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Study on Hot Dip Galvanizing with High Corrosion Resistance of Al-RE-Zn Alloy Coatimg

Author: LiuYao
Tutor: ZhouRong
School: Kunming University of Science and Technology
Course: Materials Processing Engineering
Keywords: hot dip galvanizing Al-RE-Zn alloy corrosion resistance organization structure flux passivation
CLC: TQ153
Type: Master's thesis
Year: 2012
Downloads: 81
Quote: 0
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Abstract


Hot-dip galvanized alloying process is one of the most important means to improve the formability of galvanized parts, quality of appearance and corrosion resistance. Considering the rich rare earth resources, the paper based on the research of hot-dip plated alloy, studied the microstructure and corrosion resistance of Al-RE-Zn alloy coating, developed a flux suited for the alloy coating and examined the effect of conventional chromate passivation solution on the alloy coating.The paper analyzed the impact of alloy element changes on corrosion resistance and organizational structure by the means of optical microscope, SEM, EDS, electrochemical test, neutral salt spray test and immersion test. Increase in Al content lead to skip plating and decrease in the surface quality, orthogonal experiment was used to develop flux applicable to the Al-RE-Zn alloy coating. Neutral salt spray test and immersion test were used to research the corrosion resistance of chromate passivation compared pure zinc with Al-RE-Zn alloy.The research indicates that the corrosion resistance of coating incresed with the increase in RE content. The corrosion resistance was best at0.2%RE content. When coating RE content continued to increase, the corrosion resistance decreased instead. The main reason is that the proper amount of RE in zinc bath can combine with harmful impurities and purify liquid zinc, and RE dispersion at grain boundary can refine grains, then the corrosion resistance of coating is improved. But excessive RE segregation and maldistribution result in poor organization performance and reduced corrosion resistance.The addtion of Al element improve coating homogeneity, thickness and apparent quality et al. The comprehensive performance is best at1%Al content. The main reason is that Al element can enable the zinc bath surface to form a layer of continuous Al2O3film, the zinc bath surface is not easy to be oxidized to obtain a light coating. The outer η phase get thinner by adding Al element. At the same time, Al element can inhibite Fe-Zn layer growth, the brittle ξ phase will not grow excessively. But excessive Al element will cause leakage.Results show that1% Al-0.2%RE-Zn alloy coatings exhibite the best comprehensive performance on appearance quality, corrosion resistance and organization structure.Orthogonal experimental results show that the ingredients of flux applied to1% Al-0.2%E-Zn alloy plating coating are ZnCl2-125g/L, NH4Cl-145g/L, and after adjusted pH to4,15g/L Na3AlF6,15g/L SnCl2,4g/L NaF are added; The plating process is proceeded at70℃for40s. By the help of flux, plating coating surface get lighter and smoother, the amounts of zinc dust in galvanizing bath and attached zinc dust to the surface are decreased, whole surface quality is improved.Passivation experimental results show that1%Al-0.2%RE-Zn alloy can be passivated by traditional chromate passivation solution, and the corrosion resistant performance is better than pure zinc plating chromic acid salt passivation.

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