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

• Applicant: Professor Dr.-Ing. Thomas Lampke
• Subject Area: Coating and Surface Technology
• Term: from 2014 to 2017
• Project identifier: Deutsche Forschungsgemeinschaft (DFG) - Project number 258050305

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.

• DFG Programme: Research Grants
• International Connection: Austria
• Participating Person: Dr. Wolfgang Hansal