镁合金丝状腐蚀动态生长过程的原位研究

发布时间：2018-09-13 16:31

【摘要】：镁合金具有许多优异的性能,但化学性质活泼,易发生腐蚀破坏。镁合金部件的腐蚀失效几乎全部是由局部腐蚀引起的,丝状腐蚀作为一种常见的局部腐蚀形态,破坏性和隐患性非常大。如果镁合金的丝状腐蚀得到抑制,它的使用安全性将得到极大改善,这就需要对镁合金丝状腐蚀的机制有清楚的认识。本文以Mg-3Zn和ZG21两种镁合金为研究对象,使用扫描电子显微镜(Scanning Electron Microscope,SEM)、扫描振动电极技术(Scanning Vibrating Electrode Technique,SVET)、X射线光子能谱分析(X-ray Photoelectron Spectroscopy,XPS)、电化学测试和浸泡等实验方法确定了微观结构和腐蚀介质对腐蚀丝的萌生和发展的影响,明确了镁合金丝状腐蚀发展的控制因素,并澄清了腐蚀丝沿水平方向发展的原因。通过研究微观结构和腐蚀环境对丝状腐蚀的影响发现,第二相比较小的镁合金,它的腐蚀屏蔽作用比较小,导致腐蚀丝遇到第二相后会穿过第二相继续发展。丝状腐蚀的发展主要与Cl~-有关,加入F-,或提高pH值能延缓腐蚀丝的出现。通过SVET原位观察,发现丝状腐蚀过程中的腐蚀微电池随腐蚀的发展是动态变化的;外加阳极电位,表面有较多腐蚀丝出现,且外加的阳极电位越正,腐蚀越严重。相反,外加阴极电位,没有腐蚀出现。由此确定,镁合金丝状腐蚀生长过程为阳极控制。对比了在NaCl溶液中的丝状腐蚀和Na_2SO_4溶液中的点蚀,发现Cl~-会在腐蚀丝的丝头处聚集,而SO_4~(2-)在点蚀坑底部聚集,这与在两种溶液中形成的表面膜有关。Cl~-的穿透性比较强,会在表面膜比较薄的地方优先吸附,导致腐蚀萌生。腐蚀萌生后,会使该处膜层破坏失效,并生成很疏松的腐蚀产物,腐蚀产物具有毒化效应,大量的Cl~-会更易在该处聚集。相对于破损膜层下面的镁基体,破损膜层周围的完好膜层处也会更容易聚集一些Cl~-,因此腐蚀更易沿着完好膜层水平扩展。然而,在膜层破损区域的分布并不均匀,会出现某处Cl~-浓度高的情况,腐蚀就会沿着Cl~-浓度高的方向向前扩展,一旦腐蚀丝向某个方向扩展后会形成丝尾,Cl~-很容易沿着由疏松腐蚀产物组成的丝尾处向丝头处传输,而未被腐蚀的区域Cl~-相对更难传输到丝头处,因此腐蚀丝就会沿着丝尾的方向向前生长,最终导致腐蚀丝沿水平方向发展。
[Abstract]:Magnesium alloys have many excellent properties, but their chemical properties are lively and easy to corrode. Almost all the corrosion failure of magnesium alloy parts is caused by local corrosion. As a common local corrosion form, filamentary corrosion is very destructive and hidden. If the filiform corrosion of magnesium alloy is restrained, the safety of magnesium alloy wire will be greatly improved, which requires a clear understanding of the mechanism of magnesium alloy wire corrosion. In this paper, two kinds of magnesium alloys, Mg-3Zn and ZG21, are studied. By means of scanning electron microscope (Scanning Electron Microscope,SEM), scanning vibrating electrode (Scanning Vibrating Electrode Technique,SVET), X-ray photon spectroscopy (X-ray Photoelectron Spectroscopy,XPS), electrochemical measurement and immersion, the effects of microstructure and corrosion medium on the initiation and development of corrosion wire were determined. The controlling factors for the development of magnesium alloy wire corrosion were clarified, and the reasons for the development of the corrosion wire along the horizontal direction were clarified. By studying the effect of microstructure and corrosion environment on the filamentous corrosion, it is found that the magnesium alloy with smaller second phase has less corrosion shielding effect, which leads to the further development of the second phase after the corrosion wire encounters the second phase. The development of filamentous corrosion is mainly related to Cl~-. Adding F-or increasing pH value can delay the appearance of corrosion wire. In situ observation by SVET, it is found that the corrosion microcells in filamentous corrosion process change dynamically with the development of corrosion, and that there are more corrosion wires on the surface of the applied anode potential, and the more positive the additional anode potential, the more serious the corrosion is. On the contrary, no corrosion occurs when the cathode potential is applied. Therefore, the corrosion growth process of magnesium alloy wire is controlled by anode. By comparing the filamentous corrosion in NaCl solution with pitting corrosion in Na_2SO_4 solution, it is found that Cl~- aggregates at the wire head and SO_4~ (2-) aggregates at the bottom of the pitting pit, which is related to the penetrability of the surface film formed in the two solutions. It will preferentially adsorb on the surface where the film is thinner, resulting in corrosion initiation. The corrosion initiation results in the failure of the film and the formation of very loose corrosion products. The corrosion products have toxic effect and a large number of Cl~- will easily accumulate in the area. Compared with the magnesium substrate beneath the damaged film, it is easier for the intact film to gather some Cl~-, around the damaged film, so the corrosion is easier to expand along the level of the intact film. However, the distribution of the damaged area of the film is not uniform, and there will be a situation where the concentration of Cl~- is high, and the corrosion will spread along the direction of the high concentration of Cl~-. Once the corroded wire expands in a certain direction, it forms the tail of the wire, which is easily transported along the tail of the loose corrosion product to the wire head, and the Cl~- in the uncorroded area is relatively difficult to transmit to the wire head. Therefore, the corrosion wire will grow forward along the tail of the wire and eventually lead to the development of the corrosion wire along the horizontal direction.
【学位授予单位】：沈阳工业大学
【学位级别】：硕士
【学位授予年份】：2017
【分类号】：TG178

【参考文献】