Studies Of Steel Corrosion Protection By High-Entropy Alloy Coatings Al_xFeCrCoNiCu

Steel is indispensable in engineering building materials with good mechanical properties and low price, but its poor corrosion resistance has limited the application in the most corrosive environments. Surface coating technology is the effective way to improve the material corrosion resistance. AlFeCrCoNi high entropy alloy was confirmed that it had good oxidation and Cl" resistance combined theory and experiment in this paper. High entropy alloys (HEA) coating was plated on steel surface using electron beam evaporation and laser cladding methods through theoretical results in this thesis, and the research on the effect of the corrosion properties of the HEA coatings has been done. The main contents of the present paper are as following:Firstly, in order to study corrosion mechanism of HEA the interactions of the element composition and molecular adsorption behavior have been analyzed by density functional theory method for the steel matrix and HEA. However, it was important to find a relatively stable Fe3iMnC structure and cut a Fe31MnC (001) surface. For example, O2six placeholders had happened on Fe-termination. The FCC hollow site was found to be the most stable adsorption site with the maximum adsorption energy of1.989eV. At the same time, the atomic adsorption phenomenon had taken placed, which had strong adsorptions that were around3.5eV. C-termination was more advantageous to occur the oxygen adsorption than other terminations. Furthermore, the influence of the adsorbed NaCl molecules on corrosion behavior of the surface was also examined. Results indicated that the stability of the Cl atom structure preferred to occupy the FCC hollow sites around Fe atoms, with adsorption energy of3.007eV. In three terminations surface, C-termination had the largest adsorption energy compared with Fe-termination and Mn-termination. The surface structure (B2) of high entropy alloys coating AlCrFeCoNi (001) was not much change after convergence. An O2molecule was placed to the coating surface with adsorption energy of less1.0eV, while adsorption energy for NaCl on coating surface was1.23eV. Compared with the steel substrate adsorption results, it was clear that the interaction between the surface and the adsorbed molecule was weak, which was less influence of the surface oxidation and corrosion. Therefore, we concluded that it would have the effect on oxidation and corrosion resistance if high entropy alloys as a coating on the surface of steel.Secondly, Al、Fe、Cr、Co、Ni and Cu of high entropy alloys composition elements were designed using the first-principle method. The AlxFeCrCoNiCu (x=0.25,0.5and1.0) alloys were prepared by vacuum arc-smelter with larger than99.99%of purity element on the basis of the first principle predicted results. The mechanical properties and microstructure of high entropy were investigated by hardness test, compression, friction and wear, XRD, SEM and EPMA method. The results showed that hardness value of Al1.0FeCrCoNiCu was485HV with high compressive strength and wear resistance. With a low content of aluminum (x=0.25,0.5), the coatings were mainly presented as only one simple FCC solid-solution structure, respectively. As the x value reached1.0, it was composed of FCC and BCC phases by XRD method. The high entropy alloys had typical dendrite structure and the interdendrites were rich in Cu and Ni.Finally, the HEA thin coatings were deposited on the steel substrate surface using two different methods. The HEA ingots were used as the precursor in evaporation and steel as matrix. The mechanical properties of coatings were investigated by hardness and nano indentation test. The morphology and mass fraction were investigated by XRD, AFM, SEM and EPMA methods. The corrosion resistance of coating was analyzed using electrochemical corrosion experiment. The hardness of Al-1.0coating was reach to481HV that was about two times higher than matrix materials. The maximum loads were10.25mN,12.39mN and14.78mN for Al-0.25, Al-0.5and Al-1.0, respectively. The residual depth after unloading was nearly100nm with larger plastic deformation. High entropy coatings were keeping solid solution structure, which was noted to have a composition similar as originally designed alloy excluded large content of Fe and O element. However, the surfaces of all coatings were very smooth and uniform. Compared high entropy coating with304ss for electrochemical test, the HEA coating had better overall general corrosion behaviour, with a larger Ecorr and smaller ICorr than304ss in0.5mol/L H2SO4solution. Thus, the corrosion resistance of the coating was superior to that of304ss. The Al-1.0coating had the most positive effect on the pitting corrosion with a large corrosion potential and a small corrosion current density, while the pitting corrosion resistance of304ss was the worst among them.