Effect of Temperature on the Corrosion Behavior of Biodegradable AZ31B Magnesium Alloy in Ringer’s Physiological Solution

Quasi-"In Situ" Analysis of the Reactive Liquid-Solid Interface during Magnesium Corrosion Using Cryo-Atom Probe Tomography

The early stages of corrosion occurring at liquid-solid interfaces control the evolution of the material's degradation process, yet due to their transient state, their analysis remains a formidable challenge. Here corrosion tests are performed on a MgCa alloy, a candidate material for biodegradable implants using pure water as a model system. The corrosion reaction is suspended by plunge freezing into liquid nitrogen. The evolution of the early-stage corrosion process on the nanoscale by correlating cryo-atom probe tomography (APT) with transmission-electron microscopy (TEM) and spectroscopy, is studied. The outward growth of Mg hydroxide Mg(OH)2 and the inward growth of an intermediate corrosion layer consisting of hydrloxides of different compositions, mostly monohydroxide Mg(OH) instead of the expected MgO layer, are observed. In addition, Ca partitions to these newly formed hydroxides and oxides. Density-functional theory calculations suggest a domain of stability for this previously experimental unreported Mg(OH) phase. This new approach and these new findings advance the understanding of the early stages of magnesium corrosion, and in general reactions and processes at liquid-solid interfaces, which can further facilitate the development of corrosion-resistant materials or better control of the biodegradation rate of future implants.

Experimental Apparent Stern–Geary Coefficients for AZ31B Mg Alloy in Physiological Body Fluids for Accurate Corrosion Rate Determination

The corrosion behavior of AZ31B Mg alloy exposed to Ringer’s, phosphate-buffered saline (PBS), Hank’s, and simulated body fluid (SBF) solutions for 4 days was investigated using electrochemical impedance spectroscopy (EIS), potentiodynamic polarization, weight loss, and surface characterization. Changes in corrosion rates with immersion time determined by weight loss measurements were compared with EIS data to determine the possibility of obtaining quantitative electrochemical information. In addition, changes in the protective properties of the corrosion product layer calculated from the EIS parameters were evaluated as a function of their surface chemical composition as determined by X-ray photoelectron spectroscopy (XPS) and visual observations of the corroded specimen’s surface. Apparent Stern–Geary coefficients for the AZ31B Mg alloy in each test solution were calculated using the relationship between icorr from weight loss measurements and the EIS data (both Rp and Rt). This provided experimental reference B′ values that may be used as a useful tool in independent investigations to improve the accuracy of corrosion rates of AZ31B Mg alloy in simulated body solutions.

Characterization of an Extruded Mg-Dy-Nd Alloy during Stress Corrosion with C-Ring Tests

This study focuses on the characterization of the failure behavior of an extruded Mg10Dy1Nd alloy during stress corrosion. The microstructure, hardness, strength and corrosion behavior of binary alloys Mg10Dy and Mg1Nd are compared to those of the ternary alloy system. The ternary alloy Mg-Dy-Nd that is not fully recrystallized has the highest hardness but lacks ductility. The investigated alloys twin during plastic deformation. Static C-ring tests in Ringer solution were used to evaluate the stress corrosion properties, and stress corrosion could not be found. None of the alloys failed by fracturing, but corrosion pits formed to various extents. These corrosion pits were elliptical in shape and located below the surface. Some of the pits reduced the remaining wall thickness significantly, but the stress increased by the notch effect did not lead to crack initiation. Furthermore, the C-ring specimen was subjected to compressive loading until fracture. Whereas the Mg1Nd alloy showed ductile behavior, the alloys containing Dy fractured on the tensile side. The crack initiation and growth were mainly influenced by the twin boundaries. The Mg10Dy1Nd alloy had an inhomogeneous microstructure and low ductility, which resulted in a lower fracture toughness than that of the Mg10Dy alloy. There were features that indicate hydrogen-assisted fracture. Although adding Nd decreased the fracture toughness, it reduced the grain size and had a positive influence on the corrosion rate during immersion testing.