Characterization and corrosion studies of fluoride conversion coating on degradable Mg implants

Fluoride conversion coating was synthesized on magnesium (Mg) by immersion treatment in hydrofluoric acid (HF) at room temperature, with the aim of improving the corrosion resistance of Mg in applications as degradable implant material. After an immersion period of 24 h in 48% HE, the samples carried a bronze color, and the conversion coating was dense and free of cracks. Field-emission scanning-electron microscopy (FE-SEM) of the cross-section revealed a coating thickness of about 1.5 mu m. Atomic-force microscopy (AFM) recorded an average surface roughness of similar to 21 mn for the coated sample, similar to that of the untreated one (similar to 17 nm). The coating was mainly composed of magnesium fluoride (MgF2) as identified by thin-film X-ray diffractometry (TF-XRD), consistent with compositional analysis using X-ray photoclectron spectroscopy (XPS). The MgF2 was in the form of crystallites of a few nm. A small amount of oxygen was present inside the coating, suggesting that some F- ions are replaced by hydroxyl (OH-) ions in the MgF2, structure, or that a small amount of Mg(OH)2 was present. The corrosion resistance of untreated and conversion coated Mg in Hanks' solution was studied using electrochemical impedance spectroscopy (EIS), potentiodynamic polarization tests, and immersion tests. EIS results showed a polarization resistance of 0.18 k Omega cm(2) for the untreated Mg and 5.2 k Omega cm(2) for the coated sample, giving an improvement of about 3 0 times. Polarization tests also recorded a reduction in corrosion current density from 400 mu A/cm(2) to 10 mu A/cm(2), showing an improvement of about 40 times. The galvanic effect between untreated and fluoride-coated Mg samples was small. himiersion tests in Hanks' solution also resulted in a much milder and more uniform corrosion damage on the fluoride-coated samples. The results of the present study showed that fluoride coating by conversion treatment is a simple and promising way of enhancing the corrosion resistance of Mg in Hanks' solution, or that it may be employed as a pretreatment step for subsequent coating. (c) 2007 Elsevier B.V All rights reserved.