Investigating the corrosion behaviour and pressureless sintering of FeM n-alloys for biodegradable implant applications

Abstract

FeMn alloys (Mn wt.% 30-35), have emerged as a particularly attractive option as a solution for the limitations presented by pure Fe as a biodegradable metal for temporary implants. Such alloys are antiferromagnetic and have shown in early publications that they could offer adequate mechanical properties and biocompatibility for orthopaedic applications. Another favoured approach involves alloying FeMn with noble elements like Ag, in order to create micro-galvanic couples, which in turn enhance corrosion. Despite the promise shown by these alloys in several publications, the understanding of their degradation mechanism remains limited, with multiple publications sharing similar materials and methodologies, reaching contradictory conclusions. This generally results either from short-span testing providing an incomplete picture of material behaviour, or limited consideration of the impact of testing parameters on the test outcomes. In this work, the first of two major sections were aimed at the study of powderprocessed Fe35Mn, and in select tests also (Fe35Mn)5Ag, using a variety of techniques to determine their behaviour over the first 24 h of testing. Potentiodynamic testing (PDP), electrochemical impedance spectroscopy (EIS) and in situ pH and dissolved oxygen (DO) micro-probe measurements at the sample surface, were some of the techniques used to gather this information. Tests were carried out in HBSS, HBSS containing Ca2+ (HBSS+Ca) and the latter with added bovine serum albumin protein (HBSS+BSA). Static immersion tests of Fe35Mn in the same electrolytes were carried out to provide information regarding the alloy’s long-term degradation behaviour whereas in vivo testing in GAERS rats for 6 months was aimed at demonstrating how considerable the gap between in vitro and in vivo findings, really is. All results pointed towards increased corrosion for both FeMn and FeMnAg when compared to Fe. Outcomes across all testing phases highlighted the impact of Ca2+ ions in the HBSS on the degradation of Fe-based alloys as the same ions interact with phosphates in the solution and metal ions originating from the sample to create Ca/P-rich precipitates that act as a partially-protective barrier to further degradation. Although the same precipitates were unstable on the microgalvanically corroding FeMnAg surface, the charge transfer resistance for this material after 24 h was similar to that of FeMn. This and other supporting results indicated that noble-phase additions, while effective in accelerating corrosion over the short term, might not hold much promise for long-term effectiveness in the body. The same applies to the impact of MnO-inclusions, typically present in powder-processed FeMn samples, on corrosion. Whereas EIS measurements indicated that MnO inclusions could be behaving as micro-cathodes within the austenitic matrix, the behaviour of samples with and without MnO was indistinguishable after the first 24 h. When investigating the influence of BSA over the degradation of FeMn, results indicated that protein reduces the corrosion resistance in vitro. This was likely due to the tendency of BSA to chelate Ca2+ ions, which prevented or delayed the precipitation of Ca/P-precipitates and encouraged localised corrosion as opposed to the uniform corrosion observed in HBSS+Ca. Despite these findings, analysis of FeMn and FeMnAg pin surfaces tested in rat vertebrae for 6 months showed only signs of very limited uniform corrosion, even though FeMnAg accumulated a slightly thicker layer of corrosion products. Moreover, whereas Ca/P-precipitates and metal hydroxides were present on sample surfaces tested in vivo, the major product detected via XRD was CaCO3; a product absent in all in vitro analyses. This highlights the need for further efforts to bridge the gap between in vitro and in vivo testing, as is the aim of international testing standards in the pipeline. Apart from corrosion studies, FeMn is also the centre of development of multiple novel processing methods that allow this material to be processed into implants with the desired shapes and sizes. With most of these methods including the use of powder metallurgy, issues related to the high vapour pressure and high temperature reactivity of Mn need to be addressed. In fact, in the second part of the thesis, blended elemental (B35; blended Fe and Mn elemental powders), milled (M35; Fe and Mn intricately mixed but still present in their elemental form) and alloyed (A35; fully alloyed Fe35Mn) powders, were prepared using high energy ball-milling. Analysis was carried out through microscopy of powders and pressed-and-sintered coupons, carbon analysis and thermal analysis techniques including thermogravimetry with mass-spectroscopy (TG-MS) and differential thermal analysis (DTA). Results indicated that the alloyed powder exhibited a lower tendency to oxidise compared to M35 and B35 powder. Carbon analysis also showed that an increase in ball milling time resulted in an increase in C content in the pre-processed powders due to enhanced diffusion of the organic process control agent over time. This led to the alloyed powders additionally having better carbothermal reduction capacity, allowing for further reduction of oxides as observed both in thermal analyses of powders as well as cross-sectional analysis of pressed-and-sintered samples. However, the high percentage of carbon content also led to precipitation of a higher amount of interconnected M3C carbides at grain boundaries. The same materials were then used in the preparation of cubic scaffolds using a modified replication method wherein the polyurethane foam used as a template in the traditional replication method, was replaced with a 3D-printed customised acrylate-based template, allowing for more control on the structure of the implant. The process was conducted in either of two configurations; exposed, where the sample was exposed to the sintering N2-5H2 atmosphere, and shielded, wherein the sample was covered by a stainless steel shield creating a micro-atmosphere around the sample surface. Results showed that the microstructures of the resulting scaffolds were less affected by the material used, be it M35 or A35, and more impacted by the processing configuration used. In general, samples contained an austenitic matrix from which in excess of 7 wt.% Mn was lost to sublimation and formation of mixed metal oxides and interconnected carbides. All these phases led to rather brittle structures. Shielded structures had superior microstructural homogeneity as a result of the micro-atmosphere formed whereas exposed structures had cores rich in carbides and edge struts richer in oxides. Carbothermal reduction assisted oxide removal from the surface, but kinetic limitations persisted leading to strut cores littered with oxides. This work highlighted the challenges in achieving the right balance to preserve the antiferromagnetic characteristic of these FeMn alloys, reduce oxidation and have effective carbothermal reduction.