Cathodic activation and inflammatory species are critical to simulating in vivo Ti-6Al-4V selective dissolution.

In vivo retrievals of metallic orthopedic implants have shown selective dissolution of Ti-6Al-4V, where the vanadium-rich ß phase preferentially corrodes from the surface. This damage, typically observed in crevices, is not directly caused by wear mechanics and the underlying electrochemical mechanism remains poorly understood. Previous studies show that fretting corrosion can cause negative potential drops, resulting in a decrease in surface oxide passivation resistance and the electrochemical generation of reactive oxygen species (ROS) at metallic surfaces. In this study, we combine cathodic activation and hydrogen peroxide to induce selective dissolution in vitro. After a 600 s -1 V hold and 4 h recovery in 20 °C 1 M H<sub>2</sub>O<sub>2</sub> solution, the Ti-6Al-4V ß phase was preferentially dissolved. An initial activation threshold of -0.5 V induced a significant increase in ß dissolution (p = 0.000). Above this threshold, little selective dissolution occurred. In an Arrhenius-like fashion, decreasing solution concentration to 0.1 M required 72 h to generate ß dissolution instead of 4 h at 1 M. Heating 0.1 M solution to body temperature (37 °C) resulted in a decrease in the time needed to replicate a similar level of ß dissolution (>90%). Electrochemical impedance shows that both cathodic activation and inflammatory species are necessary to induce selective dissolution, where the combinatorial effect causes a significant drop in oxide passivation resistance from 10<sup>6</sup> to 10<sup>2</sup> (p = 0.000). STATEMENT OF SIGNIFICANCE: Though hip arthroplasties are considered a successful procedure, revision rates of 2-4% result in tens of thousands of additional surgeries within the United States, subjecting patients to increased risk of complications. Corrosion is associated with implant failure and retrieval studies show that titanium and its alloys can severely corrode in vivo in ways not yet duplicated in vitro. Here, we reproduce selective dissolution of Ti-6Al-4V ß phase simulating key characteristics of in vivo degradation observed in orthopedic retrievals. We establish both cathodically activated corrosion, a relatively unexplored concept, and the presence of inflammatory species as prerequisites, furthering our understanding of this clinically relevant damage mode. We introduce an Arrhenius-based approach to assess the concentration-temperature-time interactions present.

Corrosion properties of low carbon CoCrMo and additively manufactured CoCr alloys for dental applications.

OBJECTIVES: Additive manufacturing (AM) is being applied to metallic biomaterials and dental alloys, including CoCrMo. CoCrMo mechanical properties and corrosion resistance are vital to the structural integrity of implants and dental appliances. The goal of this work is to assess the resistivity of AM cobalt chromium alloys by comparing them with traditional CoCrMo, regarding electrochemical properties resulting from microstructural and oxide film differences. METHODS: In this work, selective laser melting (SLM), was used to manufacture CoCrMoW. The corrosion characteristics of AM alloy were compared to that of wrought LC CoCrMo (ASTM F-1537) in both phosphate buffered saline (PBS) and PBS with 10 mM H2O2 to simulate increased inflammatory conditions. Anodic polarization and electrochemical impedance spectroscopy (EIS) were performed. RESULTS: Both alloys were substantially similar in corrosion behavior in both solutions. They exhibited changes with the different solutions. Polarization resistances were statistically lower (RpAM = 1.4 MΩcm2 (PBS) vs. 0.72 MΩcm2 (H2O2), RpLC = 1.86 MΩcm2 (PBS) vs. 0.55 MΩcm2 (H2O2)), and open circuit potentials (OCP's) were statistically higher in 10 mM H2O2 for both alloys (0.20 V (in H2O2) vs. - 0.09 V in PBS). Chemistry variations were revealed by the corrosion tests indicating that wrought LC CoCrMo retained its casting-based chemical heterogeneity, while AM CoCrMoW had sub-cell structures within the solidified grains. SIGNIFICANCE: As novel production methods like AM arise, it is necessary to understand any microstructural differences that may diminish the corrosion resistance properties. AM CoCrMoW alloys hold significant promise for use in dentistry where complex geometries are required.

Synthetic periprosthetic synovial fluid development for in vitro cell-tribocorrosion testing using the Taguchi array approach.

Synovial fluid is dynamic in vivo with biological components changing in ratio and size depending on the health of the joint space, making it difficult to model in vitro. Previous efforts to develop synthetic synovial fluid have typically focused on single organic-tribological interactions with implant surfaces, thus ignoring interplay between multiple solution components. Using a Taguchi orthogonal array, we were able to isolate the individual effects of five independent synovial fluid composition variables: ratios of (1) hyaluronic acid to phospholipids (HA:PL) and (2) albumin to globulin (A:G), and concentrations of (3) hydrogen peroxide (H2 O2 ), (4) cobalt (Co2+ ) and (5) chromium (Cr3+ ) ions on macrophage viability and reduced glutathione production, local solution pH and the comprehensive CoCrMo alloy electrochemical response. While no single synovial fluid variable significantly affected the collective response, HA:PL ratio resulted in the largest impact factor (Δ) on 12 of the 13 measured responses with significant effects (p < .05) on the average macrophage survival rate and electrochemical capacitive state of the CoCrMo surface. Cluster analysis separated significant responses from all trials into three groups, corresponding to healthy, mild, or severely inflamed fluids, respectively; with the healthy synovial fluid composition having mid-range HA:PL ratios with no Co2+ ions, and the severely inflamed fluids consisting of low and high HA:PL ratios with H2 O2 and Co2+ ions. By utilizing the Taguchi approach in combination with cluster analysis, we were able to advance our knowledge of complex multivariate synthetic synovial fluids influence on macrophage and electrochemical behavior at the cell-solution-metal interface.

Fretting corrosion of Si3 N4 vs CoCrMo femoral heads on Ti-6Al-V trunnions.

Fretting corrosion at the head-neck taper junction was compared between silicon nitride (Si3 N4 ) and commercially available cobalt chrome (CoCrMo) femoral heads on titanium (Ti-6Al-4V) trunnions. An electrochemical setup was used to capture the fretting currents (characterized by oxide abrasion and repassivation) during cyclic loading. Onset load, pull-off force (disassembly load), short term and long term (1 million cycles) fretting currents were used to compare the fretting corrosion performance between the test group (Si3 N4 /Ti-6Al-4V) and the control group (CoCrMo/Ti-6Al-4V). Incremental cyclic fretting corrosion tests showed that the Si3 N4 /Ti-6Al-4V combination had statistically lower (P < .05) average fretting current of 0.189 µA (SD = 0.114 µA) compared to 0.685 µA (SD = 0.630 µA) for CoCrMo/Ti-6Al-4V for cyclic load of 3200 N. Similarly, for the one million cycle fretting corrosion tests, the Si3 N4 /Ti-6Al-4V couples had statistically lower (P < .05) average current (0.048 µA, SD = 0.025 µA) vs CoCrMo/Ti-6Al-4V couples (0.366 µA, SD = 0.143 µA). The Si3 N4 heads also had higher onset loads (P < .05) for fretting (vs CoCrMo, 2200 N vs 1740 N) indicating a difference in surface contact mechanics between the two groups. Scanning electron microscopy with energy dispersive spectroscopy confirmed material transfer from the trunnions to the heads for both groups tested, and from head to trunnion for the CoCrMo heads. Minimal Si3 N4 transfer was noted. The electrochemical, mechanical, and microscopic inspection data supported the hypothesis that Si3 N4 /Ti-6Al-4Vcombination had better fretting corrosion performance compared to CoCrMo/Ti-6Al-4V.