Interface Reaction Study For Hot-dipping Zn-Al Coating And Thermodynamic Study Of Zn-Al Bath

Hot dip galvanizing is widely used to improve the atmospheric corrosion resistanceof the steel sheet. The corrosion resistance of the hot-dipping Zn-Al coating （for exampleZn-55%Al, Zn-5%Al coating et al.） is superior to a single coating （Zn, Al coating）.Because of the presence of aluminum, resulting in the strong and rapid exothermicreaction between the steel matrix and the A1-Zn bath, rapid growth of alloy layer, andlarge amounts of Zn-Al dross forming in the bath. To control the violent reaction occursbetween the iron substrate and bath, the alloying elements, such as Si and Cu, will beadded in the A1-Zn bath. Focusing on the interaction between Fe, Zn, Al and Si, therelated phase diagrams were determined and the effect of alloying elements on coatingswere carried out. The interfacial reaction and forming of intermetallic dross phases inhot-dipping Zn-Al bath have been studied, which is important to control themicrostructure of Zn-Al coating.The450°C、620°C and800°C isothermal section of the Zn-Fe-Cu ternary systemwas experimentally determined. No new ternary compound was found in the system.Eight three-phase regions exist in the450°C isothermal section. isothermal section.Experimental results indicated that the solubility of Cu in the Γ （Fe3Zn10） and the δ_Fephase reaches as high as17.9at.%and15.2at.%, respectively, and the solubility of Cu inliquid and ζ phase is lower. The results confirm, in galvanizing, the addition of Cu in bathup to1.0at.%would favor the formation of the Γ phase and δ_Fephases, the other wayround, it inhibits the development of the ζ phase. Four three-phase regions have beenidentified in the Zn-Fe-Cu ternary system at620°C, and three three-phase regions havebeen confirmed in the800°C. The γ （Cu5Zn8） and Γ phases form a continuous solidsolution at620°C, which is designated as the γ/Γ phase. Binary phases δ_Feand δ_Cuextendinto the ternary system at620°C, the three-phase triangle of (γ/Γ+δ_Fe+δ_Cu) is small andnarrow.Eight three-phase regions have been identified in the Al-Ni-Zn ternary system at600°C. No new ternary compound was found in the system. The solubility of Ni in theLiq. phase is low, which is no more than0.8at.%. Binary phases AlNi and NiZn extendinto the ternary system, and they co-exist in the Al-Ni-Zn ternary system at600°C. Thethree-phase triangles of (NiZn+AlNi+Ni₃Zn₁₄) and （AlNi₃+AlNi+NiZn） are constructedin this ternary system. As is shown in this research result, the three-phase equilibriumrelationship of the phases AlNi, Al3Ni₅and AlNi₃has been confirmed in the present study.According to this result, when a proper amount of Ni is added into the Al-Zn bath, thebath composition does not remain at Al-Zn liquid region, but enter two-phase orthree-phase region. Consequently, a great many intermetallic compounds in Al-Zn bath reduce the fluidity of melt.The interface reaction process between the iron-base and different baths was studied.The controlled mechanism of alloy layer formation by the diffusion path change ingalvanizing was proposed. In the beginning of the alloy layer formation in galvanizing（diffusion layer）, the diffusion path crosses the two-phase region of melting bath andrelevant Fe-Al compound following a tie-line. A corresponding continuous lamellarFe-Al compound forms firstly on the iron-based surface. with the immersion（diffusion）time increases, the diffusion path trends to move gradually to the line connecting withtwo diffusion component points. Once the diffusion path cuts the tie-line of the two-phaseregion of melting bath and the relevant Fe-Al compound, the preferential forming Fe-Alcompound layer is steadiness losing and breaking. At the same time, the liquid channelsforms in the alloy layer （diffusion layer） as the diffusion path cut the tie-line, whichleaded Zn-Al liquid to connect with iron-base directly, the reaction is controlled byinterfacial reaction, which brings the thickness of coating alloy layer （diffusion layer）increased rapidly.Cu is an effective bath additive for controlling the Fe-Al reactivity in Galvalumeprocess. The addition of Cu in the Galvalume baths promotes the formation of the τ₅phase and hinders the Fe₂Al₅phase. When0.5¹.0wt.%Cu is added in Galvalume baths,the presence of Cu makes Si be enriched in the reaction region during the hot-dipping.The liquid phase becomes in equilibrium with the τ₅phase, initially in equilibrium withFeAl3as the enrichment of Si in the coating. the diffusion path is ironsubstrate/Fe₂Al₅/FeAl3/τ₅/overlay. The compact τ₅will form on the outer intermetalliclayer, this phenomenon of the liquid phase eroding the α-Fe phase directly will beavoided. As a result, the intermetallic layer thickness greatly reduces, and the Fe₂Al₅phase layer becomes very thin.The intermetallic dross phases in galvalume baths containing1.0～3.0wt.%Si inthe temperature range of580～610°C have been studied detailedly. The intermetallicdross phases are considered to be the FeAl3phase and the τ₅phase containing Zn. TheAl-Zn-rich corners of (Al_0.55Zn_0.45)_1-x-yFe_xSi_ysection in the Al-Fe-Si-Zn system at580～610°C have been constructed. It is intuitionistic for prediction of the change in the natureof the intermetallic dross phases present in galvalume baths for variations of Si contentand temperature. For Si content dose not exceed1.3wt.%, the dross is the FeAl3phasesolely, and the most Si content in the FeAl3phase is1.93wt.%for1.0～1.6wt.%Si inbaths. When Si content up to2.0wt.%, the intermetallic dross phase is τ₅phase solely. Inour study, FeAl3and τ₅can co-exist when Si content is about1.6wt.%in baths at thetemperature range of580～610°C.