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

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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.