RESUMEN
Different electrochemical approaches were proposed in this study to introduce nanoparticles to the coating layers, aiming at their in situ incorporation into the coating layers fabricated via plasma electrolytic oxidation (PEO). The addition of nanoparticles to the coating layers provided an electrochemical pathway to generate the functionalized coatings with a wide range of compositions and constituent phases as well giving the appearance of sealing the pores. In this study, the microstructure, chemical composition, and electrochemical response of the composite coating formed via one-stage PEO were compared with those obtained by means of structural modification of PEO coatings together with either impregnation or pre-deposition. For the combination of PEO and pre-deposition, the coating layer demonstrated less porous and better corrosion performance in the conditions used in this study, which were attributed to the denser and/or thicker layer resulting after incorporating the nanoparticles, such as SiO2 and ZrO2. In these methods, the nanoparticles were detected mainly not only near the coating surface, but also within the micro-defects inside the coating layers. Accordingly, the electrochemical analysis based on potentiodynamic polarization tests in 3.5 wt% NaCl solution clearly showed that the corrosion resistance of Mg alloy would be enhanced significantly due to the incorporation of SiO2 and ZrO2 or ZrO2 nanoparticles.
RESUMEN
Plasma electrolytic oxidation (PEO) was a promising surface treatment for light metals to tailor an oxide layer with excellent properties. However, porous coating structure was generally exhibited due to excessive plasma discharges, restraining its performance. The present work utilized ethylenediaminetetraacetic acid (EDTA) and Cu-EDTA complexing agents as electrolyte additives that alter the plasma discharges to improve the electrochemical properties of Al-1.1Mg alloy coated by PEO. To achieve this purpose, PEO coatings were fabricated under an alternating current in silicate electrolytes containing EDTA and Cu-EDTA. EDTA complexes were found to modify the plasma discharging behaviour during PEO that led to a lower porosity than that without additives. This was attributed to a more homogeneous electrical field throughout the PEO process while the coating growth would be maintained by an excess of dissolved Al due to the EDTA complexes. When Cu-EDTA was used, the number of discharge channels in the coating layer was lower than that with EDTA due to the incorporation of Cu2O and CuO altering the dielectric behaviour. Accordingly, the sample in the electrolyte containing Cu-EDTA constituted superior corrosion resistance to that with EDTA. The electrochemical mechanism for excellent corrosion protection was elucidated in the context of equivalent circuit model.
RESUMEN
The electrochemical roles of electron-donor and -acceptor agents in surface reforming of magnesium alloy were investigated via plasma electrolysis. The surface modification was performed in an aluminate-based electrolyte, having urea and hydrazine with inherent molecular structures, which might act as electron acceptor and donor during plasma-assisted electrochemical reaction. The presence of hydrazine working as donor would promote the formation of magnesium aluminates in the oxide layer, resulting in superior compactness of the oxide layer to that when urea was used as the working as acceptor since the precipitation of MgCO3 was favored in the electrolyte with urea. The thickness of the oxide layer formed by a combination of urea and hydrazine was higher than urea, while the porosity was higher than hydrazine. The electrochemical performance was enhanced in the order of hydrazine, urea and hydrazine combined, and urea, which was discussed on the basis of impedance interpretation.