Study Of Magnesium Alloy In Physiological Environment

Biodegradable magnesium alloy, a lightweight metal, by virtue of its desirable mechanical properties, non-toxicity, good inherent biocompatibility, osteoconductivity and good degradability, is potentially a desirable degradable orthopaedic implant. As we all known that the main application of magnesium alloy is the industrial material, the biodegradable implant application is a new research direction.The relevant studies of magnesium alloy as degradable implants in physiological environment are fairly smaller compared with the studies of magnesium alloy for industrial applications in typical corrosion solutions such as chloride solutions. Basing on the fact that magnesium alloy is a promising biodegradable material in orthopaedic implant that the paper focuses on the essential corrosion behavior of magnesium alloy in physiological environment and making some references for the further clinic application. The main studied content is as follows:1. The first research point is focused on the essential corrosion behaviors of magnesium alloy in physiological environment. Common simulated body fluid (c-SBF, a widely applied test solution for in vitro corrosion studies of biomedical materials) and aerated artificial saliva (AS) are as the physiological environment and studied in this paper. Neutral 3.5 wt-% NaCl solution, as a typical standard widely-applied corrosive solution, is introduced to as a reference for our measurement techniques for investigating the different corrosion behaviors of magnesium alloy in different environments (physiological environment and industrial environment) and the aggressiveness of c-SBF and AS and making some references for the future clinic application. X-ray diffraction, Fourier transform infrared spectroscopy, optical microscopy and scanning electron microscopy techniques were utilized respectively to characterize the composition and surface morphology of the specimens after immersing in the three different media. Electrochemical test was conducted to evaluate corrosion resistance of magnesium alloy by means of electrochemical impedance spectroscopy (EIS) and polarisation tests in physiological environment and neutral 3.5 wt-% NaCl solution at 36.5±0.5℃. Results indicate that the corrosion resisting property of AZ91D magnesium alloy in AS is better than that in NaCl and c-SBF which is attributed to the lighter aggressiveness of artificial saliva and formation of a more protective and biocompatible surface film consisting of Ca8H2(PO4)6-5H2O. The differences unfolded here mainly result from the ion species and ion concentrations of the three different solutions. Owing to the buffer paired Tris-HCl, c-SBF is more aggressive than NaCl solution, a deeper pitting corrosion was finally observed on the immersed sample while a localised shallow pitting corrosion for AZ91D magnesium alloy in neutral 3.5 wt-% NaCl solution. Based on these investigations, different corrosion morphologies and corrosion performs observed are further discussed for illustrating the corrosion mechanisms. This study makes a reference for magnesium alloy as implants in the future clinic application.2. The second part is focused on the surface modification of magnesium alloy as the essential corrosion behavior of magnesium alloy in physiological environment has been studied in the first part. For better improving the corrosion resistance of AZ91D magnesium alloy in c-SBF, a new biocompatible and biodegradable coating was firstly prepared on AZ91D magnesium alloy surface by the straightforward hydrothermal (HT) strategy via one-step method in the three different and composition-simple alkaline solutions (sodium bicarbonate (SBC), disodium hydrogen phosphate (DSHP) and sodium carbonate (SC)), respectively. For better illustrating the corrosion resistance of the HT-film coated samples, the conventional alkaline treatment (AT) was also studied in the experiment for comparison. The films obtained respectively from the HT and AT methods were all studied by the composition characterization (XRD and FT1R), surface morphology analysis (SEM) and corrosion tests in c-SBF. Results show that the HT is an extremely simple treatment as the finally formed film has a growth-well morphology with a uniformly distributed micrometer structure. It is investigated by the electrochemical tests and long-term immersion tests conducted in c-SBF at 37℃that the corrosion resistance of AZ91D magnesium alloy coated with the HT-film was better than with the AT-film. This paper is focused on the surface modification method studying of AZ91D magnesium alloy as to provide some basal references for the future clinic medical and biotechnological application.