Zn-(0.3~0.9)Cu-0.3Ti合金组织演变与耐蚀性能研究

Microstructure Evolution and Corrosion Resistance of Zn-(0.3~0.9)Cu-0.3Ti Alloys

设计熔炼了Zn-(0.3~0.9)Cu-0.3Ti(%,质量分数)合金,采用X射线衍射仪(XRD)、扫描电镜(SEM)、电化学腐蚀测试等研究了不同含量Cu添加对Zn-Cu-0.3Ti合金组织与耐蚀性能的影响。在Zn-Cu-0.3Ti合金的凝固组织中发现析出相有CuZn_(5), TiZn_(3)和TiZn_(15)。CuZn_(5)纳米颗粒(15~50 nm)弥散分布在Zn基体内部,微米级TiZn_(3)颗粒(0.8~2.5μm)存在于相界面附近,条状的TiZn_(15)相存在于晶界处。随着Cu含量的增加,Zn基体由不发达枝晶或等轴晶形貌演变为等轴晶形貌,并且析出的CuZn_(5)纳米颗粒明显增多。通过分析Zn-(0.3~0.9)Cu-0.3Ti合金的极化曲线和交流阻抗(EIS)曲线,讨论了合金的腐蚀机制。

Recently,it has been reported that Zn-Cu-Ti alloy for anticorrosion can replace Zn-Al alloy in the future.In this work,Zn-(0.3~0.9)Cu-0.3Ti(%,mass fraction)alloys were designed to investigate the influence of Cu content on the microstructure and corro⁃sion properties of the alloys.Meanwhile,the corrosion behavior of the alloys in 3.5%NaCl solution was studied.Zn-x Cu-0.3Ti alloys with three components were smelt and the Cu content increased from 0.3%to 0.9%.Optical microscope(OM),X-ray diffractometer(XRD),scanning electron microscope(SEM),transmission electron microscope(TEM)and electron probe microanalysis(EPMA)were used to analyze the phase composition and microstructure characteristics of the alloys.The dynamic potential polarization and al⁃ternating current impedance(AC impedance)of Zn-x Cu-0.3Ti alloys were tested in three-electrode system.As Cu content increased from 0.3%to 0.6%,the three strong peaks of Zn matrix in Zn-x Cu-0.3Ti alloys tended to shift to low angle direction,which indicated that the crystal plane spacing of Zn matrix became large.As it was known that,there was almost no solid solubility of Ti in Zn matrix,so the existence form of Ti element should be intermetallic compound.However,Cu had a large solid solubility in Zn matrix.As Cu en⁃tered into Zn lattice in the form of replacing atom,Zn lattice would expand,and the plane spacing of Zn would also become large.OM results showed that Zn matrix mainly presented undeveloped dendritic morphology and the dendrite characteristic was not obvious.A part of Zn matrix exhibited equiaxed crystal characteristic and a large number of black precipitates could be observed in the interden⁃dritic and grain boundary region,and the size of precipitates was 0.8~2.5μm.Energy dispersive spectroscopy(EDS)analysis showed that the particles were mainly composed of Zn and Ti elements,and the atomic ratio of Zn and Ti was close to 3∶1,hence,the particle phase should be TiZn_(3) phase.In Zn matrix,a small number of black particles precipitated,and the size distribution was 15~50 nm.TEM analysis showed that the particle phase was CuZn_(5) phase with close-packed hexagonal structure.The phase interface of Zn matrix was not a clear line,but a transition zone with certain thickness.The structure characteristic in grain boundary zone was obviously dif⁃ferent from that in Zn matrix.The strip-like phases were observable in this zone.Based on the results reported in similar alloys,it could be inferred that the strip phase at the grain boundary was TiZn_(15) phase.TEM results showed that a small amount of black particle phase precipitated in Zn matrix.The measured particle size was 15~50 nm.After calibration,the black particle phase should be CuZn_(5) phase with close-packed hexagonal structure.Based on the above analysis,TiZn_(3) with equiaxed crystal feature precipitated first form Zn-0.3Cu-0.3Ti alloy melt below 650℃.As the temperature decreased to a certain extent,Zn-Cu homogeneous transformation oc⁃curred,leading to the formation of Zn phase containing Cu atoms.During the growth process of Zn phase,Ti element that did not par⁃ticipate in the formation of TiZn_(3) phase enriches at the front of Zn crystal interface,promoting the dendritic growth of Zn crystal.Final⁃ly,the remaining melt between Zn dendrites underwent peritectic reaction and TiZn_(15) phase precipitated.TiZn_(15) phase was restricted by the growth space and grew into strip shape.As the temperature continued to decrease,Cu atoms dissolved in Zn lattice precipitates in the form of CuZn_(5) phase.With the increase of Cu content,the corrosion potential(Ecorr)of the alloys decreased gradually,and the self-corrosion current density(I_(corr))also decreased.As Cu content was 0.6%,I_(corr) was the smallest,which was 2.76×10^(-3) A·cm^(-2).After 20 d corrosion for 20 d,although the Cu content was different,I_(corr) tended to be the same,which was about 6.31×10^(-4) A·cm^(-2).In the AC impedance curves of Zn-x Cu-0.3Ti alloy,there was only one capacitive impedance in the low frequency and high frequency re⁃gions.The impedance arc radius of Zn-0.6Cu-0.3Ti alloy was larger than that of the other two alloys,which indicated that the corrosion products produced by the corrosion reaction in 3.5%NaCl neutral solution were relatively dense.This was a better protective oxide film and played an important role in the corrosion prevention process.Therefore,the resistance of the corrosion products was large,which was not conducive to charge transfer in solution.The impedance of the corrosion product layer increased first,and then decreased,fi⁃nally,the impedance increased.This indicated that corrosion product showed growth trend first and then detachment,and then growth again.In the solidified microstructure of Zn-x Cu-0.3Ti alloys,there were CuZn_(5),TiZn_(3),TiZn_(15) and Zn matrix phases.With the in⁃crease of Cu content,the morphology of Zn matrix changed from undeveloped dendrite or equiaxed crystal.I_(corr) of Zn-x Cu-0.3Ti alloys decreased with the increase of Cu content.With the extension of corrosion time,I_(corr) of Zn-x Cu-0.3Ti alloy overall showed a downward trend.After corrosion for 20 d,although the content of Cu was different,I_(corr) of the alloy tended to be the same.The AC impedance curve of Zn-x Cu-0.3Ti alloy showed that only one capacitive arc existed in both low frequency and high frequency regions.With the in⁃crease of Cu content,the impedance of corrosion product layer increased first and then decreased.As Cu was 0.6%,the corresponding resistance was 75.6Ω·cm2.With the extension of the corrosion time,the impedance value first increased and then decreased,which indicated that corrosion product on the surface of the alloy underwent the process of growth-exfoliation-growth.