合金元素对镁腐蚀行为的作用及其性能的影响研究

IMR OpenIR

	合金元素对镁腐蚀行为的作用及其性能的影响研究
	胥珊娜
学位类型	博士
导师	柯伟 ; 董俊华
	2012
学位授予单位	中国科学院金属研究所
学位授予地点	北京
学位专业	材料学
关键词	镁合金恒电位负差数效应耐蚀性应力腐蚀 Magnesium Potentiostatic Negative Difference Effect Corrosion Resistance Stress Corrosion Cracking
摘要	"镁合金是实用金属中最轻的金属材料，开发利用镁合金产品是当今世界发展的潮流，已从航空航天业转向汽车、电子信息、民用家电等领域，具有广阔的应用前景和开发潜力。镁合金具有一系列的优点，如具有高的比强度和比刚度；具有很好的阻尼性能和优良的切削加工性能，其热导率仅次于铝合金且无磁性；尺寸稳定性高，收缩率小，不易因环境温度变化而改变。但镁合金也存在许多缺点，如耐腐蚀性低等，这些在很大程度上限制了镁合金的推广应用。通过添加合金元素来提高镁合金的耐蚀性，是解决镁合金耐蚀性差的重要途径，也是最本质的方法。本论文利用电化学、浸泡、慢拉伸、杯突、盐雾等实验方法，系统研究了合金元素添加对镁合金腐蚀性能的影响。研究了纯镁的电化学腐蚀特征，阴极反应除析氢反应外还存在氢化镁的生成反应，并且随电位降低反应加速；在阳极区则为阳极溶解反应并伴随负差数效应，反应随电位升高而明显加快。原位激光拉曼光谱实验结果表明，在阴极极化区形成的氢化镁（MgH2，Mg2H4）量随电位的负移而逐渐增加，随极化时间的延长逐渐增加。而在阳极区未检测出氢化镁的存在，这是因为氢化镁的分解随电位升高而加速，而其生成速度却随电位升高而减慢引起的。研究了合金元素对负差数效应及阴极析氢、氢化物生成的影响，结果表明在镁铝合金中，Al添加量为3 wt.%时促进了Mg-3Al的阴极析氢和生成氢化物的速度，抑制了负差数效应。当Al添加量增大至9 wt.%时进一步促进了Mg-9Al的阴极析氢和生成氢化物的速度，负差数效应也有所增加。这与形成β相有关。Al的添加加速了镁合金的阳极溶解，含量变化对这种加速作用的影响不显著。在镁锌合金中，Zn添加量为1 wt.%抑制了Mg-1Zn的阴极析氢和生成氢化物的速度以及负差数效应。当Zn添加量增大至5 wt.%时Mg-5Zn合金的阴极析氢和生成氢化物及负差数效应有所增大，这与形成MgZn相有关。Zn的添加加速了镁合金的阳极溶解。在镁镉合金中，Cd的添加抑制了Mg-Cd合金的阴极析氢、氢化物生成以及负差数效应，且这种抑制作用随Cd含量的增加越来越显著。提出了添加合金元素Al、Zn、Cd影响镁合金阴阳极反应的电偶模型。 XRD检测结果表明，本研究所炼制的Mg-Cd 合金仍为hcp结构，并未出现新的第二相。Cd含量较低时，为粗大的等轴晶；当Cd含量增加至4.38 wt.%出现明显的晶内偏聚。Cd的添加提高了极化电阻值、降低了自腐蚀电流密度、降低了浸泡中的总析氢量，改善了纯镁的耐蚀性能。但当Cd含量为4.38 wt.%时，由于晶内偏聚反而降低了耐蚀性。在所研究成分中2.04 wt.%Cd的耐蚀性最佳。外加阳极电位下由于存在点蚀及负差数效应会加速合金的应力腐蚀敏感性。阴极电位下由于析氢及生成氢化物而导致氢脆也会产生应力腐蚀，但较阳极电位下应力腐蚀敏感性弱。Mg-Cd合金体系中，Cd的添加会降低应力腐蚀敏感性，当Cd含量为4.38 wt.%时应力腐蚀敏感性反而升高，这与Cd晶内偏聚引起的电偶作用加速腐蚀有关。在腐蚀介质中的拉伸断口主要是以解理片层为特征的脆性断口，且由断口所展现出的延伸率变化特征与曲线中的延伸率特征相一致。在AZ31及AZ91的基础上添加合金元素Cd未改变合金的相结构，Cd在合金中呈均匀分布。Cd的添加降低了AZ31及AZ91合金阴极的析氢、生成氢化物及阳极的负差数效应，使得阳极溶解略有增加。Cd的添加明显减弱了AZ31及AZ91合金在浸泡实验中的析氢速度，电化学阻抗谱显示其极化电阻值明显增加，因此Cd的添加显著改善了AZ31及AZ91合金的耐蚀性。 Cd的添加提高了AZ系列合金在空气中的拉伸强度及延伸率；明显降低了AZ体系合金的应力腐蚀敏感性。同时，合金在阳极极化电位下的应力腐蚀敏感性显著高于开路电位及阴极极化电位下的应力腐蚀敏感性。AZ31及AZ31+Cd合金，在空气中的拉伸断口呈明显的韧窝状塑性断裂特征，在0.1MNaCl腐蚀介质中则呈现以解理片层为特征的脆性断裂特征。AZ91合金由于延伸率较低，其在不同介质中的拉伸断口特征区别不明显，均以呈脆性解理片层为特征。交叉轧制板材较单向轧制基面织构弱，杯突值较大，板材冲压成型性能好；Cd添加使得相同轧制方式的板材基面织构减弱，杯突值增大，改善了板材冲压成型性能；经氟化磷化处理的涂装板材的耐蚀性明显优于纯磷化处理的涂装试样，在合金中添加Cd后，可以进一步的改善涂装试样的耐蚀性能。"
其他摘要	"Magnesium is the lightest metal in practical metallic materials, development and utilization of magnesium alloy products is the world's development trend shifted from the aerospace industry to the automotive, electronic information, consumer appliances and other fields, and it has broad application prospects and potential for development. Magnesium alloys have a number of advantages, such as high specific strength and stiffness; good damping properties and excellent cutting performance, the thermal conductivity second only to aluminum and non-magnetic, dimensional stability, shrinkage rate is small and difficult to change due to environmental temperature changes. But there are also many disadvantages of magnesium alloy, such as low corrosion resistance limiting the promotion of the use of magnesium alloy. By adding alloying elements to improve the corrosion resistance of magnesium alloy, is the most essential way. In this