Electrochemical techniques and electron microscopy were used to investigate the effects of rapid solidification processing (RSP) and aluminum content on the repassivation of binary magnesium-aluminum alloys in aqueous solutions of potassium chromate and sodium chloride (pH 9.1). This electrolyte resembles environments causing pitting corrosion and stress corrosion cracking in Mg-A1 alloys, where chromate ions, applied as a corrosion protection measure, coexist with chloride ions indigenous to sea water and other natural electrolytes. Microcrystalline alloys containing 1 and 9 weight percent (w/o) aluminum were prepared using a melt-spinning process which yields continuous ribbons 15-25 ~m thick. Ribbon microstructures were characterized using optical and transmission electron microscopy. Breakdown of passivity was initiated by a potential pulse or physical scratch, and repassivation was then evaluated using transient current measurements and scanning electron microscopy. The rapidly solidified alloys experienced relatively uniform attack, and repassivated faster and more completely than their as-cast counterparts. In both the as-cast and RSP conditions, increasing the aluminum content improved the repassivation rate in chromate/chloride solution of Mg-A1 alloys containing 1 and 9 w/o A1.
For many applications, implants overtake body function for a certain time. Bioresorbable implants reduce patient burden as they prevent adverse consequences due to remaining implants or operations for removal. Such materials are in clinical use but do not fulfill the requirements of all applications. Iron (Fe) is promising to develop further bioresorbable materials as it offers biocompatibility and good mechanical properties. Alloying, e.g., with manganese (Mn), is necessary to adapt the mechanical behavior and the degradation rate. However, the degradation rate of FeMn is too low. The creation of phases with high electrochemical potential evokes anodic dissolution of the FeMn, increasing the degradation rate. Therefore, silver (Ag), which is insoluble with Fe, has high potential, is biocompatible, and offers antibacterial properties, can be used. Powder-based processes such as laser beam melting (LBM) are favorable to process such immiscible materials. A degradable Ag alloy has to be used to enable the dissolution of Ag phases after the FeMn. This study reports first about the successful processing of FeMn with 5 wt.% of a degradable Ag–calcium–lanthanum (AgCaLa) alloy and enables further targeted adaption due to the gained understanding of the effects influencing the morphology and the chemical composition of the Ag phases.
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