Corrosion of Silver-Cored Co-20Cr-35Ni-10Mo Composite for Networked Neuroprosthetic System

Abstract

Corrosion behavior of a silver-cored composite wire with Co-based alloy Co-20Cr-35Ni-10Mo outer tube and a silver core in vitro conditions was investigated. The electrochemical effect of the component materials on the overall corrosion behavior of the composite wire was elucidated. The formation of silver chloride corrosion product on the corroding silver core limited ionic transport therefore restricted further dissolution at the silver metal surface. Galvanic action between a Co-20Cr-35Ni-10Mo cathode and a silver anode was examined. Dissolution rate of the silver core in a composite wire obtained from immersion tests in a physiological solution of 9 g/L NaCl at 37°C was approximately 22 µm/y, i.e., 0.28 µg/y silver dissolved for a 37-µm-diameter silver core. Nearly all of the silver dissolved precipitate as silver chloride, i.e., approximately 0.37 µg/y. KEY WORDS: galvanic corrosion, networked neuroprosthetic, silver chloride, silver corrosion ISSN 0010-9312 (print), 1938-159X (online) 11/000053/$5.00+$0.50/0 © 2011, NACE International CORROSION ENGINEERING SECTION 046002-2 CORROSION-Vol. 67, No. 4 FIguRe 9. SEM micrograph of the 2-mm-long silver-cored composite specimens after immersion in 9 g/L NaCl solution at 37°C for different periods of time: (a) 1, (b) 5, and (c) 25 weeks. FIguRe 10. Depth of penetration at the silver core of the 2-mm-long silver-cored composite specimens after different immersion time in 9 g/L NaCl solution at 37°C. FIguRe 11. Calculated and experimental potentiodynamic polarization curves of the silver-cored composite wire in 9 g/L NaCl solution at room temperature. FIguRe 12. Evans diagram of the Co-20Cr-35Ni-10Mo outer tube and the silver core couple in Ringer's solution at 37°C.

The effect of silver chloride formation on the kinetics of silver dissolution in chloride solution

“…The effect of the AgCl corrosion product layer on limiting the corrosion of the silver substrate has been reported [22]. This effect of the AgCl layer is due to the retardation of the ionic transport through the layer.…”

“…Then, the wire was pulled inside the tube to form a cylindrical cavity of 0.5 mm diameter and 1 mm depth. The purpose of this treatment was to help the formation of a uniform AgCl layer rather than a mushroom cap on top of the Ag wire cross section as observed on flush mounted sample [22]. To obtain AgCl layers with different thicknesses for resistance measurement, the specimens were anodically oxidized to pass different amounts of charges up to 10 C/cm 2 at different applied current densities of 0.1, 0.2, 0.5, 1 and 2 mA/cm 2 .…”