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Item type:Publication, Access status: without files , Influence of molybdate ion and pH on the fretting corrosion of a CoCrMo – Titanium alloy coupleRecent findings suggest that during wear CoCrMo alloys develop a protein-rich protective tribochemical film formed by interaction of released molybdate ion with proteins in the lubricant. The purpose of this study was to determine the effect of adding molybdate ions directly to the medium on the fretting corrosion of a high carbon CoCrMo alloy in contact with a Ti6Al4V alloy. We also examined the effect of lowered pH to simulate acidification that may occur in enclosed areas, as within a modular joint. High-carbon CoCrMo cylindrical pins underwent fretting (± 50 μm, 17 N, 60 MPa) against a Ti6Al4V rod in a flat-on-cylinder configuration in a custom-designed fretting corrosion set-up that included a standard three-electrode configuration for electrochemical measurements. Four electrolytes were used, consisting of diluted bovine serum (30 g/L of proteins), with or without added sodium molybdate (32 mM), and at pH 4.5 or 7.6. All the tests were performed potentiostatically at − 0.25 V versus SCE. Total Co, Cr, and Ti release to the media was determined by ICP-MS. The addition of molybdate ion decreased the release of cobalt by 32% and increased the after-to-before ratio of the polarization resistance by 8 × at pH 4.5, but had not effect on these parameters at pH 7.6. Film deposits in the wear scar were more pronounced in the presence of molybdate ion. The addition of molybdate ion to the medium had a significant protective effect for the CoCrMo only at pH 4.5, possibly because the lower pH allowed for more molybdate-induced protein film formation.Publicationtype: Journal Article105 - Some of the metrics are blocked by yourconsent settings
Item type:Publication, Access status: without files , A servoelectric apparatus with potentiostat to study the fretting corrosion of cobalt-chromium-titanium alloy couples(ASTM International, 2015); ; ; ; The study of fretting degradation in prosthetic modular junctions requires a combination of tribological testing and electrochemical measurements. The goal of this study was to design a testing apparatus that offers this combination using servoelectric actuator technology because such technology provides ease of installation and handling and is cost-effective. A line-contact configuration that more closely simulates in vivo conditions than the usual point-contact geometry was used. The operation of the apparatus was demonstrated in a study of fretting of a high carbon CoCrMo-Ti-6AI-4V couple. A custom-made fretting corrosion apparatus using a servoelectric universal testing machine (THELKIN, Winterthur, Switzerland) in combination with a standard three-electrode electrochemical system was built. Line contact between the test surfaces was achieved by pressing the flat surfaces of two CoCrMo pins against a vertical Ti-6AI-4V rod attached to the machine actuator. The pins and rod served as the working electrode in an electrochemical cell controlled by a potentiostat. Fretting corrosion conditions were as follows: ±50 μm, 17N normal force, diluted bovine serum (30 g/l proteins) medium at pH 4.5 or 7.6, 5000 cycles test duration, under potentiostatic conditions (-0.25V versus SCE). The servoelectric actuator system was shown not to interfere with the electrochemical measurements. The fretting loops were consistent with a gross slip regime and indicated good system rigidity. The amount of Co and Cr released to the medium was significantly greater at pH 4.5 than at pH 7.6, whereas release of Ti was independent of pH. The reason for the higher wear and corrosion rate of the CoCrMo under acidic conditions appears to be connected with the decreased formation or stability of a chromium-rich protective tribofilm. Significance: A novel fretting corrosion apparatus that goes beyond point contact was constructed and validated to advance the study of fretting degradation in prosthetic modular junctions.Publicationtype: Conference Paper36