Abstract
In this paper, corrosion and tribocorrosion tests have been conducted on three biomaterials: Ti–6Al–4V and 316L steel that are well-known commercial biomaterials and Ti–10Zr–10Nb–5Ta, a new biomaterial elaborated in a levitation-melting furnace. Tests have been conducted in five different electrolytes: NaCl (9g/l), Ringer's solution, phosphate buffered saline solution (PBS) with and without an addition of bovine serum albumin (BSA). Two amounts of BSA were used: 1 and 5 g/l. In NaCl solution, Ringer's solution and PBS solution, Ti–10Zr–10Nb–5Ta presents the best corrosion resistance. Conversely, in the presence of proteins, 316L steel shows better resistance to corrosion in comparison with Ti–10Zr–10Nb–5Ta and Ti–6Al–4V. During tribocorrosion tests, the surfaces have been submitted to friction against an alumina ball at open circuit potential (OCP). Measurement of wear rate at the end of the tests indicates clearly that 316L steel shows better resistance to material removal in comparison with Ti–10Zr–10Nb–5Ta and Ti–6Al–4V.
Introduction
316L steel, Co–Cr based alloys and titanium alloys are the most used metals for medical implants due to their biocompatibility and their high ability to form passive and protective films.
Depending on the type of use, the implants may suffer corrosion alone (e.g. cranial, chin and jaw implants, interbody fusion cages) or corrosion and wear (e.g. dental implant, knee and hip replacement implants, spinal implants) [1–3].
Understanding mechanisms of degradation (corrosion, wear or both of them) of theses metals needs to replicate the real environment as closely as possible and to conduct tests in electrolytes whose composition is as close as possible to that of the fluid body. Instead of tests conducted in simple saline solution or ringer's solution, it is necessary to add proteins that may play a significant role in corrosion and wear of the metallic implants [4,5].
Proteins can influence corrosion resistance by interacting with: (1) the charged double layer at the interface between the aqueous solution and the metal, (2) the oxide/hydroxide surface layer that leads to modify both composition and structure of the passivating oxide/hydroxide layer and consequently its protective quality.
Action of protein (bovine serum albumin (BSA)) on corrosion resistance of metallic biomaterials has been studied in the past and mixed results have been reported. Indeed, numerous studies have reported that adsorption of proteins on the material surface may lead to increase [6–11] or to reduce corrosion rate [9,12–15] depending on the nature of the materials and the electrolytes used.
In [6] the authors reported an acceleration of corrosion rate of 316L and CoCrMo when tested in bovine calf serum (presence of proteins) in comparison with tests conducted in NaCl solution. In another study it has been pointed out that albumin molecules can decrease the surface potential and accelerate the corrosion process of CoCrMo alloy [7]. In [8] corrosion resistance of CoCrMo has been studied in 0.9% NaCl solution with and without BSA. The authors of this study reported that the corrosion current increases from 2.1 μA in 0.9% NaCl electrolyte to 32 μA in 0.9% NaCl + BSA indicating that addition of BSA enhances the corrosion rate for CoCrMo. Other authors [12] reported that Ti–13Nb–13Zr was more corrosion resistant than Ti–6Al–7Nb which was more corrosion resistant than Ti–6Al–4V in phosphate buffered saline (PBS) solution whereas after protein addition corrosion resistance of Ti–13Nb–13Zr and Ti–6Al–7Nb was reduced and that of Ti–6Al–4V increased.
In [9] electrochemical behaviour of 316L and CoCrMo has been studied in NaCl, NaCl + albumin, PBS, and PBS + albumin solutions. The authors of this work reported that albumin acts as a cathodic inhibitor or accelerating the metallic dissolution depending on solution chemistry and biomaterial. It has also pointed out that immersion time increases the resistance of the passive film in the phosphate-containing solutions while decreasing the resistance in the albumin solution. Conversely, in the case of 316L, no significant change of the passive film properties has been observed.
Corrosion behaviour study of titanium [10] and niobium [11] has been conducted in phosphate buffered saline (PBS) solution with and without BSA. Both studies showed that addition of proteins lowered the open circuit potential (OCP) indicating that the adsorption of proteins on the surface of the two materials leads to reduce the protective quality of the passivating layer that forms in PBS solution.
