The Materials Design And Oxidation Kinetics For Hot-dip Coated Steel

Every year, the direct economic loss caused by steel corrosion is up to severalhundred billion dollars around the world. How to extend the service life of steelthough alloy coating has been much concerned in domestic and international. The55wt.%Al-Zn-1.6Si alloy coating is a widely used steel protective coating. However,the mechanism that the addition of alloy element Ti causes the changes inmicrostructure and oxidation kinetics is still unclear. Some people thought that theaddition of Ti lead to the precipitation of Fe₄Al₁₃which acts as nucleation sites torefine the microstructure. But other researchers thought that Ti would combine withAl to form TiAl3which acts as nucleation sites. The problem is which intermetalliccompounds would form. This can be clarified by Al-Zn-Si-Ti-Fe thermodynamicdatabase. For the oxidation kinetics of coated steels, most of experimental worksdetermined repeatedly the relationship between reaction fraction and time inisothermal conditions recently. Few researches investigated the influence of heatingrate, sample shape and size and the volume change of scale. Therefore, this thesis willstudy the thermodynamic phase diagram and oxidation kinetics of the55wt.%Al-Zn-Si-Ti coating system.The Al-rich corner of the Al-Zn-Si-Ti-Fe system was optimized and improvedthrough CALPHAD method. The precipitated phases and precipitated quantities wereinvestigated through calculating the isothermal sections at different Ti and Si content.The effects of the Ti addition on the intermetallics in the alloy layer and themicrostructure of the55wt.%Al-Zn-Si coating were investigated by experiments. TheX-ray diffraction （XRD）, scanning electron microscopy （SEM） and thermogravimetry（TG） were used to investigate the isothermal oxidation and non-isothermal oxidationof55wt.%Al-Zn-Si and55wt.%Al-Zn-Si-Ti coatings. The evolution of the coatingmicrostructure during isothermal oxidation process was analyzed based on theexperimental results. A kinetic model with physical meaning was proposed, discussing quantitively the effect of oxidation induced volume change, temperature, oxygenpressure, heating rate, sample shape and size.The main results in this thesis can be summarized as following:1) The crystal structure of the ternary intermetallic τ-Ti26Al55Zn19was determinedthrough HR-XRD and HR-TEM, which showed that τ is an ordered face centeredcubic with Ti atoms occupying1a（0,0,0） positions and Al/Zn atoms occupying3c（0,0.5,0.5） positions. The Al-Zn riched side of Al-Zn-Ti system, Al-Fe riched sideof Al-Zn-Fe system and Al riched corner of Al-Si-Ti system at the temperature rangeof450⁷00oC were obtained by diffusion couple method and equilibrium alloys.2) Based on the experimental data, the Al-Zn-Ti, Al-Zn-Fe and Al-Si-Ti systemwere optimized through CALPHAD method and Pandat software. Then the reliable Alriched corner of Al-Zn-Si-Ti-Fe system was assessed. The calculated phase diagramsillustrated the effect of Ti addition on the55wt.%Al-Zn-Si coating: on the one hand,The TiAl₃forming in the bath makes the Si content decrease in the bath, leading to thesolubility of Fe in bath decrease and the amount of precipitated Fe4Al3increase. Onthe other hand, both the Fe4Al3and TiAl3can be the nucleation sites of （Al）, leadingto the nucleation density increase and refining the grains.3) The isothermal oxidation of55wt.%Al-Zn-Si and55wt.%Al-Zn-Si-Ti coatedsteels at550,650,700and750oC were investigated by XRD and SEM. The evolutionof the coating microstructure during isothermal oxidation process was observed. Theeffect of temperature, partial gas pressure and particle size on the reaction rate ofcoated steel, zinc powders and nickel powders were systematically studied by TG.4) The kinetic model was deduced to describe the isothermal and non-isothermalgas-solid reaction of metals in the form of sphere and flat plate with consideringoxidation induced volume change when diffusion is the rate controlling step. Theeffect of oxidation induced volume change, temperature, partial gas pressure, heatingrate and particle size on the oxidation reaction fraction were quantitively analyzedthrough an explicit function. Using the model to analyze the kinetics of isothermaloxidation of coated steel at550⁷50°C, non-isothermal oxidation at350⁹50°C,isothermal oxidation of zinc and nickel powders, the results showed that a good agreement has been obtained.In this thesis, the study of material design and oxidation kinetics on the hot-dipcoating system is significant and useful for practical application. The Al-Zn-Si-Ti-Fesystem database not only can be used to design the hot-dip coatings, but also can beused in designing other aluminum alloys. The proposed gas-solid model makes thecalculated results more accurate because of considering the oxidation induced volumechange and avoiding multi-fitting. This model can be used to other gas-solid reactionsin metallurgy and material fields to obtain an accurate prediction.