Corrosion behaviour of medical CoCr alloy after nitrogen plasma immersion ion implantation

Abstract

Surface treatment of medical CoCr alloys L605 by nitrogen plasma immersion ion implantation (PIII) leads to the formation of a hard and wear resistant surface layer, consisting of nitrogen in solid solution. However, a detailed investigation of the corrosion properties by potentiodynamic polarization and electrochemical impedance spectroscopy shows that even at processing temperatures of 350 °C, where no CrN precipitates are observed, no complete passivation of the surface by formation of a protective Cr₂O₃ layer is possible leading to enhanced corrosion rates further increasing with increasing PIII processing temperature. It is postulated that the enhanced affinity of chromium for nitrogen leads to a reduced mobility inside the alloy, thus prohibiting a timely surface passivation. Nevertheless, a surface modification where a moderate decrease in corrosion resistance coupled with a significant reduction in generated wear particles should be feasible for biomedical applications.

Introduction

Advances in metallic biomaterials have improved considerably the quality of life and life expectancy over the last fifty years, with applications ranging from endoprostheses, fracture fixation devices to intravascular implants [1]. Besides a beneficial combination of mechanical strength and ductility, high yield strength, low to moderate elastic modulus, excellent corrosion properties are required for these materials, with titanium alloys, CoCr, austenitic stainless and NiTi commonly used [2], [3]. However, a persistent low release rate of metallic cations as well as wear particles is still present in these systems. Accordingly, a large amount of literature exists detailing the interaction of these by-products with human and animal tissue, as well as several surface treatment methods to assuage these problems.

Wear particles itself in the immediate vicinity of protheses stimulate macrophages and lymphocytes to produce proinflammatory mediators, which in turn enhance osteoclast formation, thus inducing osteolysis [4], [5], [6]. In addition, macrophages can transform towards osteoclast-like cells capable of resorbing bone after having phagocytosed wear particles [7]. Furthermore, spreading of wear particles through the whole body and accumulation in organs, as spleen, liver and lung with concurrent negative effects have been observed [8], [9], [10]. These processes are observed for different kinds of wear particles, indicating only a minor influence of the chemistry for nanoparticles [11], [12], [13].

In contrast, metallic cations act highly specific when inducing a tissue response. Ni ions induce cell death in endothelial cells by necrosis [14]. However, the release rate depends strongly on the materials chemistry with NiTi itself showing rather low release rates compared to stainless steel or CoCr alloys [15]. For Co²⁺ ions, multiple pathways have been identified depending on the local concentration, including apoptosis, increased expression of HIF-1α for endothelial cells [16] and increased osteoclasts activity [17].

Nevertheless, CoCr alloys are routinely employed in biomedical applications as they combine outstanding yield strength with a moderate elastic modulus of about 200 MPa and rather low wear rates [18]. The formation of a passive surface film on these alloys, consisting of an oxide highly enriched in Cr, with additions of Mo having a further beneficial effect, is responsible for this behaviour [19], [20]. Nevertheless, generation of nanoparticles by mechanical wear processes is still observed in some total hip replacements (THRs) after explantation [21]. Besides, fretting corrosion may lead to the release of toxic ions such as Co, Cr and Ni, with Co predominantly found in the serum [22], [23].

The number of publications reporting on surface modification of CoCr alloys by energetic ions is rather limited. Some information on plasma surface alloying [24], conventional beamline nitrogen ion implantation [25] or high intensity plasma ion nitriding [26], [27] is available. All these experiments, including previous experiments on nitriding by plasma immersion ion implantation (PIII) [28], [29], typically show an increase of the surface hardness to 15–20 GPa in combination with a wear rate reduced by a factor of 10–100. These improved mechanical properties are caused by the insertion of up to 35 at.% nitrogen in the near surface region, extending up to 10 μm below the surface. Below 400–450 °C, a supersaturated expanded austenitic structure with nitrogen on interstitial sites is present while the precipitation of CrN and Cr₂N is observed at higher temperatures, together with a remaining austenitic CoNi-rich phase [24], [30].

However, a selective increase in the cobalt ion release rate, while no significant change in nickel or chromium release was obtained even for samples showing nitrogen in solid solution [31]. In contrast, a threshold for the onset of an increased corrosion current corresponding to the formation of CrN precipitates is observed for nitrogen implantation into austenitic stainless steel [32]. However, no detailed investigation of corrosion properties itself has been published until now. In this study, results on electrochemical investigations of medical CoCr alloy with and without surface modification are presented.