Understanding the Factors Influencing the In Vitro Corrosion Behaviour of Biodegradable Zinc

Abstract

Biodegradable metals (BMs), such as magnesium (Mg), zinc (Zn) and iron (Fe), are envisioned to be the main material that will be used to create the next generation of temporary medical devices. These metals appear to be viable substitutes for permanent metals, such as stainless steel and titanium, in treating temporary medical issues to lower the risk of adverse long-term implant effects. At the core of the application of BMs is a deep understanding of the BMs’ corrosion behaviour in the physiological environment. The purpose of this thesis is to gain a deeper and more comprehensive understanding of the factors affecting the in vitro corrosion behaviour of pure Zn and, to a lesser extent, Fe. A key contribution of this thesis is the examination of the influence of biologically-relevant ions, such as chlorides, carbonates, phosphates, and sulphates, on the corrosion of pure Zn and Fe in simulated physiological fluids. Electrochemical and immersion corrosion techniques coupled with surface analysis tools were employed for this analysis. With respect to corrosion, the most aggressive ions are chlorides, which induce localised corrosion damage on both Fe and Zn. Carbonates and phosphates reduce the corrosive attack of chloride ions on Zn and Fe while inducing a shift to uniform corrosion, though the potency of corrosion-inhibition varies between the two metals. Carbonates induce better corrosion protection on Zn due to the formation of Zn, while phosphates were more effective for Fe. Sulphates reduce the corrosion rate by disrupting the metal’s passive layer generated by carbonate and phosphate ions interaction. The overall corrosion rate of Zn and Fe changed in each electrolyte depending on the composition of the solution and the nature of the corrosion product formed. The influence of electrolyte deaeration (consisting of nitrogen purging) on Zn and Fe degradation in three test solutions, namely, 0.13 M NaCl, Hanks’ balanced salt solution (HBSS) and phosphate buffered saline (PBS) solution, was examined. Deaeration reduced Zn and Fe’s corrosion in the respective electrolyte, inducing an increase in the charge transfer resistance of the metal surface. This is due to the elimination of dissolved oxygen necessary to initiate the oxygen reduction reaction, which is the main cathode reaction driving the corrosion of Zn and Fe in an aqueous environment. Deaeration of the HBSS induced a significant increase in pH, resulting in the passivation of Zn and Fe and the reduction of corrosion rate. Zn in the deaerated HBSS exhibited an atypical polarisation curve consisting of multiple corrosion peaks. This is unwanted as it complicates the proper analysis of the corrosion rate from the polarisation curve. The influence of operating test factors on the measured corrosion rate of Zn from the polarisation test was investigated. Zn’s corrosion rate increased with increasing specimen surface roughness due to the enhanced effective surface area and rise in residual stresses with increasing roughness. The specimen storage time beyond one hour was found to have minimal influence on the corrosion rate of Zn, likely caused by the formation of a stable surface condition during storage. Increasing HBSS storage time increased the corrosion rate of Zn in the solution. The mechanism for the solution's enhanced aggressiveness after aging is still unexplained, though this may be linked to the increase in solution pH at the 30-day aging time or bacterial action. Increasing stirring speed enhances the corrosion rate of Zn in HBSS, particularly above a critical agitation level. Stirring accelerates Zn corrosion by removing corrosion products from the specimen's surface and reducing the associated concentration polarisation effect. High amounts of sodium bicarbonate (NaHCO3) in HBSS enhanced the corrosion rate of Zn. There was an observed delay in passive film formation, causing the surface of Zn to stay active. The mechanism for this delayed passivity still needs to be understood, though it is proposed that the bicarbonate ions were just adsorbed at the surface and did not proceed to form the carbonate film. Using hydrochloric acid (HCl) for pH modification of HBSS enhances the corrosion of Zn, while lactic acid has a less severe influence. Corrosion severity is found to be acid concentration-dependent. The addition of HCl augments the chloride concentration of HBSS and increases the solution’s aggressivity towards Zn. The influence of some test parameters on the measured corrosion rate of Zn from the immersion test was also studied. The corrosion rate of Zn in PBS was significantly higher than that in HBSS, consistent with polarisation experiments. This is likely due to the presence of other ionic components besides phosphates, including carbonates, magnesium (Mg) and calcium (Ca) cations. Zn exhibited a decreasing corrosion rate trend with increased immersion time, consistent with the progressive formation of corrosion-inhibiting surface deposits. The degradation rate of Zn in HBSS increases with decreasing test solution volume (V) at a constant specimen surface area (SA). The influence of SA:V ratio on Zn’s corrosion rate may be associated with the specimen’s access to oxygen during the corrosion test.