Layer formation mechanisms during plasma-anodising of magnesium in dependence of the electrolyte composition

镁材料的潜在应用领域远远大于今天所涵盖的应用范围，有助于进一步降低移动系统的能耗。然而，其较差的耐腐蚀性和耐磨性是许多应用的缺点，这可以通过表面改性来克服。等离子体电解阳极氧化（PAO）允许在镁基体上形成陶瓷氧化物涂层，提高了耐腐蚀性，即使大多数情况下需要后处理（例如密封）。在PAO过程中，缺乏关于电解质和镁基体相互作用的知识，这阻碍了不需要后处理的耐磨涂层或耐腐蚀涂层的发展。在目前的研究状态下，电解质、镁衬底和形成的氧化层之间的相互作用还不够充分，特别是在单一电解质成分及其不同组合的影响方面。因此，PAO的电解质完全是在经验基础上设计的，成功与否高度依赖于实验者的经验。用于镁PAO的电解质几乎全部含有低浓度（< 50 g/l）的电解质成分。本项目的申请人认为这是不利的，因为氧化镁具有硬度、强度和耐化学性低等不利特性，主要是在低浓度水溶液中PAO产生的。此外，在高极化下，衬底和涂层材料在水电解质中的溶解与涂层形成相竞争。初步试验表明，在高浓度电解质（单一组分浓度为100克/升）中，镁基体和形成的氧化层有很强的钝化作用。因此，在工艺的第一秒内的电压上升得到促进，并且可以获得更大范围的电气工艺参数。此外，文献和初步测试结果都表明，有利于形成非氧化镁或尖晶石，从而导致更好的涂层特性。本课题旨在研究镁基质在单一电解质组分及其不同组合溶液中的钝化和溶解行为。利用极化和电化学阻抗技术，系统地分析了电解质和衬底之间的相互作用，并提出了对涂层生长机制（衬底钝化，放电演变，溶解）的影响的理解。所获得的知识允许为PAO工艺的电解质组成，旨在获得定义的涂层特性。

The potential field of application of magnesium materials is far greater than the application range covered today facilitating a further reduction of energy consumption in mobile systems. However, its poor corrosion and wear resistance is a drawback to many applications, which can be overcome by surface modification. Plasma electrolytic anodic oxidation (PAO) allows for the formation of ceramic oxide coatings on the magnesium substrate improving the corrosion resistance even though a post-treatment (e.g. sealing) is mostly required. There is a lack of knowledge with regard to the interactions of the electrolyte and the magnesium substrate during the PAO process precluding the development of wear resistant coatings or corrosion resistant coatings, which do not require a post-treatment. The interactions between electrolyte, magnesium substrate and formed oxide layers are insufficiently established at the present state of research especially regarding the effect of single electrolyte components and different combinations of them. Thus, electrolytes for PAO are solely designed on an empirical basis and success is highly dependent on the experience of the experimenter. Electrolytes used for PAO of magnesium almost exclusively contain electrolyte components in low concentrations (< 50 g/l). This is perceived as unfavourable by the applicants of the present project because magnesium oxide, which has unfavourable characteristics like low hardness, strength and chemical resistance, is mainly produced during PAO in low-concentrated aqueous solutions. Further, the dissolution of substrate and coating material in the aqueous electrolyte under high polarisation is competing against the coating formation. Preliminary tests showed a strong passivation of the magnesium substrate and the formed oxide layer in high concentrated electrolytes (> 100 g/l of a single component). Thus, the voltage rise in the first seconds of the process is promoted and a wider range of electrical process parameters becomes accessible. Further, both literature and preliminary test results indicate the favourable formation of non-magnesium-oxides or spinels, which leads to better coating characteristics. The proposed research project aims to investigate the passivation and dissolution behaviour of magnesium substrate in the solutions of single electrolyte components and different combinations of them. Using polarisation and electrochemical impedance techniques, the interactions between electrolyte and substrate are systematically analysed and the understanding of the influence on coating growth mechanisms (substrate passivation, discharge evolution, dissolution) is raised. The gained knowledge allows for the composition of electrolytes for a PAO process that aims at attaining defined coating characteristics.