Novel coatings obtained by plasma electrolytic oxidation to improve the corrosion resistance of magnesium-based biodegradable implants

• Published in: Surface & coatings technology 2018-11, Vol.354, p.28-37
• Main Authors: Echeverry-Rendon, Monica, Duque, Valentina, Quintero, David, Harmsen, Martin C., Echeverria, Felix
• Format: Article
• Language: English
• Subjects: Additives; Anodic coatings; Anodizing; Barrier layers; Biodegradability; Catalytic oxidation; Coating effects; Corrosion resistance; Corrosive wear; Current density; Electric potential; Electrolytes; Hexamethylenetetramine; Industrial applications; Magnesium; Mannitol; Morphology; Multilayers; Oxidation resistance; Parameter modification; Plasma; Potassium hydroxides; Protective coatings; Sodium; Sodium fluoride; Surgical implants; Wear resistance
• ISSN: 0257-8972; 1879-3347
• DOI: 10.1016/j.surfcoat.2018.09.007

Due its good properties, Magnesium (Mg) is a metal widely used in industrial applications and in biomedical field. However Mg is highly reactive and its wear and corrosion resistance are low. Plasma electrolytic oxidation (PEO), also known as microarc oxidation (MAO), is a favorable technique which can be used to produce coatings of MgO/Mg(OH)2 in a controlled way with specific morphology, thickness and composition by the modification of the operation parameters of the system such as voltage, current density, time and electrolytic solution. The aim of this paper was to study the effect of some additives in the base electrolyte solution composed of silicate and potassium hydroxide. As additives, sodium fluoride was compared with two unreported organics compounds, hexamethylenetetramine and mannitol. Both galvanostatic and potentiostatic modes were used for anodization. Voltage-time and current density-time curves were correlated with SEM images of the surfaces and cross-sections in order to know the effect of anodizing parameters on the thickness and morphology of the coatings. Finally it was possible to conclude that coatings obtained by addition of sodium fluoride were compact and for hexamethylenetetramine and mannitol the coatings consisted of interconnected pores. Additionally it was observed that anodization in electrolytes containing NaF induced the formation of unreported multi-layered films whilst for the organic additives the anodic coatings had the typical bilayer morphology, an internal barrier layer beneath an outer porous layer. •Current, time, conductivity and composition of the electrolyte are crucial in the design of Mg coatings by anodization.•The morphology of the surfaces obtained by PEO are highly influenced by the composition of the used electrolyte.•Conductivity of the solution has a direct influence on the ion transport in the electrolytic solution.•Fluoride, hexamethylenetetramine and mannitol were used as additives to the electrolyte solution for the anodization of Mg.•Uniformity and thickness of the coatings depends directly of the charge applied and the mode operation.