paper, by use of electrochemical methods, immersion, slow strain rate tensile, cupping, salt spray and other experimental methods, the effects of alloying elements added to the magnesium alloy on the corrosion resistance have been systematically studied. The main results are followed: The electrochemical corrosion of pure magnesium has been characterized. The results show that in the cathodic region magnesium hydride forms besides the formation of hydrogen, and the formation of magnesium hydride accelerates with the decrease of potential. While in the anodic region, anodic dissolution and the negative difference effect (NDE) concurrently occur, and the reaction accelerates with the increase of potential. By using in-situ Raman spectra, it is showed that the amount of magnesium hydride (MgH2, Mg2H4) increases when the potential shifts towards the negative direction, and increases with the duration of polarization. In the anodic region no magnesium hydride could be detected by in-situ Raman spectra as the decomposition of magnesium hydride accelerates while its formation decelerates with the increase of potential. The influence of alloy content on NDE, cathodic hydrogen evolution and formation of magnesium hydride has been studied. The results show that, in Mg-Al alloy, addition of 3 wt.% Al can enhance the cathodic hydrogen evolution, the formation of hydride and suppress NDE. Further addition of Al to 9 wt.% could further enhance the cathodic hydrogen evolution and the formation of hydride, but the negative difference effect increases too probably due to the formation of b phase. The addition of Al can accelerate the anodic dissolution of Mg alloy in comparison with pure Mg; however the influence of Al content on the acceleration is not obvious. In Mg-Zn alloy, addition of 1 wt.% Zn can suppress the cathodic hydrogen evolution, the formation of magnesium hydride and the NDE. After further addition of Zn to 5 wt.%, the cathodic hydrogen evolution, the formation of magnesium hydride and the NDE increase slightly probably due to the formation of MgZn phase. The addition of Zn also accelerates the anodic dissolution of Mg alloys. In Mg-Cd alloy, addition of Cd can suppress the cathodic hydrogen evolution, the formation of magnesium hydride and the NDE. And the suppressing effect increases with the content of Cd. The mechanism of reduction of NDE in Mg-Cd alloys is elucidated by a model based on the Mgn+-Cdd- galvanic couple surface. XRD results show that the as-cast Mg-Cd alloys in the present study are of single phase with crystal structure of HCP, no other phases are detected. When the content of Cd is small, the Mg-Cd alloy has a microstructure with equiaxed coarse grains; when the Cd content increases to 4.38 wt.%, segregation of Cd in the grain interior occurs. The addition of Cd raises the polarization resistance, lowers the self-corrosion current density, lowers the total amount of hydrogen evolution in immersion test and improves the corrosion resistance of Mg. In the present study, the Mg alloy with 2.04 wt.