Dmitry L. Merson

The work investigated the anisotropy of corrosion properties of extruded low-alloy magnesium alloy ZX10 in two orthogonal sections (longitudinal TD and transverse ED). Using the EBSD and SEM methods, it was established that the TD section is characterized by the dominance of grains with a basal orientation and a stringer-like distribution of second-phase particles (relative area ~0.1%) oriented along the extrusion direction. In the ED section, the prismatic-pyramidal texture predominates, the particles are distributed more evenly, and their relative area is smaller than in the longitudinal section (~0.065%). Corrosion tests in 0.9% NaCl at 37°C for 168 h showed that the integral corrosion rates (determined by mass and volume loss) for both sections are similar, despite the different texture. However, detailed quantitative analysis of damage morphology using confocal laser scanning microscopy (CLSM) revealed significant differences in the nature of pitting corrosion. In the TD section, a larger relative area of pits was observed (14% versus 6% in ED), but their average depth was smaller (90 versus 186 µm) and their axial growth rate was two times lower. It has been shown that the initiation and growth of pits correlate with the location of second-phase particles. It is suggested that the slight differences in the integral corrosion rate of the two sections of the extruded rod are likely explained by the opposite influence of two factors: a favorable basal texture but a high density of string-distributed particles in the TD section and, conversely, an unfavorable texture but a uniform distribution of particles in the ED section. The paper demonstrates the effectiveness of CLSM for quantitatively separating the contribution of different types of corrosion.

Bioresorbable magnesium alloys are a promising material for the manufacture of self-dissolving surgical implants. Since the process of biological resorption is essentially corrosion, it is important to have a complete understanding of the influence of various factors on corrosion. The scientific literature contains numerous works devoted to the influence of such factors as the crystallographic orientation of grains, the presence and nature of the distribution of secondary phase particles in the volume of the metal, and their electrode potential on the corrosion properties of magnesium. Unfortunately, the influence of local deformation of the material on its corrosion characteristics has not been studied in sufficient detail. In some of our previous works, the spread of filiform corrosion on the surface of ZX10 alloy was observed along scratches that marked the area under study. This observation led to the hypothesis that locally deformed material might be vulnerable to this type of damage. This work is devoted to confirming this hypothesis. Using a Vickers diamond pyramid, scratches were applied to the surface of extruded ZX10 alloy samples with a force of 5, 15, and 30 N, after which the samples were tested by immersion at a temperature of 37°C in solutions simulating the ionic composition of human blood plasma: Ringer, SBF, and PBS. During the tests, video monitoring of the sample surface was used, as well as recording of the open circuit potential. After testing, the composition of corrosion products and the morphology of corrosion damage were analyzed in a scanning electron microscope column. The hypothesis was fully confirmed: in all three solutions, the initiation and spread of filiform corrosion in the deformed scratch material was recorded; however, no correlation was found between the onset time of filiform corrosion and the scribing force. A study of the filiform corrosion products showed that the mechanism of its propagation is probably the same for all three solutions, but in the case of SBF and PBS solutions, the surface area affected by filiform corrosion is significantly smaller than in Ringer’s solution. This indicates that the presence of phosphate ions in the solution has a significant effect on the formation of filiform corrosion products and the inhibition of its spread.

In this paper, the microstructure and phase composition of magnesium alloys obtained by casting from a furnace charge mixture of the Mg–2.6Y–1Zn–0.5Gd–0.2Zr–0.1Yb system are characterized. It is found that it contains about 10% of the long-period stacking ordered phase. Low-amplitude nonlinear damping and softening of the elastic modulus are studied using the composite piezoelectric resonator method. The time dependence of elasticity and microplasticity is revealed, which is explained by the redistribution of point defects in the elastic fields of dislocations. It has been established that the activation temperature of this mechanism is 227 K, this is confirmed by the presence of the stress relaxation peak on the temperature dependence of damping, as well as the absence of its time dependence at low-temperature deformation of 163 K.

