| The corrosion resistance,formability and coating property of the hot-dip Galfan(Zn-5%Al-0.1%RE)coating is more excellent than that of hot-dip galvanizing coating,thus the applications of the hot-dip Galfan are becoming more and more extensive.The effects of RE addition,RE content and cooling rate on the properties of the coating are all worthy of study,which due to the development of Galfan coating technology in the batch hot-dip galvanizing in recent years.The corrosion resistance of Galfan coating mainly depends on the solidification structure of Zn-Al alloy in the coating.Therefore,in order to simplify the test process,the Galfan alloy used for hot-dip galvanizing is studied,which lays a foundation for the subsequent coating research.In this thesis,the eutectic point of Al-rich Al-RE alloys were studied and the effects of Nd content and cooling rate on the microstructure of Al-Nd master alloys were analyzed.And then,after the optimized Al-Nd master alloy being added to the Zn-5%Al alloy,the effects of Nd content and cooling rate on microstructure and corrosion resistance of Zn-5%Al alloy were studied systematically.Finally,the mechanism of Nd in Zn-Al alloy and the corrosion mechanism of Zn-5%Al-?Nd alloy in 3.5% NaCl solution were discussed.The Al-rich Al-RE alloys with the controversial compositions were investigated systematically by scanning electron microscope(SEM),energy dispersive spectroscopy(EDS),X-ray diffraction(XRD),differential scanning calorimetric(DSC)and cooling curve tests.The results show that the similar eutectic composition(~12 wt.% RE)and eutectic temperature(~ 645 °C)on the Al-rich region were observed in the Al-La,Al-Ce,Al-Pr and Al-Nd alloys,and the eutectic structure of the above four alloys is consisted of ?-Al and ?-Al11RE3 phases.In the study of Al-Nd alloys,the microstructure showed a typical transition process of hypoeutectic,eutectic and hypereutectic with the increase of Nd content.The melting point of hypoeutectic Al-Nd alloy,which is between 645 and 660 °C,is close to the eutectic's.While the composition of Al-Nd alloys belong to the hypereutectic,the melting temperature increases rapidly with the increase of Nd content.In addition,the primary phase and eutectic structure of Al-Nd alloy can be refined with the increase of cooling rate.The microstructure and DSC analysis of Al-Nd alloy showed that the microstructure of Al-Nd alloys under the water cooling condition with the Nd content below 12 wt.% was finer.And these alloys with lower melting point is more suitable for use as a master alloy in Zn-Al alloy.The study on the microstructure of Zn-5%Al-?Nd alloy revealed that the growth of the primary ?-Zn phase can be effectively inhibited and more and compact eutectic structure of the alloy can be obtained due to the addition of Nd.In addition,the lamellar spacing of eutectic structure of the alloy with the optimal concentration of Nd can be decreased.It also found that Nd could react with Zn and form the intermetallic phase Nd2Zn17 particles easily in the alloy bath,which will reduce the effective content of element Nd in the alloy bath.Moreover,the Nd and Al elements are easy to enrich the surface of the alloy and promote the formation of Nd2O3 and Al2O3.Besides,the addition of Nd can improve the integrity and coverage of the Al2O3 film on the alloy surface,which is beneficial to inhibit the oxidation of Zn.The microstructure and the eutectic lamellar spacing of Zn-5%Al-?Nd alloys were refined successively with the increase of the cooling rate.According to the mismatch theory,the Nd does not process the function of heterogeneous nucleation in Zn-Al alloys.The refining effect is mainly achieved by the high melting point of the Nd-containing phase,which can hinder the grain growth during the solidification,and increase the undercooling of the alloy to promote nucleation.The corrosion resistance of Zn-5%Al-?Nd was investigated by electrochemical impedance spectroscopy(EIS),electrochemical polarization curve and neutral salt spray(NSS)tests,etc.The results showed that the corrosion resistance of the Zn-5%Al-?Nd alloys would be changed with Nd addition,and the best corrosion resistance of the alloy was attained when Nd addition was 0.06%.The corrosion resistance of the alloy increases initially and decreases afterwards with the increase of the cooling rate,and the alloy under the air cooling addition obtained the best corrosion resistance.Through the study on corrosion process of Zn-5%Al-?Nd alloys,it found that the eutectic structure of Zn-Al alloys showed good corrosion resistance at the early stage of corrosion with the cooperation of corrosion products and Al2O3 film.Then the Al2O3 film is continually destructed by Cl-and OH-in solution,which cause the preferential corrosion of ?-Al phase appeared.The corrosion resistance decreased with increasing immersion time due to the destruction of protective Al2O3 film and the defects(e.g.cracks,inhomogeneity)in the corrosion product layer,and the presence of selective corrosion accelerates the corrosion of the alloys.Moreover,it should be noted that minor amount of Nd addition have positively impacted on corrosion resistance,which can be ascribed to the modification of microstructure and corrosion products,improvement of the Al2O3 film coverage and inhibition of the selective corrosion of ?-Zn and ?-Al phases. |
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