Influence of molybdate ion and pH on the fretting corrosion of a CoCrMo – Titanium alloy couple

Recent 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.

Subjects

CoCrMo alloys

Corrosion

Fretting

Molybdate

Ti6Al4V titanium alloy

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Influence of molybdate ion and pH on the fretting corrosion of a CoCrMo – Titanium alloy couple