Magnesium and its alloys are promising materials for medical use due to their ability to dissolve safely in the human body. However, the rate of dissolution of bioresorbable implants should be in a narrow enough range. The difficulty in ensuring this condition is that the corrosion process in magnesium alloys is influenced by many factors, including natural (single-crystal) and technological (production scheme) anisotropy. By carrying out technological operations on thermomechanical treatment, it is possible to control the process of formation of the semi-finished product texture and to create artificially a preferred orientation of crystallites in the structure of magnesium alloys and thus control their corrosion resistance. This requires precise knowledge of the relationship between corrosion processes and certain crystallographic directions, which can be most reliably obtained in experiments on single crystals. In this work, mechanical (compression and tension) and corrosion tests were carried out for the first time on the same magnesium single crystal on samples with different crystallographic orientations. The Kearns coefficients calculated from the X-ray diffraction patterns of the single crystal specimen faces by the inverse pole figure method were used as a quantitative criterion of the natural texture. The specimens were subjected to compression tests in the , and directions, and to tension tests in the direction. The specimen surfaces with orientations close to the (0001), (10−10), (2−1−10), and (10−11) crystallographic planes were subjected to corrosion testing. It was found that the Young’s modulus and the Kearns coefficient for the basal and pyramidal faces were 48.6 GPa and 0.81; 45.3 GPa and 0.04, respectively. The shape of the stress curves depended significantly on the sample orientation and was determined by the degree of involvement of various mechanisms in the overall plastic deformation process. The rate of corrosion in a physiological aqueous solution of 0.9 % NaCl on a 72-h basis for the (0001), (10−10), (2−1−10), and (10−11) surfaces was 0.51, 0.76, 0.71 and 0.98 mm/year, respectively. In this case, the (2−1−10) plane experienced only uniform corrosion, the (0001) plane experienced uniform corrosion with minor localised corrosion; the most intense localised corrosion is observed in the (10−10) direction, and the maximum intensity of the combination of localised and uniform corrosion is in the direction.

The microstructural, phase, acoustic, and elastic properties of nine cast magnesium alloys with an LPSO structure (X phase) were studied within a concentration range of Y, Gd, Zn, and Zr that are promising for practical applications, considering their subsequent thermal dispersion strengthening (Y ≤ 7.6, Zn ≤ 2.78, Gd ≤ 4.9, and Zr ≤ 0.68 wt %). A comparison of the experimental data revealed that the propagation rate of longitudinal and transverse waves in the alloys decreases, while the attenuation coefficient increases proportionally to the total weight percentage of alloying elements forming the X phase. Furthermore, the elastic and acoustic properties correlate more significantly with the total weight percentage of alloying elements in the Mg alloy rather than with the atomic parameters of the phase-forming alloying elements (Y/Zn) commonly used in metallurgy. It was shown that the variation in wave propagation rate in Mg alloys with the X phase predominantly correlates with an increase in their density. At a low content of the secondary phase, wave attenuation is determined by grain size, while in the presence of a secondary phase in the form of conglomerates at grain boundaries, it is influenced by the ratio of grain size, secondary phase size, and wavelength. It was found that the parameter of the ratio of secondary phase size to wavelength, introduced by analogy with the conventional acoustic parameter of the ratio of grain size to wavelength, weakly correlates with the acoustic properties of alloys with the X phase. Additionally, there is a discrepancy between the dependences of the attenuation coefficient on grain size and wave frequency. This discrepancy may be due to the unaccounted for influence of the X phase in the form of banded insertions into α-Mg grains, as well as the method of calculating the secondary phase size, which requires further investigation.

Low corrosion resistance of magnesium alloys is a challenging problem that hinders their wide implementation in industry and medicine. In this regard, the study of the mechanisms and patterns of corrosion processes in magnesium and its alloys, including the analysis of the kinetics of these processes, is an urgent task. However, the set of methods available for studying the kinetics of corrosion with sufficient time resolution is very limited. Several studies have been published that demonstrated the high sensitivity of the acoustic emission (AE) method to corrosion processes occurring on the surface of magnesium alloys. Although these studies suggested that AE is associated with the release of hydrogen bubbles accompanying corrosion, no direct relationship has yet been established between the amount of hydrogen released and the AE characteristics. The present study aims at filling this gap. To conduct the study, a special setup with a corrosion cell was developed that allows monitoring changes in the volume of hydrogen released from the corroding surface of the sample, concurrently with recording AE signals and changes in the open-circuit potential (OCP) accompanying the corrosion process. Using this technique, the corrosion of ZK60 alloy in a 0.9% NaCl solution was examined. It was found that intense AE accompanied the corrosion process of this alloy from the beginning to the end of the test. A correlation was found between the AE characteristics, the volume of released hydrogen, and the OCP values at various intervals of the test. In particular, a linear relationship was discovered between the number of AE signals and the volume of hydrogen released during the corrosion process. The sensitivity of the method based on AE registration to the released hydrogen volume is shown to be several orders of magnitude higher than that of the conventional method of collecting hydrogen using a burette.

