Magnesium is a lightweight material that has been increasingly used in numerous applications. As an alternative material, it has led to a significant reduction in fuel consumption and carbon dioxide emission, especially in automobile and aerospace fields. However, its challenging anti-corrosion protection is a big threat to the development and deployment of magnesium alloys.
Several methods of preventing magnesium corrosion including surface treatments and coating have been proposed. However, most of these methods require post-processing to enhance their corrosion resistance ability which may be expensive in the long run. Recently, plasma electrolytic oxidation (PEO) has been identified as a promising solution for corrosion and wear protection. It forms crystalline ceramic surfaces on the magnesium alloys which exhibits good hardness properties and thus further treatment is not required.
Furthermore, German scientists Dr. Anna Buling and Joerg Zerrer from Eloxalwerk Ludwigsburg Helmut Zerrer GmbH successfully developed a high corrosion resistance nanocrystalline plasma electrolytical oxidation coating. This technique utilized an asymmetric potential pulse waveform to the nanocrystalline coating on magnesium substrates. The main objective was to increase the corrosion resistance of magnesium alloys and enhance their applicability in various fields. Also, they investigated the correlations between the voltage-time behavior and plasma electrolytical oxidation performance during the coating process. The work is currently published in the research journal, Surface and Coatings Technology.
Nanocrystalline plasma electrolytical oxidation coatings were analyzed on different magnesium alloys sheet plates, characterized by electrochemical analysis i.e. electrochemical impedance spectroscopy. This entailed varying the coating thickness taking into consideration their morphology and resistance wear. The produced coatings exhibited better corrosion and wear resistance on all the three alloys: AM50, AZ31, and E-Form analyzed. It was necessary to investigate the role of particle addition in the electrolyte as well as the influence of electrolyte concentration and electrical parameters. For instance, increasing the coating thickness by increasing the processing time resulted in larger pore discharge which led to low corrosion protection.
Therefore, the ultraceramic® process was optimized by an on-top modulation of the process signal to smoothen the S-PEO coating thus decreasing the pore diameter at the surface. This led to significantly higher corrosion and wear resistance despite low coating thickness. According to the authors, the above characteristics could further lower energy consumption during the coating process.
The new asymmetric waveforms are of great benefit as compared to the older waveforms by reducing the overall process time and energy consumption. On the other hand, it was observed that the charge quantity ratio was a key consideration in coating formation. Additionally, the negative pulse and pause between the pulses helped in quenching the micro-discharges by inhibiting their transition.
In summary, the presented optimization of the coating process by Anna Buling and Joerg Zerrer proved an efficient and cost-effective approach for protecting magnesium alloys from corrosion and wear. As such, their study is expected to motivate an increase in the application of magnesium alloys in numerous fields considering its light, low density, and high strength properties.
Buling, A., & Zerrer, J. (2019). Increasing the application fields of magnesium by ultraceramic®: Corrosion and wear protection by plasma electrolytical oxidation (PEO) of Mg alloys. Surface and Coatings Technology, 369, 142-155.Go To Surface and Coatings Technology