Ivana Leticia Lehr

Interest in cerium-based coatings has increased in recent years due to their low toxicity, biocompatibility, and improved corrosion protection performance. The formation of a coating from a solution containing cerium nitrate (Ce(NO₃)₃) has been investigated as a good surface modification strategy for the production of protective coatings on 316L SS. The effect of various additives in the treatment solution (sodium molybdate, sodium salicylate, and ascorbic acid) on the corrosion protection properties of the coatings was evaluated. The protection performance of the coated samples in a physiological simulated fluid (Ringer solution) was examined by electrochemical methods. The composition and morphology of all coatings were analyzed by scanning electron microscopy (SEM) and energy dispersive spectroscopy (EDX).

RESUMEN Debido a su biocompatibilidad, biodegradabilidad y características mecánicas similares a las del hueso humano, las aleaciones de Mg constituyen una alternativa prometedora como materiales para la fabricación de implantes temporales. Sin embargo, su alta velocidad de degradación en ambiente fisiológico es un impedimento para estas aplicaciones. Con la finalidad de incrementar la resistencia a la corrosión de la aleación de Mg AZ91D en ambiente fisiológico simulado, se generaron recubrimientos dobles de cerio y epoxi modificados. Como primer paso se sintetizó químicamente polipirrol (PPy) modificado con nanopartículas de plata a partir de pirrol (Py), AgNO3 y Ce(SO4)2. Se sintetizaron dos tamaños de nanopartículas variando la concentración de AgNO3 empleada para la síntesis de los polvos. Se verificó que los compuestos tienen propiedades bactericidas contra la bacteria Gram negativa Escherichia cole (E. cori). A partir de estos resultados, se formó un recubrimiento doble sobre la aleación y éste fue modificado con los polvos sintetizados. El recubrimiento consistió en una primera película base cerio y una segunda película epoxi, a la cual se le adicionó un 1% p/p de los compuestos bactericidas. Análisis electroquímicos como polarizaciones anódicas, curvas de polarización de Tafo, variación del potencial de circuito abierto (PCA) y espectroscopia de impedancia electroquímica (EIS) verificaron que los recubrimientos modificados fueron capaces de proteger al sustrato contra la corrosión en solución fisiológica simulada. A su vez, la adición del compuesto de PPy y plata confiere propiedades antibacteriales al recubrimiento, las cuales fueron evaluadas mediante la técnica de Kirby-Bauer contra la bacteria E. coli.

In order to improve the corrosion resistance of die-cast AZ91D magnesium alloy in simulated physiological solution, two cerium-based coatings were synthesized. The coatings were electrodeposited from solutions containing Ce(NO₃)₃, H₂O₂ and C₆H₈O₇ or C₄H₄O₆Na₂. The influence of the presence of sodium tartrate and citric acid in the preparation solution on the morphology, composition and anticorrosive performance of the generated coatings was evaluated. The corrosion properties were examined in Ringer solution by polarization studies, open circuit measurements and faradaic impedance spectroscopy. Results showed that the incorporation of additives improves the anticorrosive properties of the films formed without sodium tartrate or citric acid. The coating formed in the presence of sodium tartrate showed the best anticorrosive performance. Subsequently, the possibility of forming duplex coatings employing the cerium-based films as inner layers and polypyrrole as top coating was evaluated. At this point, it was found that polypyrrole can be electropolymerized on top of the formed cerium-based film modified with sodium tartrate.

A duplex coating was formed under potentiostatic conditions on magnesium alloy AZ91D in order to improve its corrosion resistance in a simulated physiological environment. The first layer was formed by anodization at low potentials in molybdate solution. The outer layer was a PPy film electrosynthesized in sodium salicylate solution. The conditions of the formation were determined to obtain a layer with globular morphology. The bilayer was characterized by scanning electron microscopy (SEM) and X-ray diffraction (XRD). The corrosion protection properties of the coatings were examined in Ringer solution by monitoring the open circuit potential (OCP), polarization techniques, and electrochemical impedance spectroscopy (EIS). The obtained results showed that the bilayer improves the corrosion resistance of the substrate. Moreover, the duplex coating presented better anticorrosive properties than a single PPy film. Afterwards, the bilayer was modified by cementation of silver ions from a solution containing AgNO3. The modified electrode exhibited good antibacterial properties.

Die-cast AZ91D magnesium alloy (8.9 wt.% Al, 0.6 wt.% Zn, 0.2 wt.% Mn, and balance Mg), as novel alternative biodegradable material, has received great attention due to their potential use in biomedical implants. However, their poor corrosion resistance in physiological fluids restricts practical applications. Cerium-based coatings have been studied as an environmental friendly option to enhance the corrosion resistance of magnesium alloys. In order to control the biodegradation rate of AZ91D magnesium alloy in simulated physiological solution, the formation of a coating from a solution containing cerium nitrate (Ce(NO3)3) was studied. The effect of different additives in the treatment solution (ascorbic acid, citric acid, and sodium citrate) on the anticorrosive properties of the coatings was evaluated. The characterization of the coatings was done using electrochemical techniques and SEM/EDS, XRD, and XPS analyses. The corrosion properties were examined in Ringer solution by polarization studies, open circuit measurements, and faradaic impedance spectroscopy. Results showed that the incorporation of additives improves the anticorrosive properties of the Ce-based film. The coating modified with ascorbic acid provides the best corrosion resistance. According to XPS results, the film is mainly composed by Mg oxides or hydroxides and Ce oxides.