Cobalt-chromium-molybdenum alloys are commonly used for biomedical applications such as dental implants and joint implants. Once the material is implanted into the body it is exposed to the corrosiveness of biological fluids and, in some cases, to mechanical loading that can lead to the combined action of wear and corrosion; better known as tribocorrosion. The effect of four different simulated body fluids on the tribocorrosion behaviour of a CoCrMo alloy has been investigated. The degradation of the studied CoCrMo alloys due to tribocorrosion shows a great dependence on the chemical composition of the media. Phosphate-buffered saline (PBS)-based solutions tend to show higher mass loss than the solutions prepared with distilled water. Phosphates present in PBS tend to accumulate on the surface of the alloy and change its tribological performance. In addition, proteins show a lubricating effect reducing the coefficient of friction of the system in the boundary lubrication regime.

CoCrMo alloys used for biomedical applications are often exposed to the combined effect of mechanical wear and electrochemical corrosion. Tribocorrosion tests have been performed in this work to investigate the repassivation kinetics of a HC CoCrMo alloy in four different solutions commonly used to simulate body fluids. The repassivation kinetics are analysed by fitting two different exponential equations. The comparison of the different equations reveals that that a second order exponential equation models the repassivation currents more closely than a first order exponential equation. A repassivation model based on a second order exponential equation is suggested. The repassivation currents are divided in two main phases, a ’coverage’ phase and a ’film thickening’ phase. At the initial stage, when part of the surface is exposed to the corrosive media, higher potentials lead to faster repassivation rates. By contrast, potential does not have a clear effect at the thickening phase, when the material is protected by the oxide film formed on top of the surface.Conclusions The repassivation kinetics of a CoCrMo alloy have been investigated.

Metals are commonly used in various biomedical applications due to their excellent mechanical properties, such as high strength, ductility and toughness.However, the main drawback of metallic biomaterials is their high reactivity, which makes them especially susceptible to corrosion when exposed to aqueous environments such as body fluids. In addition to the corrosiveness of body fluids, metallic biomaterials canbe exposed to mechanical loading and wear. The combined effect of corrosion and wear, also known as tribocorrosion, can lead to enhanced release ofmetallic ions and particles into the surrounding fluids and tissue, which can cause different adverse biological reactions and limit the lifetime of metallicimplants. The overall goal of this study was to investigate the tribocorrosion behaviourof different metallic biomaterials. Firstly, the corrosion and tribocorrosionresistance of a novel candidate material, hafnium, have been studied and compared with titanium. Secondly, the tribocorrosion behaviour of Cobalt-Chromium-Molybdenum alloys has been investigated.The study of the corrosion and tribocorrosion behaviour of hafnium and titanium revealed that both metals form a stable oxide layer that provides highprotection to corrosion. Although the oxide layer can be damaged due to frettingand wear, it rapidly reforms when the mechanical damage ceases. However,hafnium showed a tendency to suffer from pitting, especially when the material was subjected to fretting, which could be a major drawback that mightlimit the application of hafnium in biomedical applications.The behaviour of CoCrMo alloys was also investigated. The analysis of the repassivation kinetics of CoCrMo revealed that a second order exponentialdecay can be used to model the current transient after wear damage. This suggests that the repassivation process can be divided in two phases, first thedepassivated area is rapidly recovered by an oxide layer; then, the thickness of the oxide film grows and stabilises. In addition, it was observed that thechemical composition of the environment can affect not only the corrosion but also the tribological performance of the system. This work has provided an insight into the degradation processes and the parameters affecting the corrosion and tribocorrosion behaviour of different metals in simulated body fluids.

