**5. Mechanisms and in vitro degradation behavior of amorphous and crystalline magnesium alloys**

In the case of biomedical engineering, corrosion is the main factor determining the usefulness of implant materials. The tendency of biomaterials to corrode in the human body is, in fact, closely connected to their biocompatibility. Before placing in the human body, the material must be examined for the effects on the body and its properties. Ensuring such experimental conditions is difficult, as it is difficult to recreate the environment of the human tissues. Many parameters related to the production of magnesium-based materials and test parameters have an impact on the degradation results (**Figure 5**).

The alloying elements and the processing parameters of Mg have a strong impact on its degradation properties (microstructure of the material described by the grain size, impurity content, type of phases etc.). Calcium as alloying element to Mg alloy is an extremely reactive metal and spontaneously reacts with water generating hydrogen [55].

#### **Figure 5.**

*Parameters influencing the course of magnesium alloy corrosion process (in vitro) divided into subgroups: Research conditions and material factors [54].*

Moreover, the research methods and conditions can significantly change the corrosion rate, as well as the formation and composition of the degradation layer, and thus determine the corrosion type [56].

Living microorganisms play an important role in the process of implant degradation. Such metabolic activity may directly or indirectly reduce the quality of the implant due to the corrosion process. Cells can act as an electrolyte on the metal surface, thus changing the corrosion resistance of the implant surface or even its composition [6].

Corrosion of magnesium in an aqueous environment occurs as a result of an electrochemical reaction with water, resulting in the formation of magnesium hydroxide, Mg(OH)2 and hydrogen gas, according to reactions (1–3) [54]:

$$\text{Anodic reaction}: \text{Mg} \to \text{Mg}^{2+} + \text{2} \,\text{e} \tag{1}$$

$$\text{Cathodic reaction} :: 2H\_2O + 2e^- \to H\_2 + 2OH^- \tag{2}$$

$$\text{Overall reaction} \left(\text{summarry reaction}\right) \colon \text{Mg} + 2\text{H}\_2\text{O} \to \text{Mg} \left(\text{OH}\right)\_2 + \text{H}\_2 \tag{3}$$

In the initial phase of immersion, the surface of the material is exposed to the electrolyte and the anodic and cathodic reactions begin. Magnesium grains act as an anode and the cathodic reaction takes place in noble regions of alloy, which are grain boundaries, phase separation and precipitation. This leads to the exchange of electrons between the two regions, wherein the magnesium is degraded at the same rate, at which hydrogen is generated as a gas (H2). The cathodic reaction increases the pH by releasing H2 gas, while hydrolysis lowers it [54, 57].

When the concentration of Mg2+ and the increase of pH reach the solubility limit, magnesium hydroxide Mg(OH)2 is precipitated on the surface of alloy Mg [1]. In an environment, such as body fluids, where the concentration of chloride ions is greater than 30 mmol/dm3 , the hydroxide formed on the surface of the magnesium alloy converts to highly soluble magnesium chloride. This reduces the level

*Amorphous and Crystalline Magnesium Alloys for Biomedical Applications DOI: http://dx.doi.org/10.5772/intechopen.94914*

of protection of the surface layer by increasing its activity [58]. The formation of soluble magnesium chloride is described by the reaction (4):

$$\mathrm{Mg} \left( \mathrm{OH} \right)\_2 + 2 \mathrm{Cl}^- \rightarrow \mathrm{MgCl}\_2 + 2 \mathrm{OH}^- \tag{4}$$

This process accelerates the degradation of the material and increases the pH of the environment [6]. The presence of Cl<sup>−</sup> ions initiates pitting corrosion.

It should be mentioned that the structure of magnesium alloys mainly affects the course and rate of the degradation process. The analysis of corrosion tests results and studies of degradation products on the surface of the amorphous Mg64Zn32Ca4 alloy allow to distinguish and link the probable stages of the degradation process for the tested alloy in selected micro-areas, which include [59]:


It should be noted, that the specified steps are not consecutive, but may occur simultaneously during the immersion of the amorphous Mg64Zn32Ca4 alloy. Therefore, the degradation of the amorphous Mg64Zn32Ca4 alloy can be considered as a total result of the following processes: the release of alloy components and the formation of protective layers. When the sample is immersed in a chloride solution, degradation occurs directly at the surface, due to the rapid release of active Mg and Ca. On the other hand, this results in enrichment of the sample's surface with less active zinc. With the progress of degradation, zinc is oxidized and accumulates in the vicinity of disturbed chlorides, and therefore protects against further progression of degradation [60, 61]. However, the protective layer is not dense enough to completely prevent degradation. Chlorine ions damage the zinc oxide layer. Damage to the protective layer facilitates the transition to the Ca and Mg ion solution. These mechanisms are repeated until the amorphous magnesium alloy has degraded completely [62].
