热镀Zn-0.2%Al镀层中Fe-Al抑制层失稳机理及其热力学评估

DESTABILIZATION MECHANISM OF Fe-Al INHIBITION LAYER IN Zn-0.2%Al HOT-DIP GALVANIZING COATING AND RELATED THERMODYNAMIC EVALUATION

在450℃镀锌条件下,锌浴中加入质量分数为0.2%的Al,采用SEM观察镀层的结构特征,利用EDS定量分析相的微区成分,利用其线扫描和面扫描定性分析镀层截面元素变化情况.借助Miedema模型和Toop模型,计算了镀层中各二元Fe-Al,Fe-Zn和三元Fe_2Al_5Znx(h)金属间化合物(IMC)的热力学值,分析了随镀锌时间的延长,出现Fe_2Al_5抑制层失稳破坏而产生Fe-Zn反应的根本原因.结果表明,因为Fe-Al IMC比Fe-Zn IMC具有更稳定的热力学性质,钢基体与锌浴界面优先产生连续的Fe_2Al_5金属间化合物抑制层,抑制Fe-Zn反应,但随镀锌时间的延长,Fe_2Al_5的失稳破坏丧失对Fe-Zn反应的抑制作用,生成Fe Zn10(d)相.Fe_2Al_5抑制层的失稳机制有两种:一种是Fe_2Al_5/锌浴界面处Al的局部贫化导致Zn对Fe_2Al_5的侵蚀,形成Fe_2Al_5Znx,造成系统热力学稳定性降低,从而导致Fe_2Al_5被Zn侵蚀分解,同时在Fe_2Al_5/锌浴界面产生Fe Zn10(d)相;另一种是Zn通过Fe_2Al_5晶界向钢基体扩散,直接在Fe_2Al_5/钢基体界面产生d相,并引起Fe_2Al_5的迸发失稳.

The formation of a uniform Fe-Al inhibition layer with a proper thickness at steel interface during continuous hot-dip galvanizing process is a crucial issue for industrial production. The inhibition layer prohibits the nucleation and growth of brittle Fe-Zn intermetallic compounds which deteriorate the adhesion of the galvanizing coating and result in an inhomogeneous distribution of the coating. The inhibition layer was identified to be Fe_2Al_5 with some Zn dissolved in it. But Fe_2Al_5 inhibition layer was damaged with galvanized time increasing, will lose the inhibition to the Fe- Zn reaction. Nevertheless, there is no systematic and comprehensive investigation the causes of the inhibition layer is damaged. The aim of this work is to clarify the destabilization mechanism of Fe_2Al_5inhibition layer. In the present study, the mass fraction of 0.2%Al was added into the zinc bath at 450 ℃ for hotdip galvanizing. SEM was used to observe the structure characteristics of the hot-dip galvanized coating. EDS was used to quantitatively analyze the micro area components of phases and also used its line scan and mapping scan to qualitatively analyze the element change of the coating cross section. By means of the Miedema model and the Toop model, the thermodynamic values of the binary Fe- Al, Fe- Zn and ternary Fe_2Al_5Znx（h） intermetallic compounds（IMC） in the coatings were calculated. The fundamental reason for the Fe-Zn reaction caused by Fe_2Al_5 destabilization with galvanized time increasing was analyzed. The results show that because Fe-Al IMC which is generate preferentially had more stable thermodynamic property than Fe-Zn IMC, the continuous Fe_2Al_5 intermetallic compound inhibition layer was produced preferentially at steel and zinc bath interface which inhibit the Fe-Zn reaction. However, with the galvanized time increasing, Fe_2Al_5 destabilization which led to the loss of inhibitory effect of Fe-Zn reaction and produced Fe Zn10（d）. There are two kinds of destabilization mechanism of Fe_2Al_5 inhibition layer, one is that the local depletion of Al at Fe_2Al_5 and zinc bath interface result in erosion of Fe_2Al_5 by Zn and the formed Fe_2Al_5 Znxcaused the decrease of the systematic thermodynamic stability which led to erosion and decomposition of Fe_2Al_5 by Zn. At the same time, Fe Zn10（d） phase was produced between the Fe_2Al_5 and zinc bath interface.The phase transformation process can be described as： Fe_2Al_5→h→L＋h→L＋h＋d→L＋d. The other kind of destabilization mechanism is Zn diffused to the steel substrate by Fe_2Al_5 grain boundaries and directly produced d phase between Fe_2Al_5 and steel substrate interface, which caused outburst of Fe_2Al_5. The two kinds of Fe_2Al_5 destabilization mechanism are mutual coexistence and mutual competition, in particular conditions may be a mechanism to occupy absolute advantage.