【作者】 贾瑞灵；

【导师】 王福会；

【作者基本信息】 哈尔滨工程大学 ， 材料学， 2008， 博士

【摘要】 纯镁因为力学性能差而不能被用作结构材料,用于工程结构材料的是添加了合金元素的镁合金。合金元素的加入,在提高镁合金力学性能的同时,往往会形成第二相。镁一铝合金中的第二相β相(Mg₁₂Al₁₂)对其腐蚀行为有着重要影响。大气环境是镁合金最为常用的应用环境之一,在大气环境中存在污染微粒的情况下镁.铝合金中β相的腐蚀作用如何,β相体积分数的增加,是会加剧β相与基体α相之间的微电偶作用还是会增加镁-铝合金表面膜的稳定性,是本文关注的主要问题。采用普通铸造方法制备了3种不同Al含量的镁合金,Al的添加量均超过Al在Mg中的室温溶解度,因此制备的合金中均有第二相β相(Mg₁₇Al₁₂)。随着Al含量的增加,β相的体积分数增多。对具有不同体积分数β相的镁-铝合金进行室内模拟污染微粒（选用NaCl）作用下金属大气腐蚀实验,采用扫描电镜（SEM）对暴露不同周期的腐蚀产物的表面形貌,截面形貌以及去除腐蚀产物后基体的腐蚀形貌进行观察发现,NaCl盐粒作用下的镁-铝合金的大气腐蚀始于盐粒沉积的位置,表明通过沉降方式沉积到镁合金表面的盐粒诱发腐蚀时不存在相的选择性。β相的体积分数不同,腐蚀形貌存在比较大的差异。腐蚀增重结果表明随着合金中β相体积分数的增加,镁合金的腐蚀速率增加。这是因为起到阴极作用的β相,其体积分数的增加相当于增大了微电偶电池的阴极面积,从而加速了镁合金的腐蚀速率。金属表面的腐蚀产物膜对于金属基体起到一定程度的保护作用,而腐蚀产物膜的保护能力取决于膜层组成、结构和形态。采用X射线衍射（XRD）、电子探针（EPMA）、扫描电镜（SEM）等技术研究了镁合金中β相体积分数的增加对其大气腐蚀产物膜的组成、结构和性能的影响。镁合金的大气腐蚀产物主要组成为Mg（OH）₂,MgCl₂,MgCl₂·2H₂O,Mg₂（OH）₃Cl·4H₂O等。随着β相的增加,镁合金腐蚀产物中出现富铝相（MgAl₂O₄）,然而富铝相的生成没有增加腐蚀产物膜的稳定性,相反增加了腐蚀产物膜组成和元素分布的不均匀性。β相与α相之间的微电偶作用随β相体积分数的增加而加强,导致更多的富铝相及腐蚀脱落的β相进入腐蚀产物并随着腐蚀进行不断发生迁移,从而引起腐蚀产物分布不均匀、结构疏松、多孔,降低了镁合金大气腐蚀产物膜的保护性。交流阻抗能够定性测试镁.铝合金大气腐蚀产物膜的保护性,随着合金中β相的增加,在相同暴露周期内腐蚀产物膜的阻抗值下降,表明膜层保护性降低。采用原位电化学阻抗研究了镁.铝合金在NaCl盐粒作用下的初期大气腐蚀行为。原位阻抗谱与腐蚀形貌相结合能很好的反映镁-铝合金初期的大气腐蚀过程。镁-铝合金的高频容抗弧对应为反应信息,低频容抗弧半径逐渐增加,表明腐蚀产物膜的保护性在增强,阻抗谱上的扩散尾对应腐蚀产物的迁移信息。原位阻抗谱揭示了由NaCl诱发的镁合金初期大气腐蚀,在早期的10h内β相体积分数较高的镁合金的电荷转移电阻相对较大,这与β相的相对耐蚀有关。一旦突破β相的阻碍作用,Mg-21Al合金仍表现为较快的腐蚀速度。随着暴露时间的延长,Mg-3Al合金腐蚀产物膜的保护性明显更好。原位阻抗谱的测试结果与增重结果基本吻合,表明电化学阻抗技术用于研究非连续水膜作用下镁合金的初期大气腐蚀具有可行性。β相的电化学性质是决定其与基体α相的电偶行为以及整个表面腐蚀的关键。根据β相的成分特点制备了β相的模型合金,在不同浓度的Cl^-溶液环境中研究其表面膜的组成、结构以及电子特性。Cl^-浓度增加会降低β相的自腐蚀电位和点蚀击破电位,加速β相表面膜的形成和溶解,明显影响了β相表面膜的半导体特性。这是由于Cl^-浓度的增加,Cl^-过多掺杂了β相表面膜的组成,使表面膜性质发生改变。β相在溶液中的电化学行为可以揭示大气环境中镁-铝合金微区上β相的腐蚀作用。高浓度的Cl^-降低了β相表面膜的稳定性是NaCl污染因素作用下镁-铝合金中β相体积分数增加没有增加合金表面的稳定性反而使微电偶作用突出的原因所在。更多还原

