The corrosion of magnesium : the influence of inorganic ions and selected organic compounds
Magnesium (Mg) and its alloys have been extensively investigated for engineering, energy storage, and biomedical applications owing to their suitable properties. However, the intrinsic high corrosion rate of Mg alloys restricts their general applicability. In the recent years, a number of new-type alloys, surface modification methods, processing routes, and corrosion inhibitors have been developed in order to obtain Mg materials with suitable anticorrosion properties that meet all requirements for the targeted application. For this sake, corrosion tests are frequently employed to evaluate the corrosion resistance and degradation behavior of the investigated materials prior to real application. However, this is complicated by the fact that there are no generally accepted protocols for Mg corrosion tests, especially for testing Mg as implant biomaterials. Until now, a variety of media has been used in these tests and the applicability of these media has not been fully described until now impeding the comparability of results from different research groups. In this thesis, the influence of media components, (e.g. inorganic ions, synthetic pH buffer and selected organic compounds) on the degradation of Mg is systematically studied to provide more insight into the influence of the media composition on Mg corrosion tests. The results show that the synthetic pH buffer (Tris/HCl) accelerates Mg corrosion in simulated body fluid (SBF). Its acceleration effect can be attributed to three factors: the consumption of OH- (caused by the buffering effect), which is needed for the formation of corrosion products, the complexation ability of Tris for Ca2+ and Mg2+, and the increased concentration of Cl-. Moreover, in SBF without synthetic pH buffer, the synergy between Ca2+, carbonates and phosphates plays a significant role in the corrosion protection of Mg. In Tris-free electrolytes, the corrosion rate of Mg in SBF (with Ca2+, HCO3- and HPO42-) is significantly lower than that in the electrolytes missing one of the three ions. The trio of ions leads to the continuous growth of the protective co-precipitation layer above the normal MgO/Mg(OH)2 corrosion products on corroded Mg surface and then slows down the Mg corrosion. In terms of bio-relevant organic compounds, amino acids, vitamins, and saccharides at low concentration have no critical influence on Mg corrosion in NaCl solution and SBF electrolyte. The antibiotics, penicillin and streptomycin, at operating concentration (around 10-4 M), also have no significant influence on Mg corrosion in minimum essential medium (MEM) and SBF. While streptomycin, as a Ca2+ chelating agent, at higher concentration (10-2 M and 10-3 M) increases the corrosion rate of Mg in MEM and SBF, the importance of free Ca2+ concentration on the protection of Mg corrosion in HCO3- and HPO42--containing media is emphasized. In addition, the influence of albumin on Mg corrosion is investigated. The results demonstrated that the influence of albumin on Mg corrosion is susceptible to the albumin sources, test methods, and test conditions. Its influencing mechanism can be summarized in three aspects, adsorption, Ca2+/Mg2+ chelation and pH buffering effects. The results of this work highlight the significant influence of the composition of the used media on Mg corrosion tests. Based on these finding, a general discussion about the advantages, shortcomings, and applicability of the commonly used media is provided and potential medium selection criteria for Mg corrosion tests for various research purposes are suggested. The systematic investigation of the corrosion behavior of Mg in pseudophysiological media presented in this work provides a deeper understanding of the underlying mechanism and lays the foundation for the standardization of the corresponding testing protocols.
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