Magnesium and its alloys are a well-explored type of material with a multitude of applications ranging from biomedical prosthetics to non-biological tools such as automotive components. The use of magnesium and its alloys are highly desired for such applications mainly because magnesium is lightweight and possesses a high strength to weight ratio, which reduces the amount of energy required for the operation of an apparatus. In particular, the biomedical industry uses magnesium as orthopedic implants because of its strength properties that are similar to organic bone structures. Additionally, the highly corrosive or degrading nature of magnesium makes it suitable for degradable implants or medical devices. Cast magnesium alloys are also used as components in modern engines and automobiles, as magnesium's lightweight and high strength properties permit for faster automotive speeds, acceleration, and reduced energy consumption. Magnesium produces a quasi-passive hydroxide film that offers little to no inhibition of corrosion processes. Although the degree of film passivity can be increased through metallurgical techniques like alloying, the highly oxidizing nature of magnesium remains the single most important challenge to its widespread use. This chapter provides a detailed explanation of the most successful mechanisms used to control the corrosion of magnesium and its alloys and highlights the benefits and challenges for using them.
Top1. Introduction
Magnesium is a material with great potential for applications ranging from device creation in the biomedical industry to automotive components. The universally attractive feature for the use of magnesium in these industries is its lightweight and high strength to weight ratio. However, magnesium remains a highly combustible and reactive material under physiological conditions. Furthermore, the challenge remains to control the susceptibility and readiness of magnesium to corrode when exposed to other metallic elements or aqueous environments. These are the two main shortcomings of magnesium materials that hinder its widespread use apart from more specific challenges in each industry such as manufacturing techniques. Magnesium has a hexagonal close-packed (HCP) crystal structure (Figure 1). This form in itself subjects the structure of magnesium to be relatively unstable. The HCP stacking structure of magnesium can only allow for approximately 74% of the available crystal structure volume to be filled by the Mg+2 atoms. Thus, the voids in the geometric space of the structure do not permit for extensive atom slippage which leads to the creation of localized internal stresses within the material at impacting grain boundaries (Edgar, 2000). Alternatively, cubic structures like the BCC and FCC arrangements can accommodate atomic slippage because of the misaligned stacking layers. This property is what makes BCC and FCC structuralized materials more ductile and less brittle than HCP materials like magnesium.
Figure 1. Crystal structure of magnesium metals
The Pilling-Bedworth (RPB) ratio is commonly used to determine whether a material will form a protective corrosion passivation layer on its surface. It is defined as the ratio of the volume of the metal oxide, which is produced by the reaction of metal and oxygen, to the volume of consumed metal (Pilling, 1923). The RPB for magnesium is 0.81, which means that its oxide coating layer is too thin and does not provide sufficient protection against its oxidation. Exemplary corrosion resistant materials like aluminum, titanium, and chromium all have RPB values between 1 and 2; meaning that the air formed passive film provides adequate protection against corrosion.
When exposed to physiological conditions, magnesium produces a film of magnesium hydroxide as the dominant form of its protection. This nearly instantaneously film has also been reported to consist mainly of calcium, phosphorus, and sodium when immersed within biological and simulated fluids (Persaud-Sharma D. B., 2013; Keim, 2011). The composition of the film will greatly vary with the nature of the aqueous solution(s) to which magnesium is exposed. In many instances the protective film reduces the extent of corrosion, but, fails to meet acceptable standards for bare metal applications. Thus, the need for enhanced magnesium alloys with surface treatments, coatings, and other processing techniques are needed.