Abstract
AbstractPlasma electrolytic oxidation is a powerful technique allowing the formation of ceramic coatings with a high degree of functionalization. Plasma–chemical interactions, resulting from the application of a high voltage in a conductive electrolyte, like 0.5 M NaOH, favor the development of a porous surface, which deserves to be sealed to obtain proper corrosion resistance. In the present work, the effect of temperature employed during sealing in calcium acetate is investigated, finding 85°C as an optimum temperature to allow pores closure. Furthermore, a comparison with sealing performed in hot water is presented, which demonstrated similar sealing efficiency. Scanning electron microscope and electrochemical impedance spectroscopy are the tools selected to investigate sealing effects. Impedance spectra are fitted according to feasible electrochemical equivalent circuits finding two‐time constants when applied to sealed samples indicative of the presence of barrier and porous layer, respectively.
Highlights
Plasma electrolytic oxidation is a process which allows to form an oxide film on a metal surface with excellent corrosion resistance. [1,2,3] the surface morphology of the oxide layer is irregular and porous, and doen not stop effectively the diffusion of aggressive substances towards the substrate, making the corrosion resistance properties highly dependent of only the barrier layer present above the metallic substrate
Surface improvements will be evaluated according to electrochemical impedance spectroscopy (EIS) where the parameters extracted according to the use of feasible electrochemical equivalent circuit will be validated according to scanning electron microscope images (SEM)
The semicircle observed in the Nyquist plot increases its diameter, suggesting that a higher impedance to charge transfer was encountered with the use of higher temperature
Summary
Plasma electrolytic oxidation is a process which allows to form an oxide film on a metal surface with excellent corrosion resistance. [1,2,3] the surface morphology of the oxide layer is irregular and porous, and doen not stop effectively the diffusion of aggressive substances towards the substrate, making the corrosion resistance properties highly dependent of only the barrier layer present above the metallic substrate. Chemical sealing is widely applied with good results but it's often too much energetically expensive and time ee consuming, like HWS (Hot Water Sealing) on aluminum [8] requiring a huge amount of thermal power to be provided to the system. To overcome this limitation few exceptions were advanced, like cold nickel fluoride sealing. In the present paper the effectiveness of some common chemical sealing techniques as hot water (HWS) and calcium acetate (CAS) is evaluated on titanium oxides produced by PEO carried out in 0.5 M NaOH. Surface improvements will be evaluated according to electrochemical impedance spectroscopy (EIS) where the parameters extracted according to the use of feasible electrochemical equivalent circuit will be validated according to scanning electron microscope images (SEM)
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