| Hot dip galvanizing is widely used to improve the corrosion resistance of the steel sheet, add certain elements in the molten zinc bath have a great influence on the coating. Especially in the field of continuous hot galvanizing, the Al element is the most important and most widely used one of the add alloying elements, the content of Al plays an important role in the quality control of the coating, but the Al element joined in hot dip galvanizing the influence mechanism of the interfacial layer growth is not clear, which not only affects the corrosion resistance of the coating, but also forming of the plating parts post-processing and mechanical properties a greater impact. The increase of Al content in the zinc bath can seriously affect the service life of galvanized equipment, therefore low aluminum hot-dip galvanizing has a good application prospect. Because of the industrial demand, low Al Galvanized(GI) has also been used in batch galvanization field, but due to a long time batch hot dip galvanized, Al of Galvanized(GI) inhibited the growth of thick layer will be a degree of failure, but research of the interface coating growth mechanism of low aluminum hot dip galvanized still exist many disputes and confused, for example, the cause of Fe-Al inhibition layer first generation and destabilization decomposition, Fe-Zn IMC reaction position and the way, the entire lower Al hot dip galvanizing system phase transformation dynamic nature and so on, all of which need to be further studied. The related researches are carried out about the above problems in the present work, select common Q235 steel substrate, Zn-0.2%Al bath, research on the evolution law of the interface coating reaction and IMC each phase transition mechanism at 450, 470 and 490 °C. SEM was used to observe the structure characteristics of the hot-dip galvanized coating surface of the cross section and stripping free zinc layer. EDS was used to quantitatively analyze the micro area components of phases and also used its line scan and mapping scan to qualitatively analyze the element change of the coating cross section. By means of the Miedema model and the Toop model, the thermodynamic values of the binary Fe-Al, Fe-Zn and ternary Fe2Al5Znx(η) intermetallic compounds(IMC) in the coatings were calculated. The fundamental reason for the Fe-Zn reaction caused by Fe2Al5 destabilization with galvanized time increasing was analyzed, and to explore the differences in the form of instability inhibition layer at different temperatures, put forward the more complete theory of Fe2Al5 inhibition layer destabilization. This study mainly achieves results as follows:In the steel substrate immersed in Zn-0.2%Al bath, the Fe2Al5 inhibition layer formed at the steel substrate interface preferentially. Due to the steel substrate / zinc bath interface Fe-Al IMC reaction consumes high concentrations of Al, the adjacent to the Fe2Al5 layer of zinc bath Al local poor, even lower than the concentrations of Al in the main body of the zinc bath. Because Fe2Al5 hinder Fe to dissolve the zinc bath, this will lead to Al through long-range diffusion can react with steel substrate Fe to generation Fe2Al5, makes the reaction controlled by diffusion, namely dynamic control. With the consumption of Al at the interface and Fe2Al5 generated by dynamic control, reaction rate will decline, or even stagnant. Thus Fe-Zn reaction on the thermodynamics disadvantage will weaken, Fe-Al reaction will compete with Fe-Zn reaction, because this time the interface extremely poor Al but Zn content is very high, makes Fe-Zn reaction to take advantage of the dynamics, and appear obvious Fe-Zn reaction.The inhibition layer is destroyed with the increase of the immersed time, and the loss of the function of inhibiting the Fe-Zn reaction. Fe2Al5 inhibition layer instability for two reasons, respectively corresponding to two different positions of the δ phase formation: one is that the local depletion of Al at Fe2Al5 and zinc bath interface result in erosion of Fe2Al5 by Zn and the formed Fe2Al5 Znx caused the decrease of the systematic thermodynamic stability which led to erosion and decomposition of Fe2Al5 by Zn. At the same time, FeZn10(δ) phase was produced between the Fe2Al5 and zinc bath interface. The phase transformation process can be described as: Fe2Al5â†'ηâ†'L+ηâ†'L+η+δâ†'L+δ. The other kind of destabilization mechanism is Zn diffused to the steel substrate interface by Fe2Al5 grain boundaries, can also cause stress corrosion cracking to Fe2Al5 by Zn, and then directly produced δ phase between Fe2Al5 and steel substrate interface, which caused outburst.Under different temperature, for the position of the δ formation will vary. At 450 °C, no direct evidence that zinc diffusion through inhibition layer effect and the local depletion of Al effect which makes the delta phase generated first. At 470 °C, the two effects both compete with each other, the beginning is the local depletion of Al effect occupying a dominant, and then produce zinc diffusion through inhibition layer effect. Similar situation under 490 °C and 470 °C, just IMC generated to relatively early on at 490 °C.Thermodynamic calculations show that, the Fe-Al IMC than Fe-Zn IMC has a lower reaction Gibbs free energy, steel substrate immersed in Zn-0.2%Al bath Fe-Al IMC as first generation phase, consistent with the experimental results. Because of the solid solution of Zn, Fe2Al5 Znx worse than Fe2Al5 thermodynamic stability, the results can be directly confirmed the local depletion of Al theory, while outburst the Fe2Al5 crystal will also eroded by Zn, and the outburst of fragmentation Fe2Al5 are more likely to be decomposition of Zn erosion. indirectly anastomosis zinc diffusion through inhibition layer theory. Visible, two Fe2Al5 destabilization mechanism of mutual coexistence, mutual competition, and will be subject to temperature, time, and Al content and the influence of the nature of the steel substrate surface itself, under certain conditions may be one will occupy absolute advantage.Single thermodynamic factor is not sufficient to explain at different temperature, the time differences of the formation of Fe2Al5 inhibition layer, namely the higher the temperature, Fe2Al5 harder to form. But in fact Fe2Al5 continuous inhibition layer has been generated in 10 s and above Fe-Al inhibition layer has started Fe-Zn reaction at 490 °C. In contrast, in 30 s was noticeable Fe-Zn reaction at 450 °C and to appear Fe-Al inhibition layer at 470 °C. This shows that although the rise of temperature in thermodynamic hindered the production of Fe2Al5 inhibition layer, but it is not entirely decided to single factor. Because of the temperature rise is relatively large enough, the effect of the dynamics will be more obvious, greater degree of temperature increase makes atom diffusion rate sped up and the steel substrate Fe solubility increased obviously. Temperature increases influence on the thermodynamics and kinetics of competition which not resulted in the emergence of regular positive or negative correlation time Fe2Al5 inhibition layer appears and temperature. |
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