Other results indicating an enhancement or a decrease in corrosion resistance due to the presence of BSA are summarized in [13]. Beneficial or detrimental effect of proteins depends on the nature of materials and electrolytes. In addition, when the experiments have not been conducted in the same experimental conditions contradictory results may be obtained and reported by the authors.
Proteins can also influence greatly wear resistance of metal surface in aqueous solution by adsorbing and forming a lubricating protective biofilm [6,16]. When materials are submitted to corrosion and wear (tribocorrosion), as in the case for numerous implanted biomaterials, complex mechano-electrochemical reactions and interactions occur between the rubbing surfaces and the aqueous environment. The synergistic effect between corrosion and wear may lead to an important material removal whose extent depends on several factors: chemical composition of the electrolytes, chemical composition and mechanical properties of the materials, surface energy, topography, hydrophilic or hydrophobic properties, applied electrochemical potential (open circuit potential, cathodic or anodic applied potential), wear tests conditions. In [14] the authors have studied the effect of proteins on corrosion rates of 316L stainless steel, pure titanium and Ti–6Al–4V alloy in static and freeting modes. They found that, in static mode, proteins increased the corrosion rate of stainless steel and titanium but did not have any effect on Ti–6Al–4V alloy. Conversely, in the fretting mode proteins decreased the corrosion rate of the stainless steel but did not have any significant effect on the other materials. The obtained results reported above, concerning Ti–6Al–4V, has been confirmed in another freeting study [17] where the authors reported that proteins do affect neither the wear rate nor friction nor wear-accelerated corrosion of the material. Similar results have been reported elsewhere [18,19].
Even though the presence of proteins may have a detrimental effect on corrosion resistance of many metals, their adsorption on the surface may lead to the formation of a protective lubricating layer that reduces friction and wear. Such effect has been reported in previous studies [6,16]. In [6], the authors have pointed out that when submitting 316L and CoCrMo alloys to tribocorrosion tests in NaCl solution with and without proteins, these proteins have a beneficial effect by lubricating (protecting) the contact between the rubbing surfaces but has also a detrimental effect by accelerating corrosion. The authors concluded that the total material degradation was enhanced due to the increased corrosion process.
In the present paper, we conducted a comparative study of corrosion and tribocorrosion behaviour, in various electrolytes, between 316L stainless steel, Ti–6Al–4V and Ti–10Zr–10Nb–5Ta that is a new beta titanium alloy. The aim of this work is to compare corrosion and tribocorrosion behaviour of this new alloy with those of the two former ones and with those reported in the literature.
Materials and methods
Ti–6Al–4V and 316L stainless steel were obtained from Stainless (France), whereas Ti–10Zr–10Nb–5Ta was provided by R&D Consulting & Services (Bucharest, Romania). Vickers hardness values of these materials are as follows: 316L (195 HV), Ti–6Al–4V (410 HV), Ti–10Zr–10Nb–5Ta (425 HV).
Metal samples were first grinded with SiC papers down to grad 1000 and then polished with diamond solution down to 1 μm. An average roughness of Ra = 0.12 µm was obtained. Ultimately, the samples were cleaned in an ultrasonic bath and rinsed with ethanol.
Compositions of the solutions used.
Tribocorrosion behaviour has been investigated at a temperature of 37°C ± 1°C using an apparatus presented in a previous paper [20]. It consists of a teflon cell mounted on a ball-on-disc tribometer. Friction tests may be conducted at a controlled applied electrochemical potential. The samples were submitted to friction against a polycrystalline alumina ball with a diameter of 5 mm under an applied load of 5 N, at open circuit potential (OCP) in order to simulate the materials behaviour when it is used in vivo. The motion was linearly reciprocating at a constant speed with a frequency of 1 Hz which corresponds to a mean speed value of 9 mm.s−1. The electrochemical potential of the specimen (the working electrode) was measured against a saturated calomel electrode (SCE). A platinum wire constituted the counter electrode. Friction coefficient was recorded during sliding and wear rates were deduced from wear tracks surface topography measurements.
Results and discussion
Potentiodynamic polarization curves recorded in NaCl and phosphate buffered saline (PBS) solution + bovine serum albumin (BSA) solution are shown in Figure 1. One can see that the current increases gradually with increasing the applied potential and stabilizes when a protective passive film forms. If the applied potential continues increasing and reaches a critical value, film breakdown occurs and transpassive dissolution takes place.
