Study On Microstructures Of Hot Dip Aluminized Coatings And Effects Of La

The microstructures of hot dip aluminized coatings for steel 20 before and after diffusion treatment have been systematically studied with Optical Microscope, Transmission Electron Microscopy (TEM), Scanning Electron Microscopy (SEM), Electron Probe Microanalyzer(EPMA),Energy Disperse Spectroscopy (EDS) and X-ray Diffraction (XRD). The growth kinetics of diffusion layers for steel 20 was investigated and its growth kinetic equation was calculated with computer. The mathematical model of the interfacial layer growth between alloy layer and matrix was established. The morphology, distribution, formation mechanisms and growth kinetics of interfacial voids during high temperature oxidation were investigated, and the orientation relationship between Fe3Al and AlFe3C0.5 phase and crystallography of Al4C3 phase were observed and analyzed. The effects of rare earth La on hot dip aluminizing processes and the properties of aluminized steel were also discussed.The experimental results showed that, the surface coating of hot dip aluminized steel consisted ofα-Al, FeAl3 and amorphous phase. When La was added, the outer surface of the coating was smooth and continuous, and the tooth shapes of the aluminum compound of alloy layer became thin and close. While the Al-Fe alloy liquids used in hot dip aluminizing were quickly solidified and slowly solidified respectively,amorphous phase was found both Al-Fe alloy samples with or without rare earth La. While the Al-Fe alloy liquids were solidified slowly, amorphous phase was only found in sample with rare earth La. Spinodal decomposition structures were found in Fe2Al5 and FeAl2, and from the surface of diffusion layer to inner coating, the pattern of spinodal decomposition structures changed from parallel beeline toparallel across striation and to lattice shape. The wavelengths of the spinodal decomposition at initial stages were analysed by the theory of static lattice waves. The forming course of spinodal decomposition structures was cleared from continuous phase transformation to discontinuous phase transformation. The formation mechanism of spinodal decomposition structures was analyzed by means of 15% rule.AlFe3C0.5, Al7Fe6La, twin Al4C3 compounds were found in diffusion layer by TEM. The Al4C3 phases is a 180o secondary rotation twin with(003)as twinning plane and with [001] as twinning axis, and its long axis direction of [210] parallels to(003)and perpendicular to [001]. The orientation relationship between Fe3Al and AlFe3C0.5 phase is as follows:The research results show that the relation between growth rate of diffusion layer and time is a para-curves, and the RE La has influence on all diffusion stage. The effects of RE La are stronger at 750℃and 850℃, but weaker at 950℃. The growth rate of interfacial voids between Al2O3 and diffusion layer is controlled by oxidation process and approximately accords with a parabola growth rule. The average depth of voids is linear with the void diameter, and the number of the voids in unit area first increases and then decreases.The formation mechanism of interfacial voids between the subsurface layer and the transition layer is owing to the difference of the diffusion rate between Al and Fe, and the growth rate of the voids can be divided into two stages named fast growth stage and stable growth stage. The growth of interfacial voids leads to the internal oxidation of diffusion layer during high temperature oxidation. The kinetic curves of oxidation deviates form square parabola relation into parabola—straight relation. RE addition La can accelerate the formation of Al2O3 films, delay formation and aggregation of vacancies, and inhibit the voids in diffusion layer aggregating into a lineal crack. Experiments prove that the main reason why RE La restrains the formation and growth of the voids is that the concentration gradient of aluminum decreased at high temperature due to the decrement in thickness of aluminum coating after hot dip RE-aluminizing. The optimum technical parameters of hot dip aluminized coatings for steel 20 are diffusion at 900℃for 2 hours,and RE addition La can enhance the corrosion resistance of aluminized coatings based on experiment of H2S and CO2 corrosion.