Self-dissolving medical implants, such as screws for bone fracture fixation or vascular stents, represent a promising application of magnesium alloys. Magnesium-based bioresorbable materials are currently not only the subject of research by scientific groups worldwide but also the raw material for producing commercial products—medical metallic implants that are actively used in patient treatment. Nevertheless, many technological issues remain unresolved. Chloride-containing fluxes are widely used in casting magnesium alloys. It is unclear whether the presence of flux particles in materials for bioresorbable implants poses a risk of corrosion damage to the surface of the product. This study investigates the processes of initiation and development of filiform corrosion caused by the presence of a chloride-containing particle on the metal surface. Energy-dispersive spectroscopy was used to determine the composition of corrosion products, and Kelvin probe atomic force microscopy was employed to measure their electrode potential relative to the magnesium matrix. It was shown that, under ambient temperature of 25°C and 30% humidity, filiform corrosion is initiated near the chloride-containing particle. Despite the shallow depth of damage (2–3 µm), corrosion spreads over a large area and is characterized by a high propagation rate (tens of microns per day). Analysis of the chemical composition of the corrosion products revealed that the process involves reactions leading to formation of hydroxide and its breakdown under the influence of chloride and CO₂. The corrosion products exhibit a positive potential relative to the metal, enabling the activation of anodic dissolution of the matrix. Placing the material in a vacuum completely halts progression of corrosion, which resumes upon exposure to air. This demonstrates the necessity of avoiding the use of chloride-containing fluxes in the production of bioresorbable alloys and storing finished products in a moisture-free environment whenever possible.

Insufficient corrosion resistance of magnesium alloys limits their technical and medical applications. It is known that the corrosion rate of these materials depends, among other things, on the orientation of the samples relative to the workpieces, for example, rods or sheets obtained by thermomechanical processing. There is an opinion in the literature that this is primarily due to the influence of crystallographic texture. In this work, the corrosion resistance of samples cut from extruded rods of industrial alloys ZK60 and AZ31 along and across the extrusion direction was investigated. The corrosion rate was determined by the loss of weight and volume of the samples, as well as by the volume of hydrogen released when they were dissolved in a 0.9% NaCl solution. The microstructure and surface of the samples after corrosion tests were investigated using scanning electron microscopy, energy-dispersive X-ray microanalysis, and electron backscatter diffraction. It is shown that the rod has a pronounced basal texture, and therefore the crystallographic orientation of the grains to the lateral surface of the transverse and longitudinal samples is very different. It was found that samples of both alloys cut across the rod had higher corrosion resistance than longitudinal samples. According to published data, if the anisotropy of corrosion properties were caused by crystallographic texture, then longitudinal samples, on the contrary, should have a lower corrosion rate than transverse ones. Therefore, it is concluded that, in the case of medium- and high-alloy magnesium alloys, the anisotropy of corrosion properties is associated mainly with the nature of the volume distribution of second-phase particles, and not with the crystallographic texture. It is shown that the row arrangement of particles on the surface of longitudinal samples leads to deterioration of corrosion resistance.

Magnesium-based alloys are a modern material for the production of biodegradable (self-dissolving) surgical implants. Magnesium is a metal with the most negative electrode potential of all structural materials: −2.37 V. This means that close arrangement of implants made of magnesium and, for example, titanium alloys will lead to the occurrence of a galvanic effect and accelerated electrochemical corrosion of magnesium. However, it is unknown how the ratio of the areas of titanium and magnesium products affects the magnitude of this effect. This work covers this issue. In the presented study, cylindrical samples of biodegradable ZX10 and WZ31 magnesium alloys were placed in physiological Ringer’s solution at a distance of 3 cm from a sample of medical Ti6Al4V alloy of the same shape and size. During the test, the temperature of the corrosive environment was maintained at 37 °C. The series of experiments included corrosion tests lasting three days with the participation of one, two or four magnesium samples, thus the area ratios between the titanium and magnesium alloy were 1:1, 1:2 and 1:4. It was found that for both magnesium alloys, with an increase in the area ratio, the effect of electrochemical action decreases significantly, which is expressed in a decrease in the corrosion rate. At the same time, for the WZ31 alloy, the effect of the presence of Ti6Al4V on the corrosion rate is significantly weaker than for ZX10, which is explained by the presence of the LPSO phase in the alloy, as well as a more alloyed matrix and, accordingly, having a more positive electrode potential.