| The hot-dip Zinc-Aluminum coating is a type of environmentally friendly coatings which has numerous commercial applications in industry, because Zn-Al alloy has the advantage of the sacrifice protection of galvanize coating and the isolation protection of hot dip aluminize coating. In recent years, a large amount of discoveries have been devoted to explore Al-Zn coatings by alloying a number of elements to acquire more excellent corrosion resistant performance. Magnesium is one of the most mainly added elements derived from the excellent performance of Al-Zn-Mg coatings in corrosion resistance. The published reports of Al-Zn-Mg coatings mainly concentrate on low Al content, and few researches about Al-Zn-Mg coatings with high Al content are reported. Recently, as our research group analyzed the microstructure and corrosion behavior of the 45Al-Zn-Mg coatings, he occasionally found a typical periodic layered structure in some samples and the result of salt spraying test showed that the corrosion resistance of these samples is excellent. This interesting discovery led us to make efforts to investigate the Al-Zn coatings with Mg addition. Up to now, most of studies on periodic layered structure are found in solid-solid reaction system. However, the information about the periodic layered structure formed in solid-liquid reaction systems is still few.This study investigated the effect of Al and Mg content and immersion time on the formation of periodic layered structure during hot-dip galvanizing in Al-Zn-Mg bath. The steel substrates were immersed for 10 to 120 s in Al-Zn bath with five level of Al contents in the range of 15 to 55 wt.% and four level of Mg additions in the range of 0.5 to 3.0 wt.%.The microstructure and phase constituent of periodic layered structure were characterized by optical microscopy, scanning electron microscopy, energy dispersive analysis and x-ray diffraction. Corrosion resistance of different coatings was evaluated by means of electrochemical technology and neutral salt-spray test. The main contents are as follows:As the microstructure and phase constituent of the all samples with the periodic layered structure are similar, the sample with typical periodic layered structure is analyzed as a representative. It is found that the periodic layered structure contains alternate light gray layer(FeAl3Znx) and black gray layer(Al dendrites and Zn-rich interdendritic regions), a thin light gray layer(Fe2Al5Znx) close to substrate besides.The periodic layered structure is not formed for a short immersion time,Only when the immersion time reaches incubation period, the periodic layered structure is formed,while the immersion time is too long, the periodic layered structure is destroyed. When the Al content is in the range of 15 to 25 wt.%, the coating consists of intermetallic layers(Fe2Al5Znx and FeAl3Znx) and an overlay layer(Al dendrites and Zn-rich interdendtitic region),when the Al content is 35 wt.%,the periodic layered structure is formed,and the incubation period of the formation of periodic layered structure is reduced as the Al content increases. While the Al over a certain content, the immersion time for the periodic layered structure destroyed is reduced with the increasing of Al content. As the Al content is in the range of 35 to 55 wt.%, the incubation period of periodic layered structure is reduced as adding Mg to the bath, while Mg addition exceeds a certain scale, the periodic layered structure begins to be destroyed.The thickness of FeAl3 Znx layer and total periodic layer pairs increase with the increasing of immersion time. The growth of FeAl3 Znx layer is controlled by interface reaction. In addition, The Al and Mg content in bath have a great effect on the thickness of FeAl3 Znx layer, layer pair and total periodic layer pairs. According to the result of Electrochemical corrosion test, it is found that the corrosion resistance of coatings with periodic layered structure is better than coatings without this structure. |
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