Metallic Medical Implants: Electrochemical Characterization of Corrosion Processes

whole range of medical implants is justified by their superior mechanical properties (hardness, stiffness, etc.) compared, for example, to polymers. Other properties like biocompatibility or visibility in X-ray images also can be mentioned. One of their drawbacks is that electrochemical reactions take place on metallic surfaces in the human body. To replicate the real environment as closely as possible, implants should be tested in vivo in animal experimentation; but the possibility of monitoring electrochemical processes is then very limited and not straightforward. (Editor’s Note: See also Hiromoto’s article in this issue.) In vitro reactivity characterizations help to understand the degradation processes (failure risks) and the development of new implant materials. Different macroand microelectrochemical methods allow the investigation of uniform and localized corrosion susceptibility and its relation to material microstructure. A major difference with classical corrosion investigations is the complexity of the physiological media with the presence of proteins (and cells). The influence of solution chemistry on degradation mechanisms, as well as of the specific temperature and atmosphere (amount of O2 and CO2), has to be investigated. Electrochemical methods can also be used for implant surface functionalizing by growing tailored anodic oxide layers or deposition of coatings. There is a whole range of issues related to corrosion processes that needs to be considered and addressed experimentally. Degradation of the implants can be uniform, but for most of the standard metallic materials used (stainless steel, Co based alloys), localized corrosion related to microstructural features is observed. The metallic surface is often covered by a native anodically grown oxide layer that guarantees a uniform corrosion resistance, but does not prevent localized breakdown when an aggressive environment is present. This is typically the case when chloride ions are present in physiological media. Crevice corrosion related to the complex geometries of implants and galvanic coupling between dissimilar materials used can also occur and can be followed in model experimental devices. These two types of corrosion often occur simultaneously because when two different materials are brought into Metallic Medical Implants: Electrochemical Characterization of Corrosion Processes