Hafnium is a passive metal with good biocompatibility and osteogenesis, however, little is known about its resistance to wear and corrosion in biological environments. The corrosion and tribocorrosion behavior of hafnium and commercially pure (CP) titanium in simulated body fluids were investigated using electrochemical techniques. Cyclic polarization scans and open circuit potential measurements were performed in 0.9% NaCl solution and 25% bovine calf serum solution to assess the effect of organic species on the corrosion behavior of the metal. A pin-on-plate configuration tribometer and a three electrode electrochemical cell were integrated to investigate the tribocorrosion performance of the studied materials. The results showed that hafnium has good corrosion resistance. The corrosion density currents measured in its passive state were lower than those measured in the case of CP titanium; however, it showed a higher tendency to suffer from localized corrosion, which was more acute when imperfections were present on the surface. The electrochemical breakdown of the oxide layer was retarded in the presence of proteins. Tribocorrosion tests showed that hafnium has the ability to quickly repassivate after the oxide layer was damaged; however, it showed higher volumetric loss than CP titanium in equivalent wear-corrosion conditions

Hafnium has been suggested as an interesting material for biomedical applications due to its good biocompatibility and osteogenesis. However, its behaviour under fretting corrosion conditions, found in applications such as dental and joint implants, has not been studied in depth. A three-electrode electrochemical cell integrated with a ball-on-flat reciprocating tribometer was used to investigate the corrosion of hafnium and commercially pure (CP) titanium in simulated body fluids. An increased susceptibility to pitting corrosion was observed when hafnium was subjected to fretting. Open circuit potential measurements showed a more severe mechanical depassivation due to fretting in the case of CP titanium in comparison to hafnium. In addition, the anodic currents measured during potentiostatic tests were also higher for CP titanium.

Hafnium has been suggested as an interesting candidate for orthopaedic applicationsdue to its good biocompatibility and osteogenesis. However, there is a need to further investigate its resistance to wear and corrosion in biological environments if hafnium is to be considered for such applications.The corrosion and tribocorrosion behaviour of hafnium and commercially pure titanium in simulated body fluids were investigated using electrochemical techniques. A ball-on- plate configuration tribometer and a three electrode electrochemical cell were integrated to investigate the tribocorrosion performance of the studied materials. In addition, the effect of micro-motions on the corrosion resistance of the material was also studied. Cyclic polarisation scans, open circuit potential measurements and potentiostatic tests were performed in 0.9% NaCl solution and 25% bovine calf serum solution in order to assess the effect of organic species on the corrosion behaviour of the metal.The results showed that hafnium has a good corrosion resistance due to its passive state in the studied solutions. A tendency to suffer from localised corrosion was observed, but the electrochemical breakdown of the oxide layer was retarded in the presence of proteins. However, the pit formation was enhanced due to the presence of surface imperfections and when the surface was subjected to micro-motions. Tribocorrosion tests showed that hafnium has the ability to quickly repassivate after the oxide layer was damaged; however, it showed higher volumetric loss than CP titanium in the studied wear-corrosion conditions.This study provides an insight into the potential of hafnium for orthopaedic applications.

Metals have excellent properties, such as high strength, ductility and toughness, which make them the material of choice for many biomedical applications. However, the main drawback of metals is their general tendency to corrode, which is an important factor when they are used as biomaterials due to the corrosive nature of the human body.Titanium and titanium alloys are widely used in biomedical devices due to their excellent corrosion resistance and good biocompatibility. However, one of the disadvantages of titanium is its low wear resistance. Hafnium is a passive metal with a number of interesting properties, such as high ductility and strength, as well as resistance to corrosion and mechanical damage. Previous studies have shown that hafnium has good biocompatibility and osteogenesis. However, the behaviour of hafnium in biological environment has not been studied in great depth. Furthermore, little is known about the resistance of the passive layer under wear-corrosion conditions and the effect of proteins on its corrosion and tribocorrosion behaviour. The overall goal of this study is to assess the potential of hafnium for use in biomedical applications. The aim of this work is to investigate the corrosion resistance of hafnium in simulated body fluids as well as its behaviour in wear corrosion and fretting corrosion conditions.The results showed that hafnium presents a passive state in the presence of proteins and its oxide layer provides high protection to corrosion. In addition, although the passive layer could be disrupted due to wear and fretting, increasing the corrosion of the metal, it was rapidly rebuilt when the damaging ceased. On the other hand, the main drawback of hafnium was its tendency to suffer from localised corrosion. Although the formation of corrosion pits was retarded in the presence of proteins, it was drastically increased when hafnium was scratched or subjected to fretting.