Corrosion behavior of microarc oxidation and polycaprolactone coatings applied to AZ31 magnesium alloy evaluated in simulated body fluid and balanced salt solution

Abstract

Recent research in orthopedic implant materials has focused on the use of magnesium alloys as a base material due to its mechanical properties similar to that of human bone. Rapid corrosion of magnesium materials in aqueous environments poses a significant hurdle to their application as a biomedical implant. A variety of coatings have been shown to improve the corrosion resistance of magnesium based materials in simulated body fluid environments including microarc oxidation and polymer coatings. However, formulation and corrosion rates vary significantly between solution types. Furthermore, in vivo results have shown that many common in vitro solutions over estimate corrosion rates. In addition to variations between solutions needing to be resolved, there has been little work performed to characterize large sample corrosion under stress. This is an essential step in evaluating concept performance at a macro scale, for application as a human implant. The experiments performed and presented in this thesis primarily involve the comparison of conventional simulated body fluid (c-SBF) and Earle's balanced salt solution (EBSS). Samples evaluated in these environments are microarc oxidation (MAO) coated AZ31 magnesium alloy and polycaprolactone dip-coated AZ31. MAO coated samples were created for a range of process settings to observe the effect of processing on corrosion performance. A dependence of MAO coating thickness on process voltage was found which augmented the initial corrosion resistance values observed via electrochemical testing. Both MAO and PCL coatings were found to improve the corrosion resistance of the samples as compared to uncoated AZ31. It was found that all variations (MAO, PCL, and uncoated) showed a reduced corrosion rate in EBSS as compared to c-SBF. This corrosion reduction was apparent through potentiodynamic scanning, electrochemical impedance spectroscopy, and visual inspection. Preliminary mechanical corrosion results, in the form of constant extension testing, showed no dependence of corrosion on stress level. Future work may be aimed towards expanding modes of mechanical testing and further refining simulated body fluids to fit with in vivo test results.