% Cd has the highest corrosion resistance. However, when the content of Cd reaches 4.38 wt.%, the corrosion resistance of Mg alloy lowers due to grain interior segregation. Under applied anodic potential due to pitting and the negative difference effects, stress corrosion sensitivity of Mg alloys is accelerated. While in the cathodic region hydrogen evolution and hydride can also cause stress corrosion cracking, but less sensitive than the circumstances under anodic potential. For Mg-Cd alloys, the addition of Cd can lower the stress corrosion sensitivity when the grain interior segregation of Cd is negligible. When the content of Cd reaches 4.38 wt.%, the stress corrosion sensitivity will raise again as corrosion is accelerated by the galvanic couple caused by segregation of Cd in grain interior. The fractography of Mg alloy in a corrosion medium shows lamella of cleavage which is characteristics of brittle fracture, in accordance with the small tensile elongation. Addition of Cd in AZ31 and AZ91 does not change the phase constitution of the Mg alloys and the Cd distributes homogeneously in them. Addition of Cd lowers the cathodic hydrogen evolution, the formation of hydride and the NDE at the anodic region while slightly increases the anodic dissolution. In immersion test, the addition of Cd significantly reduces the hydrogen evolution rate. Study by electrochemical impedance spectroscopy shows that the polarization resistance of Mg alloys with Cd addition increases significantly. The addition of Cd can significantly improve the corrosion resistance of AZ31 and AZ91. Addition of Cd can raise the tensile strength and elongation of Mg alloys of AZ serials in the environment of air, and lower their stress corrosion sensitivity obviously. The fractography of AZ31 and AZ31+Cd samples in air shows obvious dimples which are features of ductile fracture. But in corrosion medium of 0.1MNaCl, both kinds of samples show brittle cleavage fracture characteristics. The fractography of AZ91 alloys in both air and corrosion medium show brittle cleavage fracture characteristics due to their small elongation. As compared with one-way rolled plate, the texture of the cross-rolled plate is weaker, resulting in reduced anisotropy and the increased cupping value. Finally, the forming properties of the sheet metal are improved by cross-rolling. By adding Cd, the base texture can be reduced, leading to the increase of cupping value and the improvement of deformation performance. The chromatic aberration of the phosphate-fluoride treated plates is significantly lower than those plates treated only by phosphate. Also, the phosphate-fluoride treated plates have better corrosion resistance. Cd addition in the Mg alloys further improves the corrosion resistance of coated specimens under cyclic salt spray test."
文献类型	学位论文
条目标识符	http://ir.imr.ac.cn/handle/321006/64426
专题	中国科学院金属研究所
推荐引用方式 GB/T 7714	胥珊娜. 合金元素对镁腐蚀行为的作用及其性能的影响研究[D]. 北京. 中国科学院金属研究所,2012.