Intergranular pitting corrosion of CoCrMo biomedical implant alloy Pooja Panigrahi,1 Yifeng Liao,1 Mathew T. Mathew,2 Alfons Fischer,2 Markus A. Wimmer,2 Joshua J. Jacobs,2 Laurence D. Marks1 1Department of Materials Science and Engineering, Northwestern University, Evanston, Illinois 2Department of Orthopedic Surgery, Rush University Medical Center, Chicago, Illinois Received 3 June 2013; revised 2 October 2013; accepted 13 October 2013 Published online 26 December 2013 in Wiley Online Library (wileyonlinelibrary.com). DOI: 10.1002/jbm.b.33067 Abstract: CoCrMo samples of varying microstructure and boundaries and grain boundaries of high energy (high-angle carbon content were electrochemically corroded in vitro and and low lattice coincidence, R11 or higher); grain bounda- examined by scanning electron microscopy and electron ries of lower energy did not appear to corrode. This sug- backscatter diffraction techniques. The rate of corrosion was gests the possibility of grain boundary engineering to 2 minimized (80% reduction from icorr 5 1396 nA/cm to icorr 5 improve the performance of metal implant devices. VC 2013 276 nA/cm2) in high-carbon CoCrMo alloys which displayed Wiley Periodicals, Inc. J Biomed Mater Res Part B: Appl Biomater, a coarser grain structure and partially dissolved second 102B: 850–859, 2014. phases, achieved by solution annealing at higher tempera- tures for longer periods of time. The mechanism of degrada- Key Words: cobalt–chromium (alloy), hip prosthesis, micro- tion was intergranular pitting corrosion, localized at phase structure, total joint replacement, corrosion How to cite this article: Panigrahi P, Liao Y, Mathew MT, Fischer A, Wimmer MA, Jacobs JJ, Marks LD. 2014. Intergranular pit- ting corrosion of CoCrMo biomedical implant alloy. J Biomed Mater Res Part B 2014:102B:850–859. INTRODUCTION passive film (Cr2O3) that forms in any oxidizing environ- A variety of conditions, most notably osteoarthritis and rheu- ment,6 cobalt and chromium cations have been routinely matoid arthritis, may result in a patient’s need for prosthetic detected in serum from patients with implants12–15; molyb- joint implants at hips and knees. Every year, as patients live denum forms water soluble ionic species that are presumed longer and younger patients become candidates for joint to readily leave the body. Complications observed clinically replacement, the demand for hip replacements rises. Hip as a result of corroding CoCrMo at hip joints include hyper- implants are becoming more common with the older popula- sensitivity, metallosis (metallic staining of the surrounding tion, with the number of total hip arthroplasties (THA) per- tissue), excessive periprosthetic fibrosis, muscular necrosis, formed annually in the U.S. expected to grow 174% from and vasculitis (patch inflammation of the wall of small 2005 to 572,000 by 2030.1 blood vessels).8 There are currently minimal ASTM/FDA The CoCrMo alloy has been approved as an implantable regulations concerning the microstructure of CoCrMo joint biomedical material since the 1970s.2 Advantages of using implants, for instance there are no requirements concerning a metal for this application are mainly in the bulk mechan- vol% of precipitates and exact temperatures for thermome- ical properties, which can be predictably modified by vari- chanical processing history. ASTM F1537 does call for a ous processing techniques such as casting, annealing, hot maximum grain size of 56.6 lm, but the effects of grain size forging, and cold working.3–7 Of the different articulating within this range (sub-micron to grain size 5) are not joint interfaces available for hip implants, metal-on-metal detailed with any recommendations with respect to corro- (MoM) is known to have one of the lowest wear rates, sion behavior.16 This study analyzes how processing and compared to systems with polyethylene.2 However, microstructure have a direct effect on the corrosion resist- recently the usage of MoM joints has diminished due to ance of this commonly used biomedical implant material the serious clinical concerns that were reported associated and also delineates that the distribution of grain boundaries with metal ions release to the surrounding tissues and have an important, perhaps critical role with high-energy host body.8 boundaries corroding substantially faster. We note that this One of the major degradation processes that inevitably suggests that classic metallurgical techniques can be used to affect all metallic implants in vivo is corrosion.9–11 While minimize corrosion, although some important unknowns cobalt–chromium is especially corrosion resistant due to the merit further study. Correspondence to: L.D. Marks (e-mail: [email protected]) Contract grant sponsor: National Institutes of Health; contract grant numbers: Grant 1RC2AR058993-01 and National Science Foundation; Grant CMMI 1030703. 