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Zn-Bi-Fe-Ni四元系相关系的测定及Bi对镀层组织和耐蚀性的影响
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摘要
锌及锌合金主要用于热浸镀锌,而热浸镀锌作为一种有效的钢铁表面防护工艺被广泛应用于钢铁构件的防护上。锌浴中的合金元素的增加会导致镀层组织及性能的改变。在锌浴中添加Ni元素能有效地控制圣德林钢的Fe-Zn反应。另外,除了Ni元素能提高锌浴的流动性外,还可以添加少量的Pb,但是由于Pb对环境有极大地危害,目前已经有许多国家禁止使用含Pb的合金。所以必须尽快找到一种能替代Pb的元素。研究发现,Bi是替代Pb加入锌浴中用于提高锌浴流动性的最佳替代品,而且它还可以降低锌液的表面张力,延长锅体的使用寿命。为了深入研究Ni和Bi在镀锌中的协同作用,已经有人研究了Zn-Bi-Ni三元系在450℃时的相关系,而本实验的研究工作是在此三元系的基础上测定含93 at. %Zn的Zn–Bi–Fe–Ni四元系富锌角450℃等温截面,为继续研究Ni和Bi对Fe-Zn反应的协同作用提供理论依据。
     本实验采用平衡合金法,利用扫描电子显微镜与能谱仪(SEM-EDS)和X射线衍射仪(XRD)等手段测定了含93 at. %Zn的Zn–Bi–Fe–Ni四元体系富锌角450℃等温截面。实验结果表明:该四元体系中有2个四相平衡区[L (Zn) + L (Bi) + T +ζ和L (Zn) + L (Bi) + T +δ–Ni],5个三相平衡区[(L (Zn) + L (Bi) +ζ, L (Zn) + L (Bi) + T, L (Zn) + L (Bi) +δ–Ni, L (Zn) + T +ζ, L (Zn) + T +δ–Ni)]和4个二相平衡区[L (Zn) +ζ, L (Zn) + T, L (Zn) +δ–Ni, L (Zn) + L (Bi)];L (Zn)和L (Bi)分别可以与T,ζ,δ–Ni,(T +ζ),(T +δ–Ni)和(ζ+δ–Ni)共存;实验中没有发现新三元相的存在。
     本工作还研究了合金元素Bi对工业纯铁热浸镀锌镀层组织及耐腐蚀性能的影响。通过在锌浴中添加适量的Bi,采用溶剂烘干法,将试样在450℃下浸镀,然后采用浸泡腐蚀法和电化学腐蚀法对浸镀后的试样进行耐腐蚀研究。通过SEM-EDS观察分析各个样品的镀层组织,通过对比分析得出不同Bi含量和不同浸镀时间对镀层组织及耐腐蚀性能的影响,研究结果表明:镀层由δ相、ζ相和η相(纯锌层)组成,在δ相和基体之间没有发现Г相的存在;随着浸镀时间的延长,δ相层,ζ相层和合金层厚度(δ相+ζ相)都有所增加;在锌浴中添加一定量的Bi元素可以抑制合金层厚度的增长;镀层的耐腐蚀性能随着Bi含量的增加而降低;本实验中镀层的腐蚀形式属于一般腐蚀。
Zn and Zn alloys are mainly used as coatings to improve the corrosion resistance of steel, and hot-dip galvanizing is one of the most effective methods for improving the corrosion resistance of steel. The addition of alloy elements into Zn-bath can result in the change of the coating microstructure and performance. The addition of Ni into Zn-bath can control the Fe-Zn reactivity in the Sandelin steel effectively. Besides the addition of Ni can improve the fluidity of liquid Zn, and the addition of Pb into Zn-bath can make the molten Zn more fluid, too. Due to the harmful of Pb to the environment, many countries have prohibited the usages of Pb in Zn-bath now. So, it is very important to find an effective element to substitute Pb as soon as possible. It is found that Bi is the best element which can substitute Pb to improve the fluidity of liquid Zn. Bi can reduce surface tension of Zn-bath, and prolong the service life of the Zn boiler. For a deeper understanding of the combined effect of Ni and Bi in the hot-dip galvanizing, the 450℃isothermal section of Zn-Bi-Ni ternary system has been investigated recently. And the present work based on this ternary system is to determine the Zn–rich corner of the 450℃isothermal section of the Zn–Bi–Fe–Ni quaternary system with the Zn being fixed at 93 at. %, which can be served as the theoretical basis to study the combined effect of Ni and Bi on the Fe-Zn reaction.
     The phase relations of Zn-Bi-Fe-Ni quaternary system at 450℃were determined by the equilibrated alloys approach, and the specimens were investigated by means of scanning electron microscopy (SEM) equipped with energy dispersive X–ray spectroscopy (EDS) and X–ray diffraction (XRD). It was found there exist 2 four–phase regions [L (Zn) + L (Bi) + T +ζ和L (Zn) + L (Bi) + T +δ–Ni], 5 three–phase regions [(L (Zn) + L (Bi) +ζ, L (Zn) + L (Bi) + T, L (Zn) +L (Bi) +δ–Ni, L (Zn) + T +ζ, L (Zn) + T +δ–Ni)] and 2 four–phase regions[L (Zn) +ζ, L (Zn) + T, L (Zn) +δ–Ni, L (Zn) + L (Bi)]. Two liquid L (Zn) and L (Bi) can coexist with T,ζ,δ–Ni, (T +ζ), (T +δ–Ni) and (ζ+δ–Ni) in this isothermal section, respectively, no new ternary phase was found in this study.
     The effect of alloying element Bi on the coating microstructure and the corrosion resistance of the galvanized industrial pure iron was investigated in the present work, too. Bi was added into Zn-bath and then the samples were galvanized at 450℃by drying solution approach. After that the samples were dipped into 5% NaCl solution for 10 days and electrochemical corrosion to study the corrosion resistance. The coatings were investigated by means of SEM-EDS. The conclusions were concluded through summarizing the experimental results as follows. The coatings includeδphase,ζphase andηphase, and there exists noГphase betweenδphase andζphase. The thickness ofδphase,ζphase and the intermetallic layer (δphase +ζphase) increases with prolonging of the immersion time, respectively. The addition of Bi can control the thickness of galvanized coating. The corrosion resistance of the coating degrades with the increase of Bi content. The mechanism corrosion of the galvanized coating is general corrosion in the present work.
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