Apart from numerous coarse dendrites（FCC）,a small number of fine equiaxial particles of FeAl₃ as the intermetallic compound were also observed in the interdendritic regions of the coatings with x=0.50,0.75,2.00.Corrosion resistance of the coatings was improved first and then decreased with the increase in x in the electrochemical tests（potentiodynamic anodic polarization tests and electrochemical impedance spectroscopy tests）carried out in 0.1 mol?L^-1 HCl solution.The coating with x=1.50 exhibited the optimum corrosion resistance,which had higher corrosion potential（-0.2936 V）,lower passive current density(2.95E-4 A?cm^-2),broader passive potential range（0.30 V）,higher breakdown potential（0.0930 V）,and higher charge-transfer resistance（3996.25??cm²）than the other coatings.The coating of the passive film was mainly composed of NiO、CoO、FeO、Fe₂O₃、Al₂O₃、and Cr₂O₃.The thermodynamics calculation also confirmed that Cr₂O₃ was highly difficult to dissolve in the acid solution rich in Cl ion.It is concluded that Cr₂O₃ plays a decisive role in improving the corrosion resistance of the coatings and the stability of the passive film.The result of thermodynamics calculation can agree well with that of the dynamics investigation.Anodic polarization tests and electrochemical impedance spectroscopy tests also confirmed that the coating with x=1.50 had the best corrosion resistance.2 AlCoCr_xFeNiTi HEAs coatings were prepared on the surface of Ti6Al4V bylaser cladding with different contents of Cr（x=0,0.50,1.00,2.00）.As the x content was increased,the dilution rate of the coating changed little by about 70%.The crystal structure of the coatings changed regularly with increasing the content of Cr.The content of Ti₂Ni phase was decreases,accompied with the increase in the content ofα（Ti）phase.When x=2.00,the coating was composed of a single solid solution withα（Ti）phase.It is concluded that the addition of Cr element is beneficial to promote the transformation of coating phase structure towards a single solid solution.Coating II（x=0.50）had the best corrosion resistance in 0.1 mol?L^-1 HCl solution.Electrochemical test and immersion experiments showed that the corrosion resistance of the coatings were improved（from x=0to x=0.50）,then decreased（from x=0.50 to x=1.00）and increased（from x=1.00 to 2.00）again with the increase of x.This may be the large amount of Cr solid solution into the coatings,which increases the corrosion potential and prevents the Cl ions from penetrating into the coatings.AlCoCr_xFeNiTi HEAs coating II（x=0.50）had the best corrosion resistance,the coating was composed ofα（Ti）and Ti₂Ni dual-phase structure,and the excellent corrosion is attributed to the uniformity of the structure and the addition of the corrosion-resistant element Cr.In addition,the wear resistance of AlCoCr_xFeNiTi high-entropy alloy coating is greater than that of Ti6Al4V substrate.