【作者】 杨丽景；

【导师】 卫英慧；

【作者基本信息】 太原理工大学 ， 材料加工工程， 2010， 博士

【摘要】 镁合金作为一种轻质工程材料,具有比重轻、比强度高、比刚度高、电磁屏蔽性能好及铸造性能优良等一系列优点,在汽车、3C、航空航天等领域有着广阔的应用前景。然而,镁合金的耐腐蚀性能较差,限制了镁合金的应用。目前对镁合金在NaCl溶液中的腐蚀行为的研究较多,但是对镁合金在大气腐蚀特别是在重污染地区等特殊环境下的腐蚀行为的研究还相对较少。镁合金产品的制造方式有铸造、压铸及半固态成形技术等,各种成形方式日趋成熟,但是针对不同成形工艺下镁合金显微组织对其腐蚀行为影响的研究还缺乏系统性,对镁及其合金在含硫酸根离子的溶液中的腐蚀行为的研究还相对较少,对触变成形铸件在热处理过程中显微组织的演变及耐腐蚀性能的研究也有待深入。因此,有必要系统研究AZ91D镁合金显微组织对其在大气及溶液等特殊环境下腐蚀行为的影响,以期丰富和发展镁合金腐蚀理论,为拓展镁合金的应用领域、充分发挥其性能潜力奠定坚实基础。本文利用金相显微镜、SEM、TEM、EDS、XRD、GDS、FTIR及电化学测试等方法,对不同显微组织的AZ91D镁合金在特殊环境下的腐蚀行为进行了研究,得到在不同环境下镁合金的腐蚀机制。对铸态和压铸态AZ91D在太原地区环境下进行室外暴露实验,研究了镁合金表面腐蚀产物层的结构特征和表面龟裂现象;通过比较压铸镁合金AZ91D在中性NaCl溶液、Na₂SO₄溶液、MgSO₄溶液及不同pH值的Na₂SO₄溶液中的腐蚀行为,系统研究了SO₄^2-离子对压铸镁合金腐蚀的影响,构建了AZ91D镁合金在含SO₄^2-溶液中腐蚀速率及腐蚀电位与pH值之间的关系图;比较了触变成形与高压压铸AZ91D镁合金在中性NaCl水溶液中的腐蚀行为,研究了不同显微组织对镁合金耐腐蚀性能的影响。详细研究了触变成形AZ91D镁合金铸件固溶、时效处理时显微组织的演变,分析了触变成形镁合金铸件时效强化的机理。镁合金AZ91D在重污染地区的大气腐蚀速率服从幂指数规律。铸态和压铸态镁合金在太原地区的大气腐蚀动力学方程分别为C=1.533t0.602及C=1.661t0.507。铸态和压铸态AZ91D在太原地区环境下暴露12个月的大气腐蚀速率分别为6.24g·m^-2·y^-1和5.97g·m^-2·y^-1。大气腐蚀产物包括Mg（OH）2、MgCO₃及Mg₂Al₂（SO₄）₅·39H₂O等,酸雨及SO2的存在是硫酸盐产物形成的重要原因。显微组织对镁合金大气腐蚀行为有较大的影响。表面缺陷主要影响镁合金初期的大气腐蚀速率,晶粒尺寸在AZ91D镁合金的大气腐蚀过程中影响着腐蚀产物层的厚度及表面裂纹分布,第二相分布情况主要影响大气腐蚀后期的腐蚀速率。压铸镁合金AZ91D在NaCl溶液中腐蚀48h后发生了点蚀,在含SO₄^2-的溶液中发生了全面腐蚀。各种溶液中的腐蚀速率大小顺序为:vNaCl>vMgSO₄>vNa₂SO₄,镁合金在含SO₄^2-的溶液中腐蚀速率相对较小,腐蚀过程主要是阴极析氢腐蚀控制。腐蚀产物主要为Mg（OH）₂及MgAl₂（SO₄）4·22H₂O。MgAl₂（SO₄）4·22H₂O的形成需要一个时间门槛值。压铸镁合金AZ91D在不同pH的Na₂SO₄的溶液中腐蚀速率大小顺序为酸性>中性>碱性溶液。随着pH值的增大,腐蚀速度呈递减趋势。在各种溶液中的腐蚀产物主要为MgAl₂（SO₄）₄·22H₂O、MgCO₃及Mg（OH）₂,不同的pH值不改变腐蚀产物类型,只改变腐蚀产物的形貌。在酸性Na₂SO₄的溶液中,浸蚀约330h后表面会发生点蚀现象。触变成形AZ91D镁合金组织中存在原固相α-Mg,其Al含量相对较低,导致共晶α-Mg中Al含量较高压压铸镁合金中共晶α-Mg要高。在腐蚀初期,触变成形铸件中原固相α-Mg作为主要腐蚀相,腐蚀一定时间后原固相α表面的腐蚀产物膜能较稳定地附着于α相表面阻止腐蚀继续发生,而后共晶α相作为主要被腐蚀相。电化学测试结果表明触变成形铸件腐蚀速率约比压铸铸件低28%。触变成形AZ91D镁合金在415℃时固溶处理导致β相溶入α-Mg固溶体,其固溶速率比高压压铸的快,在固溶初期观察到富铝晕圈。压铸态和触变成形试样在热处理过程中的晶粒长大动力学方程分别为d3-d03=588t和d3-d03=324t。触变成形铸件β相的快速溶解主要是因为其组织细小而且均匀。更多还原

