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Review of the Soft Sparking Issues in Plasma Electrolytic Oxidation

1
Department of Chemical Engineering, National Taiwan University of Science and Technology, Keelung Road 43, Section 4, Taipei 10607, Taiwan
2
Department of Mechanical Engineering, National Taiwan University of Science and Technology, Keelung Road 43, Section 4, Taipei 10607, Taiwan
*
Author to whom correspondence should be addressed.
Metals 2018, 8(2), 105; https://doi.org/10.3390/met8020105
Received: 14 December 2017 / Revised: 27 January 2018 / Accepted: 29 January 2018 / Published: 1 February 2018
(This article belongs to the Special Issue Plasma Electrolytic Oxidation)
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PDF [3483 KB, uploaded 1 February 2018]
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Abstract

A dense inner layer is highly valued among the surface coatings created through plasma electrolytic oxidation (PEO) treatment, because the PEO coating has been troubled by inherent porosity since its conception. To produce the favored structure, a proven technique is to prompt a soft sparking transition, which involves a sudden decrease in light and acoustic emissions, and a drop in anodic voltage under controlled current mode. Typically these phenomena occur in an electrolyte of sodium silicate and potassium hydroxide, when an Al-based sample is oxidized with an AC or DC (alternating or direct current) pulse current preset with the cathodic current exceeding the anodic counterpart. The dense inner layer feature is pronounced if a sufficient amount of oxide has been amassed on the surface before the transition begins. Tremendous efforts have been devoted to understand soft sparking at the metal–oxide–electrolyte interface. Studies on aluminum alloys reveal that the dense inner layer requires plasma softening to avoid discharge damages while maintaining a sufficient growth rate, a porous top layer to retain heat for sintering the amassed oxide, and proper timing to initiate the transition and end the surface processing after transition. Despite our understanding, efforts to replicate this structural feature in Mg- and Ti-based alloys have not been very successful. The soft sparking phenomena can be reproduced, but the acquired structures are inferior to those on aluminum alloys. An analogous quality of the dense inner layer is only achieved on Mg- and Ti-based alloys with aluminate anion in the electrolytic solution and a suitable cathodic current. These facts point out that the current soft sparking knowledge on Mg- and Ti-based alloys is insufficient. The superior inner layer on the two alloys still relies on rectification and densification of aluminum oxide. View Full-Text
Keywords: soft sparking; plasma electrolytic oxidation; electrolytic rectification; porosity; cathodic current; aluminum soft sparking; plasma electrolytic oxidation; electrolytic rectification; porosity; cathodic current; aluminum
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This is an open access article distributed under the Creative Commons Attribution License which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited (CC BY 4.0).
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Tsai, D.-S.; Chou, C.-C. Review of the Soft Sparking Issues in Plasma Electrolytic Oxidation. Metals 2018, 8, 105.

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