Investigation of the corrosion behaviour of bare and polypyrrole coated WE43 magnesium alloy for the development of biodegradable implants
The focus of this PhD project was to investigate the short term corrosion behaviour of bare and polypyrrole coated WE43 Mg alloy at in vitro experimental conditions closer resembling those experienced in the physiological environment. This was done by performing 5 day immersion experiments in a modified simulated body fluid (m SBF) with an ionic composition similar to that of blood plasma, at the physiological pH and average body temperature. Interfacial processes such as charge transfer, diffusion and adsorption of species were investigated by electrochemical impedance spectroscopy (EIS) The corrosion kinetics was investigated by hydrogen evolution, inductively coupled plasma optical emission spectroscopy (ICP OES) and EIS. The polypyrrole coating and corrosion products layer were characterized by scanning electron microscopy (SEM) energy dispersive spectroscopy (EDS) X ray diffraction (XRD) attenuated total reflectance Fourier transform infrared spectroscopy (ATR FTIR) and X ray photoelectron spectroscopy (XPS) The corrosion kinetics and mechanisms of WE43 Mg alloy in m SBF and their time dependence were investigated. An initial decrease in the corrosion rate was related to formation of a partially protective corrosion layer, while an increase in the corrosion rate observed after 48 h was related to onset of a localized corrosion process. Four corrosion stages were proposed involving: (i) the formation of a partially protective corrosion layer, (ii) the adsorption of Mg intermediates, (iii) the formation of an inner protective MgO layer with increasing coverage over time, (iv) the rupture of the corrosion layer, and (v) lateral growth of stable pits. Characterization of the corrosion products layer revealed the presence of an amorphous carbonated apatite/Mg(OH)2 mixed corrosion layer.The effect of daily electrolyte renewal, as a method to better simulate physiological homeostasis, on the corrosion behaviour of WE43 Mg alloy in m SBF was investigated. Electrolyte renewal was found to affect the corrosion mechanisms by promoting partial dissolution of the corrosion layer and by increasing mass transport through the corrosion layer, thereby delaying the increase in the MgO barrier film coverage and the occurrence of localized corrosion. No significant variation in the corrosion layer morphology and composition with respect to the results obtained without electrolyte renewal was observed. The corrosion behaviour of polypyrrole coated WE43 Mg alloy in m SBF with daily electrolyte renewal was also investigated. Polypyrrole (PPy) was found to effectively contribute to a decrease in both the corrosion rate of WE43 Mg alloy and in the production of hydrogen gas during the time period investigated. A temporary increase in the corrosion resistance after electrolyte renewal was related to the increase in the concentration of dissolved CO2 in the m SBF. The corrosion protection of PPy was attributed to: (i) the PPy coating barrier properties, and (ii) the adsorption of dissolved CO2 species on the PPy coating, which leads.