Enhanced Bio-Corrosion Resistance of Sn-Containing ZK60 Magnesium Alloy for Biodegradable Implants
Authors: Zakiyuddin Ahmad , Kwangmin Lee 
Published: February 2020

Magnesium (Mg) alloys have unique biodegradable property with low density, non-toxicity, and mechanical properties that are similar to human bones attracting many biomedical applications from bone to cardiovascular implants. The combination of mechanical strength and degradability of Mg makes it suitable for bone fixation. Commercial ZK60 (ZK60) alloy has been widely used for structural application due to its relatively high yield strength, excellent damping capacity and high mechanical stability. The excellent physical properties of ZK60 makes it suitable for load bearing in biomedical applications. Zinc known to be an essential element in the human body and plays an important role in bone formation. Although its function is not yet clear, zirconium also exists as a trace element in the human body. Many researchers demonstrated the feasibility of ZK60 as a good candidate for biodegradable implants. However, the rapid degradation rate in the early stage lower the mechanical properties of the alloy. Addition of alloying elements known to be an effective method to decrease the degradation rate of Mg-based alloys. Many of the alloying elements could promote a solid corrosion product that acts as passive layer on the surface of Mg-based alloys. One of the promising alloying element which also biocompatible is Sn. Many researchers demonstrated the Sn-induced passivated layer formed on the surface of Mg-based alloys. Sn addition could also control the anodic reaction of various Mg-based alloys and inhibit hydrogen gas evolution. In this research, corrosion behavior of as-cast ZK60 and ZK60-xSn (x = 1, 3, 5) alloys were investigated, and the passivated surface of the both alloys were examined. The corrosion of the ZK60-based alloys in Tas simulated physiological media was investigated through a potentiodynamic polarization test, electrochemical impedance spectroscopy (EIS) and a hydrogen evolution measurement test for biomedical applications. Potentiodynamic polarization tests confirmed that anodic current density was slightly suppressed in ZK60-1Sn alloy. The corrosion current density gradually decreased as the amount of Sn addition increases. Time-based monitoring on potentiodynamic polarization tests revealed that the passive layer slowly formed on the surface of ZK60 alloy. While on ZK60-5Sn alloy, the passivated layer identified at 15 minutes following the immersion of the alloy. The EIS spectra of ZK60 and ZK60-xSn alloys shows a capacitive loop in the high frequency region, a capacitive loop in the medium frequency region and an inductive loop in the low frequency region, suggesting the contribution of three time constants. The change in Rct and CPE1 measured as a function of immersion time reveals that ZK60-5Sn alloy offers a higher corrosion resistance than ZK60 alloy. Immersion test in a Tas-SBF solution revealed that the corrosion rate of the ZK60 alloy was significantly faster than those of ZK60-5Sn alloy. Hydrogen gas generated from the test demonstrates a clear graph where the addition of Sn to the alloy was significant in the formation of a strong and solid passivated layer on the surface of the alloy. A properly distributed MgSn2 intermetallic phase was observed along the grain boundaries. ZK60-5Sn alloy also displays a finer grain size (30-60 μm) to those of ZK60 alloy (80-120 μm). The combination of properly distributed MgSn2 intermetallic phase and finer grain size in ZK60-5Sn alloy plays important role to the formation of a stronger passivated layer. Further examination by XPS analysis shows a strong sign of Sn contribution in the formation of the passivated layer with a thickness up to 1000 nm. The results of this study suggest that the addition of Sn into ZK60 alloy significantly improved its corrosion resistance.