Potentiodynamic polarization curves of (316L:
), (Ti–6Al–4V:
), (Ti–10Zr–10Nb–5Ta:
) in NaCl solution (A) and PBS + BSA(5) (B). Similar curves were obtained whatever the electrolyte (Table 1).
Passive current densities for the three materials studied in various electrolytes (µA/cm2).
Concerning the influence of proteins, the obtained results indicate that the addition of BSA leads to increase the passive current density indicating a reduce of corrosion resistance of Ti–6Al–4V and Ti–10Zr–10Nb–5Ta alloys (Table 2). The detrimental effect of BSA addition on corrosion resistance obtained in the present study is in good agreement with previous results [10–12]. For 316L steel, the detrimental effect of BSA is still measured, but it is lower in comparison with the two other materials. After an addition of 5 g/l BSA to PBS, the passive current density measured in PBS has been multiplied by 14 for Ti–6Al–4V and by 125 for Ti–10Zr–10Nb–5Ta whereas it has only doubled for the stainless steel. The effect of proteins on corrosion resistance of stainless has been studied in a previous work that reported similar results that ours [21]. In that study, the authors pointed out that proteins were able to increase the release of Fe, Cr, and Ni from 316L stainless steel.
It is worth noting that, for a given material, the effect of the presence of proteins may have a beneficial or detrimental effect on corrosion resistance depending on the exact composition of the electrolyte used. This has been clearly showed in [9] where the authors observed a decrease of corrosion current density in NaCl + BSA solution in comparison with NaCl solution alone whereas in a solution composed with PBS + BSA an increase of corrosion rate has been reported. An enhancement of stainless steel resistance to corrosion in NaCl after addition of BSA, by stopping anodic dissolution of chromium and decreasing iron dissolution, has also been reported in [15].
The results we obtained indicate clearly that electrochemical behaviour of the three materials studied depends greatly on the nature of the electrolyte. In NaCl solution, Ringer's solution and PBS solution, Ti–10Zr–10Nb–5Ta presents the best corrosion resistance as demonstrated by a broad passivating plateau associated with a low passive current density. It is followed by Ti–6Al–4V whereas 316L steel shows the worst electrochemical behaviour. Conversely, in the presence of proteins, 316L steel shows better resistance to corrosion in comparison with Ti–10Zr–10Nb–5Ta and Ti–6Al–4V.
Steady open circuit potential (OCP) for the three materials studied in various electrolytes (mV/SCE).
In order to study the stability of passivating films formed at OCP, their kinetics of re-passivation and their resistance to wear, samples were immersed during one hour in the electrolytes and then submitted to friction. Figure 2 shows the variation of OCP during rubbing. One can see that value of OCP for 316L steel during friction is higher than those measured for Ti–6Al–4V and Ti–10Zr–10Nb–5Ta. In addition, it appears that as soon as rubbing starts, OCP abruptly decreases and rises when friction stops. This behaviour is correlated with the destruction of the passivating film during sliding and its formation again when friction stops. Identical behaviour has been obtained for the five electrolytes used. Figure 3 shows values of the potential jump at the beginning of friction for the three materials studied. Whatever the electrolyte, 316L steel shows the lowest drop of OCP. This is probably because the difference between the electrochemical potential between the bare surface and the passivation surface is lower for 316L steel than for the other two materials. When the passivating film is removed during sliding, a galvanic couple forms between the bare surface (cathode) and the wear track (anode). Consequently, metal dissolution will be as high as the cathode and the anode present an important difference in their electrochemical potentials. This seems to be the case for the titanium-based alloys in comparison with the 316L steel.
Variation of open circuit potential during friction in PBS solution (A) and Ringer's solution (B) for (316L:
Potential jump during friction for 316L steel, Ti–6Al–4V and Ti–10Zr–10Nb–5Ta in various electrolytes: (NaCl:
), (Ti–6Al–4V:
), 
).
), (Ringer's solution:
), (PBS:
), (PBS + BSA(1):
), (PBS + BSA(5):
).
