Abstract
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.
Highlights
Plasma electrolytic oxidation (PEO) was a promising surface treatment for light metals to tailor an oxide layer with excellent properties
In order to suppress the detrimental effects of plasma discharges, significant attention was paid towards the development of soft sparking regime during plasma electrolytic oxidation (PEO) process[6,7,8,9], which was associated with the intensity decrease of plasma discharges while maintaining the coating growth
The sharp increase of the responding voltage in the early stages of PEO coating was attributed to an increase in electrical resistance which resulted from the formation of the thin passive film formed initially on substrate
Summary
Plasma electrolytic oxidation (PEO) was a promising surface treatment for light metals to tailor an oxide layer with excellent properties. In order to suppress the detrimental effects of plasma discharges, significant attention was paid towards the development of soft sparking regime during PEO process[6,7,8,9], which was associated with the intensity decrease of plasma discharges while maintaining the coating growth The utilization of this regime, was limited in terms of practical application due to the need of a sophisticated voltage/current waveform with an extended coating time. To decrease the defects in the PEO coating, a more simple approach would be desirable to conduct PEO with softer plasma In this regard, electrolyte additives emerged as a potential candidate due to their apparent influence to the microstructure, composition and electrochemical behaviour of the resultant coating layer[10,11,12]. Shi et al.[13] compared the effects of EDTA and sodium tetraborate as additives within the silicate-phosphate electrolyte and they reported that the corrosion resistance of the coating with EDTA was superior due to the development of a www.nature.com/scientificreports/
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