The Effects of the Inflammatory Species Hypochlorous Acid on the Electrochemical Behavior of Co-Cr-Mo Alloys
Gregory W. Kubacki, Yangping Liu, and Jeremy L. Gilbert
Syracuse Biomaterials Institute, Department of Biomedical and Chemical Engineering, Syracuse University, Syracuse NY, 13244
Introduction: Metallic orthopedic implants mainly consist of cobalt-chromium-molybdenum, titanium, and 316L stainless steel alloys due to their
superior mechanical, corrosion, and biocompatibility properties. A primary focus of these materials has been on the reaction of the biological system to the
wear debris and corrosion products, such as metal cations, generated which may lead to adverse local tissue reactions (ALTR’s). This one-way perspective
of implant-biology interactions may miss a significant element, which is that biological processes (e.g., inflammation) may influence corrosion. Indeed,
inflammatory cells are capable of releasing a host of chemical species that are highly oxidative in nature (reactive oxygen species, ROS) which could affect
the corrosion performance of metallic implants by either damaging the oxide film, or increasing the oxidizing power of the environment. This study
investigated one such chemical, hypochlorous acid (HClO), produced by the leukocyte enzyme myeloperoxidase (MPO) during respiratory burst and its
effect on the electrochemical behavior of Co-Cr-Mo alloys.
Materials and Methods: Disks of Co-Cr-Mo alloy were polished to 600-grit with SiC paper and evaluated in a 3-electrode test setup. Simulated
inflammatory solutions comprised of phosphate buffered saline (PBS) modified with additions of up to 100 mM HClO were used. Acidic solutions,
achieved by addition of HCl to pH 3 into HClO-containing solutions, were also explored. Electrochemical tests performed were open circuit potential (OCP)
versus time for up to 1 hr, anodic polarization testing at 0.167 mV/s scan rate starting -200 mV from OCP. Electrochemical impedance spectroscopy (EIS)
was also performed at OCP using a 10 mV amplitude from 0.01 Hz to 20 kHz. A minimum of three samples per solution condition per experiment were
tested. Statistical analysis was performed using ANOVA techniques with Tukey post-hoc comparisons (P < 0.05).
Results: OCP vs time (Fig. 1) showed significant increases with increasing concentrations of HClO up to 911 mV (vs. Ag/AgCl) at 100 mM. Even
relatively small amounts (e.g., 15 mM) of HClO increased the OCP to over 550 mV. Potentials of this magnitude may be within the range where Cr6+ is
produced in Cr-alloys [1]. The anodic polarization results (Fig. 2) show that small additions of HClO (15 mM) create a significant increase (P < 0.05) in the
corrosion rates (currents) of Co-Cr-Mo alloy, along with increasing OCP. It is interesting to note that not only does the corrosion current (Icorr, the current
where oxidation and reduction reactions are balanced) increase 1 and 3 orders of magnitude over PBS for 15 and 100 mM HClO, respectively, but the shape
of the curve also changes indicating a fundamental change of the structure of the oxide. To further investigate the nature of the oxide layer, EIS (Fig. 3)
showed the reduction of impedance at low frequencies (Figure 3a) with increasing concentration, which in addition to the phase angle data (Figure 3b)
indicates an increasingly defected oxide layer as more HClO is added as the low frequency impedance decreased by up to 3 orders of magnitude (P < 0.05).
Interestingly, acidic shifts of HClO containing PBS solutions to pH 3 resulted in increased surface oxide resistance than in neutral environment for 100 mM
solutions (Fig. 3, P<0.05). Experiments are ongoing to investigate the chemical structure of the oxide using X-ray photoelectron spectroscopy (XPS) after
exposure to these different solution conditions.
Discussion: The results of this study show that additions of HClO to PBS solutions, a simulated inflammatory species, significantly affects the corrosion
behavior of CoCrMo alloys. This results from two effects: loss of the oxide film resistance and increase in the oxidizing potential of the solution. These
combine to raise the OCP and corrosion currents in CoCrMo, even in the absence of mechanical effects. HClO generation by the inflammatory system
through respiratory burst and the presence of MPO producing cells (e.g., neutrophils, leukocytes), may result in large increases in corrosion of CoCrMo
therefore, inflammatory conditions at the metal surface may strongly influence corrosion. What concentrations of HClO in the joint fluid are possible is not
known, but local levels have been cited to be as high at 100 mM [2].
Significance: This study has demonstrated that HClO additions to PBS can have a pronounced effect on the corrosion behavior of CoCrMo. Surface
oxides show lower oxide resistance and greater oxidizing power increasing corrosion rates up to 3 orders of magnitude. This results show that inflammatory
conditions may impact the corrosion of CoCrMo in vivo. This study will form the basis of our understanding on how HClO, which is an aggressive ROS
released naturally by leukocytes, can affect the performance of Co-Cr-Mo implants.
1. Beverskog B, Puigdomenech I. Revised Pourbaix diagrams for chromium at 25-300 oC. Corrosion Science 1997;39:43-57.
2. Winterbourn CC, Hampton MB, Livesey JH, Kettle AJ. Modeling the reactions of superoxide and myeloperoxidase in the neutrophil phagosome:
implications for microbial killing. J Biol Chem 2006;281:39860-69.
Figure 1. Open Circuit
potential (OCP) vs.
concentration. *, **
represent P<0.05.
Figure 2. Anodic polarization results.
* represents P<0.05 comparing Icorr.
Figure 3. Electrochemical impedance
spectroscopy (EIS) of Co-Cr-Mo alloy in
HClO solutions showing impedance (a) and
phase angle (b) vs frequency. *, ** represent
P<0.05.
ORS 2017 Annual Meeting Poster No.1220