The Theoretical Study On The Corrosion-resistance Of Iron-based Alloy

With the deep development of the condensed matter physics and quantum chemistry as well as the improvement of the level of the computer, the material design based on the atomic and molecular becomes an important direction of development for the synthesis technology of advanced materials. Moreover, the theoretical calculation of clusters plays a very important role in the study of the new material microstructure design.According to the nature of electrochemical corrosion of metal and guided by the DFT, a series of iron-aluminium, iron-silicon and iron-nickel alloy clusters are studied in order to discover their ground state structures and the growth mechanisms as well as the building blocks, to design and predict of their geometric and electronic structure in this paper. Our main purpose is to explore the corrosion-resistant property of iron-based alloy clusters by theoretical calculations in order to obtain a group of quantitative parameters combined with experimental data at the atomic and molecular level, and provide theoretical guidance for designing the most optimal corrosion-resistant materials. The main study contents and results of this paper are described in detail as follows:1. Using the BPW91 method combined with Lanl2dz basis sets, we study on the geometric, electronic properties of Fen(n≤13) clusters in this chapter. The BPW91 method is proved to be able to obtain correct information and save the computing time. Moreover, the average bonding energy per atom obtained is closer to the experimental results than that of other DFT methods. The small iron clusters favors highest-dimension. The results of the second finite difference of the total energy and the HOMO-LUMO energy gap show that n=6 and 8 are magic number which are more stable than neighboring ones. And the Fen(n≥8) cluster are easy to be dissociated to Fe6 and Fe8, which further indicate Fe6 and Fe8 clusters have special stablility. A strong peak at 238 cm-1 is found for Fe8 cluster agreed with the stretching vibration frequency of octahedron for Fe6, which indicates the octahedron structure has special stability. Moreover, the octahedral structure is part of BCC lattice structure, including atoms of six cell surface. That fully prove Fe6 cluster can very well reflect the properties of iron metal, and it can provide a theoretical model for Al, Si, and Ni doping iron clusters.2. After study the ground state and electronic properties of Al-doped Fen(n≤8) clusters by using the BPW91/Lanl2dz/6-31G(d) method, We obtain accurate geometric information in a kind of way. In order to eliminate the error of basis sets, single-point BPW91/6-311++G(d, p) calculations are performed for all the structures by using the geometries obtained from the BPW91/lanl2dz//6-31g(d) optimizations. Furthermore, the single-point BPW91 calculations for Al-doped Fen(n=2~4) are very close to those obtained by the fully optimized BPW91 results.Calculated results show an doped Al atom prefers to the surface of the iron structures and do not change the frame structures of iron atoms. Further, we analyze the stability of Fen-1Al clusters, and combined with the results of the second finite difference of the total energy as well as the HOMO-LUMO energy, the vertical ionization potential (VIP) shows that the Fe3Al has special stability. On the other hand, the Fe6 cluster is selected to model the geometric and electronic structures of FemAln with different Al content by adopting Al atom one by one. The most stable structures of FemAln(m+n=6) clusters transfer from octahedron to the capped trigonal bipyramid, growing towards low- dimension. Our calculations show Fe5Al,Fe4Al2 and Fe3Al3 have large HOMO-LUMO gap and low HOMO value, indicating these clusters have high stablility, of which the Al content of atom percentage for these cluster methioned above are within Al content of Fe3Al and FeAl intermetallic compounds. Therefore, the quantitative parameters such as HOMO-LUMO gap and HOMO can characterize Fe-Al alloy corrosion resistance at atom and molecular level.Experimentally, we present the electrochemical corrosion behaviors of Fe3Al in seawater due to vertify our theoretical results. the annual corrosion rate of the Fe3Al is compound by using weight loss method, the corresponding results indicate Fe3Al have more excellent corrosion resistance than carbon steel. Subsequently, by analyzing the surface morphology of corrosion of Fe3Al and carbon steel as well as the measurement of the open circuit potential, Fe3Al is found to have excellent corrosion-resistance which indicates that the properties of bulk materials can be obtained from a finite size cluster model.3. The geometric and electronic structures of Si-doped iron clusters are investigated by using the same method as that of Fe-Al alloy clusters. Compared with pure iron clusters, the doping of Si atom can improve the stability of iron clusters. By analyzing the stability of Fe-Si alloy clusters, the Fe3Si and Fe5Si are found to be magic clusters. The Si content of Fe5Si is 14%Si which locates in the best Si-doped scope of corrosion resistance of Fe-Si alloy. Clearly, the study on small clusters can make a better understanding of the corresponding materials.4. As a major component of stainless steel, Ni-doping steel improved its corrosion resistance greatly. Theoretical, we study the ground-state and the bonding mechanism of Fe-Ni alloy clusters with the density functional theory method. By analyzing the stability of the ground state structures, Fe3Ni,Fe4Ni,Fe5Ni和Fe6Ni are found have high stability, and the corresponding atomic ratio of Fe and Ni atoms for those clusters is consisted with the doped Ni content of Fe-Ni alloy possessing excellent corrosion resistance. Our theoretical results show Fe5Ni is most stable in those clusters, the corresponding content of Ni is 16.7%Ni. In light of the results, we predict 16.7%Ni is the optimized doped contend for the corrosion resistance of Fe-Ni alloy.