Enhancement of corrosion resistance of magnesium by alloying, fluoride treatment and nano-hydroxyapatite coating for biomedical applications

Abstract

Compared to the traditional metallic implant materials such as stainless steels, titanium alloys and cobalt chromium alloys, magnesium (Mg) has received great attention as biodegradable medical implants as it does not require second surgical procedure for removal. Mg and its alloys also possess suitable mechanical properties for orthopaedic and cardiovascular applications. However, clinical applications of Mg have been limited due to its relatively poor corrosion resistance, rapid degradation rate and hydrogen gas evolution in human body fluid. This research is aimed at decreasing the Mg degradation and corrosion rate by alloying with calcium (Ca) and zinc (Zn), surface treatment by hydrofluoric acid and coating with nanosized hydroxyapatite (HA) and brushite (DCPD) using electrodeposition method. The first stage of the research is to enhance the corrosion resistance of pure Mg by the addition of Ca (0.5 to 10 wt.%). In the second stage, Zn at different percentages (0.5 to 9 wt.%) was added to the binary Mg-Ca alloy to further enhance the corrosion properties. Both strategies were found to enhance the corrosion resistance of the alloy, however, the effect was not significant. To further enhance the corrosion resistance fluoride treatment by using different concentrations of hydrofluoric acid (35 and 40%) for the duration of 6 to 24 hrs were employed on binary Mg-Ca and ternary Mg-Ca-Zn alloys. Finally, nano-HA and Brushite were coated on the fluoride-treated specimens via electrochemical deposition (ED) method at different voltages (0.15 to 0.8 mA/cm2) and deposition times (10 to 60 min). Microstructural evolutions were characterized by XRD, AFM, FTIR, SEM, and TEM. Corrosion resistance was examined by potentiodynamic polarization and immersion test in Kokubo solution at room temperature. The results revealed that the grain size and dendrite cell size decreased with the addition of Ca and Zn contents into the binary and ternary alloys respectively. The addition of 0.5 wt.% Ca content was found to produce the lowest dissolution rate and the highest corrosion resistance. However, further addition of Ca led to an increased dissolution rate and pH value. The corrosion resistance of Mg-0.5Ca alloy was enhanced with the addition of up to 1 wt.% Zn, but further addition produced the reverse effect. Mg-0.5Ca-lZn alloy, which has ll-Mg+Ca2Mg6Zn3+M~Ca phases showed lower corrosion rate than those alloys with Zn/Ca atomic ratio higher than 1.23. After fluoride treatment the degradation rates of the alloys were significantly reduced compared to the untreated alloys. Electrochemical tests showed a significant decline in corrosion current density from 365.2 to 5.23 !!A1cm2 on Mg-0.5Ca-lZn alloys coated with composite nano-HAlMgF2. The application of composite coating of nano-Ha/Mgfe, on Mg-CaZn alloys could be used to reduce the corrosion rates ofMg alloys for biodegradable medical applications.