Corrosion Behavior of Solution-Annealed CoCrMo Alloy for Metal-on-Metal (MoM) Hip Joint Application +1Panigrahi, P; 1Liao, Y; 2Mathew, M T; 2Nagelli, C; 2Fischer, A; 2Wimmer, M A; 1Marks, L D

+1Department of Materials Science and Engineering, Northwestern University, Evanston, IL, 2Section of Tribology, Department of Orthopedic Surgery, Rush University Medical Center, Chicago, IL.

PoojaPanigrahi2007@u.northwestern.edu

INTRODUCTION Although the use of CoCrMo alloys in medical implant bearings

has been largely successful, the metal ions released through corrosion and tribocorrosive (synergy of wear and corrosion) processes sometimes result in hypersensitivity, necrosis, and other painful conditions to the patient [1]. The MoM joint replacement industry regulates the chemical composition and hardness of the alloys, but does not require any specific thermal processing history. A thorough understanding of how heat treatments affect the material’s corrosion behavior may be useful in designing a more corrosion-resistant MoM joint implant.

To determine the corrosion behavior of solution annealed CoCrMo wrought alloys, electrochemical testing was conducted on various solution-annealed high carbon and low carbon samples. It was hypothesized that the microstructural changes that arise from solution-annealing [2] would have a beneficial effect on the alloy’s corrosion behavior. METHODS

High carbon and low carbon wrought CoCrMo pins (12 mm dia, 7 mm thickness) were solution-annealed at 1150oC or 1230oC for 2 or 24 hours, followed by water quenching. The wrought alloy pins in the absence of heat treatment were used as a control. Samples were then mechanically polished using colloidal diamond abrasive solutions to a mirror finish (Ra = 10 nm). Electrochemical tests (n = 3) were performed at 37oC with bovine calm serum (BCS) as the electrolyte, and monitored using a potentiostat (Gamry Inc, USA) following a standard protocol [3]. During the initial stabilization period, the open circuit potential (OCP) was determined, followed by an electrochemical impedance spectroscopy (EIS) test at Ecorr and a frequency range of 100 kHz to 0.005 Hz. A cyclic polarization test was conducted from -0.8 V to 1.8 V (vs SCE) at scan rate of 2 mV/sec. The open circuit potential (Eoc) was measured at the end of the test sequence. Impedance data was fit to a Randles equivalent circuit and the corrosion potential (Ecorr) and current density (icorr) were extrapolated from the potentiodynamic curve using the Tafel method.

The solution-annealed pins were microstructurally characterized prior to electrochemical testing by a Hitachi S3400 scanning electron microscope (SEM) and a Zygo white light interferometer. SEM and interferometry were used once more after corrosion to characterize the morphology. RESULTS

SEM micrographs (Fig. 1 a-b) revealed that the solution heat treatments coarsened the microstructural grains, with more dramatic results at the higher temperature and the longer annealing time. Partial dissolution of the intermetallic and carbide second phases, only present in the high carbon alloy, was seen after the 24 hour solution anneals.

Figure 1 – SEM images of high carbon wrought CoCrMo alloy, (a) in

absence of heat treatments and (b) after a 1230oC/24h/WQ solution anneal. The polarization curves for the solution-annealed high carbon

CoCrMo alloy are shown in Fig. 2a. The Cr2O3 passive film is seen to have a similar tendency to corrode across experimental conditions, as seen in the -0.25 V – 0.5 V passive region. A significant reduction in the icorr values (Fig. 2b) shows an improvement in the corrosion resistance from solution-annealing. The polarization resistance (Rp) and double-layer capacitance (Cf) data from EIS models exhibited similar values, confirming that the corrosion kinetics and passive layer formations are not very different.

Figure 2 – (a) Cyclic polarization curves for high carbon CoCrMo alloys,

and (b) current density (icorr) at corrosion potential for wrought and solution-annealed CoCrMo.

SEM micrographs taken after electrochemical testing (Fig. 3 a-c)

revealed that corrosion preferentially targeted phase boundaries and certain grain boundaries in high carbon alloys, resulting in fewer corrosion pits for solution-annealed CoCrMo. Low carbon samples did not exhibit any grain boundary corrosion.

 Figure 3 – SEM images of corroded high carbon wrought (a) CoCrMo, (b)

1150oC/24h/WQ solution-annealed CoCrMo, and (c) 1230oC/24h/WQ solution-annealed CoCrMo. DISCUSSION

The high carbon CoCrMo alloys exhibited a lower rate of corrosion than their low carbon counterparts, confirming that this alloy composition is indeed preferable for medical implants [3, 4]. Solution-annealing the high carbon alloy results in a reduction of grain boundaries due to grain coarsening and dissolution of second phases. These microstructural changes may be responsible for the reduced rate of corrosion, since pitting corrosion targets phase boundaries and some grain boundaries.

Preferential grain boundary corrosion may be due in part to elemental (Cr, C) segregation at the intergranular interfaces, possibly evidenced by the lack of grain boundary corrosion in the low carbon alloy. In the high carbon alloy, the absence of targeted corrosion at certain grain boundaries indicates that this phenomenon may depend on the interfacial energy between adjacent grains, which is greater at higher misorientation angles.

The next step is to obtain orientational image maps of the grain structure and compositional profiles of the grain boundaries to confirm the predicted corrosion behavior. Further studies on solution-annealed CoCrMo are needed to measure the wear properties after heat treatment. SIGNIFICANCE

There is a growing concern about corrosion products at the site of metal-on-metal (MoM) joint implants since they are known to cause adverse reactions in patients’ periprosthetic tissue [1]. This study demonstrates that solution-annealing the wrought CoCrMo alloy to alter the microstructure may decrease the rate of corrosion, ultimately making the material more suitable for biomedical applications.

REFERENCES: [1] Hallab, N et al., J Bone Joint Surg Am 2001;83:428. [2] Clemow, A J T and Daniell, B L, J Biomed Mater Res 1979;13:265-279. [3] Mathew, M T et al., Orthopedics Transactions 2010;35:2263. [4] Montero-Ocampo, C and Martinez, E L, ECS Transactions 2009;19:37. ACKNOWLEDGMENTS: This study was funded by a NIH RC2 (1RC2AR058993-01) grant. The CoCrMo wrought alloy pins were kindly donated by ATI Allvac, US.

(a) (b)

(a) (b) (c)

(a) (b) Reduction in the icorr

Difference in corrosion kinetics

Poster No. 2068 • ORS 2012 Annual Meeting