Magnesium biodegradable alloys are a promising material for self-dissolving surgical implants. Magnesium is known to be sensitive to electrochemical corrosion due to the galvanic effect between the matrix and particles of secondary phases and inclusions. Another important factor is the pH level. The behavior of certain chemical reactions depends on the pH level, so one can assume that the pH level of a corrosive medium at the material surface is a factor determining what chemical reactions can occur there. Finally, there is evidence that variability of the crystallographic orientation of the grains may be a cause of anisotropy of corrosion properties. The purpose of this work is to reveal the influence of the electrode potential of the microstructural elements, the crystallographic orientation of the grains, and the pH level of the near-surface volume of the corrosion solution on the corrosion process. In the study, sections of 2×1.5 mm were marked on the ZX10 alloy samples, for which maps of the distribution of crystallographic orientations and chemical composition were drawn. To assess the influence of the electrode potential of the particles, the authors carried out a Kelvin probe mapping in the 90×90 µm area. Next, corrosion tests were carried out with video filming of the surface on the marked area. To determine the pH level influence, the solution circulation in the cell was varied. Upon completion of the tests, corrosion products and corrosion damage were examined in detail. According to the results, the pH level in the liquid near-surface micro-volumes has a greater influence than the electrode potential of the particles as it provokes the formation of corrosion products of a different composition, which leads to passivation of the surface areas around the particles. The authors identified two different types of filiform corrosion. For filiform corrosion, a correlation between the corrosion direction and the crystallographic orientation of the grains was established.

Biodegradable magnesium alloys are one of the most promising materials for osteosynthesis surgical implants due to the combination of unique properties: high strength, low weight, Young’s modulus close to the bone’s one, and low cytotoxicity. The most important performance characteristic is the corrosion rate, which determines the lifetime of an implant. At the moment, the main efforts of the researchers are aimed at finding a material with optimal corrosion properties ensuring the preservation of the operational properties of an implant during the bone healing period. Most of the works on this issue cover the study of the influence of the alloy chemical composition. At the same time, it is widely known that the structure of a material can also have a great effect on corrosion, for example, grain refinement can even change its type. Besides, it is important that the materials with the same quantitative parameters of corrosion can be substantially different in terms of the corrosion process staging. The authors studied the WZ31 and ZX10 magnesium alloys in two states: as-cast (coarse-grained) and after multi-axial isothermal forging and pressing (fine-grained), using the up-to-date in-situ methods that allow monitoring the dynamics of changes in the corrosion rate, as well as the staging of the corrosion damage development on the sample surface. Such methods are the corrosion rate measuring by hydrogen evolution and the sample’s surface video-monitoring during the corrosion attack. The authors carried out tests within the conditions similar to the human body conditions, such as temperature, the corrosion environment composition, and pH level. The obtained results show that the type of corrosion of the WZ31 alloy changes with the decrease in the grain size from a relatively uniform to a highly localized corrosion. In contrast, the ZX10 alloy showed a decrease in the corrosion rate with the decreasing grain size, but the corrosion type did not change.

The preliminary exposure of Mg alloys to corrosion solutions can cause their embrittlement. The phenomenon is referred to as pre-exposure stress corrosion cracking (PESCC). It has been reported that relatively long storage in air after pre-exposure to the corrosion solution is capable of eliminating PESCC. This effect was attributed to the egress of diffusible hydrogen that accumulated in the metal during pre-exposure. However, recent findings challenged this viewpoint and suggested that the corrosion solution retained within the side surface layer of corrosion products could be responsible for PESCC. The present study is aimed at the clarification of the role of hydrogen and the corrosion solution sealed within the corrosion products in the “healing” effect caused by post-exposure storage in air. Using the slow strain rate tensile (SSRT) testing in air and detailed fractographic analysis of the ZK60 specimens subjected to the liquid corrosion followed by storage in air, we found that PESCC was gradually reduced and finally suppressed with the increasing time and temperature of air storage. The complete elimination of PESCC accompanied by recovery of elongation to failure from 20% to 38% was achieved after 24 h of air storage at 150–200 °C. It is established that the characteristic PESCC zone on the fracture surface is composed of two regions, of which the first is always covered by the crust of corrosion products, whereas the second one is free of corrosion products and is characterised by quasi-brittle morphology. It is argued that the corrosion solution and hydrogen stored within the corrosion product layer are responsible for the formation of these two zones, respectively.

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.

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-resorbable implants made of magnesium alloys, unlike the traditional implants made of titanium alloys and stainless steels, have the ability to completely dissolve in the human body, which makes it possible to eliminate the need for a recurrent operation to extract them. The issue of the possibility of using magnesium implants in the combination with products made of titanium alloys remains insufficiently studied at the moment. At the same time, it is widely known that the elements such as titanium and iron, with a potential more positive than magnesium, have a disastrous influence on the corrosion of magnesium alloys, since magnesium dissolves much faster due to the galvanic effect. This work is aimed to determine how the distance to a titanium implant affects the corrosion rate of a ZX10 magnesium alloy sample with an ultra-fine grain structure. As it is an issue of medical application, the authors carried out the corrosion tests within the conditions simulating the human body conditions: the corrosion medium circulation and keeping temperature within 37±1 °C. The authors used physiological solution as a corrosion medium. During corrosion testing, a titanium implant was placed in three, six, and twelve centimeters from the magnesium alloy sample; and the control tests were also carried out without a titanium implant. According to the obtained data, at a distance of 3 cm, the galvanic effect between titanium and magnesium manifests itself strongly, increasing the corrosion rate and the size of corrosion damage, but at a distance of 6 cm, the titanium implant does not have a visible effect on the corrosion of a sample.