Anna Igual Muñoz

Influence of the sliding velocity and the applied potential on the corrosion and wear behavior of HC CoCrMo biomedical alloy in simulated body fluids.

The corrosion and tribocorrosion behavior of an as-cast high carbon CoCrMo alloy immersed in phosphate buffered solution (PBS) and phosphate buffered solution with bovine serum albumin (PBS+BSA) have been analyzed by electrochemical techniques and surface microscopy. After the electrochemical characterization of the alloy in both solutions, the sample was studied tribo-electrochemically (by open circuit potential, OCP measurements, potentiodynamic curves and potentiostatic tests) in a ball-on-disk tribometer rotating in different sliding velocities. The influence of solution chemistry, sliding velocity and applied potential on the corrosion and tribocorrosion behavior of the CoCrMo alloy has been studied. Anodic current density increases with sliding velocity but wear rate does not change at an applied anodic potential; on the other hand, BSA modifies the wear debris behavior (by agglomerating the debris formed by mechanical removal of particles) thus increasing the mechanical wear volume. Under cathodic conditions, cathodic current density also increases during mechanical contact while the wear rate decreases with sliding velocity and BSA lubricates the contact thus reducing the total wear volume with respect to the non-containing BSA solution. The work shows how the electrode potential critically affects the corrosion and tribocorrosion rates by increasing the wear coefficients at applied anodic potentials due to severe wear accelerated corrosion.

Electrochemical Aspects in Biomedical Alloy Characterization: Electrochemical Impedance Spectrosopy

Metals and alloys are widely used as biomedical materials and are essential for orthopaedic implants, bone fixations, artificial joints, external fixations... since they can substitute for the function of hard tissues in orthopaedic. In particular, toughness, elasticity, rigidity, and electrical conductivity are important properties for metallic materials used in medical devices. Because the most important property of biomaterials is safety and biocompatibility, corrosion-resistant materials such as stainless steel, cobalt-chromium-molybdenum alloys and titanium alloys are commonly employed. However, there is still a significant concern associated with biomedical alloys related to the production of metal particles and ions (Fleury et al., 2006; Okazaki & Gotoh, 2005) which can lead to cellular toxicity (Germain et al., 2003; Catelas et al., 2001; Horowitz et al., 1998), metal hypersensitivity (Granchi et al., 2005; Hallab et al., 2000), and chromosomal changes (Masse et al., 2003). Corrosion of orthopedic biomaterials is a complex multifactorial phenomenon that depends on geometric, metallurgical, mechanical and physico-chemical parameters, thus a firm understanding of these factors and their interactions is required in order to comprehend how and why implant materials fail (corrode, degrade). Electrochemical measurements are powerful in situ methods that allow analyzing the interface properties and corrosion behaviour between metal biomaterials (passive oxide film) and the involved body fluids. Within this group of techniques, the Electrochemical Impedance Spectroscopy (EIS) is a useful tool which provides information about the interface, its structure, passive film properties and the reactions taking place on the interface electrolyte/oxide passive film. The impedance spectroscopy is a technique that permits the measurement of uniform corrosion and passive dissolution rates, the elucidation of reaction mechanisms, the characterization of surface films and it is also used for testing coatings or surface modifications. The aim of the present chapter is to describe the EIS technique and its potentiality in the fundamental understanding of the processes occurring at the metal/human body interface in bio-systems. The chapter will be mainly focused on its application in characterizing CoCrMo biomedical alloys.

Influence of caffeine and temperature on corrosion-resistance of CoCrMo alloy

The inhibitory activity of caffeine (1,3,7-trimethyl xanthine) on artificial saliva was studied on a CoCrMo alloy using different electrochemical methods: open circuit potential (OCP), potentiodynamic measurements and electrochemical impedance spectroscopy (EIS). The results show that caffeine produces an inhibitory effect on the anodic currents due to its adsorption on the surface of the alloy. Temperature is another parameter with an influence on corrosion processes, so thermodynamic data were obtained from Arrhenius plots and Langmuir adsorption isotherms. The protective action of caffeine is enhanced at high temperatures at OCP, while for potentiodynamic experiments high temperatures block the inhibitory activity of caffeine and the corrosion rate increases. The process may also be studied by a simulation, determining the functional dependence between OCP, corrosion current density (icorr), corrosion potential (Ecorr), breakdown potential (Ebd) and temperature and amount of caffeine in artificial saliva, for Heraenium® CE. The neural network-based methodology applied in this work provides accurate results, thus proving to be an efficient modelling technique.

Rationalizing the In Vivo Degradation of Metal-on-Metal Artificial Hip Joints Using Tribocorrosion Concepts

In this study, the in vivo degradation of metal-on-metal (MoM) artificial hip joints was assessed based on the present state of the art concepts of tribocorrosion. A recently developed tribocorrosion model, based on the combination of mechanical and corrosion concepts, was used in order to rationalize experimental observations and clinical outcomes. This analysis permitted one to identify and assess the relevance of crucial mechanical (load, velocity), material (hardness, Young’s modulus), chemical (oxidation valence, passivation charge density, electrode potential), and geometrical (head radius, clearance) parameters and synovial fluid viscosity affecting hip joint degradation. Moreover, the tribocorrosion approach taken here shows that in vivo degradation is highly dependent on individual patient features and reveals that in vivo characterization of the corrosion and chemical reactivity of implant materials as well as of synovial fluid properties are needed for predicting degradation of MoM implants.

Effect of Bactericidal Elements Addition on the Microstructure and Mechanical Properties of Ti34Nb Alloy

The functionalization of β-Ti alloys by the addition of small amounts of bactericidal elements is interesting for biomedical applications. Thus, alloying pure titanium with highly biocompatible elements such as Nb or Ta, stabilizes the β phase of the resulting alloy although they can also include difficulties during the fabrication process due to their refractory nature. This work studies the effect of small additions of Ag and Cu (1.5 to 3 wt.%) on the microstructure and mechanical properties of the Ti34Nb (wt.%) alloy processed by powder metallurgy. The blend elemental powders were mixed (30 rpm during 30 min). The samples were compacted at 600 MPa and sintered at 1250 oC during 3 hours. The microstructure was analyzed by X-Ray Diffraction (XRD) and Field Emission Scanning Electron Microscope with X-Ray Spectroscopy (FE-SEM/EDS). The mechanical properties were obtained by bending tests; the elastic modulus was measured by ultrasonic methods and the porosity by Archimedes test. Cu addition generates the appearance of α phase sheets inside the β phase grains. Cu also decreases the open porosity and increases the closed porosity of the material. On the contrary, Ag addition does not influence the stabilization of the β phase and it does not modify the density, thus the total porosity of the resulting material. With respect to the influence of the alloying elements on the elastic modulus (E) of the alloys, the E of the Ti34Nb (76.8 GPa) increases with the Cu addition (92.6 GPa) and decreases with the Ag one (68.9 GPa). Therefore, silver addition, which does not modify the microstructure and slightly decrease the mechanical properties of the Ti34Nb, can be considered a good alloying element to provide antibacterial features to the titanium alloy without losing performance.