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The Effects of Mn, Ti on the Microstructure and Corrosion Behaviour of Zn-Al Alloy and the Hot-dip Coating

Author: WangHuaiTao
Tutor: LiuZiLi
School: Nanjing University of Aeronautics and Astronautics
Course: Materials Science
Keywords: Zn-5Al alloy Mn Ti Microstructure Electrochemical test Corrosion resistance
CLC: TG174.44
Type: Master's thesis
Year: 2013
Downloads: 12
Quote: 0
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Abstract


Hot dip galvanizing technology is the most widely used for anti-corrosion technology of steel. Hotdip Zinc-aluminum coating is usually used because of its strong bonding strength, weldability,corrosion resistance, simple process, low cost and so on. In this present work, the effects andmechanism of Al, Mn and Ti on microstructure and corrosion resistance of Zn alloy were studied bymany analysis and testing means, such as optical microscope （OM）, X-ray diffractometer （XRD）,scanning electron microscope （SEM） with energy dispersive spectrometer （EDS）, polarization andelectrochemical impedance spectroscopy measurement.The results show that, large amounts of the primary β-Zn phase as the sacrificial anode is corrodedpreferentially in the Zn-4Al alloy; the eutectic structure around the primary α-Al phase is corrodedfirstly in Zn-6Al alloy. When the content of aluminum is5%, the proportion of eutectic composition isthe most in the zinc alloys, and the corrosion rate reachs their minimum value, which is3.88μg·cm-2·d-1, and the corrosion resistance of the Zn-5Al is the best.As the increase of Mn contents in Zn-5Al alloys, the number and volume fraction of the β-Zn phaseare both getting smaller and smaller. When the content of Mn increases to0.3%, a new three-elementcompound （Al, Zn and Mn atomic ratio of8:2:3） appeares in Zn-5Al alloy, but the shape and numberof β-Zn phase do not change significantly from that in Zn-5Al-0.2Mn alloy. When the content of Mnis0.2%, the corrosion rate and corrosion current density reach their minimum value simultaneously,which are1.17μg·cm-2·d-1and1.4μA·cm-2, and the value of high frequency and low frequencyImpedance reach their maximum. With the increasing of Mn content, the number and volume fractionof β-Zn phase which acts as anode decrease, and the three-element compound does not corrodedpreferentially.A small addition of Ti to Zn-5Al alloy refines grains and inhibits the precipitation of primaryβ-Zn phase. When the content of Ti is0.15%, the Zn-5Al alloy is composed by full eutectic structure.But a small amount of a new three-element compound （Al, Zn and Ti atomic ratio of15:4:6） appearsin Zn-5Al alloy with the increase of Ti content up to0.2%. Both the corrosion current density(1.1μA·cm-2) and the corrosion rate (0.85μg·cm-2·d-1) of the Zn-5Al-0.15Ti alloy reach their minimum,and the value of high frequency and low frequency impedance reach their maximum. And both theimpedance of Zn-5Al-xTi alloy increases with the immersion time. The Zn-5Al alloy composed byfull eutectic structure is corrosed uniformly, and the corrosion resistance of Zn-5Al-0.15Ti alloy is the best in Zn-5Al-xTi alloy.Zn-5Al, Zn-5Al-0.2Mn and Zn-5Al-0.15Ti coating are prepared on Q235based using by hot-diptechnology. Some primary zinc phase appears in The Zn-5Al coating, but disappears with the additionof0.2%Mn or0.15%Ti. The thickness of Fe2Al5layer of Zn-5Al-0.2Mn and Zn-5Al-0.15Ti coating isboth bigger than that of Zn-5Al coating. The quality of Zn-5Al-0.2Mn coating and Zn-5Al-0.15Ticoating is both superior to Zn-5Al coating in terms of quality of the coating surface, bonding strengthand corrosion resistance.

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CLC: > Industrial Technology > Metallurgy and Metal Craft > Metallurgy and Heat Treatment > Metal corrosion protection,metal surface treatment > Corrosion control and protection > Metal surface protection technology > Metal complex layer of protection
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