Characterisation and Electrochemical Evaluation of Plasma Electrolytic Oxidation Coatings on Magnesium with Plasma Enhanced Chemical Vapour Deposition Post-Treatments

Abstract

Plasma enhanced chemical vapour deposition (PECVD) post-treatment with three different precursors is employed to improve corrosion resistance of plasma electrolytic oxidation (PEO) coated Mg. Surface morphology, chemical and phase composition are characterised by SEM, EDS, and XRD analyses, whereas corrosion resistance is assessed by electrochemical tests. PECVD post-treatments can significantly affect PEO coatings, resulting in sealed pores and multi-layered inner barrier structure. Depending on the precursor, this can have both positive and negative influence on the corrosion resistance of the PEO coatings. The PECVD post-treatment with hexamethyldisiloxane precursor significantly improves corrosion resistance of PEO coatings, providing the best corrosion protection of Mg.

Effect of Plasma Oxidation Treatment on Production of a SiOx/SiOxCyHz Bilayer to Protect Carbon Steel Against Corrosion

“…Considering the as received carbon steel, is noticed a typical curve with one time constant and maximum (70°) around 10 1 Hz, representative of the interactions between the electrolyte and the substrate 24,29,99 . When the phase angle is measured in the carbon steel coated with the as deposited organosilicon film, the end of the curve at high frequencies moves to greater phase angles.…”

“…In general, there is a drop in the impedance of the samples with the increase in the frequency of the perturbation signal, typical behavior of these systems 25,29,39,66,99 . Besides, it is noted that the curves are distributed in different positions along the impedance axis.…”

“…The impedance module at low frequencies (10 -1 Hz) is associated to the total resistance of the system to the current flow, R T . The higher the R T the greater the system's opposition to the flow of oxidative species from the electrolyte, that is, the better the barrier property of the film 25,29,39,66,99 . The R T values were determined according to the method proposed by Mansfeld 29,71 and are depicted in Figure 13.…”

“…The data obtained using the EIS technique can be adjusted to a theoretical model of an equivalent circuit (EC), which allows discerning the contribution of each element of the coating-substrate system to the obtained values 24,26,66,99 .…”

Enhanced Wear Performance of Hybrid Epoxy-Ceramic Coatings on Magnesium Substrates

“…Nevertheless, the porous microstructure of PEO coatings is difficult to change considering the dielectric breakdown growth mechanism. Benefiting from the mentioned intrinsic properties of PEO coatings, PEO is an ideal pretreatment process, and several duplex surface modification strategies, for example, chemical conversion, , chemical vapor deposition, electrophoretic deposition, hydrothermal treatment, and sol–gel, , have been developed with the aim of changing the surface conditions to overcome the disadvantages associated with PEO coatings. Although most of these reported methodologies increased the corrosion resistance of PEO coatings by sufficiently reducing the number of pores and cracks by depositing a protective layer on the porous surface, few investigations have been performed regarding the adhesion strength between the duplex layer and the PEO coatings as well as their tribological properties.…”