Friction coefficient and wear rate have been also measured. The results show that friction coefficients situate between 0.4 and 0.5 for the three materials in NaCl solution, in Ringer's solution and in PBS solution (Figure 4). Conversely, in presence of PBS + BSA one can obtain a lower value of friction coefficient of 0.2 in the case of 316L steel indicating that the adsorbed film of proteins that forms on the surface has a lubricant effect. This finding is in agreement with those reported previously concerning CoCrMo and 316L steel materials [6,16]. On the contrary, no significant modification has been observed in the values of friction coefficient after BSA addition in the case of Ti–6Al–4V and Ti–10Zr–10Nb–5Ta. Similar result has been obtained in [17] where the authors have studied corrosion and tribocorrosion behaviour of Ti–6Al–4V alloy.
Friction coefficient of 316L steel (A), Ti–6A–4V (B) and Ti–10Zr–10Nb–5Ta (C) in NaCl solution (α), and PBS + BSA (5) solution (β). Tests were conducted with a 1 Hz frequency of rubbing against an alumina ball under a load of 5N.
Figure 5 shows examples of wear tracks after tests conducted in NaCl, PBS and PBS + BSA(1). Wear tracks on Ti–6Al–4V and Ti–10Zr–10Nb–5Ta are wider and deeper than the one obtained on 316L indicating that titanium-based alloys suffered larger material removal than stainless steel. When comparing Ti–6Al–4V and Ti–10Zr–10Nb–5Ta, it appears that the later wears more than the former (Figures 5–6). However, wear rate measurements (Figure 6) show that for a given material and taking into account the error bars, quite similar values of wear rates have been found in the various electrolytes.
Wear tracks recorded on 316L steel (A), Ti–6A–4V (B) and Ti–10Zr–10Nb–5Ta (C) after tribocorrosion tests in NaCl solution (α), PBS (β) and PBS + BSA(1) (γ). Wear rate measured at open circuit potential in various electrolytes after sliding against an alumina ball under an applied load of 5 N in (NaCl:
), (Ringer's solution:
), (PBS:
), 
).
It is worth noting that 316L which is the softer material (hardness = 195 HV) wears less during tribocorrosion tests than Ti–6Al–4V (hardness = 410 HV) and Ti–10Zr–10Nb–5Ta (hardness = 425 HV). This behaviour may be explained by the good protective quality of the passivating film formed on the surface of the stainless steel. This film grant the material better wear resistance as has been reported in [23,24]. The passivating film that forms on the titanium-based alloys are rich in TiO2 that is a weak compound [25,26]. This film rapidly degrades under friction and eliminated as wear debris. The repetition of this mechanism of generation and degradation of the passivating film leads to the progressive consumption of the material and increasingly severe wear.
Conclusions
Corrosion and tribocorrosion tests have been conducted on 316L steel, Ti–6Al–4V and Ti–10Zr–10Nb–5Ta alloys in five different electrolytes: NaCl, Ringer's solution, phosphate buffered saline solution (PBS) with and without an addition of bovine serum albumin (BSA, 1 g/l and 5 g/l). The following is a summary of the results obtained:
Whatever the electrolyte, the corrosion resistance of Ti–6Al–4V and Ti–10Zr–10Nb–5Ta alloys is substantially higher than that of the 316L steel. In NaCl solution, Ringer's solution and PBS solution, Ti–10Zr–10Nb–5Ta presents the best corrosion resistance. It is followed by Ti–6Al–4V whereas 316L steel shows the worst behaviour. Conversely, in the presence of proteins, 316L steel shows better resistance to corrosion in comparison with Ti–10Zr–10Nb–5Ta and Ti–6Al–4V. For Ti–6Al–4V and Ti–10Zr–10Nb–5Ta alloys, in the presence of BSA, one can obtain an important increase of the passive current density indicating a reduce of corrosion resistance. For 316L steel the detrimental effect of BSA addition is still measured, but it is lower in comparison with the two other materials. Friction tests have been conducted on the materials at OCP against an alumina ball and have been followed by measurement of wear rate. The results indicate that, whatever the electrolyte, the wear of 316L is eight times lesser than that of the two other materials. The stainless steel showed the best resistance to material removal in all the solutions used. It is followed by Ti–6Al–4V whereas Ti–10Zr–10Nb–5Ta shows the worst behaviour. The results obtained indicated that the use of Ti–10Zr–10Nb–5Ta alloy is not beneficial as it does not perform better than 316L steel and Ti–6Al–4V against tribocorrosion.
Footnotes
Disclosure statement
No potential conflict of interest was reported by the authors.