850 VC 2013 WILEY PERIODICALS, INC. ORIGINAL RESEARCH REPORT TABLE I. Exact Compositions of CoCrMo Wrought Alloy Specimens Wt% Co Cr Mo C Mn Si Ni Fe High-carbon Balance 27.63 6.04 0.241 0.7 0.66 0.18 0.14 Low-carbon Balance 27.56 5.7 0.034 0.6 0.38 0.55 0.19 MATERIALS AND METHODS A standard protocol was used to conduct the electro- Samples chemical tests. Each test began with a potentiostatic test, Both high-carbon (HC) and low-carbon (LC) samples were applying a constant cathodic potential (20.9 V), which is used in this study, with an emphasis on the former due to an electrochemical cleaning process of the exposed area to its higher current prevalence in the MoM industry.2 Twenty remove the passive film and any contaminants that may cylindrical wrought CoCrMo alloy pins, 12 mm in diameter have adsorbed on the metallic surface.20 This ensured that and 7 mm thick, were obtained from ATI Allvac, US. Fifteen all samples had similar surface conditions prior to data pins were of high-carbon (HC) content and five pins were of acquisition. An open circuit potential test (OCP initial) was low-carbon (LC) content in accordance with ASTM F 1537- run before any corrosion was performed to determine the 16 08. Exact compositions of the wrought alloy pins are given material’s open circuit potential (Eoc). Then, an electro- in Table I. chemical impedance spectroscopy (EIS) test was con- The HC and LC wrought alloy pins were annealed at ducted. The EIS measurements were performed in the solution heat treatment temperatures previously investi- frequency range from 100 kHz to 10 mHz, with an AC sine gated by several authors3,17–19 either for a short (2-h) or wave amplitude of 10 mV applied at its open circuit poten- long (24-h) anneal time. The 10 experimental conditions are tial. Results from this test were used to construct equiva- detailed in Table II, with the HC alloy samples heat-treated lent circuits (ECs) as a Randles circuit of a solution and corroded in triplicate (n 5 3) to ensure statistical reli- resistance Ru in series with a polarization resistance Rp ability of the data. All solution anneals were performed in and non-ideal capacitance (or constant phase element, an air furnace, and any surface oxides forming on the sur- CPE) Cf in parallel. The polarization resistance stems from face were mechanically removed using SiC grinding paper. the kinetics of the half cell reactions controlling the speed The solution-annealed CoCrMo alloy pins were polished of the electrochemical reaction at the interface. When the to a mirror finish (Ra 10 nm) using a mechanical polish- working electrode (CoCrMo) is polarized above or below ing sequence of polycrystalline colloidal abrasive solutions on commercially available Texmet polishing cloths. The microstructure was analyzed following a chemical etch by a TABLE III. Electrochemical Testing Sequence and Parameters 2–5 min immersion in a solution of 50 mL H2O and 50 mL Test Parameter Value HCl with 4 g of K2S2O5 as a reagent. OCP (initial) Total time (s) 1800 Electrochemical corrosion testing Stability (mV/s) 0 Sample period (s) 0.1 Samples were mechanically re-polished to remove any bulk Potentiostatic Initial E (V) vs. Eref 20.9 surface impurities and etching artifacts immediately prior to Initial time (s) 600 electrochemical corrosion testing. Each sample was then Final E (V) 20.9 incorporated into a custom made four-chamber electrochemi- Final time (s) 0 cal cell as the working electrode (WE) with a graphite counter Sample period (s) 1 2 electrode (CE) and saturated calomel electrode (SCE) refer- Limit I (mA/cm )25 Open circuit E (V) 20.232233 ence electrode (RE), connected to a potentiostat monitored Electrochemical DC voltage (V) vs. Eoc 0 by an electrochemical analysis software supplied by Gamry impedance AC voltage (mV ms) 10 Instruments. The corrosion cell was filled with 10 mL of spectroscopy Initial frequency (Hz) 100000 bovine calf serum (BCS) with a protein content of 30 g/L) Final frequency (Hz) 0.005 buffered to a basic pH 7.4 solution, and placed in a hot water Points/decade 10 bath maintained at 37C to mimic physiological conditions. Open circuit E (V) 20.367033 Cyclic polarization Initial E (V) vs. Eref 20.8 (potentiodynamic) Apex E (V) vs. Eref 1.8 TABLE II. Solution-Annealing Heat Treatments Performed on Final E (V) vs. Eref 20.8 CoCrMo Wrought Alloy Forward scan (mV/s) 2 Samples Temperature (C) Time Annealed (h) Sample period (s) 0.5 Reverse scan (mV/s) 2 1 LC, 3 HC – – Apex I (mA/cm2)25 1 LC, 3 HC 1150 2 Open circuit E (V) 20.367033 1 LC, 3 HC 1150 24 OCP (after Total time (s) 1800 1 LC, 3 HC 1230 2 polarization) Stability (mV/s) 0 1 LC, 3 HC 1230 24 Sample period (s) 0.1 JOURNAL OF BIOMEDICAL MATERIALS RESEARCH B: APPLIED BIOMATERIALS | MAY 2014 VOL 102B, ISSUE 4 851 TABLE IV.