Localized plasma electrolytic oxidation of aluminum by focusing the discharges in the electrolytic free jet process (Jet-PEO)

等离子体电解氧化(PEO)是一种制备氧化性转化膜的工艺，非常适合作为金属-塑料复合材料界面的防腐层和增粘剂。在工业规模上，到目前为止，PEO过程只在电解槽中进行。对于铝合金在无毒、无氟的电解液中进行PEO的高工艺电压约为500V，电流密度约为30A/dm？，将PEO工艺限制在选定的功能表面(例如连接表面)以节约能源是有意义的。根据目前的技术水平，这只能通过用极粘性、高度电气绝缘以及化学和热极耐受的盖子来遮盖才有可能，这些盖子很难使用和拆卸。使用封闭式无电解液喷射(Jet-PEO)的PEO为无掩膜、局部限制生产PEO层提供了一种很有前途的可能性。因此，以前不能在镀液过程中处理的成分可以通过电解液喷射的受控移动来局部或大面积地进行PEO处理。与镀液工艺相比，涂层的性能可以局部调整。Jet-PEO的可行性已经在无氟电解液的初步工作中得到了证明。然而，目前还不可能有针对性地生产具有低厚度和明显的多孔外层的增粘PEO层，其特征是大量的气孔和底切。因此，本课题旨在科学地了解Jet-PEO的工艺特性、氧化物微观结构和表面性质之间的关系。首先，在电解液设计中，通过电化学极化和点火试验，获得了稳定的Jet-PEO过程所必需的基础知识，使其具有尽可能低的点火电压和高的成膜率。建立了基于电学参数、放电特性与仪表池PEO膜层微观结构和形貌之间关系的有限元仿真模型。利用具有线形对电极的实验装置，对模拟模型进行了关于层微结构和形貌的时间和空间分辨发展的验证和校准。从流体力学、射流几何和电气过程控制等方面对自由射流过程进行了多物理有限元模拟。通过Jet-PEO工艺，最终在衬底表面实现了点状和线状氧化物结构。通过使用微毛细管电池和使用分形算法评估表面形貌来表征氧化物结构，特别是关于腐蚀保护。

Plasma electrolytic oxidation (PEO) is a process for the production of oxidic conversion coatings, which are perfectly suitable as a corrosion barrier and for adhesion promotion in the interface of metal-plastic composites. In the industrial scale, the PEO process has only been carried out in an electrolyte bath, so far. Regarding the high process voltages of around 500 V and current densities of around 30 A/dm² for the PEO of Al alloys in non-toxic, fluoride-free electrolytes, it makes sense to save energy by limiting the PEO process to selected functional surfaces (e.g., joining surfaces). According to the state of the art, this is only possible by masking with extremely adhesive, highly electrically insulating and chemically and thermally extremely resistant covers, which are difficultly applicable and removable. PEO using a closed electrolyte free-jet (Jet-PEO) represents a promising possibility for the mask-free, locally limited production of PEO layers. Components that previously could not be handled in the bath process can thus be PEO treated locally or over a large area through the controlled movement of the electrolyte jet. In contrast to the bath process, the coating properties can be adjusted locally. The feasibility of Jet-PEO has already been proven in preliminary work with fluoride-free electrolytes. However, the targeted production of adhesion-promoting PEO layers with a low thickness and a pronounced porous outer layer, which is characterized by numerous pores and undercuts, is currently not possible. Therefore, the present project aims at the scientific understanding of the relationships between the process characteristics, the oxide microstructure and the surface properties in Jet-PEO. Firstly, the necessary basic knowledge for a stable Jet-PEO process with the lowest possible ignition voltage and high layer formation rate is acquired within the electrolyte design using electrochemical polarization and ignition tests. An FEM simulation model is developed based on the relationships between electrical parameters, discharge characteristics and microstructure and morphology of the PEO layers from an instrumented bath process. Validation and calibration of the simulation model with regard to the temporally and spatially resolved development of the layer microstructure and morphology are carried out using an experimental setup with a wire-shaped counter electrode. Multiphysical FEM simulations are used to design the free-jet process with regard to fluid mechanics, jet geometry and electrical process control. With the Jet-PEO process, punctiform and linear oxide structures are finally realized on substrate surfaces. The oxide structures are characterized in particular with respect to the corrosion protection using a microcapillary cell and by evaluating the surface topography using fractal algorithms.