Corrosion and Corrosion-Assisted Cracking of a Magnesium Alloy under Appropriate Mechano-Chemical Conditions for Temporary Bioimplant Applications

In the recent years, magnesium (Mg) and its alloys have attracted great research interest as temporary implants devices such as screws, wires, plates and stents, as they possess one of the best biocompatibilities, and their degradation products are not at all harmful to human physiology. In addition, Mg has the required mechanical strength as well as its density and elastic modulus are respectively close to those of natural bone which result in alleviation of the stress shielding under load bearing conditions. However, despite their highly attractive properties, Mg alloys have rarely been used as body implant because of their high corrosion rates in the physiological environment, which may result in a loss of mechanical integrity before an implant could accomplish its purpose. Mg alloys can also suffer sudden cracking or fracture under the simultaneous action of tensile or cyclic loading and the corrosive physiological environment, i.e., stress corrosion cracking (SCC) and corrosion fatigue (CF). Therefore, it is imperative to develop a comprehensive understanding of corrosion and corrosion-assisted cracking of biodegradable Mg alloys under simulated physiological conditions before they could be used in actual service. Accordingly, this PhD thesis has attempted to evaluate corrosion and corrosion-assisted cracking resistance of one of the most common Mg alloys (AZ91D) under appropriate mechano-chemical conditions that appropriately simulate the actual human body conditions. For assessment of corrosion resistance, electrochemical impedance spectroscopy (EIS) and potentiodynamic polarization tests were conducted in Hanks’ balanced salt solution (HBSS) in the absence and presence of bovine serum albumin (BSA) at pH 7.4. Both EIS and potentiodynamic polarization results indicated that the corrosion rate of the alloy increased in the presence of BSA. SCC of the alloy was evaluated using slow strain rate testing (SSRT) at strain rate of 3.1 × 10-7. CF behaviour of the alloy was also investigated using three-point bending cyclic testing at frequency of 1 Hz for 1 million cycles, i.e., a closely simulated in-vivo conditions for body implants. A drastic reduction in mechanical integrity of the alloy was observed under tensile and cyclic loading when the alloy was tested in HBSS with and without BSA. Both the mechanical data and fractographic evidence confirmed the susceptibility of the alloy to SCC and CF.