【Abstract】 Pure magnesium is seldom used as structural material due to its poor mechanical properties,and that used as industrial structural material are magnesium alloys which were alloyed with certain elements.The addition of alloy elements not only improves the mechanical properties of magnesium alloys,but also induces the formation of second phase.The second phaseβ-Mg₁₇Al₁₂ plays an important role in corrosion performance of magnesium-aluminum alloy.It is known that magnesium alloys are frequently used in various atmospheric environments.The main subject concerned in the present paper is how the atmospheric corrosion behavior of magnesium alloys when the atmospheres were contaminated with certain salt particles,which frequently were as deposits on the alloy surface.What the article particularly concerns how the variation of the volume fraction ofβphase affects the micro galvanic effect,and the influence ofβphase on the surface films.Three magnesium-aluminum alloys with different aluminum contents were prepared by ordinary casting methods,all of which contain the secondβphase (Mg₁₇Al₁₂) because the content of additive aluminum exceeds the solubility of aluminum in magnesium at ambient temperature.The volume fraction ofβphase increases with the increase of the aluminum content.The atmospheric corrosion tests with the presence of contaminant particles in laboratory simulation was conducted to the three magnesium alloys with different volume fraction ofβphase, the surface morphologies and cross sectional morphologies of corrosion products, and the matrix after the remove of corrosion products were observed by SEM.It was found that the atmospheric corrosion of magnesium-aluminum alloys initiated directly where beneath NaCl particles deposits,that is to say,the corrosion could take place at any phase as long as it is the position where NaCl particles was deposited.The corrosion behavior varied because of the different volume fraction ofβphase in the magnesium alloys.It was indicated from the results of corrosion mass gain that the corrosion rate of magnesium alloys increased as increasing the volume fraction ofβphase,resulting in the serious corrosion of the magnesium alloy substrate.Because the increase ofβphase enlarged the cathodal area of micro galvanic cells,consequently increasing the corrosion rate of magnesium alloys,which indicated that in dual phase magnesium-aluminum alloys,the effect of micro couple betweenβphase and a phase enhanced with the increase ofβphase.The formed corrosion product fills showed protectiveness to some extent, depending on the structure and state of the corrosion product films.The influence of the increased volume fraction ofβphase in magnesium-aluminum alloys on the composition,structure and performance of atmosphere corrosion products were investigated by XRD,EPMA and SEM.There were Mg（OH）₂,MgCl₂, MgCl₂·2H₂O,Mg₂（OH）₃Cl·4H₂O in the corrosion products of the three magnesium alloys,and as theβphase in magnesium alloys increased,the Al-enriched phase（MgAl₂O₄） emerged.However,the formation of Al-enriched phase didn’t improve the stability of corrosion product fill,in contrast,the higher the volume fraction ofβphase,the more defects and pores existing in the corrosion products.The reason is that the micro galvanic effect betweenβphase andαphase in the substrate aggravates corrosion,leading to the inhomogeneous distribution,loose structure and pores of the corrosion products,which decreased the protective function of corrosion products of magnesium in atmosphere.The protective ability of corrosion product films of the three magnesium alloys could be qualitatively determined by Electrochemical Impedance Spectral,and the results showed that with the increase ofβphase,the impedance values of corrosion product films decreased when exposed for the same periods,indicating the decline of protective effect.The in-situ Electrochemical Impedance Spectra measurements were conducted to investigate the earlier stage of atmospheric corrosion behavior of the three magnesium alloys with NaCl particles using Solartron 1260 impedance/gain phase analyzer and 1296 dielectric interface.The in-situ impedance spectra combined with corrosion morphology could preferably reflect the process of atmospheric corrosion of the three magnesium alloys.The high frequency capacitive impedance arc of the three magnesium alloys stabilize gradually with the increase of exposure time.The gradually increased low frequency capacitive impedance arc reflected the slowdown of the substrate corrosion and the enhancement of the protectiveness of corrosion product films.The diffusion tale on the impedance spectra corresponds to the migrating information of corrosion products,rather than the diffusion controlled by non-oxygen factors.The in-situ impedance spectra revealed the better corrosion resistance of magnesium alloy with high volume fractionβphase during the earlier atmospheric corrosion induced by NaCl particles centralized in the earlier ten hours,and faster corrosion occurred for Mg-21Al alloy after ten hours becauseβphase is corroded.As the exposure time prolonged,the protective function of corrosion product films in Mg-3A1 alloy is significantly superior to the other two alloys.The results of in-situ impedance spectra were well consistent with those of the above results,showing the feasibility of the impedance spectra analysis technology to investigate the earlier stage of atmospheric corrosion induced by NaCl particles for magnesium alloys.The surface performance ofβphase is the key determining the galvanic couple behavior betweenβphase and a matrix phase,as well as the whole surface corrosion.A model alloy of consisted of nearlyβphase was smelt according to the composition features ofβphase,and the composition,structure and electron characteristics of the formed passive films on which were studied in solutions conditions containing different Cl^- concentration.The increase of Cl^- concentration may decrease the corrosion potential and the pitting breakdown potential ofβphase,accelerate the formation and dissolution of films on the surface ofβphase, and affect semiconductor characteristics of surface films obviously.This is because there is excess Cl^- doping the formation ofβphase surface film,resulting in the change of the properties of surface film as the Cl^- concentration increases.It is indicated from the electrochemical behavior ofβphase in the solution that theβ phase in the micro-region of magnesium-aluminum alloy plays a role of corrosion in atmosphere condition.The high concentration of Cl^- decreases the stability ofβphase surface film,attributing from the fact that the increase of volume fraction ofβphase in magnesium-aluminum alloy doesn’t increase the stability of surface, conversely,emphasizes the function of micro galvanic couple.更多还原