All materials are constituted of atoms that are bonded together by interactions. In metals, metallic atoms are closely packed in a crystal structure, and atoms are held together through a nondirectional strong metallic bond. Metals are group of materials with high corrosion resistance, biocompatibility, high wear resistance, and excellent mechanical properties such as good ductility and strength. Due to their crystal structure and strong metallic bonds with superior mechanical properties, they are used frequently as implant material. Metals have been preferred practically completely for load-bearing applications, such as joint arthroplasties and fracture fixation wires, pins, screws, and plates. Although pure metals can be used, metal alloys are preferable. Some characteristics, such as strength and corrosion resistance of metal implants, can be improved when used as a metal alloy by varying the composition or by using different manufacturing processes [7, 8].

Convenient mechanical properties, corrosion resistance, biocompatibility, and reasonable cost are the main considerations in preferring metal alloys for implant use. It is important to know the physical and chemical properties of the different metal alloys used in a surgery as well as their interaction with the host tissue of the human body, to be able to make knowledgeable decisions. Elastic modulus, yield stress, ultimate tensile stress, and fatigue stress are the most important characteristics of a metal implant defining its strength and stiffness. These properties of a metal can be seen from stress-strain curves. The strength characteristics of a metal can be influenced by the grain size, inclusion content, and surface porosity. A metal with a smaller grain size has a higher tensile and fatigue strength compared to the larger grain size. High surface porosity and too much inclusion content will weaken the metals [7, 8].

Extracellular fluid of human body contains various ions such as dissolved oxygen, chloride, and hydroxide. Therefore, the human body with a different ion concentration and pH changes in fluids is a highly corrosive environment for metals when used as implants. Corrosion is degradation of materials’ properties due to interactions with their environments, and corrosion of most metals is inevitable. While primarily associated with metallic materials, all material types are susceptible to degradation. Corrosion weakens the material; also corrosion products that enter into tissues can cause damage to cells. Three types of corrosion are common in implant materials: fatigue, galvanic, and crevice corrosion. Fatigue is the weakening of a material caused by repeatedly applied loads. It is the progressive and localized structural damage that occurs when a material is subjected to cyclic loading. If the loads are above a certain threshold, microscopic cracks will begin to form at the stress concentrators such as the surface and grain interfaces. Eventually a crack will reach a critical size, and the structure will fracture. Galvanic corrosion occurs when two different metals have physical or electrical contact with each other. An electric current is established between two metals that cause degradation. To avoid tragic galvanic corrosion, stainless steels should never be used with cobalt or titanium alloys. Crevice corrosion is a localized form of corrosion occurring in confined spaces, to which the fluid in contact with a metal becomes stagnant, resulting in a local oxygen depletion and decrease in pH. Stainless steel corrodes more readily than other alloys. Approaches available for controlling corrosion include the application of protective coatings to metal surfaces to act as a barrier or alteration of an alloy chemistry to make it more resistant to corrosion and the treatment of the surface of a metal to increase its resistance to corrosion. The chromium and molybdenum content of alloys produces a corrosion-resistant surface layer. Titanium alloys have an oxide passive film layer that provides their corrosion resistance. Nitric acid, by forming an oxide surface layer, is used to make the surface of the implant passive to corrosion [1–4].

The mechanical properties of the metal are important and should satisfy the requirements of the specific application in the body. For instance, when a metal is used to augment a bone, the elastic modulus of the metal should be ideally equivalent to that of the bone. If the elastic modulus of the metal is greater than that of bone, then the load experienced by the bone is reduced due to a phenomenon known as stress shielding. This can cause the bone to remodel to adjust to the lower load and eventually result in the loss of bone quality. Metals are passed through a series of processes to provide materials desired properties such as harder, softer, or durable.

Three material groups dominate biomedical metal implants: stainless steel, cobalt-chromium alloy, and titanium alloys. Other metals used in the biomedical industry include nitinol, tantalum, and magnesium. Nitinol, a nickel-titanium alloy, belongs to the class of shape memory alloys. At low temperature, these alloys can be plastically deformed but return back to their original predeformed shape when exposed to a high temperature. Tantalum (Ta) has been used for making biomedical implants and devices, either in its commercially pure (99.9 %) state or as an alloying element in titanium alloys. Tantalum is well known for its excellent corrosion resistance and biocompatibility because of a stable surface oxide layer. It has also been used as coatings on other metallic devices, such as 316 L stainless steel, to improve the substrate’s corrosion resistance and to enhance biocompatibility. The use of magnesium (Mg) for orthopedic applications dates back nearly half a century. Mg is well known for its light weight and biodegradability. The density, elastic modulus, yield strength, and fracture toughness of Mg are close to that of bone [1–9].