Solid solution treatment on strength and corrosion of biodegradable Mg-Ag wiresWednesday (28.09.2016) 10:00 - 10:15 Part of:
Wires out of Magnesium have gained interest in the area of biodegradable metals for biomedical applications. Pins, rods and clips for temporary fixation of bone fractures use wires with diameters in an mm range, to form stents much smaller diameters of a few 100 µm or less are needed. Binary Mg-Ag alloys are designed as antibacterial material to treat infections in an implant site. It has been indicated that Mg-Ag alloys have satisfactory corrosion properties under cell culture condition and much better mechanical properties than pure Mg. In this research a warm-drawn Mg6Ag wire with a diameter of 1.6 mm has been investigated by immersion tests in Ringer solution in two conditions: the initial as-drawn condition showing high strength and the solution heat treated less strong condition. After wire drawing heavily twinned grains have been found. Solution heat treatment causes slight grain growth, but less microgalvanic corrosion due to dissolved second phases and a significantly reduction of twin boundaries. The mean corrosion rate decreases by more than half by the heat treatment resulting in higher remaining mechanical properties, for this study evaluated by bending strength. The corrosion rate also decreases with corrosion time, slightly more for the heat treated condition. Even pitting corrosion is still present after solution heat treatment, the pit shape is less harmful. The corroded wires and corrosion products have been qualitative and quantitative analysed with focus on the exposure time. Analysis included qualitative and quantitative SEM imaging with EDX, XRD, Raman, μ-XRF and IR spectroscopy and ICP-OES.
The Mg6Ag wires of this study corrode under forming Mg(OH)2, CaCO3 and AgCl, where Mg and Ag comes from the wire and Ca from the corrosion media. The corrosion products contain more Mg(OH)2 than CaCO3 and at least AgCl, where Mg and Ag contents increase with immersion time and Ca stays constant. Furthermore REM analysis of the wires and composition analysis of the corrosion media indicate a higher corrosion resistance of heat treated Mg6Ag. A direct comparison of the corrosion media shows lower Mg contents in the solutions of the heat treated wires.