| Summary: | 碩士 === 國立臺灣大學 === 材料科學與工程學研究所 === 103 === Magnesium and its alloys have become increasingly important in the biomedical field for the last decade. Comparing to the traditional bioinert implants such as stainless steels, titanium alloys and cobalt-chromium-molybdem alloys, magnesium and its alloys have some advantages including biocompatibility, biodegradability, non-toxicity, signifying them as one of the potential biodegradable implants. However, magnesium and its alloys corrode rapidly in the physiological environment. Surface modification is thus essential to enhance the corrosion resistance of magnesium alloys.
Dicalcium phosphate (DCPD) coating can provide some degrees of protection from corrosion in the simulated body fluid and stimulate the attachment and differentiation of bone cells. The present study employed chemical conversion coating method to fabricate bioactive DCPD coating on pure magnesium. The conversion solution was mainly composed of 0.152 M calcium nitrate and 0.217 M ammonium dihydrogen phosphate. After immersion in the conversion solution for several min., the DCPD-coated magnesium was characterized by SEM/EDS and its corrosion resistance was evaluated by polarization curves, EIS, and hydrogen evolution measurement in simulated body fluid (Hank balanced salt solution, HBSS).
The results show that the DCPD coating formed in the conversion solution free of sodium nitrate at 40℃ is homogeneous and crack-free. The presence of sodium nitrate (0.8 ~ 6.4 mM) in the solution at 40℃ accelerates the nucleation of DCPD. The resulting coating thus has lower porosity and better corrosion resistance than that formed in the absent of sodium nitrate. Moreover, the coating formed in the solution with the addition of 3.2 mM sodium nitrate exhibits the lowest porosity and the smallest corrosion current density. The DCPD-coated magnesium underwent lower amounts of hydrogen evolution than the bare magnesium during the first two-day immersion in HBSS. A marked increase in hydrogen evolution was observed after three days of immersion, suggesting that the DCPD coating had been attacked locally. The amount of hydrogen evolved during the first four-day of immersion was consistent with the electrochemical characterization results. Finally, how the presence of sodium nitrate affects the formation and corrosion resistance of the DCPD coating on magnesium is discussed in detail.
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