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% HF, 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 μm. Atomic-force microscopy (AFM) recorded an average surface roughness of ∼ 21 nm for the coated sample, similar to that of the untreated one (∼ 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 photoelectron spectroscopy (XPS). The MgF2was 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 MgF2structure, or that a small amount of Mg(OH)2was 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 kΩ cm2for the untreated Mg and 5.2 kΩ cm2for the coated sample, giving an improvement of about 30 times. Polarization tests also recorded a reduction in corrosion current density from 400 μA/cm2to 10 μA/cm2, showing an improvement of about 40 times. The galvanic effect between untreated and fluoride-coated Mg samples was small. Immersion 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.