Chemical and electrical interaction mechanisms during the plasma electrolytic (PEO) mixed oxide formation on magnesium
Final Report Abstract
Magnesium, with a density of 1.7 g/cm³, is the lightest metallic construction material and therefore offers enormous weight-saving potential in automotive, aerospace, and space technology. At present, however, the range of applications for Mg alloys is limited by their low resistance to tribological and corrosive loads. Plasma electrolytic oxidation (PEO) is a promising surface-treatment process to overcome these challenges. In PEO, the component to be treated undergoes a strong anodic polarization within a passivating, aqueous electrolyte, which causes the initiation of plasma electrolytic spark discharges along the substrate surface. They lead to the conversion of the component surface into a ceramic coating, which consists of crystalline oxides of the base material, in the course of conversion processes. On Al-based materials, excellent coating characteristics are achieved with this method, which remain unattained on magnesium according to the current state of the art. In addition to the low durability of MgO compared to Al2O3 modifications, this is mainly due to the defect-rich morphology of PEO films generated on Mg substrates. Therefore, in the course of this research project, a comprehensive process-diagnostic system was developed to investigate the complex substrate/electrolyte interaction taking place during PEO. This enabled the identification of parasitic electrochemical sub-processes during the PEO of magnesium. These have a self-reinforcing effect and lead to continuous damage to the substrate/layer interface via electrolysis reactions, accompanied by localized acidification at the electrodes and the resulting substrate dissolution. The underlying processes can be influenced by the addition of passivating electrolyte components but cannot be prevented. Using high concentrations of Si and Al containing electrolyte species, the PEO layer formation equilibrium can be shifted in favor of more stable AlMg and SiMg mixed oxides. However, this does not lead to the desired improvement in the technological coating properties of wear and corrosion resistance due to the defect-rich coating morphology that still occurs. The knowledge gained in the project opens up a research strategy based on the use of metastable aluminate electrolytes to specifically prevent the self-amplification of the critical processes and, at the same time, to allow for a further reaction path for the incorporation of electrolyte components into the PEO layer. In addition, the published advancement of the PEO process-diagnostics system represents a significant scientific progress for the complete PEO process group.
Publications
-
Introduction to Plasma Electrolytic Oxidation—An Overview of the Process and Applications. Coatings, 10(7), 628.
Simchen, Frank; Sieber, Maximilian; Kopp, Alexander & Lampke, Thomas
-
Methoden der Elektrolytentwicklung und Prozessdiagnostik für die plasmaelektrolytische Oxidation am Beispiel der PEO von niedrig legiertem Stahl, WOMag 2021, 5
Simchen, F.; Mehner, T. & Lampke, T.
-
Diagnostics of Plasma Electrolytic Oxidation Prozesses on Magnesium, Posterbeitrag, Werkstofftechnisches Kolloquium, 2023
Simchen, F.
-
Identifikation parasitärer elektrochemischer Subprozesse bei der plasmaelektrolytischen Oxidation von Magnesium, ZVO-Oberflächentage, Berlin 13.–15.9.2023
Simchen, F.; Albero Rojas, C.; Schwöbel, S.; Mehner, T. & Lampke, T.
-
Specification of parasitic electrochemical subprocesses during plasma electrolytic oxidation of magnesium. Electrochimica Acta, 464, 142858.
Simchen, F.; Schwöbel, S.D.; Mehner, T. & Lampke, T.
