Currently, the demand for surface treatment methods like plasma electrolytic polishing (PeP)—a special case of electrochemical machining—is increasing. This paper provides a literature review on the fundamental mechanisms of the plasma electrolytic polishing process and discusses simulated and experimental results. The simulation shows and describes a modelling approach of the polishing effect during the PeP process. Based on the simulation results, it can be assumed that PeP can be simulated as an electrochemical machining process and that the simulation can be used for roughness and processing time predictions. The simulation results exhibit correlations with the experimentally-achieved approximation for roughness decrease. The experimental part demonstrates the results of the PeP processing for different times. The results for different types of roughness show that roughness decreases exponentially. Additionally, a current efficiency calculation was made. Based on the experimental results, it can be assumed that PeP is a special electrochemical machining process with low passivation.

Local anodization with a free electrolyte jet is a suitable solution for locally confined surface functionalization without additionally required preparation of the parts. However, the geometrical formation of the anodic oxide layer in jet-based anodization is not yet sufficiently understood. In this study, numerical calculations based on physical descriptions are used to describe the lateral and vertical oxide formation on aluminum alloy EN AW-7075. The required electrical resistance and capacitance were determined by immersion-based anodization and implemented into the numerical simulation model to evaluate the electrical conductivity of the porous layer. The simulation results showed an electrical conductivity of 2.6 × 10⁻⁶ S/m for the porous layer. Subsequently, a model for jet-based anodization was developed and the previous results were implemented to calculate the oxide formation. The simulation results showed decreasing oxide layer thickness at increasing radial distance from the center of the jet, which corresponds to experimental results. The simulation model was validated by varying the current efficiency from 5% to 90%, where similar developments of the anodic oxide layer thickness compared with experimental results were determined at 5%.

Maskless anodic oxidation with a continuous free jet of electrolyte can be used for the local surface functionalization and structuring of aluminum materials. In this study, a two-phase jet was applied with the aim of enhanced lateral confinement of the anodic oxide on the aluminum alloy EN AW-7075. The two-phase jet was realized by a coaxial arrangement. While the inner electrolyte nozzle with a diameter of 100 µm acted as the cathode and was used to provide the electrolyte with a flow rate of 10 ml min⁻¹ resulting in an average jet velocity of approximately 21 m s⁻¹, the outer nozzle with a diameter of 3000 µm was used to provide deionized water with a flow rate of 383 ml min⁻¹ resulting in an average water jet velocity of 1 m s⁻¹, which is sufficient to realize a continuous free two-phase jet. Process voltages from 10 to 60 V were investigated. The realized oxide layers were characterized by non-destructive as well as destructive methods to determine their microstructure and their thicknesses. Optically transparent anodic oxide layers were achieved in the voltage range between 10 and 25 V. Maximum total layer thicknesses between 1.1 µm and 16.9 µm were measured in these cases. Thicknesses of more than 50 µm were determined for higher voltages up to 60 V; however, burning effects and stronger discolorations were determined for voltages ranging from 30 to 60 V. Consequently, 25 V was derived as best suitable voltage for two-phase anodizing with high growth rate leading to oxide layers with low defects. Graphical abstract

During plasma electrolytic oxidation (PEO), an electrochemical oxide layer formation is essential for the substrate passivation, both during the initial stage of the PEO process, prior to the onset of discharges, and at the bottom of discharge channels during ongoing PEO treatment. Therefore, the final properties of the PEO coating are linked to the electrochemical formation of the oxide layer. This study presents a two-dimensional simulation of the barrier layer formation during the initial stage of PEO on aluminium. The high-field model was used to calculate the oxide layer growth as a function of the transferred electric charge. The model was validated by comparing the simulated results with experimentally measured current–potential curves obtained from polarisation experiments and with oxide-layer thicknesses determined by electron microscopy. Good agreement between the simulation and the experimental results was achieved when the electron current contribution was taken into account. The maximum current density was found to be 2.35 A dm^-2 in the experiment and 2.51 A dm^-2 in the simulation. The resulting barrier layer thickness is (127 ± 4) nm according to the experiments and 128.6 nm according to the simulation. Graphical Abstract