【Abstract】 Magnesium alloys have potential as structural materials for aerospace, 3C and other transport applications due to their high specific properties, low density and high damping capacity. However, the use of Mg alloys in these fields is still limited to some extent because of their susceptibility to corrosion. Despite such concern, very few studies are currently dedicated to the atmospheric corrosion of Mg alloys, especially in a polluted environment. Mg alloy parts are mainly formed by cast, high pressure die-casting and semi-solid processing. Previous studies have been extensively carried out on the corrosion behaviour of Mg alloys in NaCl solution. However, the corrosion processes of Mg alloys in solutions containing SO₄^2- ions still remain ambiguous. Although there are several works studied the corrosion behaviour and microstructure evolution of Mg alloy produced by semi-solid processes during heat treatment, there are rare reports concerning on the thixomolding Mg alloy. We aim to study the influence of microstructure on the corrosion behaviour of AZ91D magnesium alloys in particular environments in great detail. The results of our investigation may find wider applications for advancing materials.In this paper, the atmospheric corrosion behaviour of cast and high pressure die-cast AZ91D alloy in a polluted environment, the corrosion behaviour of die-cast AZ91D magnesium alloys in sulphate solutions with various pH value, the corrosion behaviors of the thixomolding and high die-casting AZ91D alloy in NaCl solution and the microstructure evolution of thixomolding AZ91D magnesium alloy during heat treatment were investigated by OM, SEM, TEM, XRD, FTIR, GDS and polarization measurements in order to provide further details regarding the corrosion processes and mechanisms of a die-cast Mg alloy in solutions containing the SO₄^2-. The results are discussed in terms of the overall Mg corrosion theories.The kinetics of atmospheric corrosion of the ingot and the die-casting AZ91D alloys could be expressed as C=1.533t0.602 and C=1.661t0.507, respectively. The corrosion rates of the ingot and the die-casting AZ91D alloys exposed to the polluted environment were about 6.24 g·m^-2·y^-1 and 5.97 g·m^-2·y^-1, respectively. SO₂ gas played an important role in atmospheric corrosion. The main corrosion products of AZ91D alloys were Mg（OH）₂, Mg₂Al₂（SO₄）₅·39H₂O, and MgCO₃. The microstructure played an important role in the corrosion process. The initial corrosion rate was mainly influenced by surface defects. The thickness of the product layer and the diameter of the crack were greatly influenced by the grain size. The distribution of theβphase acts on the later corrosion rate. The pitting takes place on the die-casting specimen surface immersed in a NaCl solution when the immersion time is less than 48 h. In contrast, in Na₂SO₄ and MgSO₄ solutions, pitting corrosion does not occur, and the corrosion appears generalized because the aggressiveness of SO₄^2- is lower than that of Cl^-. The corrosion rate on the whole surface is approximately uniform in the sulphate solution. The corrosion rate order of the die-cast AZ91D Mg alloy in the three aqueous solutions is: vNaCl > vMgSO₄ > vNa2SO₄. The control factor of the corrosion process in the sulphate solution is the cathodic hydrogen-evolution corrosion. The main corrosion products are Mg（OH）₂ and MgAl₂（SO₄）₄·22H₂O in the sulphate solution. The precipitation of the MgAl₂（SO₄）₄·22H₂O needs a threshold immersion time. When the immersion time is extended to 6 h, the concentration of MgAl₂（SO₄）₄·22H₂O in the solution gradually became supersaturated and then began to precipitate from solution. The order of corrosion rate of Mg alloy in Na₂SO₄ solutions with various pH value was acidic solutions > neutral solutions > alkaline solutions. The corrosion products of the die-cast Mg alloy were mainly Mg（OH）₂ and MgAl₂（SO₄）4·22H₂O. The pH value could influence the corrosion rate and morphology of the corrosion products. Pitting occurred on the specimen surface which immersed in the acidic solution after immersed for about 330 h.There was unmeltedα-phase in the thixomolding specimen, and the Al content of which was low, so it made the Al content of the eutecticα-phase become higher than the die-casting specimen. The unmeltedαphase was a major corrosion phase at the early stage, with the corrosion time prolonged, the corrosion process was prevented due to the corrosion products covering on the unmeltedα-phase. And then, the eutecticα-phase corroded and dissolved. Electrochemical testing results showed that the corrosion rate of the thixomolding specimen was 28% lower than that of the die-casting specimen. The AZ91D Mg alloy specimen processed by thixomolding exhibits a fasterβphase dissolution rate than the die-casting alloy at 415℃solution treatment. The Al-rich halos have been seen at the early stage of the solution treatment because of Al diffusing from theβphase to the unmeltedα-globules. The thixomolding AZ91D alloy has fasterβphase dissolution kinetics in comparison with the die-casting alloy, showed as d3-d03=588t and d3-d03=324t, respectively. The reason is that initial secondaryα-Mg grain in the thixomolding specimen is equiaxed and fine, which makes the dissolution rate of theβ-Mg17Al12 faster.更多还原