CONVERSION COATINGS ON PURE MAGNESIUM. CHARACTERIZATION AND CORROSION EVALUATION IN VITRO

LÜHNING F; CERE S.; TANO DE LA HOZ M.F.; GOMEZ SANCHEZ A.

Mar del Plata

Congreso; XII Latin-American Congress of Artificial Organs and Biomaterials; 2023

INTEMA

Introduction and objective: The main limitations for the application of Mg and its alloys as biomedical devices are their high corrosion rate and intense hydrogen evolution in biological media. Conversion coatings are being studied as surface modification route with the aim of controlling magnesium degradation rate. In this work are described the results of a series of conversion surface films obtained on pure magnesium and their characterization. In this initial stage, coated samples were immersed in simulate body fluid solution (SBF) for prolonged periods in order to characterize the degradation process, while electrochemical tests were conducted in the same media.Methodology: Pure magnesium (99.99 Cordés, Argentina) discs were used after polishing with #400 and #600 abrasive paper. Conversion solution was 40wt% HF. Immersion at room temperature for 5, 25, 50 and 100 hours was evaluated. The samples were studied with scanning electron microscopy, Raman spectroscopy and X-Ray diffraction spectroscopy (DRX). Samples were kept in immersion in simulated biological fluid (SBF) at 37°C for 1, 4 and 6 weeks and then observed the type and extent of degradation and surface damage. Electrochemical tests (corrosion potential, anodic polarization curves and electrochemical impedance spectroscopy) were performed in SBF.Results and discussion: After immersion of pure Mg specimens in 40wt% HF for 5 hours, an uniform opaque black coating was observed. 25 hours of immersion lead to a more intense and homogeneous black coating, as observed with optical microscopy. After 50 and 100 hours, dark grey surface color coating was observed, with distinctive tones revealing solidification characteristic patterns. No flaws or cracks were detected on the surface coatings with optical microscopy. With SEM microscopy, it resulted evident that all the conversion films covered the surface with an homogeneous layer. The elemental analysis by EDS showed a significant presence of F on the modified surface, and also O in less proportion. The analysis of detailed XPS spectra of Mg, F and O allowed the identification of MgF2 and MgO as components of the conversion layer. However, no evidence of crystalline MgF2 nor MgO was detected by normal incidence XRD and Raman spectroscopy, then supporting the amorphous structure of the conversion layer.After 7 days of immersion in SBF at 37°C, all the magnesium specimens present white precipitates and evidence of corrosion damage. However, specimens with conversion treatments evidenced less deterioration than non- treated magnesium. The same trend was observed after 14, 28 and 45 days of immersion. A slight increase in corrosion potential for conversion treatment longer than 24 hours was observed, while longer immersion periods in HF presented lower corrosion potentials than non-treated samples. Additionally, all the magnesium samples evidenced active corrosion during anodic polarization, with lower corrosion densities for 24 hours of conversion, and higher for the coatings obtained at longer periods, compared to non-treated magnesium.Conclusions: In this work the effect of immersion time in conversion treatments on Mg was studied. Homogeneous films were obtained. 24 hours of immersion in concentrated HF presented promissory results in corrosion tests. MgF2 and MgO were identified by XPS