Corrosion Behaviour of Biodegradable Mg-1Ca Alloy in Simulated Body Fluid

UoM administered thesis: Phd

  • Authors:
  • Yuxiang Liu

Abstract

The thesis focuses on investigating long-term corrosion behaviour of Mg-1Ca alloy in simulated body fluid (SBF) solution and understanding the biodegradable process of the alloy to survive for 12 weeks for orthopaedic implant applications; on the effect of excimer laser on tailoring corrosion behaviour of the alloy. The thesis contributes to establishment of corrosion mechanism of the alloy by corrosion observation and by the time evolution of electrochemical impedance spectroscopy (EIS) measurements for the as-received alloy (90 days) and for the excimer laser-treated alloy (5 days). Phenomenological study of corrosion behaviour of the as-received alloy and the laser-treated alloy has been observed by scanning electron microscopy (SEM)/energy-dispersive X-ray (EDX), X-ray diffraction (XRD) and Fourier transform infrared spectrometer (FTIR). Corrosion of the as-received alloy proceeded in three phases: in the early phase, corrosion expanded horizontally was influenced by microstructure; in the intermediate and the long-term phases, corrosion proceeded vertically resulting in a two-layer structure (the upper layer of precipitates and the bottom layer of corrosion products (MgO/Mg(OH)2)) whose composition changed with immersion time. The two layers became thicker and denser resulting in decreasing corrosion rate. Excimer laser treatment generated a ~5 μm laser-treated layer with a rippling surface, a surface MgO film and relatively homogeneous microstructure. During immersion, a layer of precipitates formed on the laser-treated layer before its dissolution. With time, the laser-treated layer and the underlying alloy dissolved to form a corrosion product layer which along with the precipitate layer developed a similar two-layer structure contributing to low corrosion rate. Overall, excimer laser treatment can provide protection to the alloy at the early stage of corrosion. Electrochemical behaviour of the alloy before and after excimer laser treatment has been studied by EIS. From the EIS response, corrosion also proceeded in three stages, characterized by differences in the values of resistances and by differences in the inductive behaviour at different measuring times. Specifically, the inductive contribution was more significant before 12 h, and it decreased gradually from 12 to 120 h. The decreasing corrosion rate had been measured by hydrogen collection, weight loss, corrosion depth and resistances extracted from the EIS response. Correlation between the resistances and corrosion rates by the experimental measurement was revealed. Among all resistances of the as-received alloy, only charge transfer resistance presented a good correlation with the corrosion rates over the whole duration of the measurement, and an apparent Stearn-Geary coefficient was calculated (85 mV). No correlation was obtained for the laser-treated alloy, which can be attributed to laser modifications and to limitations of the electrochemical impedance analysis.

Details

Original languageEnglish
Awarding Institution
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Award date31 Dec 2018